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.The spinal cord and its associated spinal nerves contain neuronal
circuits that mediatespinalreflexes.
2.The spinal cord is the siteforintegration (summing) of nerve impulses that arise locally or arrive from the periphery and brain.
3.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.
1.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.
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.
Enlargements -- The cervicalenlargement, from C4-T1, represents
the origins of spinal nerves to and from the extremities. The lumbarenlargement, 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.
Central canal -- The gray commissure is the cross-bar of the “H”
allowing communication between the two sides, and bearing in its middle the centralcanal, 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 dorsalroots and are from those neurons whose cell bodies are located in the dorsalrootganglion 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 ventralroots. 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.
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 spinalreflexes
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.Inborn
2.Unlearned
3.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.
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.Receptor
2.Sensory neuron
3.Center of integration
4.Motor neuron
5.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.
The stretchreflex 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 musclespindles 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 muscletone 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 sameside that the motor impulse leaves it, this reflex is said to be an ipsilateralreflex (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 associationneuron, 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 reciprocalinnervation.
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 patellarreflex.
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 crossedextensorreflex.
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 intersegmentalreflex.
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
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 pairs5 sacral pairs
12 thoracic pairs1 coccygeal
5 lumbar pairs
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.
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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