Muscle

MUSCLE
PHYSIOLOG
Y
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Definition
Muscle is a kind of tissue comprising of
fibers that are capable of contracting.
Latin – ‘mus’ means mouse.

In the sensorimotor framework in
which we observe man, movement is
second element. It is the movement
which characterizes animal life from one
celled organisms to the most complex.
In the specialization of cells, contractility
has become the particular property of
muscle.

To propel his skeleton man has
639 muscles,
composed of 6 billion muscle fibers.
Each fiber has 1,000 fibrils,
which means,
there are 6,000 billion fibrils at work at
one time or another.

Types Of Muscles
I. Depending on cross striations:
i. Striated muscle
- cross striations
- possess a highly organized
structure capable of relatively
fast contractions.
- e.g., skeletal and cardiac
muscles.

ii. Non striated muscle
- No striations.
- Relatively poorly organized
contractile apparatus; it is able to
make prolonged tonic contractions
of considerable extent.
- Eg., smooth muscles.

II. Depending upon control:
i. Voluntary muscle
- Activities of these muscles are
controlled at will.
- Innervated by somatic nerves.
- Eg., skeletal muscle.

ii. Involuntary muscle
- Activities cannot be controlled at
will.
- Innervated by autonomic nerves.
- Eg., smooth and cardiac muscle

III. Depending upon function:
i. Skeletal muscle
- Are in association with bones
forming the skeletal system
- Voluntary and striated.
- Supplied by somatic nerves.
- Eg., all the muscles of
mastication and facial
expression, tongue, limb
muscles etc.

ii. Cardiac muscle
- Forms the musculature of heart.
- Involuntary and striated.
- Supplied by both sympathetic and
parasympathetic divisions of
autonomic nervous system.

iii. Smooth muscle
- Muscles which are in association with
viscera.
- Involuntary and non-striated.
- Supplied by both sympathetic and
parasympathetic divisions of
autonomic nervous system.
- Forms the main contractile units of the
walls of the various visceral organs.
- Present in numerous viscera.
Eg - trachea, bronchial tube,
- ducts of digestive glands,
- walls of organs of g.i.t.,
- walls of blood vessels, etc.

Muscle Striations Control
Nerve
supply
Skeleta
l
Present Voluntary Somatic
Cardiac Present Involuntary Autonomic
Smooth Absent Involuntary Autonomic

Origin and Development
The various muscle types originate from
distinct regions of the embryo.
The whole musculature of the body,
both striated and unstriated, with the
exception of the musculature of the iris,
originates from the intra-embryonic
mesoderm.

SKELETAL MUSCULATURE
With the exception of certain muscles of
the head and neck, which are
developed from branchial mesenchyme
and the limb muscles, which develop in
situ from the mesenchyme of the limb
buds, all the skeletal muscles are
derived from the myotomes of the
paraxial mesodermal somites.

Muscles of head and neck region are
derived from branchial mesenchyme
mainly and some from myotomes of the
pre-optic and post-optic cranial somites.

CARDIAC MUSCULATURE
Cardiac muscle stems from splancho-
pleuric mesenchyme of the primitive
pericardium.

SMOOTH MUSCULATURE
The non striated muscle of the viscera
also arises in splanchopleuric cells
elsewhere, or those derived from
intermediate mesoderm.
Vascular non striated muscle may,
however, develop at any point from
unspecialized mesenchymal cells.
The non striated muscle of the iris is
said to be derived from cells of
ectodermal origin (near margin of optic
cup).

Similarly, the various myoepitheliocytes
are considered to have an ectodermal
origin, whereas the non striated ciliaris
oculi and arrectores pili stem from local
mesenchymal sources.

Skeletal Muscles
(aka striped, striated, somatic or voluntary)

Most abundant; found attached to the
skeleton.
Constitute some 40% of the body mass in
man. ( Smooth and cardiac muscle constitute
approx. 10%)
Exhibit cross striations under microscope.
Considered to be the best differentiated form
of muscle.
Supplied by somatic (cerebrospinal) nerves,
and therefore, are under voluntary control,
with certain exceptions.
Respond quickly to stimuli, being capable of
rapid contractions, and therefore, get easily
fatigued.

Help in adjusting the individual to external
environment.
Are under the highest nervous control of the
cerebral cortex.
Histologically each muscle fiber is
multinucleated cylindrical cell.
Longest muscle – satorius muscle of thigh –
24 inches.
Shortest muscles – tensor tympani – 2 cms
and the even shorter stapedius muscle.
Eg., muscles of the limbs and body wall, and
branchial muscles.

Parts of a muscle
A. Two ends:
Origin - fixed end during contraction.
Insertion - other end which moves
during contraction.
In the limb muscles, the origin is usually
proximal to insertion. However, the terms,
origin and insertion, are at times
interchangeable, and at other times
difficult to define, as in the intercostal
muscles.

B. Two parts:
Fleshy part is contractile, and is called
the ‘belly’.
Fibrous part is non-contractile and
inelastic. When cord like or rope like, it is
called ‘tendon’; when flattened, it is
called ‘aponeurosis’.

Tendons are discrete, shining white
composed of closely packed collagenous
fibers.
The tendon is a flexible, non-contractile,
immensely strong member of the muscle-
tendon unit.
In certain regions of the body, muscles are
attached by means of aponeuroses. These
are sheet-like tendons (eg., epicranial
aponeurosis of the occipitofrontalis muscle.)
Since the connective tissue elements
(tendon, aponeurosis) are commonly included
in the complete description of a muscle, the
muscular, or contractile, part is often
designated as the belly.

Physiologic Anatomy
The histological units of skeletal muscle
are the muscle fibers, each of which
can be regarded as a single cell
provided with many hundreds of nuclei.
The muscle fibers are arranged parallel
to one another with some connective
tissue in between them.

These epimysium, perimysium, endomysium
are nothing but connective tissue sheaths.
At the end of the muscle fibers, the
collagenous bundles of the endomysium and
perimysium pass directly over into those of a
tough cord of connective tissue called
‘tendon’.

The dense connective tissue fibers of the
tendon become continuous with those of the
periosteum, penetrate the bone, or blend with
the fibers of dermis, joint capsules, or other
connective tissue structures.
It is through such intimate connective tissue
relationships that muscle produces traction
on bone and other movable parts.

Each muscle fiber is cylindrical in shape.
Length – 1.0 – 300 mm. In most muscles,
the fibers extend the entire length of the
muscle. Elsewhere, they may traverse only
part of the length of the muscle.
Diameter – varying from 10 to 100 μm. The
diameter varies in a single muscle.
Almost each muscle fiber is innervated by
only one nerve ending, located near the
middle of the fiber.

The thickness of muscle fiber varies with the
degree of nourishment of the individual. The
growth of the muscle, on systemic exercise, is
due to increase in total fiber volume and
improvement of blood supply and not due to
increase in fiber number.

As told earlier, whole muscles are made up of
bundles of overlapping, interweaving, shorter,
individual muscle fibers.
Each cylindrical muscle fiber is enclosed by a
cell membrane, below the endomysium,
called ‘sarcolemma’.

At the end of the muscle fiber the
sarcolemma is fused with the collagenous
fibers of the tendon.
The flattened nuclei of the muscle fibers lie
peripherally in the zone immediately within
the cell membrane or sarcolemma.

The cytoplasm of the muscle fiber, called
‘sarcoplasm’, has numerous other structures
embedded in it:
Nuclei
Myofibrils
Golgi apparatus
Mitochondria
Sarcoplasmic reticulum
Ribosomes
Glycogen and occasional lipid droplets.

MYOFIBRILS
Each muscle fiber contains several hundred
to several thousand myofibrils. These
myofibrils are fine parallel filaments present in
the sarcoplasm of the muscle cell.

Each myofibril is around 1 μm in diameter
and runs through the entire length of the
muscle fiber.
In longitudinal sections, or surface view, light
microscopic studies of the myofibrils show
that each myofibril consists of a number of
alternating light and dark bands.

The light bands are called isotropic or ‘I’
bands, because if rays of polarized light are
passed through the muscle fiber at this area,
the plane of polarization is rotated slightly i.e.
rays are refracted at same angle.
‘I’ bands – aka ‘J’ bands.

The dark bands are called anisotropic or ‘A’
bands, because if rays of polarized light are
passed through the muscle fiber at this area,
the plane of polarization is strongly rotated
i.e. light rays are refracted at different
directions.
‘A’ bands – aka ‘Q’ bands.

