Peripheral Nervous System
SOURCE:
training.seer.cancer.gov/
The peripheral nervous system (PNS) refers those nerves (neurons)
that are not part of the brain
or spinal cord. These nerves
form the communication network between the CNS and the body
parts. Nerves inside the brain and spinal cord are
part of the central nervous system
(CNS). PNS nerves have their cell bodies (ganglia)
inside the CNS but their length outside it. The PNS consists
of neurons with both myelinated
and un-myelinated axons.
All the cranial nerves (the 12 pairs of nerves leaving the base of
the braincase) are part of the peripheral nervous system
although the second cranial nerve (the optic nerve) can be
considered as part of the brain.
The
peripheral nervous system is further subdivided into the somatic
nervous system and the autonomic nervous system. The somatic
nervous system consists of nerves that go to the skin and
muscles and is involved in conscious activities. The autonomic
nervous system consists of nerves that connect the CNS to the
visceral organs such as the heart, stomach, and intestines. It
mediates unconscious activities.
There are two kinds of nerve in the Peripheral Nervous
System:
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Motor or efferent nerves. These are the nerves that connect to
muscles in the limbs, torso, face and internal organs. They
carry nerve signals from the CNS and make body parts move.
The cell bodies of the motor nerves connect to CNS in the
shaft of grey
matter in the spine called the
anterior horn. They terminate at a neuromuscular
junction known as the motor plate or motor end plate.
Neurotransmitters released by the motor nerves into the
motor plate cause the muscle to contract.
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Sensory or afferent nerves. These are nerves that connect to
sensory organs and sensors in the skin, muscle and internal
organs. They take relay nerve signals to the CNS. They
convey heat, touch, position (proprioceptive), balance,
sound and other information from the limbs and sense and
internal organs. Sensory nerves terminate in the CNS at the
root ganglion.
The peripheral nervous system is divided into two parts:
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The SNS is performs all our interactions with the
physical world such as control of our limbs and receiving
of conscious information from our senses. It consists of
the 12 pairs of cranial nerves and the 31 pairs of spinal
nerves.
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The ANS consists of both motor and sensory nerves
which run both to and from the central nervous system and
fulfills "automatic" functions that we are seldom aware of
such as heart activity and the release of hormones. The
autonomic nervous system is further divided into the
sympathetic nervous system and the
parasympathetic nervous system.
Structure of a Nerve
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A nerve contains bundles of nerve fibers, either
axons or dendrites, surrounded by connective tissue.
Sensory nerves contain only afferent fibers, long
dendrites of sensory neurons. Motor nerves have only
efferent fibers, long axons of motor neurons. Mixed
nerves contain both types of fibers. A connective tissue
sheath called the epineurium surrounds each nerve.
Each bundle of nerve
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fibers is called a fasciculus
and is surrounded by a layer of connective tissue called the
perineurium. Within the fasciculus, each individual nerve fiber,
with its myelin and neurilemma, is surrounded by connective
tissue called the endoneurium. A nerve may also have blood
vessels enclosed in its connective tissue wrappings.
Cranial Nerves
Twelve pairs of cranial nerves emerge from the inferior
surface of the brain. All of these nerves, except the
vagus nerve , pass through
foramina of the skull to innervate structures in the head, neck,
and facial region.
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The cranial nerves are designated both by name and
by Roman numerals, according to the order in which they
appear on the inferior surface of the brain. Most of the
nerves have both sensory and motor components. Three of
the nerves are associated with the special senses of
smell, vision, hearing, and equilibrium and have only
sensory fibers. Five other nerves are primarily motor in
function but do have some sensory fibers for
proprioception. The remaining four nerves consist of
significant amounts of both sensory and motor fibers.
Acoustic neuromas are benign fibrous growths that arise
from the balance nerve, also called the eighth cranial
nerve or Vestibulocochlear nerve. These tumors are
non-malignant, meaning that they do not spread or
metastasize to other parts of the body. The location of
these tumors is deep inside the skull, adjacent to
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vital brain centers in the brain stem. As the tumors
enlarge, they involve surrounding structures which have to do
with vital functions. In the majority of cases, these tumors
grow slowly over a period of years. In other cases, the growth
rate is more rapid and patients develop symptoms at a faster
pace. Usually, the symptoms are mild and many patients are not
diagnosed until some time after their tumor has developed. Many
patients also exhibit no tumor growth over a number of years
when followed by yearly MRI scans.
Spinal Nerves
Thirty-one pairs of spinal nerves emerge laterally from the
spinal cord. Each pair of nerves corresponds to a segment of the
cord and they are named accordingly. This means there are 8
cervical nerves, 12 thoracic nerves, 5 lumbar nerves, 5 sacral
nerves, and 1 coccygeal nerve.
Each spinal nerve is connected to the spinal cord by a
dorsal root and a ventral root. The cell bodies of the sensory
neurons are in the dorsal root ganglion, but the motor neuron
cell bodies are in the gray matter. The two roots join to form
the spinal nerve just before the nerve leaves the vertebral
column. Because all spinal nerves have both sensory and motor
components, they are all mixed nerves.
Autonomic Nervous System
The autonomic nervous system is a visceral efferent system,
which means it sends motor impulses to the visceral organs. It
functions automatically and continuously, without conscious
effort, to innervate smooth muscle, cardiac muscle, and glands.
It is concerned with heart rate, breathing rate, blood pressure,
body temperature, and other visceral activities that work
together to maintain homeostasis.
The autonomic nervous system has two parts, the sympathetic
division and the parasympathetic division. Many visceral organs
are supplied with fibers from both divisions. In this case, one
stimulates and the other inhibits. This antagonistic functional
relationship serves as a balance to help maintain homeostasis.
Peripheral Nervous System links:
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