Functions of the muscular system

Copyright © 2004 Pearson Education, Inc., publishing as Benjamin Cummings
Muscle Physiology Dr. Ebneshahidi

Skeletal Muscle
Figure 9.2 (a)

Functions of the muscular system
 1. Locomotion
 2. Vasoconstriction and vasodilatation- constriction and
dilation of blood vessel Walls are the results of smooth muscle
contraction.
 3. Peristalsis – wavelike motion along the digestive tract is
produced by the Smooth muscle.
 4. Cardiac motion
 5. Posture maintenance- contraction of skeletal muscles
maintains body posture and muscle tone.
 6. Heat generation – about 75% of ATP energy used in
muscle contraction is released as heat.

 Striation: only present in skeletal and cardiac muscles. Absent in
smooth muscle.
 Nucleus: smooth and cardiac muscles are uninculcated (one
nucleus per cell), skeletal muscle is multinucleated (several nuclei
per cell ).
 Transverse tubule ( T tubule ): well developed in skeletal and
cardiac muscles to transport calcium. Absent in smooth muscle.
 Intercalated disk: specialized intercellular junction that only
occurs in cardiac muscle.
 Control: skeletal muscle is always under voluntary control‚ with
some exceptions ( the tongue and pili arrector muscles in the
dermis). smooth and cardiac muscles are under involuntary
control.

Innervation: motor unit
 a) a motor nerve and a myofibril from a neuromuscular
junction where gap (called synapse) occurs between the two
structures. at the end of motor nerve‚ neurotransmitter (i.e.
acetylcholine) is stored in synaptic vesicles which will release the
neurotransmitter using exocytosis upon the stimulation of a nerve
impulse. Across the synapse the surface the of myofibril contains
receptors that can bind with the neurotransmitter.

Neuromuscular junction

Skeletal muscle fiber
 1. Each skeletal muscle fiber
is a single muscle cell , which
is the unit of contraction .
 2. Muscle fibers are
cylindrical cells with many
nuclei .
 3. The cell membrane is
called . Sarcolemma, the
cytoplasm is called
sarcoplasm .
 4. The sarcoplasm contains
abundant , parallel thread like
myofibrils , that run in
parallel fashion .


 5. The myofibrils contain 2
kinds of protein filaments .
 a. Thick filaments –
composed of myosin .
 b. Thin filaments – composed
of Actin , troponin and
tropomyosin .
 c. striations are produced by
alternating light and dark
filaments .

Arrangement of the Filaments in a Sarcomere
 Longitudinal section within one sarcomere

Ultrastructure of Myofilaments: Thick Filaments
Figure 9.4 (a)(b)

Ultrastructure of Myofilaments: Thin Filaments
Figure 9.4 (c)

Striation pattern of skeletal muscles: 2 parts
 1. The I bands (The light bands) -
 Extends from the edge of one stack of thick filaments to the edge
of next stack of thick filaments .
 - The I band is composed of thin actin filaments .
 2. The A bands (The dark bands) – composed of thick myosin
filaments , overlapping thin filaments (actin) .

 - myosin filaments are held
together by Z lines (not
attached) .
 - A band consist of a region
Where the thick and thin
filaments overlap , and a
region called central region
(H zone) , consisting of only
thick filaments . In the center
of A band is a dark band
called the M line .
 Sarcomere : The segment of
myofibrils that extends from
one Z line to the next Z line.

 Cross bridge Attachment:
The activated myosin heads
are attracted to the exposed
binding sites on actin and
cross bridge attachment
occurs .
 Power stroke : The sliding
action , which occurs at the
same time for thousands of
actin and myosin molecules
is referred to as the power
stroke .

Muscle Response:
 All – or – none response
 a. if a muscle fiber contracts at all , it will contract completely .
 b. motor units respond in an all – or – none manner .
 - Threshold stimulus is the minimal stimulus needed to elicit a
muscular contraction .
 Twitch : single , short contraction reflecting stimulation of some
motor units in a muscle .
 - Latent period is the time between stimulus and responding
muscle contraction .
 - refractory period : During his period immediately following
contraction , a muscle can not respond .

The muscle twitch

Summation
Summation : A rapid series of stimuli may produce summation of
twitches and a sustained contraction .
- Forceful , sustained contraction without relaxation is a tetanic
contraction .
- Tetany is the result of low Ca2+ concentrations .

Types of Contractions:
 Isotonic : when a muscle contracts and its ends are pulled closer
together .
 Isometric : when a muscle contracts but attachments do not move
 Isokinetic : when the force a muscle generates is less than that
required to move or lift an object , the contraction is called
isokinetic .

Fast & Slow Muscles
 a. white or fast skeletal muscle fibers , have few mitochondria ,
reduced ability to carry on aerobic respiration and tend to fatigue
rapidly . (ex. extra ocular muscles) . Designed for speed , fatigue
easily.
 b. Red or slow skeletal muscle fibers , have many mitochondria ,
are designed for enduration , and can contract for long periods of
time (ex. Solues) .
 Muscle Fatigue :
 - a fatigued muscle loses its ability to contract .
 - muscle fatigue is due to accumulation of lactic acid and ATP
exhaustion.

Oxygen debt:
 a. During rest or moderate exercise , O2 is sufficient
to support aerobic respiration (using many ATP
molecules) .
 b. During strenuous exercise , O2 deficiency may
develop and lactic acid may accumulate as a result
of anaerobic respiration .
 c. The amount of O2 needed to convert accumulated
lactic acid to glucose and restore supplies of ATP
and creatine phosphate is called oxygen dept .