Electron micrograph of human gastrocnemius muscle

In an intact muscle fiber, ‘I’ band and ‘A’ band
of adjacent myofibrils are placed side by side.
Therefore the entire muscle fiber has light
and dark bands and characteristically
appears to have cross striations.

The I bands are bisected transversely by a
thin line called ‘zwischenscheibe’ or ‘Z’
line/band.
Aka Krause’s membrane.
The A band is also bisected by a paler lighter
area called Hensen’s zone or ‘H’ band.
The portion of myofibril between the two ‘Z’
lines is called sarcomere.

SARCOMERE
Sarcomere is the structural and functional
unit of the skeletal muscle.
Each sarcomere extends between two ‘Z’
lines of myofibril and is about 2 – 3 μm long
in resting muscle.
Thus each myofibril contains many
sarcomeres arranged in series throughout the
length of the myofibril.

Electron Microscopic Study of
Sarcomere
Electron microscopy shows each myofibril to
be composed of numerous longitudinally
arranged fine thread like protein filaments
called myofilaments.

Two types of myofilaments are
distinguishable in each sarcomere, viz.,
i. Actin filaments, &
ii. Myosin filaments.
It is these large polymerized protein
molecules that are responsible for muscle
contraction.

Each myofibril contains around 1500 myosin
& 3000 actin filaments.
The actin filaments are the finer ones, about
5 nm (50 ºA) in diameter while myosin
filaments are the thicker ones about 12 nm
(120 ºA) in diameter.

The actin filaments extend from either side of
the ‘Z’ lines, run across ‘I’ band and enter into
‘A’ band upto ‘H’ zone.
Myosin filaments are situated in the ‘A’ band
only.

Thus, I bands represent those regions of the
actin filaments which do not overlap with the
myosin.
The H bands are the middle region of the A
bands into which the actin filaments have not
penetrated.

Another line, the ‘M’ band, lies transversely
across the middle of the H band and close
examination shows this to consist of fine
strands interconnecting adjacent myosin
filaments.
The actin filaments are arranged around the
myosin filaments in a hexagonal pattern.

There are some lateral processes
(projections) or cross bridges arising from
myosin filaments. The enlarged structures
called myosin heads are at the tip of these
bridges. These myosin heads attach
themselves to actin filaments. These heads
pull the actin filaments during contraction of
the muscle by means of a mechanism called
sliding or ratchet mechanism.

The actin and myosin filaments are formed by
protein molecules called the muscle proteins
or the contractile elements of the muscle.

CONTRACTILE ELEMENTS OF THE
MUSCLE
The thick filaments or the myosin filaments of
the sarcomere are formed by myosin
molecules.
The thin actin filaments are formed by three
types of proteins called actin, tropomyosin
and troponin.
These four proteins together are the main
muscle proteins or the contractile elements of
the muscle.

MYOSIN MOLECULE
Each myosin filament consists of about 180
– 200 myosin molecules.
Myosin is a globulin and each myosin
molecule is made of six polypeptide chains –
two heavy and four light chains.

The two heavy chains are twisted against each
other forming a double helix.
These two heavy chains form the tail portion of
the myosin molecule. At one end, each chain
turns to one side and forms the globular head.
To each part of this head are attached two light
chains.

Each myosin head has two two sites for
attachments.
One site is for actin filament and other is for
an ATP molecule.

In the central part of the myosin filament, in
‘H’ zone, the myosin head is absent.
Myosin head has a great deal of affinity for
actin molecule. However in resting state, the
active sites on actin filaments are not
available to myosin head for attachment.

ACTIN MOLECULE
The actin filaments (f – actin) are composed
of globular sub-units of g - actin (5.5 nm
diameter).
These sub-units are attached end to end in
two longitudinal filaments wound around each
other in an extended helix.
The actin molecule has an active site where
the myosin head attaches during contraction.

TROPOMYOSIN B
Another protein, tropomyosin B is associated
with the actin filaments, and lies in the groove
between the two strands of the helix.
In relaxed condition of the muscle, the
tropomyosin molecules cover all the active
sites of F actin molecules.

TROPONIN
At 40 nm intervals on F-actin, yet another
protein, troponin, is present, bound to the
tropomyosin B.
Troponin is constituted by three sub-units.
These are:
i. Troponin I – attached to F actin.
ii. Troponin T – attached to tropomyosin B.
iii. Troponin C – attached to calcium ions.

SARCOTUBULAR SYSTEM
The myofibrils in the sarcoplasm of the
muscle fiber are surrounded by some
important structures, which are made up of
membranes. And these membranous
structures appear as vesicles and tubules in
electron microphotographs. These structures
are together called sarcotubular system.

The sarcotubular system is formed mainly
by two types of structures called:
i. T – tubules
ii. Sarcoplasmic reticulum
(aka L – tubules)

‘T’ tubules
‘T’ tubules are narrow
transverse tubules formed
by the invagination of the
cell membrane
(sarcolemma) of the
muscle fiber.
The transverse tubules
penetrate all the way from
one side of the muscle
fiber to other side. That is,
these tubules penetrate the
muscle cell through and
through.

Because of their origin
from sarcolemma, the
‘T’ tubules open to the
exterior of the muscle
cell.
Thus, these tubules
communicate with
extracellular fluid and
their lumen contains
extracellular fluid.

Sarcoplasmic reticulum –
‘L’ tubules
Sarcoplasmic reticulum is
formed by tubules, which
extend throughout the
sarcoplasm.
These tubules run along
long axes of the muscle
fibers, hence –
‘longitudinal’ tubules or
‘L’ tubules

These tubules form a
closed tubular system
around each myofibril
and do not open to the
exterior like ‘T’ tubules.
Sarcoplasmic reticulum
≈ Endoplasmic
reticulum – ribosomes.

At regular intervals,
throughout the length of
the myofibrils, the ‘L’
tubules dilate to form a pair
of lateral sacs called
terminal cisternae. Each
pair of terminal cisternae is
in close contact with ‘T’
tubule. The ‘T’ tubule along
with the cisternae on either
side is called the triad of
skeletal muscle.
This special organization of
sarcoplasmic reticulum is
extremely important in
muscle contraction.

In human skeletal muscle, the triads are
situated at the junctions between ‘A’ band
and ‘I’ band.
Calcium ions are stored in sarcoplasmic
reticulum and the amount of calcium ions is
more in cisternae.

Functions of Sarcotubular System
Functions of ‘T’ tubules –
The T tubules are responsible for rapid
transmission of impulse in the form of action
potential from sarcolemma to the myofibrils.
When the muscle is stimulated, the action
potential develops in sarcolemma and
spreads through it. As the T tubules open to
the exterior and as these are the continuation
of sarcolemma, the action potential quickly
reaches the interior of the muscle fiber.

Functions of ‘L’ tubules –
The L tubules store a large quantity of
calcium ions. When the action potential
reaches the cisternae of L tubules, these
calcium ions are released into the
sarcoplasm. These calcium ions trigger the
processes involved in the contraction of
muscle.
The process by which the calcium ions cause
contraction of muscle is called excitation
contraction coupling.

Composition of muscle

Composition of muscle
7 5 %
W a t e r
2 0 %
P r o t e in s
5 %
O r g a n ic s u b s t a n c e s o t h e r t h a n p r o t e in s
a n d s o m e in o r g a n ic s u b s t a n c e s .
i. Myosin
ii. Actin
iii. Tropomyosin
iv. Troponin
v. Myogen
vi. Myoglobulin
i. Lipids
ii. Carbohydrates
iii. Nitrogenous
substances
i. Cations –
potassium,
sodium, calcium
and magnesium
ii. Anions –
chloride,
phosphate and
sulfate

The Innervation of Skeletal
Muscle
Each skeletal muscle has both sensory and
motor innervation.
The sensory or afferent neurons carry
information from the muscle to the central
nervous system at both the spinal cord and
the higher center levels.
The type of information carried by the afferent
nerve fibers most often depends on the
sensory nerve endings.

Once the sensory information has been
received and processed by the central
nervous system, regulatory information is
returned to the muscles by way of the motor
or efferent nerve fibers.
The efferent neurons initiate the impulses for
the appropriate function of the specific
muscles that will bring about the desired
motor response.

Each skeletal muscle receives one or more
nerves of supply.
Each nerve contains both motor and sensory
fibers; the motor fibers comprise the large
myelinated efferents of ventral grey column
motor neurons (alpha-efferents) which supply
extrafusal muscle fibers, the smaller
myelinated gamma-efferents which run to the
muscle spindles, and the fine nonmyelinated
autonomic efferents which supply vascular
smooth muscle.

