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No. 21
1. Introduction of Nervous System
2. Spinal Cord (1)
PART Ⅵ THE
NERVOUS SYSTEM
Chapter 1 Introduction
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The nervous system is a master system in
the living body; it regulates and integrates
the activities of all the bodily systems for
the benefit of the organism as a whole.
Ⅰ. Divisions of the Nervous
System
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The nervous system consists of central
part (central nervous system) and
peripheral part (peripheral nervous
system).
The central nervous system is
composed of brain and spinal cord.
The peripheral nervous system includes
cranial nerves (12 pairs), spinal nerves
(31 pairs), and visceral nerves.
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The somatic and visceral nerves:
According to the functions of organs
innervated by the nerves, the peripheral
nervous system is also divided the
somatic nerves and visceral nerves.
The somatic nerves supply the body
surface, bones, joints, and skeletal muscle.
The visceral nerves are distributed in the
viscera, heart, vessels, and smooth
muscles.
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In the peripheral nerves (the somatic and visceral
nerves), there are two nerve fibers called
afferent nerves (sensory nerves) and
efferent nerves (motor nerves).
The efferent (motor) part of visceral nerves is
called the vegetative nervous system or
autonomic nervous system.
The autonomic nervous system innervates the
smooth muscle and the glands of viscera, and the
smooth muscle of blood vessels, together with
the cardiac muscle, and further divided into
sympathetic nerve and parasympathetic
nerve.
Ⅱ. Organization of nervous system
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The nervous system is composed of
nervous tissue that consists of billions of
nerve cells (neurons) and supported by a
special variety of connective tissue known
as neuroglia.
Ⅰ) Neurons
The neurons are independent structural
unit of the nervous system and are
functional specialized for reception,
integration, and transmission of coded
information.
1. Morphology
 Each neuron possesses a nucleated cell
body and two types of processes; an
axon which conducts impulse away from
the cell body and one or more dendrite
which conduct impulses towards the cell
body.
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2. Structure
① Cell body
Like other cells, the cell body of the neuron
serves as metabolic center of the entire unit and
consists of a large, pale nucleus and cytoplasm,
organelles, cell membrane, and Nissl body.
Nissl body and neurofibril:
The organelles contained within the cytoplasm
are common to other cell in the body, but there
are abundant granular endoplasmic reticulum
which constitutes the Nissl body, a protein
synthesis apparatus.
Neurofibrils have role of supporting neuron and
involved in transmission of substance in the cell.
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② Processes
The axon:
It is a slender process. It may transfer the nerve
impulses from the beginning part (axon hillock)
to the end (axon terminal). Because the
axoplasm does not contain RNA and ribosome,
proteins synthesis cannot take place in the axon.
All axonal proteins, therefore, must come from
the cell body, and the products are transported
by a perpetual axoplasmic motion. Some
organelles, structural protein and
neurotransmitters contained within cytoplasm are
carried by axoplasmic flow which moves in both
directions and with varying velocity. This
phenomenon is called axoplasmic transport.
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The dendrites:
The main or primary dendrites arise from
the cell body and then branch repeatly in
a tree –like manner to form a complex
dendrite tree. Dendrite spines are on the
dendrites, which are structures specialized
for synaptic contact, receiving nerve
impulses.
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3. Classification of neurons
①According to the number of their processes,
they are described as:
Unipolar neuron,
Bipolar neuron,
Multipolar neuron.
②According to their functions and the direction of
transportation, they are described as:
Sensory (afferent) neuron
Motor (efferent) neuron
Intermediate (association) neuron.
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4. Nerve fiber
The nerve fibers are the longer processes
of neurons which are enveloped by
myelin sheath and nerve membrane.
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5. The synapses
Within the nervous system impulses are
conducted from one part to another along a chain
of neurons. The terminal arborizations of the
axon of one neuron ramify in close contact with
the cell body or dendrites, less frequently with
axonic terminals of many others. These structural
and functional areas of contact are termed
synapses.
Chemical synapse transports the impulses
through the chemical substance
neurotransmitter. The chemical synapse is the
most common type in the mammalian nervous
system.
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The chemical synapse includes three parts:
Presynaptic element,
Postsynaptic element.
Synaptic cleft.
The presynaptic element contains
numerous synaptic vesicles in which the
neurotransmitter is present and
presynaptic membrane.
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When an impulse arrives at the
presynaptic element, the neurotransmitter
diffuse cross the synaptic cleft and bind to
the receptor molecules in the postsynaptic
membrane. As a result, the postsynaptic
neuron is activated and impulse is
conducted from one neuron to the others.
Ⅱ). Neuroglia
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It includes the central and peripheral nervous
systemic neuroglia.
1. In the central nervous system
The neuroglial cells is the interstitial cells or
supporting cells.
According to their shape, they are divided into
four types, i.e.:
Macroglia, including Astrocytes,
Oligodendrocyte,Microglia, Ependymal cell.
2. In the peripheral nervous system
Schwann cell,Satellite cell.
Ⅲ. Activated Way of Nervous
System
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The way of activity of nervous system is
reflex.
