THE SPINAL CORD AND THE SPINAL NERVES
A.
SPINAL CORD ANATOMY
The spinal cord is continuous with the brain, together forming the central nervous
system. List its three basic functions.
1.
2.
3.
1.
The spinal cord and its associated spinal nerves contain neuronal
circuits that mediate spinal reflexes.
The spinal cord is the site for integration (summing) of nerve
impulses that arise locally or arrive from the periphery and brain.
The spinal cord provides the pathways by which sensory nerve
impulses reach the brain and motor nerve impulses pass from the
brain to motor neurons.
PROTECTION AND COVERINGS
a.
VERTEBRAL COLUMN
b.
MENINGES
What protects the spinal cord?
Two types of connective tissue, bone and meninges, plus
the cushion of cerebrospinal fluid (CSF), surround and
protect the delicate nervous tissue of the brain and spinal
cord.
Where is the spinal cord located?
The spinal cord is located within the vertebral (spinal) canal
of the vertebral column. The vertebral foramina of the
vertebrae, stacked one on top of the other, form the canal.
What are the meninges?
The meninges (singular is meninx) are connective tissue
coverings that encircle the spinal cord and brain. There are
three layers:
1.
Dura mater
2.
Arachnoid membrane
3.
Pia mater
Identify the following:
Epidural space -- The epidural space is located between the
bony wall of the vertebral canal and the outer surface
of the dura mater. It is filled with fat and blood
vessels.
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Dura mater -- The dura mater, the outermost meninx, is
dense connective tissue forming a tube enclosing the
spinal cord. It extends to the S-2 vertebra, where it
closes.
Subdural space -- The subdural space lies deep to the dura
mater, between it and the arachnoid membrane. It
contains a small amount of interstitial fluid.
Arachnoid membrane -- The arachnoid membrane is the
middle meninx formed by delicate collagen and
elastin fibers. It is avascular.
Subarachnoid space -- Deep to the arachnoid membrane,
between it and the pia mater, is the subarachnoid
space. It is filled with cerebrospinal fluid (CSF).
Pia mater -- The pia mater, the inner-most meninx, is a thin
connective tissue that adheres to the surface of the
spinal cord, anchoring blood vessels to it.
What are the denticulate ligaments?
The denticulate ligaments are thin extensions of the pia
mater that anchor to the dura mater, effectively suspending
the spinal cord within the CSF of the subarachnoid space.
2.
EXTERNAL ANATOMY OF THE SPINAL CORD
Describe the following spinal cord features:
External anatomy -- The spinal cord is roughly cylindrical and
slightly flattened in its anterior-posterior dimension.
Differential growth -- Early in development the spinal cord fills the
entire vertebral canal. By the time of birth, the tip of the cord
reaches only to level L3-4. At age 4-5 the cord had reached
its adult length and ceases to grow. Differential growth of
the vertebral column, continuing until adult stature is
reached, is responsible for the disparity in length between
the vertebral canal and the spinal cord of the adult.
Adult length -- In the adult, the spinal cord extends from the
foramen magnum of the occiput, where it is continuous with
the medulla of the brain, to vertebral level L2.
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Enlargements -- The cervical enlargement, from C4-T1, represents
the origins of spinal nerves to and from the extremities. The
lumbar enlargement, from T9-T12, represents the origins of
spinal nerves to and from the lower extremities.
Describe the spinal cord as follows:
Conus medullaris -- Below the lumbar enlargement is the conus
medullaris, the conical tapering end of the adult spinal cord,
ending at L2.
Cauda equina -- Some nerves that arise from the spinal cord must
pass inferiorly through the vertebral canal before reaching
the appropriate intervertebral foramen for exit. These wisps
of nerve roots passing inferiorly through the lower vertebral
canal are collectively known as the cauda equina (horse’s
tail).
Filum terminale -- From the tip of the conus medullaris is the filum
terminale, an extension of the pia mater that attaches
inferiorly to the inside of the coccyx, thus anchoring the
spinal cord within the vertebral canal.
Segments (#) -- The spinal cord is functionally divided into 31
segments; from each “segment” emerges a pair of spinal
nerves. Therefore, there are 31 pairs of spinal nerves: 8
cervical. 12 thoracic, 5 lumbar, 5 sacral, and 1 coccygeal.
3.
INTERNAL ANATOMY OF THE SPINAL CORD
Describe the spinal cord as follows:
Internal anatomy -- In cross section, gray matter of the spinal cord
is shaped roughly like the letter “H” or a butterfly, and is
surrounded by white matter.
Gray matter -- The gray matter consists of:
1.
Neuronal cell bodies
2.
Unmyelinated axons and dendrites of association and
motor neurons
3.
Neuroglia
White matter -- The white matter consists of bundles of myelinated
axons of sensory, association, and motor neurons called
tracts.
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Central canal -- The gray commissure is the cross-bar of the “H”
allowing communication between the two sides, and bearing
in its middle the central canal, which runs the length of the
spinal cord and communicates with the fourth ventricle of the
brain.