The sensory fibers comprise a range of
myelinated fiber diameters distributed to the
muscle spindles, neurotendinous sensory
endings and terminals in the fasciae, and
non-myelinated pain afferents of uncertain
origin.

The Innervation of Masticatory
Muscles
The motor innervation of the extra-fusal fibers of
the masticatory muscles is by the alpha efferents.
As in other areas of the body, various types of
sensory receptors are located throughout the
tissues that make up the masticatory system.
Specialized sensory receptors provide specific
information. Some receptors are specific for
discomfort and pain. Others provide information
regarding the position and movement of the
mandible and associated oral structures. These
movements and positioning receptors are called
proprioceptors.

Like other systems, the masticatory system
utilizes four major types of sensory receptors
to monitor the status of its structures:
1. The muscle spindles, which are specialized
receptor organs found in the muscle tissue;
2. The Golgi tendon organs, located in the
tendons;
3. The pacinian corpuscles, located in tendons,
joints, periosteum, fascia, and subcutaneous
tissues; and
4. The nociceptors, found generally throughout
all the tissues of the masticatory system.

MUSCLE SPINDLES
Skeletal muscles consist of two types of
muscle fiber: the first is the extrafusal fibers,
which are contractible and make up bulk of
the muscle; the other is the intrafusal fibers,
which are only minutely contractile.
A bundle of intrafusal muscle fibers bound by
a connective tissue sheath is called a muscle
spindle.

Each muscle spindle is formed by about 10
intrafusal fibers. All the intrafusal fibers are
enclosed by a capsule formed by connective tissue.
The muscle spindle has a central bulged portion
and two tapering ends. The intrafusal fibers are
attached to the capsule on either end. The capsule
is attached to either side of extrafusal fibers or the
tendon of the muscle. Thus, the intrafusal fibers are
placed parallel to the extrafusal fibers.

The central portion of the intrafusal fibers
does not contract as it has only few or no
actin and myosin filaments. So, this portion
acts only as a receptor. Only the end portion
of the intrafusal fibers can contract. The
discharge from the gamma motor neurons
causes the contraction of the intrafusal fibers.

Nerve supply to muscle spindle –
The muscle spindle is innervated by both
sensory and motor nerves. It is the only
receptor in the body which has got motor
nerve supply also.
Sensory nerve supply – each muscle spindle
has two types of sensory nerve endings,
which are:
1. Primary sensory nerve ending (type Iα nerve
fiber)
2. Secondary sensory nerve ending (type II nerve
fiber)
Motor nerve supply – the motor nerve fiber
supplying the muscle spindle belongs to
gamma motor neuron.

Functions of muscle spindle
Muscle spindle gives response to change in
the length of the muscle. It has two
functions:
1. It is the receptor organ for stretch reflex.
2. It plays an important role in maintaining the
muscle tone.

THE GOLGI TENDON ORGANS
The Golgi tendon organs are located in the
muscle tendons between the muscle fibers
and their attachment to bone. They occur in
series with the extrafusal muscle fibers and
not in parallel as muscle spindles. Each of
these sensory organs consist of tendinous
fibers surrounded by lymph spaces enclosed
within a fibrous capsule.

Tension on the tendon stimulates the
receptors in the Golgi tendon organ.
Therefore contraction of the muscle
stimulates the organ. Likewise, an overall
stretching of the muscle creates a tension in
the tendon and stimulates the organ.

Nerve supply to GTO
The sensory nerve supply belongs to Ib type.
Functions:
The GTO gives response to the change in the
force or the tension developed in a skeletal
muscle during contraction. It is also receptor
for the Golgi tendon reflex and the lengthening
reaction.

1. Role in forceful contraction
When the tension is increased in the muscle
during powerful contraction the GTO is
stimulated. The impulses discharged from
the endings of branches of Ib sensory nerve
fibers are transmitted to an inhibitory
interneuron at the spinal cord. Now, the
contraction of the muscle is inhibited.

2. Role in Golgi Tendon Reflex
Via the GT reflex the GTOs provide the
nervous system with instantaneous
information on the degree of tension in
each small segment of each muscle.
3. Role in lenghtening reaction
GTO plays a role in lengthening reaction
(aka Clasp knife reflex).

PACINIAN CORPUSCLE
This is a pressure receptor situated in fascia
over the muscle, tendons, joints and
periosteum.

NOCICEPTORS
Generally nociceptors are sensory receptors
that are stimulated by injury. They are located
throughout most of the tissues in the
masticatory system.There are several general
types.
The primary function is to monitor the
condition, position, and the movement of
tissues in the masticatory system. When
conditions that are either potentially harmful
or actually cause injury to tissue these relay
this information to the CNS as sensations of
discomfort or pain.

Once the nerve has entered the muscle it
breaks up into a plexus which runs in the epi-
and peri-mysial septa before passing into the
endomysial spaces around the muscle fibers.
The alpha-efferents then branch and finally
lose their myelin sheaths as they terminate
on a variable number of individual muscle
fibers.
The autonomic fibers ramify in the
endomysium throughout the whole muscle
supplying its vasculature.

The somatic motor axons break up into a
number of branches, each of which
terminates on an individual muscle fiber in the
form of a specialized structure, the
neuromuscular termination or motor end
plate.

MUSCLE
PHYSIOLOG
Y
(contd……..)

Neuromuscular Junction
The junction between the terminal branch of
the nerve fiber and muscle fiber is called
neuromuscular junction.
Each terminal branch of the nerve fiber is
called axon terminal. While approaching
close to the muscle fiber, the axon loses the
myelin sheath. So, the axis cylinder is
exposed. The terminal portion of the axis
cylinder is expanded like a bulb. This is called
motor end plate.

The membrane of the muscle fiber below the
end plate is thickened. And, it invaginates
inside the muscle fiber forming the
depression. This depression is known as
synaptic trough or synaptic gutter.
The motor endplate fits into the trough.

The membrane of the nerve ending is called the
presynaptic membrane. The membrane of the
muscle fiber is called postsynaptic membrane.
The space between these two is called synaptic
cleft. The axon terminal contains mitochondria
and synaptic vesicles. The synaptic vesicles
contain the neurotransmitter substance,
acetylcholine.

The synaptic cleft contains basal lamina. Large
quantity of the enzyme, acetylcholinesterase is
attached to the matrix of basal lamina.
The postsynaptic membrane is thrown into
numerous folds. These folds are called subneural
clefts. The postsynaptic membrane contains the
receptor proteins called nicotinic acetylcholine
receptors.

NEUROMUSCULAR TRANSMISSION
The function of neuromuscular junction is to
transmit the impulses from the nerve to the
muscle.
The impulse from spinal cord to the muscle is
transmitted through the nerve fibers in the
form of action potential. When the action
potential reaches the axon terminal, the
neurotransmitter substance acetylcholine is
released from the vesicles by exocytosis. The
acetylcholine is released into the synaptic
cleft.

The acetylcholine molecules bind with
nicotinic receptors present in the postsynaptic
membrane. This brings about a change in the
electrical potential here, which is called the
end plate potential.
The end plate potential is not action potential
nor is it propagative. But it causes the
development of action potential in the muscle
fiber when the critical level of –60mV is
reached.
Within one millisecond after the release into
the synaptic cleft, the acetylcholine is
destroyed by the enzyme,
acetylcholinesterase.

Motor Unit
A motor unit is a functional division of a
muscle and is defined as a single alpha motor
neuron together with the muscle fibers which
it innervates.
The size of motor unit varies between
muscles, smaller units occurring where
precise control of muscular action is required.

A single motor neuron may innervate only 2 –
3 muscle fibers, as in ciliary muscles (which
precisely controls the lens of the eye).
Conversely one motor neuron may innervate
hundreds of muscle fibers as in large muscles
(e.g., rectus femoris in the leg).
Consequently the force generated by each
motor unit is inversely related to the precision
of control.

There is a similar variation in the number of
muscle fibers per motor neuron within the
muscles of mastication.
The lateral pterygoid muscle has a relatively
low muscle fiber/motor neuron ratio and
therefore is capable of fine adjustments in
length needed to adapt to horizontal changes
in the mandibular position.
By contrast the masseter has a greater
number of motor fibers per motor neuron,
which corresponds to its more gross functions
of providing the force necessary during
mastication.

A muscle can work as a whole or in portions.
This is because any muscle is divided into
numerous motor units.
E.g., when the entire temporalis contracts it
elevates the mandible and the teeth are
brought into contact. If only portions contract
the mandible is moved according to the
direction of those fibers that are activated.
Anterior portion – mandible is raised vertically.
Middle portion – mandible will elevate and retrude
Posterior portion – elevation and slight retrusion.

