Nerves and synapses

The Nervous System
Composed of a highly organized
collection of nerve cells and neurons.
It is found in all higher forms of life.

Nerve cells
Collects information from the
environment by means of
receptors.
Coordinates the information
with the internal activities of
the organism in a process
called integration.
Stores information in terms of
memory and generate adaptive
patterns of behavior.

neurons
• The neuron is the functional unit
of the nervous system. Humans
have about 100 billion neurons in
their brain alone! While variable in
size and shape, all neurons have
three parts. Dendrites receive
information from another cell and
transmit the message to the cell
body. The cell body contains the
nucleus, mitochondria and other
organelles typical of eukaryotic
cells. The axon conducts messages
away from the cell body.

• Three types of neurons occur. Sensory neurons
typically have a long dendrite and short axon,
and carry messages from sensory receptors to
the central nervous system. Motor neurons have a
long axon and short dendrites and transmit
messages from the central nervous system to the
muscles (or to glands). Interneurons are found
only in the central nervous system where they
connect neuron to neuron.

Structure of a neuron and the direction of
nerve message transmission.

Some axons are wrapped in a myelin sheath formed from
the plasma membranes of specialized glial cells known
as Schwann cells. Schwann cells serve as supportive,
nutritive, and service facilities for neurons. The gap
between Schwann cells is known as the node of Ranvier,
and serves as points along the neuron for generating a
signal. Signals jumping from node to node travel
hundreds of times faster than signals traveling along the
surface of the axon. This allows your brain to
communicate with your toes in a few thousandths of a
second.

Cross section of myelin sheaths that
surround axons

Synapses
• At rest the outside of the membrane is more positive
than the inside.
• Sodium moves inside the cell causing an action
potential, the influx of positive sodium ions makes the
inside of the membrane more positive than the outside.
• Potassium ions flow out of the cell, restoring the resting
potential net charges.
• Sodium ions are pumped out of the cell and potassium
ions are pumped into the cell, restoring the original
distribution of ions.

synapse
• The junction between a nerve
cell and another cell is called
a synapse. Messages travel
within the neuron as an
electrical action potential.
The space between two cells
is known as the synaptic cleft.
To cross the synaptic cleft
requires the actions of
neurotransmitters.
Neurotransmitters are stored
in small synaptic vessicles
clustered at the tip of the
axon.

• Neurotransmitters tend to be small molecules, some are even hormones. The time for
neurotransmitter action is between 0,5 and 1 millisecond. Neurotransmitters are either
destroyed by specific enzymes in the synaptic cleft, diffuse out of the cleft, or are
reabsorbed by the cell. More than 30 organic molecules are thought to act as
neurotransmitters. The neurotransmitters cross the cleft, binding to receptor molecules
on the next cell, prompting transmission of the message along that cell's membrane.
Acetylcholine is an example of a neurotransmitter, as is norepinephrine, although each
acts in different responses. Once in the cleft, neurotransmitters are active for only a
short time. Enzymes in the cleft inactivate the neurotransmitters. Inactivated
neurotransmitters are taken back into the axon and recycled.

• Diseases that affect the function of signal transmission can have serious consequences.
Parkinson's disease has a deficiency of the neurotransmitter dopamine. Progressive
death of brain cells increases this deficit, causing tremors, rigidity and unstable posture.
L-dopa is a chemical related to dopamine that eases some of the symptoms (by acting
as a substitute neurotransmitter) but cannot reverse the progression of the disease.

• The bacterium Clostridium tetani produces a toxin that prevents the release of GABA.
GABA is important in control of skeletal muscles. Without this control chemical,
regulation of muscle contraction is lost; it can be fatal when it effects the muscles used
in breathing.

• Clostridium botulinum produces a toxin found in improperly canned foods. This toxin
causes the progressive relaxation of muscles, and can be fatal. A wide range of drugs
also operate in the synapses: cocaine, LSD, caffeine, and insecticides.

Three basic functions are performed
by nervous systems:
• Receive sensory input from internal and external
environments
• Integrate the input
• Respond to stimuli

Sensory Input
• Receptors are parts of the nervous system that sense changes in
the internal or external environments. Sensory input can be in
many forms, including pressure, taste, sound, light, blood pH, or
hormone levels, that are converted to a signal and sent to the
brain or spinal cord.
• Integration and Output
• In the sensory centers of the brain or in the spinal cord, the
barrage of input is integrated and a response is generated. The
response, a motor output, is a signal transmitted to organs than
can convert the signal into some form of action, such as
movement, changes in heart rate, release of hormones, etc.

