Download Fundamentals on Peripheral Nerves

Fundamentals on Peripheral Nerves
Introduction
Objectives- After you have completed your study of this material you should be able to:
1. Classify nerve fibers into four basic types: afferent, efferent to skeletal muscle, preganglionic
efferent, and postganglionic efferent. Give the location of the cell bodies of each of these fiber
types and tell where each type of fiber terminates.
2. List the types of nerve fibers found in each of the following nerves: dorsal roots of spinal
nerves, ventral roots of spinal nerves, branches to skeletal muscles, gray rami, white rami,
pelvic splanchnic nerves, and all other branches of spinal nerves.
3. List the cranial sensory ganglia and know with which cranial nerve each ganglion is
associated.
4. List the cranial nuclei that give rise to efferent fibers to skeletal muscle.
5. List the cranial nerves that contain preganglionic efferent fibers, give the nuclei from which
these fibers arise, and give the autonomic ganglia in which these fibers terminate.
6. List the cranial autonomic ganglia and give the cranial nerve from which each ganglion
receives preganglionic fibers.
Types of Nerve Fibers in Peripheral Nerves
Although there are many different ways of classifying nerve fibers, in this course we will use only
a very simple method based primarily on the direction of impulse transmission. Fundamentally, nerve
fibers can be divided into AFFERENT FIBERS which conduct impulses toward the central nervous
system and EFFERENT FIBERS which conduct impulses away form the central nervous system. The
efferent fibers can be further subdivided into three types: 1) EFFERENT FIBERS TO SKELETAL
MUSCLE, 2) EFFERENT FIBERS TO AUTONOMIC GANGLIA, and 3) EFFERENT FIBERS
TO SMOOTH MUSCLE, GLANDS, or CARDIAC MUSCLE. The latter two types of efferent fibers
are more commonly referred to as PREGANGLIONIC EFFERENT FIBERS and
POSTGANGLIONIC EFFERENT FIBERS.
Afferent
Afferent
Efferent
Efferent to skeletal muscle
Efferent to Autonomic Ganglia (preganglionic
efferent)
Efferent to smooth muscle, cardiac muscle, or
glands (postganglionic efferent)
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Only rarely are all four of these types of nerve fibers found in a single nerve. Most nerves have only two
or three of these fiber types. For each nerve that you study in this course, I will expect you to know 1) the
types of fibers that make up the nerve, 2) the location of the cell bodies from which each different type of
fiber arises, and 3) where each different type of fiber terminates peripherally. With this information you
will be able to predict the results of stimulation of the nerve and the deficits that will occur if the nerve or
any of its components are damaged.
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Afferent Fibers
Afferent fibers conduct impulses toward the central nervous system. They are processes (axons)
of nerve cell bodies located in the dorsal root ganglia of spinal nerves or in equivalent ganglia on cranial
nerves (V, VII, VIII, IX, and X). (Although cranial nerves I and II are special sensory nerves, they are
special cases and will not be considered here). These ganglion cells have an axon that divides into a
peripheral process that terminates peripherally in a receptor and into a central process that terminates
centrally by forming synapses on other nerve cells within the CNS. Although all sensory fibers are
afferent fibers, not all afferent fibers transmit impulses that are registered at a conscious level. Therefore,
not all afferent fibers are sensory (e.g., Vagal reflex fibers).
Location of the Cell Bodies of Afferent Fibers
Dorsal Root Ganglia
Trigeminal Ganglion (CN V)- Mesencephalic Nucleus of the Trigeminal nerve
(proprioceptive [position sense] afferent fibers in the muscular branches of the
trigeminal nerve and in other cranial nerve branches to skeletal muscles in the head
arise from nerve cells located in the mesencephalic nucleus of the trigeminal nerve, an
exception to the rule that all afferent fibers come from nerve cells located in ganglia).
Geniculate Ganglion (CN VII)
Vestibular Ganglion (CN VIII)
Spiral Ganglion (CN VIII)
Superior Ganglion (CN IX)
Inferior Ganglion (CN IX)
Superior Ganglion (X)
Inferior Ganglion (X)
Efferent Fibers to Skeletal Muscle
Efferent fibers to skeletal muscle arise from nerve cell bodies located in the ventral gray column of
the spinal cord and in equivalent nuclei in the brain associated with cranial nerves III, IV, V, VI, VII, IX,
X, XI, and XII. These fibers all terminate in skeletal muscles.