Physiology

General and Molecular
Mechanism of Muscle
Contraction
The initiation and execution of muscle
contraction occurs in the following sequential
steps:
The decision to contract a muscle is executed by
the CNS in the form of an impulse through an
alpha motor neuron to the specific muscle.
An action potential travels along a motor nerve to
its endings on muscle fibers.

(excitation contraction
coupling)
(muscular contraction)

The action potential
depolarizes the muscle fiber
membrane (sarcolemma)
and rapidly travels deeply
within the muscle fiber (via
the T tubules).
When the action potential
reaches the cisternae of the
sarcoplasmic reticulum (L
tubules) the cisternae are
excited.

Now, the calcium ions
stored in the cisternae are
released into the
sarcoplasm.
The calcium ions from the
sarcoplasm move towards
the actin filaments to
produce the contraction

The loading of troponin
C with calcium ions
exerts a pull on the
tropomyosin molecule
away from the F actin.
A large number of
calcium ions bind with
the troponin C of the
actin filament.

Sliding theory
Due to this movement
of tropomyosin, the
active site of F actin
becomes uncovered
and immediately the
head of myosin gets
attached to it.

Sliding Theory or Ratchet
Theory
This theory explains how the
actin filaments slide over the
myosin filaments forming an
acto-myosin complex during
muscular contraction. This is
also called walk along theory.
Each cross bridge from the
myosin filament has three
components: hinge, arm, head.

After binding with the active
site of F-actin the myosin
head is tilted towards the
arm so that the actin
filament is dragged along
with it. This tilting of head is
called power stroke.
After tilting the head
immediately breaks away
from the active site and
returns to the original
position.

It now combines with a
new active site and again
the tilting movement
occurs.
Thus the head of the cross
bridge bends back and
forth and pulls the actin
filament towards the centre
of the sarcomere.

In this way all the actin
filaments of both the ends
of the sarcomere are
pulled, so the actin
filaments of the opposite
side overlap and form
actomyosin complex
causing contraction.

Changes which take
place in sarcomere:
The length of all the
sarcomeres is reduced
as the ‘Z’ lines come
closer to each other.
The length of the I
bands is reduced since
the actin filaments on
the opposite side
overlap.
The H zone
disappears.
However the length of
A band is not altered.

Relaxation
This occurs when the calcium ions are
pumped back into the sarcoplasmic reticulum.
When calcium ions enter the sarcoplasmic
reticulum, the calcium content decreases
leading to detachment of calcium ions from
the troponin.
This causes the release of myosin from actin,
thus causing relaxation.

FATE OF ACETYLCHOLINE
Within one millisecond after the release
into the synaptic cleft the Ach is destroyed by
the enzyme acetylcholinesterase.
Rapid destruction of Ach has got an important
functional significance i.e. it prevents the
repeated excitation of muscle fiber.

Properties of Skeletal Muscles
Elasticity
Excitability
Contractility
Muscle Tone

Elasticity
The muscle fiber has an inherent property of
some degree of elasticity.
The process of the material returning to its
original shape after being stretched illustrates
elasticity.
However, a relaxed muscle can withstand
only a certain amount of elongation (about
6/10th
of its natural length).

Excitability
Excitability means the reaction or response of
a tissue to the irritation or stimulation.
It is a physico-chemical change.
The muscle can be excited both by direct
stimulation and indirect (through its nerve)
stimulation.

A stimulus is an agent or influence or act
which brings about the response in an
excitable tissue.
Types of stimulus:
1. Mechanical (pinching)
2. Electrical (electric shock)
3. Thermal (by applying heated glass rod or
wire)
4. Chemical (by applying chemical substances
like acids)

To excite a tissue, the stimulus must posses
two characters namely:
1. Intensity or strength
2. Duration.
Only if the the stimulus is of sufficient
strength (threshold stimulus) and is applied
for sufficient period will it be able to cause
excitation.

Contractility
The skeletal muscle gives response to a
stimulus in the form of contraction.
The contraction can be defined as the internal
events of the muscle which are manifested by
shortening or development of tension or both.

Individual muscle fibers have no variable
contraction status but they are relaxed or they
are in maximum contraction only on the basis
of the stimulus. This is termed as the “all or
none” law, i.e., if they contract at all, they
contract maximally within the limits imposed
by their initial length and conditions of
loading.

Whole muscles, however, exhibit
considerable gradation in their contraction
and this is achieved by differential activity of
the motor units.
Individual units vary in their twitch frequency,
and the number of units that are active also
fluctuates.
In small contractions only a few units are
operative, but with increasing contraction
more are recruited until many or all are
active.

The sum of these activities results in a steady
contraction of the whole muscle even though
the individual units are twitching repetitively
(but in an asynchronous manner).
Thus what looks steady contraction on
superficial examination will show fine
oscillations on closer examination with
sensitive recording devices.

Two types:
1. Isotonic contraction
(iso = same; tonic = tension)
Tension remains same, length of muscle
changes.
2. Isometric contraction
(iso = same; metric = length)
Length remains same, tension of muscle
increases.
Types of Contraction

The motor unit can carry out only one action
- contraction or shortening.
The entire muscle, however, has three
potential functions.
1. When a large number of motor units in the
muscle are stimulated, contraction or an
overall shortening of the muscle occurs.
This type of shortening under a constant
load is called isotonic contraction.
Simple flexion of arm.
Isotonic contraction occurs in the masseter when
the mandible is elevated, forcing the teeth
through as bolus of food.

2. When a proper number of motor units
contract opposing a given force, the
resultant function of the muscle is to hold or
stabilize the jaw. This contraction without
shortening is called isometric contraction.
Pulling any heavy object.
Isometric contraction occurs in the masseter
when an object is held between the teeth (e.g., a
pipe or pencil).

3. A muscle also can function through
controlled relaxation. When stimulation
of the motor unit is discontinued, the fibers
of the motor unit relax and return to their
normal length. By control of this decrease in
motor unit stimulation, a precise muscle
lengthening can occur that allows smooth
and deliberate movement.
When decelerating a limb segment at the
termination of a movement.
Controlled relaxation is observed in the
masseter when the mouth opens to accept a
new bolus of food during mastication.

In the first type of contraction (isotonic), the
actin-myosin cross bridges are active in
causing a mutual sliding of filaments.
In the second (isometric), the cross bridges
are made and broken repetitively to maintain
length under conditions of external loading.
In the third case (controlled relaxation), the
precise behaviour of the filaments has not
been established, but it is probable that the
cross bridges interact in the same manner
whilst the filaments are sliding apart.

In practice there are, of course, many
combinations of the foregoing three ‘types’ of
contraction, with variations in the conditions
of external loading, initial length, etc.

Simple Muscle Contraction or Twitch
or Curve
The contractile property of the muscle is
studied by using the frog’s gastrocnemius-
sciatic preparation. This is also called
muscle-nerve preparation.
The simple contraction is called simple
muscle twitch and the curve is called simple
muscle curve.

Four points are to be noted in this curve.
These are:
1. Point of stimulation - PS
2. Point of contraction - PC
3. Point of maximum contraction - PMC
4. Point of maximum relaxation - PMR

The above four points divide the entire
simple muscle curve into 3 periods called,
1. Latent period - LP
2. Contraction period - CP
3. Relaxation period - RP

Time duration of different periods:
Latent period : 0.01 second
Contraction period : 0.04 second
Relaxation period : 0.05 second
Total twitch period : 0.10 second

Contraction period is always shorter than
relaxation period. This is because, the
mechanical process of contraction is active
and that of relaxation is passive.

CONTRACTION TIME
The contraction time or the total twitch period
in the simple muscle varies from species to
species.
In warm blooded (homeothermic) animals it is
less than in cold blooded (poikilothermic)
animals.
In the same animal, it varies in different
groups of muscles.
The skeletal muscle fibers are divided into two
types, depending on the basis of contraction
time namely:
Type I (slow) fibers
Type II (fast) fibers

Every muscle of the body is composed of a
mixture of fast and slow muscle fibers, with
still other fibers graded between these two
extremes.
The muscles that react rapidly are composed
mainly of the fast fibers with only small
numbers of slow variety.
E.g., hand muscles and ocular muscles.
Conversely, the muscles that respond slowly
but with prolonged contraction are composed
mainly of slow fibers.
E.g., back muscles.