Reflex:
A reflex is an automatic, stereotyped
reaction, such as movement, that is
performed without conscious volition in
response to an appropriate stimulus.
The basic structure is reflex arc.
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Reflex Arc:
The reflex arc, a linkage of afferent and efferent
neurons, is defined as the entire neural pathway
that is involved in a reflex. The effector, e. g. a
muscle, is supplied by an efferent nerve, and
between the afferent and efferent components
there may be one or more connector or
interneuron. These elements-afferent neurons,
interneurons and efferent neurons-are the
basis of reflex nervous activities.
Receptor→afferent (sensory) nerve
→center→efferent (motor) nerve→effector.
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3. Gray matter and white matter
Gray matter:
In the CNS the part that contains
aggregations of nerve cell bodies
embedded in a network of delicate nerve
processes is known as gray matter, it has
a gray color during the fresh condition.
White matter:
In the CNS the part that contains mainly
bundles of nerve fibers are white matter
and the white color is due to a rich
content of fatty myelin sheath.
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4. Cortex and medullary substance
Cortex:
The cortex is the outermost layer of gray
matter in the cerebral hemispheres or in
the cerebellum. The cell bodies in the
cortex are arranged in more or less welldefined laminae or layers.
Medullary substance:
It a central core of white matter beneath
the cortex of the cerebrum and cerebellum.
Ⅳ. Some Usual Terminology of
nervous system
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The neuroanatomical terms in common usage are
as follows:
1. Nucleus and ganglion
Nucleus:
Nerve cells with the same shape, function and
connections within the central nervous system
(CNS) are grouped together into nucleus.
The nucleus may originate, relay, modify, or
amplify neural signal within the nervous system.
Ganglion:
Nerve cells with the same shape, function and
connections outside the CNS often are grouped
together into ganglion.
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2. Nerve and nerve fiber
Nerve fiber:
Nerve fibers are mainly axons, some of which are
enveloped by myelin sheath. Fasciculus: In
CNS, a distinct collection of nerve fibers with
common origins, destinations and functions are
referred to fasciculus, or tract.
Nerve:
In the peripheral nervous system (PNS), the
nerve fibers are grouped into bundles to form the
nerve trunk. Most of nerves have a whitish
appearance because of their myelin content.
Chapter 2 The Central
Nervous System
Section 1 The Spinal Cord
Ⅰ. Location and Length
 The spinal cord, a long cylindrical
structure, is located in the vertebral canal
and invested by meninges.
 It extends from the foramen magnum,
where it continues with the medulla
oblongata, to the lower border of the first
lumbar vertebra, about 40～45 cm in
length. Diameters of the spinal cord are
not equal at various levels.
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Ⅱ. External Features
Ⅰ) Two enlargements and conus medullaris
The spinal cord displays two prominent
enlargements.
The cervical enlargements,
The lumbosacral enlargements.
Each enlargement associates with the nerve roots
that make up the brachial plexus and lumbosacral
plexus, which innervate the upper and lower
extremities, respectively. Caudal to the
lumbosacral enlargement, the spinal cord tapers
gradually and becomes the conical termination
known as conus medullaris.
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Ⅱ) Film terminale and cauda equina
A condensation of pia mater forms the film
terminale which descends the conus medullaris
to the level of the second sacral vertebra, from
here it is enveloped by the dura mater and
continues to the posterior surface of the coccyx.
Since the spinal cord is markedly shorter than the
vertebral column, the lumbosacral roots descend
for varying distances within the terminal cisterna
before reaching their corresponding intervertebral
foramina. They form a divergent sheaf of spinal
roots surrounding the film terminale, and is
called the cauda equina.
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Ⅲ) Fissure and sulci
Six longitudinal sulci are shown on the surface of
the naked spinal cord.
The anterior median fissure is on the median
line of the anterior surface, where the anterior
spinal artery and the companion vein are lodged.
On the posterior surface there is the shallow
posterior median sulcus.
On each side of the posterior median sulcus there
is a pair of posterior lateral sulci into which
filaments of the posterior roots enter the spinal
cord and a pair of posterior spinal arteries and
veins run along the sulci.
On each side of anterior median fissure there is a
pair of anterolateral sulci which mark the exit
of the anterior root fibers.
Ⅲ. External segments
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Thirty-one pairs of spinal nerves arise from the
spinal cord and pass through the intervertebral
froamina between adjacent vertebrae of the
vertebral column.
Each portion of the cord that gives rise to a pair
of spinal nerves is called a spinal segment. Each
spinal segment is functionally correlated with the
related cutaneous area, skeletal musculature and
viscera.
Because the vertebral column grows at a faster
rate than the spinal cord, so the spinal segments
are not equal to the vertebral segments except
for the CS1-4.
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Table 1. The correlation between spinal
segments (PS) and vertebrae
Spinal segments (SS)
Vertebrae
CS1-4
CV1-4
CS5-8, ST1-4
-1
TS5-8
-2
TS9-12
LS1-5
SS1-5, CoS
-3
TV10-12
TV12, LV1