Divisions of the Nervous System
• The nervous system monitors and controls almost every organ system
through a series of positive and negative feedback loops.The Central Nervous
System (CNS) includes the brain and spinal cord. The Peripheral Nervous
System (PNS) connects the CNS to other parts of the body, and is composed
of nerves (bundles of neurons).

• Not all animals have highly specialized nervous systems. Those with simple
systems tend to be either small and very mobile or large and immobile. Large,
mobile animals have highly developed nervous systems: the evolution of
nervous systems must have been an important adaptation in the evolution of
body size and mobility.

• Coelenterates, cnidarians, and echinoderms have their neurons organized into
a nerve net. These creatures have radial symmetry and lack a head. Although
lacking a brain or either nervous system (CNS or PNS) nerve nets are capable
of some complex behavior

nervous systems in radially
symmetrical animals

• Bilaterally symmetrical animals have a body plan
that includes a defined head and a tail region.
Development of bilateral symmetry is associated
with cephalization, the development of a head
with the accumulation of sensory organs at the
front end of the organism. Flatworms have
neurons associated into clusters known as
ganglia, which in turn form a small brain.
Vertebrates have a spinal cord in addition to a
more developed brain.

Some nervous systems in bilaterally
symmetrical animals

The brain
• During embryonic
development, the brain first
forms as a tube, the anterior
end of which enlarges into
three hollow swellings that
form the brain, and the
posterior of which develops
into the spinal cord. Some
parts of the brain have
changed little during
vertebrate evolutionary
history

The Peripheral Nervous System
(PNS)
• contains only nerves and connects the brain and
spinal cord (CNS) to the rest of the body. The
axons and dendrites are surrounded by a white
myelin sheath. Cell bodies are in the central
nervous system (CNS) or ganglia. Ganglia are
collections of nerve cell bodies. Cranial nerves in
the PNS take impulses to and from the brain
(CNS). Spinal nerves take impulses to and away
from the spinal cord.

major subdivisions of the PNS motor
pathways
• The Somatic Nervous System (SNS) includes all
nerves controlling the muscular system and
external sensory receptors. External sense
organs (including skin) are receptors.
• The Autonomic Nervous System is that part of
PNS consisting of motor neurons that control
internal organs. It has two subsystems. The
autonomic system controls muscles in the heart,
the smooth muscle in internal organs

Central nervous system
• The Central Nervous System (CNS) is
composed of the brain and spinal cord. The
CNS is surrounded by bone-skull and vertebrae.
Fluid and tissue also insulate the brain and spinal
cord.

Spinal cord
• The spinal cord runs along the dorsal side of the body
and links the brain to the rest of the body. Vertebrates
have their spinal cords encased in a series of (usually)
bony vertebrae that comprise the vertebral column.

• The gray matter of the spinal cord consists mostly of
cell bodies and dendrites. The surrounding white matter
is made up of bundles of interneuronal axons (tracts).
Some tracts are ascending (carrying messages to the
brain), others are descending (carrying messages from
the brain). The spinal cord is also involved in reflexes
that do not immediately involve the brain.

Senses
• Input to the nervous system is in the form of
our five senses: pain, vision, taste, smell, and
hearing. Vision, taste, smell, and hearing input
are the special senses. Pain, temperature, and
pressure are known as somatic senses. Sensory
input begins with sensors that react to stimuli in
the form of energy that is transmitted into an
action potential and sent to the CNS.

Sensory receptors
• Hearing involves the actions of the external ear, eardrum, ossicles, and cochlea. In
hearing, sound waves in air are converted into vibrations of a liquid then into
movement of hair cells in the cochlea. Finally they are converted into action potentials
in a sensory dendrite connected to the auditory nerve. Very loud sounds can cause
violent vibrations in the membrane under hair cells, causing a shearing or permanent
distortion to the cells, resulting in permanent hearing loss.

• Orientation and Gravity
• Orientation and gravity are detected at the semicircular canals. Hair cells along three
planes respond to shifts of liquid within the cochlea, providing a three-dimensional
sense of equilibrium. Calcium carbonate crystals can shift in response to gravity,
providing sensory information about gravity and acceleration.

• Photoreceptors Detect Vision and Light Sensitivity
• The human eye can detect light in the 400-700 nanometer (nm) range, a small portion
of the electromagnetic spectrum, the visible light spectrum. Light with wavelengths
shorter than 400 nm is termed ultraviolet (UV) light. Light with wavelengths longer
than 700 nm is termed infrared (IR) light.

Nerves and synapses

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