The difference between these two types of
fibers are the following:
Type I (slow) fibers Type II (fast) fibers
1. Smaller & narrower fibers
Much larger fibers for
greater strength of
contraction
2.
Poorly defined myofibrils,
irregular in size, with thick
Z bands.
Better defined and regular
myofibrils, with thin Z
bands.
3.
Contraction is less
powerful.
Contraction is more
powerful.
4.
Myoglobin content is
more. So it is red (red
muscle).
Myoglobin content is less.
So it is pale (white
muscle).

5.
Sarcoplasmic reticulum is
less extensive.
Extensive sarcoplasmic
reticulum for rapid release
of calcium ions to initiate
contraction
6.
More extensive blood
vessel system and
capillaries to supply extra
amounts of oxygen.
Less extensive blood
supply because oxidative
metabolism is of
secondary importance.
7.
Greatly increased
numbers of mitochondria,
also to support high levels
of oxidative metabolism.
Fewer mitochondria, also
because oxidative
metabolism is secondary.
8.
Response is slow with
long latent period.
Response is rapid with
short latent period.

9.
They are associated with
small, low-tension, slowly
contracting motor units.
They are associated with
large, high-tension,
rapidly contracting units.
10.
These units are very
resistant to fatigue and
richly supplied with
capillaries.
They may be fatigue
resistant (IIA) or fatigue
sensitive (IIB) and posses
either a good (IIA) or a
poor (IIB) capillary
circulation.
11.
These units posses
excellent endurance at low
forces, which makes them
well suited for
maintenance of posture
(of the mandible).
The type IIA units are
thought to be recruited for
maximum effort of long
duration and type IIB for
maximum effort of short
duration.www.indiandentalacademy.com

The significance of these differences lies
partly in their respiratory metabolism; fast
fibers obtain energy primarily by glycolytic
respiration but are quite easily fatigued,
whereas slow fibers also have a well-
developed aerobic metabolism and are highly
resistant to fatigue.

On the basis of histochemical studies it has
been found (Eriksson) that the temporalis, the
masseter, the anterior medial pterygoid, and
the lateral pterygoid muscles are 75%
composed of type I fibers (based on cross
sectional areas).
While this suggests that these muscles are
primarily responsible for the posture of the
mandible, it is quite likely that these fibers
also perform most of the modest work
entailed in mastication of a modern soft diet.

Type IIA fibers are found in significant
proportions (30%) only in digastric muscle.
Type IIB fibers, which are found in all jaw
muscles, are present in the highest proportion
(45%) in the superior posterior temporalis,
posterior medial pterygoid, and anterior
digastric muscles.
The mix of fiber types is quite different
between the digastric and lateral pterygoid
muscles, both of which are jaw depressors,
but not between the two heads of the lateral
pterygoid.

Factors Affecting Force of
Contraction
A. Strength of stimulus,
B. Number of stimuli,
C. Temperature, and
D. Load

A. Effect of Strength of Stimulus
If a series of electrical stimuli are applied by
increasing the strength (voltage of current)
each time, the force of contraction increases.
Thus, curves of different amplitude are
obtained.
The strength of stimuli are of 5 types namely:
Subminimal stimulus - No response
Minimal (threshold) stimulus – Minimum
contraction occurs.
Submaximal stimulus – Force of contraction
increased.
Maximal stimulus – Force of contraction reaches
maximum.
Supramaximal stimulus – No further increase in
force of contraction.

B. Effect of Number of Stimuli
The force of contraction of the muscle is
affected due to application of two stimuli or
more than two (multiple) stimuli.

Effect of Two Successive Stimuli
When two stimuli are applied successively
to a muscle three different effects are
noticed depending upon the interval
between the two stimuli. These effects are:
i. Beneficial effect,
ii. Superposition,
iii. Summation.

1. Beneficial effect
When the second stimulus falls after the
relaxation period of the first curve, two separate
curves are obtained and the force of second
contraction is greater than that of first one.
During the first contraction the increase in
temperature decreases the viscosity of muscle.
So, the force of the second contraction is more.www.indiandentalacademy.com

2. Superposition
If the second stimulus falls during relaxation
period of the first twitch, two curves are
obtained. However, the first curve is
superimposed by the second curve. This is
called superposition or incomplete summation.
Here also, the second curve is bigger than the
first curve because of beneficial effect.www.indiandentalacademy.com

3. Summation
If second stimulus is applied during contraction
period, (or during the second half of the latent
period) the two contractions are summed up and, a
single curve is obtained. This is called summation or
complete summation.
The summation curve is different from the simple
muscle curve because, the amplitude of the
summation curve is greater than that of simple curve
as the two contractions are summed up. The base of
the curve is also broader.www.indiandentalacademy.com

Effect of multiple stimuli
In a muscle-nerve preparation, the multiple
stimuli cause two types of effects depending
upon the frequency of stimuli. These effects
are:
i. Fatigue and
ii. Tetanus.

1. Fatigue -
Fatigue is defined as the decrease in
muscular activity due to repeated stimuli.
When stimuli are applied repeatedly, after
some time, the muscle does not show any
response to the stimulus. This condition is
called fatigue.

Causes of fatigue:
1. Exhaustion of acetylcholine in motor end plate.
2. Accumulation of metabolites like lactic acid and
phosphoric acid.
3. Lack of nutrients like glycogen.
4. Lack of oxygen.
The fatigue is a reversible phenomenon.
The fatigued muscle can be made to
recover after being given rest and nutrition.

2. Tetanus
When multiple stimuli are applied at a higher
frequency, i.e. duration between two stimuli
is very short and thus the stimuli are applied
during contraction period of previous twitch
this leads to the muscle remaining in a
contracted state. This is called tetanus.
Thus, tetanus is defined as apparent
sustained contraction of muscle due to
repeated stimuli of high frequency.

When the frequency of stimuli is not sufficient
to cause tetanus, the fusion of contractions is
not complete. This is called incomplete
tetanus or clonus.

In humans the frequency required to produce
tetanus is
Slow muscle - 30/sec
Fast muscle - 100/sec

Muscle Contractions of Different Force –
Force Summation
Summation means the adding together of
individual twitch contractions to increase the
intensity of overall muscle contraction.
Summation occurs in two ways:
1. Multiple fiber summation (aka quantal
summation) – this occurs by increasing the
number of motor units contracting
simultaneously.
2. Frequency summation (aka temporal
summation) – this occurs by increasing the
frequency of stimuli, and can lead to
tetanization.

C. Effect of Variations in Temperature
If the temperature of the muscle is altered the
force of contraction is also affected.
If warm Ringer’s solution (40°C) is applied
over the muscle-nerve preparation the force
of contraction is increased and all the periods
are shortened because of:
The excitability of muscle is increased.
The chemical processes involved in muscular
contraction are accelerated.
The viscosity of muscle is decreased.

Cooling of the muscle nerve preparation with
Ringer’s solution (10°C) produces the
reverse effect.

D. Effect of Load
The load acting on muscle is of two types:
After load
Free/Fore load
After load is the load which acts on the
muscle after the beginning of muscular
contraction. E.g., lifting any object from the
ground.
Free load is the load which acts on the
muscle freely even before the onset of
contraction of the muscle. E.g., filling water
from a tap by holding the bucket in hand.

Among these two types of loads, the free load
is more advantageous because, in free
loaded condition, the force of contraction as
well as the work done by the muscles are
more than in after loaded condition. This is
because, in free loaded condition the muscle
fibers are stretched and the initial length of
muscle fiber is increased. And as per Frank
Starling’s law, ‘the force of contraction is
directly proportional to the initial length of
muscle fibers within physiological limits’.

Muscle Tone
The muscle fibers always maintain a state of
slight contraction with certain degree of vigor
and tension. This property of muscle is called
tone or tonus.
All skeletal muscles show tonus to some
extent. Anti-gravity muscles like extensors of
lower limb, trunk muscles and neck
muscles show tonus to a greater extent.

MAINTENANCE OF TONE IN SKELETAL
MUSCLE
It is neurogenic and is due to continuous
discharge of impulses from gamma motor
neurons in anterior grey horn of spinal cord
which are in turn controlled by higher centres in
brain.

Role of muscle spindle in maintaining muscle
tone:
Muscle spindle is the organ which helps the
higher centres to maintain the muscle tone in
the skeletal muscles.
The gamma motor neurons innervate the
intrafusal fibers of the muscle spindle.

The impulses from the gamma motor neurons
cause contraction of end portions of intrafusal
fibers. So, the central portion of the intrafusal
fibers is stretched and activated. This leads to
the discharge of impulses from the primary
nerve endings.

The impulses stimulate the alpha motor
neurons of the spinal cord. The alpha motor
neurons in turn, send impulses to extrafusal
fibers and cause contraction of the muscle
fibers.