Describe the spinal cord as follows:
Nuclei -- The gray matter on each side of the cord is subdivided into
regions called horns. Within the gray matter are clusters of
neuronal cell bodies called nuclei (centers); each nucleus
has a specific function.
Dorsal Horns -- The dorsal horns are those sections of the spinal
cord gray matter that project dorsally or posteriorly. They
contain nuclei that receive sensory information from the
spinal nerves and are therefore involved in sensory functions
only. The axons entering the dorsal horns from the dorsal
roots and are from those neurons whose cell bodies are
located in the dorsal root ganglion found just outside the
spinal ford within the intervertebral foramen.
Ventral horns -- The ventral gray horns of the spinal cord project
ventrally or anteriorly. They contain nuclei composed of
motor neurons whose axons leave the spinal cord as the
ventral roots. These neurons control somatic motor
functions only.
Lateral horns -- The lateral gray horns, found between the dorsal
and ventral horns and only in spinal segments T1-L2, and
S2-4, contain nuclei of motor neurons. The axons of these
neurons exit the spinal cord via the ventral roots. They are
involved in autonomic motor functions only.
Describe the spinal cord as follows:
Columns -- The white matter is also arranged into three broad
regions called columns: anterior (ventral), posterior (dorsal),
and lateral.
Tracts -- Each column is subdivided into distinct bundles of nerve
fibers, called tracts, each having a common origin or
destination and carrying similar information.
Ascending tracts -- Ascending tracts are sensory tracts, consisting
of axons that conduct impulses, and therefore information,
up the spinal cord to the brain.
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Descending tracts -- Descending tracts are motor tracts, consisting
of axons that conduct impulses down the spinal cord to the
ventral gray horns.
B.
SPINAL CORD PHYSIOLOGY
The spinal cord had two essential functions:
1.
Convey impulses between the periphery and the brain
2.
Provide integrating centers for spinal reflexes
1.
REFLEXES
What are reflexes?
Reflexes are fast, predictable, autonomic responses to changes in
the environment that help maintain homeostasis.
What are the three essential characteristics of a reflex?
1.
2.
3.
Inborn
Unlearned
Unconscious
Describe the following:
Roots -- Spinal nerves are the paths of communication between the
CNS and most of the periphery of the body. There are two
separate points of attachment called roots that connect a
spinal nerve with its segment of the spinal cord.
Dorsal root -- The dorsal (posterior) root contains sensory neuron
axons that conduct impulses from the periphery into the
dorsal gray horn of the spinal cord.
Dorsal root ganglion -- Each dorsal root has a swelling located
within the intervertebral foramen called the dorsal root
ganglion. It contains the nerve cell bodies of all the sensory
neurons found in that spinal nerve.
Ventral root -- The ventral (anterior) root contains motor neuron
axons and conducts impulses away from the spinal cord to
the appropriate effectors in the periphery. The nerve cell
bodies of origin for these fibers are within appropriate nuclei
found either in the ventral gray horns for somatic motor
effectors or in the lateral gray horns for visceral motor
effectors.
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2.
REFLEX ARC AND HOMEOSTASIS
What is a pathway?
The route followed by a series of nerve impulses from their origin in
one part of the body to their arrival elsewhere is called a pathway.
Pathways are specific neuronal circuits and may include only a
single synapse (monosynaptic) or more than one synapse
(polysynaptic).
The simplest kind of pathway in the nervous system is the reflex arc.
Regardless of complexity, all reflex arcs must include what five functional
components?
1.
2.
3.
4.
5.
Receptor
Sensory neuron
Center of integration
Motor neuron
Effector
Describe the following parts of a reflex arc:
Receptor -- The receptor is the distal end of a sensory neuron or an
associated sensory structure that responds to a specific
stimulus (change in the environment) by initiating a nerve
impulse.
Sensory neuron -- The sensory neuron passes the nerve impulse
generated by the receptor to the axon terminals located
within the gray matter of the CNS. Its cell body is located in
the dorsal root ganglion.
Center -- The center of integration is that region of the CNS gray
matter where the synapse(s) associated with the reflex are
located.
Motor neuron -- Impulses triggered by the integrating center are
carried by the motor neuron, whose cell body lies within the
gray matter, to the part of the body that will respond.
Effector -- The effector (muscle or gland) is that structure stimulated
by the motor neuron and which provides the response of the
body to the change in the environment that stimulated the
receptor.
a.
PHYSIOLOGY OF THE STRETCH REFLEX
Describe the stretch reflex.
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The stretch reflex is a monosynaptic reflex involving only two
neurons. It results in the contraction of a skeletal muscle
when the muscle is suddenly stretched.
The receptors, found within all skeletal muscles, are called
muscle spindles and constantly monitor changes in muscle
length.
In response to a stretch, a muscle spindle produces one or
more action potentials that are propagated along the
dendrite of its associated sensory neuron.