Locations of the Cell Bodies of Efferent Fibers to Skeletal Muscle
Ventral Gray Column
Oculomotor Nucleus (CN III)
Trochlear Nucleus (CN IV)
Motor Nucleus of the Trigeminal Nerve (CN V)
Abducens Nucleus (CN VI)
Facial Nucleus (CN VII)
Nucleus Ambiguus (CN IX and X)
Spinal Accessory Nucleus (CN XI)
Hypoglossal Nucleus (CN XII)
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Preganglionic Efferent Fibers (Efferent Fibers to Autonomic Ganglia)
Preganglionic efferent fibers arise from nerve cell bodies located in the Intermediolateral cell
column of the thoracic and upper lumbar regions of the spinal cord, the intermediate gray matter of the
sacral spinal cord (sacral autonomic nucleus), and in four equivalent nuclei located in the brain stem and
associated with cranial nerves III, VII, IX, and X. Almost all of these fibers terminate peripherally in
autonomic ganglia by forming synapses with the nerve cell bodies of postganglionic efferent fibers. A
small percentage of the preganglionic efferent fibers terminate on cells in the adrenal medulla.
Locations of the Cell Bodies of Preganglionic Efferent Fibers
Intermediolateral Cell Column (T1-L2)
Intermediate Gray Column of the Sacral Spinal Cord (Sacral Autonomic Nucleus S2-S4)
Nucleus of Edinger-Westphal (CN III)
Superior Salivatory Nucleus (CN VII)
Inferior Salivatory Nucleus (CN IX)
Dorsal Motor Nucleus of the Vagus (CN X)
Postganglionic Efferent Fibers (Efferent fibers to smooth muscle, glands, and cardiac muscle)
All postganglionic efferent nerve fibers arise from nerve cell bodies located in autonomic ganglia.
They terminate on smooth muscle cells, glands, or cardiac muscle cells. The postganglionic efferent
fibers found in spinal nerves and their branches all come from nerve cells located in the ganglia of the
sympathetic trunk. The preganglionic fibers associated with these ganglia all arise from nerve cells in the
Intermediolateral cell column of the thoracic and upper lumbar (T1-L2) segments of the spinal cord. There
are also four named autonomic ganglia in the head that supply postganglionic efferent fibers to certain
named smooth muscles and glands in the head, and there are a very large number of autonomic (terminal
ganglia in the walls of the thoracic and abdominal viscera which supply postganglionic efferent fibers to
smooth muscles and glands in the viscera.
Locations of the Cell Bodies of Postganglionic Efferent Fibers
Ganglia of the Sympathetic Trunk (paravertebral Ganglia)
Prevertebral Ganglia (Collateral Ganglia)
Celiac Ganglion
Superior Mesenteric Ganglion
Inferior Mesenteric Ganglion
Cranial Autonomic Ganglia
Ciliary Ganglion
Pterygopalatine Ganglion
Submandibular Ganglion
Otic Ganglion
Terminal Ganglia (in the walls of viscera)
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Fiber Composition of the Roots and Rami of Spinal Nerves
Dorsal Roots1. Afferent fibers only.
Ventral Roots1. Efferent Fibers to skeletal muscle (all ventral roots)
2. Preganglionic efferent fibers (T1-L2 and S2-S4 only)- note, some investigations have
found afferent fibers in some ventral roots- Coggeshall et al., Neurosurgery 4:443-448,
1979).
Branches to Skeletal Muscles- all branches to skeletal muscles and all nerves that supply such
branches (e.g., anterior primary rami, posterior primary rami, etc.) have three types of fibers:
1. Efferent fibers to skeletal muscle
2. Afferent fibers
3. Postganglionic efferent fibers (to smooth muscle in the walls of the blood vessels that
supply the muscle)
Gray Rami
1. Postganglionic Efferent Fibers
White Rami
1. Preganglionic Efferent Fibers
2. Afferent Fibers (from viscera, i.e., visceral afferents)
All other Branches of Spinal Nerves- all other branches of anterior and posterior primary rami
(i.e., cutaneous branches, branches to bone, branches to joints, recurrent rami to the meninges)
have two kinds of fibers:
1. Afferent fibers
2. Postganglionic efferent fibers (to smooth muscle or glands)
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