When the frequency of discharge from
gamma motor neurons increases, the activity
of muscle spindle is increased and the
muscle tone also increases.
Cardiac muscle maintains its tone
myogenically i.e. by itself. The tone is not
under nervous control.

Higher centres of muscle tone maintenance
The hindbrain maintains muscle tone by:
Maintenance of axial tone of the body for the
purpose of standing.
Continuous modification of different degrees of
tone in the different muscles in response to
continuous information from the vestibular
apparatuses for the purpose of maintaining
equilibrium.

Importance of Muscle Tone in TMJ
Stability
In the TMJ the articular surfaces have no
structural attachment or union, yet contact
must be constantly maintained for joint
stability.
Stability of TMJ is maintained by constant
tonicity of the muscles that pull across the
joint, primarily the elevators.
Thus muscle tonus plays an important role in
the mandibular rest position as well as in
resistance to any passive displacement of the
mandible.

Muscles that are in full contraction fatigue
rapidly because of decreased blood flow and
eventual buildup of metabolic byproducts in
the muscle tissues.
By contrast, muscles in tonic contraction
allow proper blood flow to bring needed
metabolic products to the muscle tissues.
Therefore normal muscle tonus does not
create fatigue.

ABNORMALITIES OF MUSCLE TONE
1. Hypertonic state
In hypertonic states the muscle becomes
spastic (rigid or stiff). This condition of the
muscle is called spasticity.
2. Hypotonic state
In hypotonic states, i.e., when the muscle
tone is decreased or lost, the muscle
becomes flaccid and the condition is called
flaccidity.

The Staircase Effect (Treppe)
When a muscle begins to contract after a
long period of rest, its initial strength of
contraction may be as little as half its strength
10 to 50 muscle twitches later. That is the
strength increases to a plateau, a
phenomenon called the staircase effect or
treppe.

Although all the possible causes of the
staircase effect are not yet known it is
believed primarily to be caused increase of
calcium ions in the cytoplasm because of
release of more and more ions from the
sarcoplasmic reticulum with each muscle
action potential and failure to recapture the
ions immediately.

Clinical Significance

Reflex Action
A reflex action is the response resulting from
a stimulus that passes as an impulse along
an afferent neuron to a posterior nerve root or
its cranial equivalent, where it is transmitted
to an efferent neuron leading back to the
skeletal muscle.

Myotatic (Stretch) Reflex
When a muscle is stretched it contracts
reflexly. This is called the stretch reflex or
myotatic reflex.
It is the only mono-synaptic jaw reflex and the
quickest of all.
The muscle spindle plays a crucial role in this
reflex action.

The intrafusal fibers which form the muscle
spindle are situated parallel to the extrafusal
fibers of the skeletal muscle. These fibers are
attached to the tendon of the muscle by
means of capsule.

So when the muscle is stretched, the muscle
spindle is also stretched and stimulated. The
sensory impulses are discharged from muscle
spindle and are transmitted via the primary
and secondary sensory nerve fibers to the
spinal cord.

The sensory nerve fibers end directly on the
alpha motor neurons of the spinal cord. Now,
the impulses from the alpha motor neurons
cause contraction of extrafusal fibers.

The myotatic reflex can be demonstrated by
observing the masseter when a sudden
downward force is applied on the chin. This
sudden downward tap on the chin will cause
the jaw to be reflexly elevated resulting in
tooth contact.

The myotatic reflex occurs without specific
response from the brain and is very important
in determining the resting position of the jaw.
The myotatic reflex is a principal determinant
of muscle tonus in the elevator muscles.

As gravity pulls down on the mandible, the
elevator muscles are passively stretched,
which also creates stretching of the muscle
spindles.
This information is reflexly passed from the
afferent neurons originating in the spindles to
the alpha motor neurons that lead back to the
extrafusal fibers of the elevator muscles.
Thus passive stretching causes a reactive
contraction that relieves the stretch on the
muscle spindle.

This reflex is used in myofunctional
appliances like activator.
The appliance is trimmed loosely and the
patient is conditioned to bite into the
appliance to keep it in position.
When the mandible moves mesially so that
the teeth can engage the appliance, the
elevator muscles are stretched.
The myotatic reflex is activated, the muscles
contract and the forces elicited help in
causing skeletal and dento-alveolar changes.

Nociceptive (Flexor) Reflex
This is a polysynaptic reflex to noxious stimuli
and is therefore considered to be protective.
E.g. reflex in the larger limbs, as in withdrawal
of hand as it touches a hot object.
In the masticatory system this reflex becomes
active when a hard object is suddenly
encountered during mastication.

As the tooth is forced down on the hard
object a noxious stimulus is received by the
tooth and surrounding periodontal structures.

The associated sensory receptors trigger
afferent nerve fibers, which carry the
information to the interneurons in the
trigeminal motor nucleus.

The afferent neurons stimulate both
excitatory and inhibitory interneurons. The
interneurons synapse with the efferent
neurons in the trigeminal spinal tract nucleus.

Inhibitory interneurons synapse with efferent
fibers leading to the elevator muscles. The
message carried is to discontinue contraction.

The excitatory interneurons synapse with the
efferent neurons which innervate the jaw depressing
muscles. The message sent is to contract, which
brings the teeth away from the noxious stimulus.

The action taken during this reflex is more
complicated than the myotatic reflex in that
the activity of several muscle groups must be
co-ordinated to carry out the desired motor
response.
Not only the elevator muscles be inhibited to
prevent further jaw closure on the hard
object, but the jaw muscles must be activated
to bring the teeth away from potential
damage.

Clasp Knife Reflex
Aka Phillipson’s reflex
Autogenic inhibition
If one attempts to flex the spastic limb of a
patient forcibly, resistance is encountered as
soon as the muscle is stretched. This
resistance is, of course, due to hyperactive
reflex contraction of the extensor muscles in
response to stretch. If flexion is forcibly
carried out further, at a point, all resistance to
flexion melts and rigid muscle collapses.
This resembles a spring loaded folding knife
blade. Hence the name.

The receptors responsible for Clasp Knife
Reflex are located in Golgi tendon organ.
When the Golgi tendon organs of a muscle
are stimulated by increased muscle tension,
signals are transmitted into the spinal cord to
cause reflex effects in the respective muscle.
This reflex is entirely inhibitory and provides a
negative feedback mechanism that prevents
the development of too much tension on the
muscle.

When tension on the muscle and, therefore
on the tendon becomes extreme, the
inhibitory effect from the tendon organ can be
so great that it leads to a sudden reaction in
the spinal cord and instantaneous relaxation
of the entire muscle.
This effect is called lengthening reaction.
This is a protective mechanism to prevent
tearing of muscle or avulsion of the tendon
from its attachment to the bone.
Functional significance is to protect the over
load by preventing damaging contractions
against strong stretch forces.

Golgi Tendon Reflex
The GTO is present in the tendon of the muscles.
It gets stimulated by the tension produced in the
muscles.
The main function of GTO is to detect tension.
It has both a dynamic and a static response,
responding intensely when the muscle tension
suddenly increases (the dynamic response) but
within a small fraction of second settling down to a
lower level of steady-state firing that is almost
directly proportional to the muscle tension (the
static response).
Thus the GTOs provide the nervous system with
instantaneous information on the degree of
tension in each small segment of each muscle.www.indiandentalacademy.com

Protective role of reflexes
The myotatic reflex protects the masticatory
system from sudden stretching of the muscle.
The nociceptive reflex protects the teeth and
supporting structures from damage created
by sudden and unusually heavy functional
forces.
The Golgi tendon reflex protects the muscle
from overcontraction by eliciting inhibition
stimuli directly to the muscle that they
monitor.

Reciprocal Innervation
The control of antagonistic muscles is of vital
importance in reflex activity.
As in other muscle systems, each muscle that
supports the mandible and in part controls
function has an antagonist that counteracts
its activity. This is the basis of muscle
balance.
There are certain groups of muscles that
primarily elevate the mandible as well as
others that primarily depress.

Regulation of Muscle Activity
Various conditions of masticatory system
greatly influence mandibular movement and
function.
The sensory receptors in the periodontal
ligament, periosteum, TMJs, tongue and
other soft tissues of the mouth continuously
feed back information, which is processed
and used to direct muscle activity.
Noxious stimuli are reflexly avoided so
movement and function can occur with
minimal injury to the tissues and structure of
masticatory system.

