The peripheral nervous system (PNS) consists of all neurons
that exist outside the brain and spinal cord. This includes long
nerve fibers containing bundles of axons as well as ganglia made of
neural cell bodies. The peripheral nervous system connects the
central nervous system (CNS) made of the brain and spinal cord to
various parts of the body and receives input from the external
environment as well.
Functionally, the PNS is divided into sensory (afferent) and
motor (efferent) nerves, depending on whether they bring
information to the CNS from sensory receptors or carry instructions
towards muscles, organs or other effectors. Motor nerves can be
further classified as somatic or autonomic nerves, depending on
whether the motor activity is under voluntary conscious
control.
The primary function of the peripheral nervous system is to
connect the brain and spinal cord to the rest of the body and the
external environment. This is accomplished through nerves that
carry information from sensory receptors in the eyes, ears, skin,
nose and tongue, as well as stretch receptors and nociceptors in
muscles, glands and other internal organs.
When the CNS integrates these varied signals, and formulates a
response, motor nerves of the PNS innervate effector organs and
mediate the contraction or relaxation of skeletal, smooth or
cardiac muscle.
Thus, the PNS regulates internal homeostasis through the
autonomic nervous system, modulating respiration, heart rate, blood
pressure, digestion reproduction and immune responses.
One functional division of the PNS is the somatic nervous
system. It controls voluntary muscular movement of skeletal muscles
in the limbs, back, shoulders, neck and face. It also mediates
reflex actions, where an afferent nerve fiber is nearly directly
connected to a motor nerve fiber, to quickly generate a response to
stimulus.
These include protective responses, like the movement of the
body away from acute injurious stimuli like extremes in
temperature, as well as those like the patellar ‘knee-jerk’
response when the patellar ligament is struck.
The autonomic nervous system is related to all the involuntary
visceral activity of the body. It consists of the sympathetic and
parasympathetic nervous systems, and their effector organs include
cardiac muscle, smooth muscle, and various glands.
The autonomic nervous system controls the cardiovascular
system. It can alter the force and rate of heart contractility, as
well as the constriction and dilation of blood vessels. Therefore,
it also influences blood pressure.
The rate of breathing can also be changed by the ANS. It
affects both skeletal and smooth muscle fibers across the body,
whether it is changing the metabolism of glucose in skeletal
muscles or causing pupil dilation in the eye. The ANS can influence
digestive efficiency, altering the secretion of enzymes from glands
and the rate of peristaltic movement.
Activation of the sympathetic nervous system slows down
digestion and diverts blood flow towards skeletal muscle. It can
impair sexual arousal and shut down most non-essential functions of
the body.
On the other hand, the parasympathetic nervous system enhances
digestive secretions, peristaltic movements, encourages normal
cycles of circadian activity, deep sleep and activates the repair
mechanisms of the body.
In most cases, a physiological response by the parasympathetic
nervous system is in direct opposition to the results mediated by
the sympathetic nervous system.
Colloquially, the sympathetic nervous system is said to
influence the fight-or-flight response, and the parasympathetic
nervous system is related to the feed-and-breed, or rest-and-digest
responses.
With sympathetic nervous responses, the body speeds up, tenses
up and becomes more alert. Functions that are not essential for
survival are shut down. Following are the specific reactions of
sympathetic nervous system:
increase in the rate and constriction of the heart
dilation of bronchial tubes in the lungs and pupils in the
eyes
contraction of muscles
release of adrenaline from the adrenal gland
conversion of glycogen to glucose to provide energy for the
muscles.
shut down of processes not critical for survival
The parasympathetic nervous system counterbalances the
sympathetic nervous system. It restores the body to a state of
calm. The specific responses are:
decrease in heart rate
constriction of bronchial tubes in the lungs and pupils in the
eyes
relaxation of muscles
saliva production: the stomach moves and increases secretions
for digestion.
increase in urinary output
The parasympathetic nervous system is a slower system and moves
along longer pathways. Preganglionic fibers from the medulla or
spinal cord project ganglia close to the target organ. They create
a synapse, which eventually creates the desired response.
The sympathetic nervous system is a faster system as it moves
along very short neurons. When the system is activated, it
activates the adrenal medulla to release hormones and chemical
receptors into the bloodstreams.
The target glands and muscles get activated. Once the perceived
danger is gone, the parasympathetic nervous system takes over to
counterbalance the effects of the sympathetic nervous system's
responses.