Role of Ca+ in muscle contraction:
 1. promotes neurotransmitter release .
 2. Triggers Ca+ release from SR .
 3. Triggers sliding of my filaments and ATpase activity .
 4. promotes glycogen breakdown & ATP synthesis .

Sliding Filament Theory
 1. A myofiber , together with
all of its myofibrils , shortens
by movement of the insertion
towards the origin of the
muscle .
 2. shortening of the
myofibrils is caused by
shortening of the sarcomere
(The distance between Z
lines is reduced) .
 3. shortening of the
sarcomere is accomplished
by each filament remains the
same during contraction .

 4. sliding is produced by power
strokes of myosin cross bridges ,
which pull the thin actin over the
thick myosin .
 5. The A band remains the same
length during contraction , but
are pulled toward the origin of
the muscle .
 6. Adjacent A bands are pulled
closer together as the I bands
between them shorten .
 7. The H band shorten during
contraction as the thin filaments
on the sides of the sarcomeres
are pulled towards the middle .

Major Events of muscle contraction :
1. The distal end of a motor neuron releases Acetylcholine .
2. Acetylcholine diffuse across the gap at the neuromuscular junction .
3. The sarcolemma is stimulated , and a muscle impulse travels over the
surface of the muscle fiber and deep into the fiber through the transverse
tubules and reaches the sarcoplasmic reticulum .
4. Ca2+ ions diffuse from the sarcoplasmic reticulum into the sarcoplasm
bind to troponin molecules .

Neuromuscular Junction:

5. Tropomyosin molecules move and expose specific sites on actin
filament .
6. Actin and myosin filaments form linkages .
7. Actin filaments are pulled inward by myosin cross – bridges .
8. muscle fiber shortens as a contraction occurs .

Muscle contraction: Role of Ca+

Sliding of actin filament over myosin

Major events of muscle relaxation :
1. Acetylcholinestrase decomposes acetylcholine , and the muscle fiber
membrane is no longer stimulated .
2. Ca2+ ions are actively transported into the sarcoplasmic reticulum .
3. ATP causes linkage between actin and myosin filaments to break .
4. Cross–bridges re–open .
5.Troponin & tropomysin molecules inhibit the interaction between
myosin and actin filaments .
6. Muscle fiber remain relaxed , yet ready until stimulated again .

1. Smooth muscles contain filaments of actin and myosin .
2. Lack transverse tubules and S.R. is not well developed .
3. Display rhythmieity (spontaneous repeated contractions) , responsible
for peristalsis (alternate contraction and relaxation) .
4. Lack troponin (protein that binds to Ca2+) , instead calmodulin binds to
Ca2+ .
5. Both Acetylcholine & norepinephrine are neurotransmitters for smooth
muscles .
6. Hormones and stretching affect smooth muscle contractions .
7. Can contract for a long period of time .
Smooth Muscle Contraction

Cardiac muscle
 a) unique arrangement of actin and myosin filaments produces
the cross- striations (an optical illusion the microscope)‚ and rapid
contraction with powerful forces involved.
 b) muscle cells are joined by intercalated disks‚ and allow
muscle groups to form branching networks - both features are
necessary for cardiac muscle to function as a unit (″ sancytium″ ).
 c) SR and T tubules are well developed‚ so a large amount of
calcium can be released rapidly through the T tubules.
 d) contains more mitochondria in each muscle cell than skeletal
and smooth muscles‚ providing more ATP energy for continuous
contraction.

Cardiac Muscle
 self- exciting muscle fibers form ″pacemakers″ which initiate
spontaneous nerve impulses for autorthymic contraction .
These pacemakers can be influenced by the autonomic
nervous system and hormones.

Cardiac Muscle:
 1. Contracts for a longer time than skeletal muscle
because transverse tubules supply extra Ca+2 ions .
 2. intercalated disc connects the ends of adjacent
muscles and hold cells together as a unit (syncytium) .
 3. Fibers contracts as a unit .
 4. Muscle fibers are self – exiting , rhythmic , and
remain refractory until a contraction is completed.
 5. No Tetanic contractions.

Electromyogram (EMG):
 a) Latent period – chemical reactions and physical changes
that occur preceding the actual contraction of a skeletal muscle.
 b) Period of contraction – actin causing the shortening of
macromere and the contraction of muscle.
 c) period of relaxation- actin returns to its original position,
causing the lengthening of sarcomeres and the relaxation of
muscle.

Clinical Terms:
 Convulsion : series of involuntary contractions of various
voluntary muscles .
 Fibrosis : Degenerative disease in which connective tissue
replaces skeletal muscle tissue .
 Myalgia : pain resulting from any muscular disorder .
 Myasthenia gravis : an autoimmune , chronic disease
characterized by muscles that are weak and easily fatigue . it
results from the immunes systems attack on neuromuscular
junctions .
 Paresis : partial or slight paralysis of the muscle .
 Muscular dystrophy : progressive muscle weakness and atrophy
caused by deficient dystrophin protein .

Clinical Terms
 Myopathy : Any muscular disease .
 Paralysis : loss of ability to move a body part .
 Myotonia : prolonged muscular spasm .
 Myositis : inflammation of skeletal muscle tissue .
 Spasm : A sudden , involuntary smooth or skeletal muscle twitch ,
can range from mild to very painful irritation .
 Tics : spasm of eye–lid or facial muscles .
 Cramp : a prolonged spasm that cause a muscle to become taut
and painful .

Functions of the muscular system

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