Posture and Equilibrium
As dentists, we tend to think of certain of our
muscles primarily as masticating elements. The
dental student learns first that the masseter,
temporalis, external and internal pterygoid muscles
are “muscles of mastication”. This is only one part of
the picture. These muscles, as well as other facial
muscles with which they are intimately associated,
have other functions that are equally important or
more so. The average person eats three meals a
day, but he swallows all day long, and he breathes
constantly and talks a good part of that time. In
addition to mastication, deglutition, respiration and
speech, there is an even more important role of the
musculature – that of posture.

Subconscious adjustment of tone in different
muscles in regard to every movement is
known as posture.
The significance of posture is to make the
movements smooth and accurate and to keep
the body in equilibrium with the line of gravity.
Posture is not the active movement; it is a
passive movement associated with
redistribution of tone in different groups of
related muscles.

Basic phenomena for maintenance of posture
are:
Muscle tone
Stretch reflex
The muscle tone is present in all the muscles
but, is well pronounced in the extensor
muscles i.e., anti-gravity muscles.
Stretch reflex is normally present and serves
particularly to maintain the body in an upright
position. Such reflexes are therefore more
pronounced in the extensor muscles.

Mandibular Posture
The simplest concept of neural control of
posture of the mandible is its maintenance
against gravity by the stretch reflex in the
mandibular elevators.

This concept of posture of the mandible
involving posture of other structures can be
extended to the head.
Brodie conceived of head position as
determined by a chain of muscles anterior to
the vertebral column opposed by another
chain posterior to the cervical vertebrae. The
anterior chain would include the mandibular
elevators, the muscles connecting the
mandible to the hyoid bone and the muscles
connecting hyoid to the sternum.
Extension of head results in an increase in
freeway space while flexion results in
decrease. Changes in the head position also
affect the antero-posterior position of
mandible in postural position.

The most important application of this field
concept of muscle action relates to its effect
on development of jaws.
The effect of postural activity is more
important than that of synergies such as
mastication and swallowing.
The alteration of mandibular, tongue, hyoid
position in mouth breathing changes the
environment of both the mandible and the
maxilla and alters the way they grow.
The long-face syndrome which is associated
with mouth breathing is a good example.

Mouth Breathing

Since postural position is clearly determined
by muscle contraction, it is important to know
the reflexes accounting for that muscle
contraction.
The usual reflex citied as the basis for
postural position of the mandible is the tonic
stretch reflex of mandibular elevators (i.e.
myotatic reflex).
Because the levator muscles of the mandible
are richly supplied by muscle spindles and
since the monosynaptic reflex arc has been
demonstrated both anatomically and
physiologically, there seems to be little doubt
that the tonic stretch reflex plays a role in
postural position.

Receptors in the TMJ are also involved in
monitoring the position of the mandible.
An example for this is seen in patients in
which the mandible postured away from a
painful joint.

Postural position of the mandible is also
determined by the demands for a patent
upper airway.
An example for this is when nasal breathing is
impossible and the patient is forced to breath
from the mouth, the posture of the mandible
changes along with posture of the tongue and
hyoid complex.
Similar changes in mandibular posture are
seen when the pharyngeal tonsils are
inflamed and enlarged.

Thus, we conclude that postural position of
the mandible is decided by,
Muscle spindles (gravity induced stretch reflex)
TMJ receptors
Demand for patent upper airway.

Postural position is used in orthodontics in
diagnosis and in taking the bite for functional
appliance.
Postural position may be used in the
differential diagnosis of functional
malocclusions from dental or skeletal
malocclusions.

For the mandible to be elevated by the
temporalis, medial pterygoid or masseter
(elevators), the supra-hyoid muscles
(depressors) should relax and lengthen.
The neurogenic control mechanism for these
antagonistic groups is known as reciprocal
innervation.
This phenomenon enables smooth and exact
control of mandibular movements to be
achieved.

Strength of Contraction in
Various Different Positions of
the Mandible
The dentist must know that the greatest
strength of contraction when the muscle
approximates its resting length.
The strength diminishes as muscle shortens
or lengthens beyond the optimal or resting
length.

Activities of the Masticatory
System
Activities of the masticatory system can be
divided into two types:
Functional (which include chewing, speaking,
swallowing)
Parafunctional (which include clenching or
grinding of the teeth.
Parafunctional activity is also known as
muscle hyperactivity.

Factor
Functional
activity
Parafunctional
activity
Forces of tooth
contact
17,200 lb-sec/da
57,600 lb-sec/da,
possibly more
Direction of
applied forces to
teeth
Vertical (well
tolerated)
Horizontal (not well
tolerated)
Mandibular
position
Centric occlusion
(relatively stable)
Eccentric
movements
(relatively unstable)
Type of muscle
contraction
Isotonic
(physiologic)
Isometric
(nonphysiologic)
Influences of
protective
reflexes
Present Absent
Pathologic
Unlikely Very likelywww.indiandentalacademy.com

Parafunctional activity is more likely responsible for
structural breakdown of the masticatory system and
TM disorders.
This is an important concept to remember since
many patients come to the dental office
complaining of functional disturbances such as
difficulty in eating or pain during speaking. It should
be remembered that functional activities often bring
to the patient’s awareness the symptoms that have
been created by parafunctional activities. Therefore
treatment should be primarily directed toward
controlling parafunctional activity.

Role of Musculature in
Deciding Tooth Position
Forces due to tongue musculature and labial
musculature (the buccinator mechanism) are
normally in equilibrium which leads to the
eruption and maintenance of the teeth in a
stable position called the neutral zone.
Even after eruption any change or disruption
in the magnitude, direction, or frequency of
these muscular forces will tend to move the
teeth into a position where the forces are
again in equilibrium.

A common example of an abnormal muscular
pattern is tongue thrusting during swallowing.
In normal swallow the tongue does not invade
the neutral space.
In tongue thrust the tongue is positioned
forward and presses lingually on the maxillary
anterior teeth. As a result proclination and/or
open bite results.

Similarly abnormal
perioral muscle
function results in
malocclusions by
shifting the neutral
zone.
For e.g. a
hyperactive mentalis
(‘golfball chin’)
causes retroclination
of the lower anteriors.
A hypotonic upper lip
is seen in many class
II div. I
malocclusions.

The correction of the tooth positions in such
cases will surely fail if the etiology of the
position is not eliminated i.e. if the abnormal
muscle activity is not controlled.
In fact, force elimination is one of the
treatment principles of functional appliances.

Functional appliances do not act on the teeth
like conventional appliances, using
mechanical elements such as springs,
elastics or ligatures, but rather transmit,
eliminate, or guide natural forces (muscle
activity, growth, or tooth eruption).
In force elimination the abnormal and
restrictive environmental influences are
eliminated, allowing optimal development.
Primarily function is rehabilitated and is
followed by a secondary adaptation in form.

Cerebral palsy
It is the paralysis of or lack of muscular co-
ordination attributed to an intracranial lesion.
As far as the dentist is concerned effects of
this neuromuscular disorder may be seen in
the integrity of occlusion.
Unlike cleft palate, where there are abnormal
structures, the tissues are quite normal but
the patient, because of his comparative lack
of motor control does not know how to use
them properly.

Varying degrees of
abnormal muscular
function may occur in
mastication, deglutition,
respiration and speech.
The uncontrolled or
aberrant activities upset
the muscle balance that
is necessary for
establishment and
maintenance of a
normal occlusion, it is
obvious that abnormal
pressure habit that
result would create
malocclusions.

Torticollis
The far-reaching effects of abnormal muscle
forces are visible also in torticollis, or ‘wry
neck’.
The foreshortening of the
sternocleidomastoid muscle can cause
profound changes in the bony morphology of
the cranium and face.
Torticollis provides an example of the thesis
that in the struggle between muscle and
bone, bone yields.
Bizarre facial asymmetries with uncorrectable
dental malocclusions may be created if this
problem is not treated fairly early.

Left - preoperative torticollis right side. Photograph shows severe
facial asymmetry.
Right – postoperative photograph. Mandibular position has been
improved, although the midline still deviates to the right.

Unfavorable Sequelae of
Malocclusion – Improper or
Abnormal Muscle Habits
Even as abnormal muscle function may be
causative, or atleast contributory in the
formation of a malocclusion, it may also be
resultant.
Even today, in some situations, the line
between ‘cause’ and ‘effect’ is blurred.
In a number of instances a single factor may
operate as both. It is likely that muscle activity
is in this category.

Associated with class II malocclusions
particularly, are certain abnormal habits.
Tongue thrust and sucking occur with greater
frequency in children that have class II
division I.
In these cases is this habit etiologic,
symbiotic or resultant?