From the sensory dendrite the impulse passes along the
sensory axon (dorsal root), enters the dorsal gray horn, and
synapses with the appropriate motor neuron in the ventral
gray horn.
This excitatory synapse stimulates the motor neuron and a
second action potential is created that passes from the
spinal cord (ventral root), through the appropriate spinal
nerve, to innervate the appropriate motor unit.
Stimulation of the motor unit by the motor neuron causes it
to contract, thus counteracting the stretch. It is through this
mechanism that muscle tone is maintained.
Describe how the reciprocal innervation occurs so that the prime
mover of the stretch reflex can accomplish its task.
Since this reflex enters the spinal cord on the same side that
the motor impulse leaves it, this reflex is said to be an
ipsilateral reflex (all monosynaptic reflexes are ipsilateral).
Although the stretch reflex itself is monosynaptic, an
associated polysynaptic reflex to the antagonistic muscle
must also be activated.
The incoming sensory information from the stretch also
stimulates an association neuron, which in turn inhibits the
appropriate motor neurons to the antagonistic muscles.
Without this neuronal activity, the prime mover muscle of the
stretch reflex could not counteract the stretch.
This type of neuronal circuit, which provides for the
simultaneous contraction of one muscle while inhibiting the
antagonistic muscle(s), is called reciprocal innervation.
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The reflex adjusts muscle tone, adjusts muscle performance
during exercise, and helps prevent overstretching of
muscles. The best-known example of the stretch reflex is
the patellar reflex.
b.
PHYSIOLOGY OF THE FLEXOR (WITHDRAWAL) REFLEX AND
CROSSED EXTENSOR REFLEXES
Describe the flexor reflex.
The flexor (withdrawal) reflex is a polysynaptic reflex that
usually produces and occurs with a crossed extensor reflex.
Suppose you step on a tack: the pain stimulus is received
by a pain receptor and the resulting action potential passes
along a sensory neuron to the CNS.
At the integration center, the sensory neuron synapses with
an association neuron that does several things
simultaneously.
Ipsilaterally, the association neuron stimulates the
appropriate motor neurons to cause flexion, so that the
offended foot can be removed from the tack.
Since many different flexors are required to move the leg,
the association neuron must stimulate motor neurons in
several segments of the spinal cord. This is an
intersegmental reflex.
At the same time, there is reciprocal innervation of the
appropriate motor neurons so the ipsilateral extensor
muscles are inhibited, thus allowing the flexion movement.
What is the crossed extensor reflex?
The sensory impulses that initiate flexor reflex also initiate a
crossed extensor reflex so that balance can be maintained.
The axons of association neurons decussate (cross the
midline of the spinal cord) through the gray commissure to
stimulate motor neurons of the opposite side that initiate
extension of the other extremity.
C.
SPINAL NERVES
1.
COMPOSITION
2.
DISTRIBUTION OF SPINAL NERVES
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What are spinal nerves?
Spinal nerves connect the receptors in the periphery to the CNS via
sensory neurons, and the CNS to muscles and glands via motor
neurons.
How are they named?
The 31 pairs of spinal nerves are named and numbered according
to the region and level of the spinal cord from which they emerge.
Where do they emerge from the vertebral column?
The first cervical pair emerges between the atlas (C-1) and the
occiput; all other spinal nerve pairs emerge from the vertebral
column through the intervertebral foramina between adjoining
vertebrae,
What is their distribution?
8 cervical pairs
12 thoracic pairs
5 lumbar pairs
5 sacral pairs
1 coccygeal
How is a spinal nerve formed?
A typical spinal nerve is formed by the union of the dorsal and
ventral roots from the same side of the spinal cord segment. The
union of the spinal roots occurs within the intervertebral foramen
through which that spinal nerve passes
What is a mixed nerve?
The spinal nerve is a mixed nerve since it contains all sensory and
all motor components of that particular spinal cord segment.
Identify the following:
Endoneurium -- Each individual nerve fiber, either sensory dendrite
or motor axon, is surrounded by a connective tissue wrap
called the endoneurium.
Perineurium -- Groups of nerve fibers are arranged into fascicles
(fasciculi) and each bundle is surrounded by another
connective tissue wrap called the perineurium.
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Epineurium -- All fascicles of a spinal nerve are bound together by
an outermost connective tissue called the epineurium. This
layer is a continuation of the dura mater.
3.
DERMATOMES
What are dermatomes?
The skin over the entire body, with the exception of the face and
top of the head, is supplied by spinal nerves that carry somatic
sensory nerve impulses into the spinal cord. All spinal nerves,
except C-1, serve a specific and constant segment of the skin. The
area of skin that provides sensory input to the dorsal roots of one
pair of spinal nerves or to one spinal cord segment is called a
dermatome. Because the nerve supplies of adjacent dermatomes
overlap to some degree, there may be little loss of sensation if only
a single nerve supplying one dermatome is damaged.
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