The peripheral nervous system consists of the nerves that
branch out from the brain and spinal cord. These nerves form
the communication network between the CNS and the body parts.
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.
Structure of a Nerve
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.
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A connective tissue sheath called the epineurium surrounds
each nerve. Each bundle of nerve 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.
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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 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.
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