BRUXISM
There is a strong co-relation seen clinically
between malocclusion and incidence of night
grinding or bruxism.
Occlusal disharmonies and excessive overbite
are associated most frequently with these
functional aberrations.
Though nervous tension is thought to be a
primary causative factor, a ‘high’ filling, a
malposed tooth, or a deep overbite is
frequently contributory.

IMPROPER DEGLUTITION
Abnormal swallowing is usually corollary to
abnormal muscle function.
But in swallowing additional muscle groups
are involved and the process is more
complex.
In children with cleft palate the bolus of food
is handled differently.
Hence by inductive reasoning we can think of
certain types of malocclusions that could be
attributed atleast partly to abnormal
deglutition.

MOUTH BREATHING
Also intimately associated with abnormal muscle
function is the mouth breathing habit.
Long considered a primary causative factor in the
creation of dental malocclusion, this habit has, in
recent times, been deemed by many as more of an
associated or symbiotic factor and, to a lesser
degree, a result of the inherent malocclusion.
Respiratory ailments, enlarged turbinates, enlarged
tonsils and adenoids, ‘adenoid facies’ appearance.
Orthodontist must assist where he can.
In many cases elimination of excessive overjet and
the establishment of normal perioral muscle function
reactivates the upper lip, makes the lip closure
possible and stimulates normal nasal breathing.

IMPROPER MASTICATION
The inability to chew properly is largely an
associated factor or a result of malocclusion.
Irregular or missing teeth often initiate a
particular pattern of chewing. Most people
favor one side more than the other and
seldom distribute the bolus evenly – unilateral
mastication. The buccal segment that does
not get adequate exercise and massage may
show periodontal abnormalities more readily.
Coupled with improper deglutition, the
combined abnormal function may increase
the severity of the malocclusion.

SPEECH DEFECTS
Certain malocclusions may also cause certain
speech defects, such as improper
pronunciation of certain sounds.

Remodeling of Muscle to
Match Function
All muscles of the body are continually being
remodeled to match the functions that are
required.
Their diameters are altered, their lengths are
altered, their strengths are altered, their
vascular supplies are altered, and even the
type of muscle fibers are altered atleast
slightly.
This remodeling process is quite rapid, within
a few weeks.

Adjustment of Muscle Length
When muscles are stretched to a greater than
normal length, new sarcomeres are added at
the ends of muscle fibers where they attach
to the tendons.
When a muscle remains shortened
continuously to less than its normal length,
sarcomeres at the ends of muscle fibers
disappear.
It is by these two processes the muscles are
continually remodeled to have the appropriate
length for proper muscle contraction.

Following Orthognathic
Surgery
Orthognathic procedures are designed to correct
skeletal imbalances and to create an improved
occlusal relationship.
Procedures usually involve osteotomy,
repositioning of bone, segmentation, etc.
Either of these procedures often causes
concomitant alterations in the associated
musculature and other soft tissues, thereby
altering the previously stable and balanced
functional relationship.
If a homeostatic relationship is not re-established
through muscular alterations and other soft
tissue components, relapse may occur.www.indiandentalacademy.com

Mechanism of Muscle
Adaptation
It is understood that any muscle or muscle
group which has been elongated within
physiological limits will seek to establish
functional homeostasis.
McNamara gives the following four types of
adaptations:
1. Within the central nervous system
2. Within muscle tissue
3. At the muscle bone interface
4. Within or between bony attachments

In the CNS, there occurs adaptations in the
muscle tone and postural activity of the muscles is
altered.
In the muscle tissue, adaptation occurs by
geometric rearrangement of fibers, change in
sarcomere number and length, and change in the
muscle physiology I.e. change in contractile
property and oxidative capacity of the muscle.
At the muscle bone interface, adaptations occur
by the migration of the muscle at point of origin or
insertion by bone remodeling.
Adaptive changes within or between osseous
attachments occurs either by osseous
displacement (i.e. spatial reorientation of a bone
in relation to one or more adjacent bones) or by
localized bony remodeling.www.indiandentalacademy.com

Clinical Implications
When designing an orthognathic surgical
procedure it is better to maintain the original
length of associated musculature.
It is always better to detach a muscle than to
lengthen a muscle. Detaching a muscle at its
origin or insertion at the time of surgery
results in an elimination of the disrupting
forces produced by that muscle.

If a muscle is detached, it is better to
surgically reposition the muscle than to allow
it to reattach spontaneously. This always
prevents overshortening.
The experimental results show that surgical
repositioning of a muscle at a length
approximately or slightly longer than its
original resting length provides a more
predictable post surgical result than if the
muscle is allowed to reattach spontaneously.

EMG
Electromyography is a procedure used for
recording the electrical activity of the muscles.
The resting potential of a muscle fiber is 85 –
90 mV i.e., the membrane of each fiber is
electrically charged with positive outside and
negativity of 85 – 90 mV inside. Upon
receiving a stimulus, there is a reversal of this
potentiality resulting in muscle contraction. This
is called action potential and denotes the
mechanical activity of the muscle.
The electromyograph is a machine which is
used to receive, amplify and record the action
potential during muscle activity.

Electromyogram is a record obtained by such
a procedure.
The action potential is picked up by
electrodes which are of two types:
Surface electrodes – when muscle is superficially
placed.
Needle electrodes – deep muscles e.g. pterygoid
muscles.
Having picked up the action potential with
electrodes, it is recorded either with the help
of a moving pen in the form of a graph or
recorded in the form of sound with the help of
a magnetic tape recorder.

EMG is used to detect abnormal muscle
activity associated with certain forms of
malocclusion.
In severe class II division 1 malocclusion the
upper lip is hypofunctional. Thus during
swallowing, the lower lip extends upwards and
forwards to force the maxilla labially and a strong
mentalis activity is seen. EMG can be used to
study such a condition.
Abnormal buccinator activity in class II, division 1.
Overclosure of jaws is associated with
accentuated temporalis muscle activity.
Children with cerebral palsy.
EMG can be carried out after orthodontic therapy
and orthognathic surgery to see if muscle balance
is achieved.

Retention and Muscle
Physiology
Alfred Coleman (1865) wrote about
restoration of various conditions by muscular
pressure — in other words, the first cause of
relapse. More than a century later, clinicians
still refer to abnormal muscular pressure as a
dominant factor in the cause of relapse.
The musculature school (1922) suggested
that the proper functional muscle balance
was necessary for maintenance of stability.
Theorem 2: Elimination of cause of
malocclusion will prevent recurrence.

Muscle as a Source of
Anchorage
and
Use of Muscular Force for
Space Regaining

Muscle Hypertrophy
When the total mass of the muscle enlarges
due to increase in size of the muscle fibers
only it is called muscle hypertrophy.
Virtually all muscle hypertrophy results from
increase in number of actin and myosin
filaments in each muscle fiber thus causing
enlargement of individual muscle fibers which
is called fiber hypertrophy.
Hypertrophy occurs at much greater extent
when the muscle is simultaneously stretched
during the contractile process (i.e. free loaded
conditions).

Muscle Atrophy
When the total mass of muscle decreases, it
is called muscle atrophy.
When a muscle remains unused for a long
period, the rate of decay of contractile
proteins as well as the number of myofibrils
occur more rapidly than the rate of
replacement which leads to muscle atrophy.

Hyperplasia of Muscle Fibers
Under rare conditions of extreme muscle
force generation, the actual number of
muscle fibers have been observed to
increase.
This increase in fiber number is called fiber
hyperplasia.
Fiber hyperplasia occurs only by a few
percentage points compared to fiber
hypertrophy.

Myasthenia Gravis
In this, grave weakness of the muscle occurs. This
is due to inability of the neuromuscular junction to
transmit impulses from nerve to the muscle.
It is an autoimmune disease. The body develops
antibodies against its own acetylcholine receptors.
These antibodies destroy the acetylcholine
receptors. So, though the Ach release is normal, it
cannot act due to destruction of the receptors.
Symptoms
Muscular contraction is very slow and weak.
When repeated contractions are attempted by the
patient, fatigue occurs quickly.
In severe conditions, there is paralysis of muscles. The
patient dies mostly due to the paralysis of respiratory
muscles. www.indiandentalacademy.com

Conclusion

Those who are enamored of practice without
science are like a pilot who goes into a ship
without rudder or compass and never has any
certainty where he is going. Practice should
always be based upon a sound knowledge of
theory.
- Leonardo da Vinci

References
Text books on anatomy
Gray
Woodburne
Text books of physiology
Guyton
Sembulingam
Text books on orthodontia
Moyers
Profitt
Graber
Graber vanersdall
Graber rakoski petrovic

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