The Nervous System
Organization
Central Nervous System (CNS)= Brain + Spinal Cord
Peripheral Nervous System (PNS) = All the nerve fibers and nerve cells located outside the central
nervous system; the bundles of afferent and efferent nerve fibers and their associated ganglia which make
up 1) the Cranial Nerves, 2) the spinal nerves and 3) the Autonomic Nervous System.
Central
Nervous
System
Brain
Spinal Cord
Afferent Fibers
"Sensory"
Convey Information from
receptorsto the central
nervous system
Peripheral
Nervous
System
Efferent Fibers
convey information from
the central nervous system
to effectors (muscles and glands)
Somatic Nervous System
Conveys information from the CNS
to skeletal muscles
Autonomic Nervous System
Conveys information from the CNS to
smooth muscle,cardiac muscle and glands
Sympathetic
(T horaco-lumbar)
Nervous System
Parasympathetic
(Cranio-sacral)
Nervous System
Definitions
NEUROLOGY- the study of the nervous system
NERVE- a bundle of nerve fibers (axons) held together by connective tissue. (Be sure that you
can distinguish between a nerve and a nerve fiber. A nerve can be compared to a cable made
up of individual wires. Each wire corresponds to a nerve fiber.) Nerves are located outside
the central nervous system. Bundles of nerve fibers within the central nervous system are
called TRACTS.
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NERVE FIBER - an axon (a process of a nerve cell) and its sheath.
myelin sheath
neurolemma
axon
AFFERENT FIBERS- nerve fibers that conduct impulses toward the CNS.
EFFERENT FIBERS- nerve fibers that conduct impulses away from the CNS.
SOMATIC - on or relating to the wall of the body or the framework of the body and not to the
viscera; i.e., relating to the skin and the skeletal muscles.
VISCERAL - relating to the organs of the ventral body cavities (the viscera) and also to smooth
muscle and glands outside these cavities.
AUTONOMIC NERVOUS SYSTEM- the system of nerves and ganglia concerned with the
distribution of nerve impulses to 1) Cardiac Muscle (located only in the heart), 2) Smooth
Muscle (located in essentially all of the viscera, in the walls of blood vessels, and in the skin
[arrector pili muscles]} and 3) Glands.
NEURON - a nerve cell consisting of a cell body and all its processes.
AXON- a single, usually elongated process that conducts impulses away from the nerve cell body.
DENDRITES - multiple, shorter, branched processes that conduct impulses toward the nerve cell
body.
PERIKARYON (soma)- the nerve cell body; the part containing the nucleus.
PERIPHERAL PROCESS - the part of the axon of a sensory ganglion cell that conducts nerve
impulses from the receptor to the perikaryon. Some textbooks refer to this as a dendrite,
although morphologically it is an axon.
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dendrites
dendrites
perikaryon
peripheral process
perikaryon
axon
central process
synapses
synapses
CENTRAL PROCESS - the part of the axon of a sensory ganglion cell that conducts impulses
from the perikaryon to the central nervous system.
RECEPTOR - any structure which detects a change in its environment and causes the
transmission of a nerve impulse in an afferent fiber.
EFFECTOR - a muscle or gland.
SYNAPSES - functional contacts between neurons.
GRAY MATTER- that part of the nervous tissue composed mainly of nerve cell bodies and
dendrites, unmyelinated axons, and neuroglia.
WHITE MATTER - aggregations of myelinated axons.
NUCLEUS- a collection of nerve cell bodies within the CNS
GANGLION - a collection of nerve cell bodies outside the CNS, i.e., within the PNS.
AUTONOMIC GANGLION- a ganglion made up of the cell bodies of efferent fibers to smooth
muscle, to cardiac muscle, or to glands (postganglionic efferent fibers).
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Cranial Nerves- There are 12 pairs of cranial nerves numbered and named as follows: They are
primarily sensory neurons (but some do provide innervation to muscles).
I Olfactory
II Optic
III Oculomotor
IV Trochlear
V Trigeminal
VI Abducens
VII Facial
VIII Vestibulocochlear
IX Glosspharyngeal
X Vagus
XI Accessory
XII Hypoglossal
Gross Features of Cranial Nerves
Unlike the spinal nerves, cranial nerves do not have a regular pattern of dorsal and ventral roots
and primary rami.
Spinal Nerves
There are 31 pairs of spinal nerves
divided regionally as follows:
1.
Cervical- 8 pairs (C1-C8)
Thoracic- 12 pairs (T1-T12)
Lumbar- 5 pairs (L1-L5)
Sacral- 5 pairs (S1-S5)
Coccygeal- 1 pair (Cc1)
3.
4.
2.
5.
Gross Features Common to all Spinal Nerves
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All spinal nerves have the following roots and branches
(rami):
1.
2.
3.
4.
5.
Dorsal root
Ventral root
Posterior (dorsal) primary ramus (branch)
Anterior (ventral) primary ramus
Gray ramus
Special: splanchnic nerve (#6) – Only for S2-S4
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Special Branches Found Only in Thoracic (T1-T12), Upper Lumbar (L1,L2) or Mid-Sacral
(S2-S4) Nerves
In addition to the roots and rami listed above, thoracic and upper lumbar nerves have
1. White rami (T1-L2) (think thoraco-lumbar)
Mid-sacral nerves have
2. Pelvic Splanchnic nerves (S2-S4)
Note: White and Gray rami together are called COMMUNICATING RAMI because they
are the structures through which the spinal nerve communicates with the sympathetic
trunk. Both white and gray rami connect with sympathetic ganglia.
SPLANCHNIC NERVES, on the other hand, are visceral nerves (nerves to viscera) which
contain preganglionic fibers. Since the pelvic splancnic nerves do not connect to the
sympathetic trunk, they are not communicating rami.
S2, S3 or S4
T1-L2
Pelvic
splanchnic
nerve
White
Ramus
Plexus Formation
Plexus: a network of interlacing nerves. Nerves tend to interchange fibers by branching of
the fiber bundles and subsequent rejoining, branching, and joining again so that peripheral nerves
contain fibers from more than one spinal nerve. There are four main plexuses:
1)
2)
3)
4)
the cervical plexus
the brachial plexus
the lumbar plexus
the sacral plexus.
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Overlap
C1-C5- Cervical
plexus
C5-T1- Brachial
plexus
L1-L4- Lumbar
Plexus
L4-S4- Sacral
Plexus
Dermatomes- The area of skin supplied by the
afferent fibers in the dorsal root of a single spinal
nerve.
Redundancy or “overlap” lessens
the possibility of sensory loss due to injury.
Because of the phenomenon of plexus formation,
the fibers entering any single nerve to skin or to
muscle generally come from more than one
spinal nerve. Also adjacent cutaneous nerves
tend to overlap areas supplied by each other.
Therefore, sectioning of a single dorsal root will
not result in total loss of skin sensation, nor will
sectioning of a single ventral root result in
paralysis of a muscle.
Referred pain
There are two types of pain: somatic and
visceral. Somatic pain is caused by stimulation
of receptors in skin (superficial somatic pain) or
in skeletal muscles, joints, tendons (deep somatic
pain). Visceral pain is caused by stimulation of
receptors in viscera. Our awareness of the
location of the painful stimulus is generally
accurate and precise for somatic pain. In many
instances of visceral pain, however, the pain is
experienced in some site other than where the
stimulus occurred. Frequently, it will be
experienced as a diffuse, poorly localized pain in
some part of the body surface innervated by
afferent fibers from the same spinal level that
supplies the viscus that is the origin of the pain.
This phenomenon is called “referred pain.”
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