l>BIOL 237 Class Notes - The Spinal Cord and Spinal Nerves
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The Nervous System - Spinal Cord and Peripheral Nerves
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The spinal cord is the connection center for the reflexes as well asthe afferent (sensory) and efferent (motor) pathways for most of thebody below the head and neck. The spinal cord begins at thebrainstem and ends at about the second lumbar vertebra. Thesensory, motor, and interneurons discussed previously are found inspecific parts of the spinal cord and nearby structures. Sensoryneurons have their cell bodies in the spinal (dorsal root) ganglion.Their axons travel through the dorsal root into the gray matter ofthe cord. Within the gray matter are interneurons with which thesensory neurons may connect. Also located in the gray matter arethe motor neurons whose axons travel out of the cord through theventral root. The white matter surrounds the gray matter. Itcontains the spinal tracts which ascend and descend the spinal cord.Surrounding both the spinal cord and the brain are the meninges, athree layered covering of connective tissue. The dura mater is thetough outer layer. Beneath the dura is the arachnoid which is like aspider web in consistency. The arachnoid has abundant space withinand beneath it (the subarachnoid space) which containscerebrospinal fluid, as does the space beneath the dura mater(subdural space). This cerebrospinal fluid supplies buoyancy forthe spinal cord and brain to help provide shock absorption. The piamater is a very thin layer which adheres tightly to the surface of thebrain and spinal cord. It follows all contours and fissures (sulci) ofthe brain and cord.
Terms:ganglion - a collection of cell bodies located outside the Central Nervous System. Thespinal ganglia or dorsal root ganglia contain the cell bodies of sensory neurons enteringthe cord at that region.nerve - a group of fibers (axons) outside the CNS. The spinal nerves contain the fibers ofthe sensory and motor neurons. A nerve does not contain cell bodies. They are located inthe ganglion (sensory) or in the gray matter (motor).tract - a group of fibers inside the CNS. The spinal tracts carry information up or downthe spinal cord, to or from the brain. Tracts within the brain carry information from oneplace to another within the brain. Tracts are always part of white matter.gray matter - an area of naipublishers.comyelinated neurons where cell bodies and synapses occur. Inthe spinal cord the synapses between sensory and motor and interneurons occurs in thegray matter. The cell bodies of the interneurons and motor neurons also are found in thegray matter.white matter - an area of myelinated fiber tracts. Myelination in the CNS differs fromthat in nerves.
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At 31 places along the spinal cord the dorsal and ventral roots cometogether to form spinal nerves. Spinal nerves contain both sensoryand motor fibers, as do most nerves. Spinal nerves are givennumbers which indicate the portion of the vertebral column in whichthey arise. There are 8 cervical (C1-C8), 12 thoracics (T1-T12), 5lumbar (L1-L5), 5 sacral (S1-S5), and 1 coccygeal nerve. Nerve C1arises between the cranium and atlas (1st cervical vertebra) and C8arises between the 7th cervical and 1st thoracic vertebra. All theothers arise below the respective vertebra or former vertebra in thecase of the sacrum. Since the actual cord ends at the second lumbarvertebra, the later roots arise close together on the cord and traveldownward to exit at the appropriate point. These nerve roots arecalled the cauda equina because of their resemblance to a horsestail.
The dermatomes are somatic or musculocutaneous areas served by fibers from specificspinal nerves. The map of the dermatomes is shown by Figure 13.11.This map is useful indiagnosing the origin of certain somatic pain, numbness, tingling etc. when thesesymptoms are caused by pressure or inflammation of the spinal cord or nerve roots.Referred pain is caused when the sensory fibers from an internal organ enter the spinalcord in the same root as fibers from a dermatome. The brain is poor at interpretingvisceral pain and instead interprets it as pain from the somatic area of the dermatome. Sopain in the heart is often interpreted as pain in the left arm or shoulder, pain in thediaphragm is interpreted as along the left clavicle and neck, and the "stitch in your side"you sometimes feel when running is pain in the liver as its vessels vasoconstrict. (SeeFigure 14.8)
Spinal nerves join together in plexuses. (See Figure 13.5) A plexus is an interconnectionof fibers which form new combinations as the "named" or peripheral nerves. There arefour voluntary plexuses (there are some autonomic plexuses which will be mentionedlater): they are the cervical plexus, the brachial plexus, the lumbar plexus, and thesacral plexus. Each plexus gives rise to new combinations of fibers as the peripheralnerves. The nerves and plexuses you need to know are:Cervical Plexus (See Figure 13.7, Table 13.3) - the phrenic nerve travels through thethorax to innervate the diaphragm.Brachial Plexus (See Figure 13.8, Table 13.4) - Axillary nerve - innervates the deltoid muscle and shoulder, along with the posterioraspect of the upper arm. Musculocutaneous nerve - innervates anterior skin of upper arm and elbow flexors. Radial nerve - innervates dorsal aspect of the arm and extensors of the elbow, wrist,and fingers, abduction of thumb. Median nerve - innervates the middle elbow, wrist and finger flexors, adducts thethumb. Ulnar nerve - innervates the medial aspect wrist and finger flexors.Lumbar Plexus (See Figure 13.9, Table 13.5) genitofemoral - to the external genitalia obturator - to the adductor muscles femoral - innervates the skin and muscles of upper thigh, including the quadriceps.Sacral Plexus (See Figure 13.10, Table 13.6) gluteal nerves (superior and inferior) - superior innervates the gluteus medius andminimus, inferior innervates the gluteus maximus. sciatic nerve - the body"s largest nerve, consisting of two major branches, the tibialand common peroneal. Together they innervate most all of leg including the flexors ofthe knee, part of adductor magnus, muscles for plantar flexion, dorsiflexion, and othermovements of the foot and toes.
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Structure of a nerve:A peripheral nerve is arranged much like a muscle in terms of itsconnective tissue. It has an outer covering which forms a sheatharound the nerve, called the epineurium. Often a nerve will runtogether with an artery and vein and their connective coverings willmerge. Nerve fibers, which are axons, organize into bundles knownas fascicles with each fascicle surrounded by the perineurium.Between individual nerve fibers is an inner layer of endoneurium.

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The myelin sheath in peripheral nerves consists of Schwann cellswrapped in many layers around the axon fibers. Not all fibers in anerve will be myelinated, but most of the voluntary fibers are. TheSchwann cells are portrayed as arranged along the axon likesausages on a string. (A more apt analogy would be like jelly rolls!)Gaps between the Schwann cells are called nodes of Ranvier.These nodes permit an impulse to travel faster because it doesn"tneed to depolarize each area of a membrane, just the nodes. Thistype of conduction is called saltatory conduction and means thatimpulses will travel faster in myelinated fibers than in naipublishers.comyelinatedones.The myelin sheath does several things:1) It provides insulation to help prevent short circuiting betweenfibers. Diseases which destroy the myelin sheath lead to inability tocontrol muscles, perceive stimuli etc. One such disease is multiplesclerosis, an autoimmune disorder in which your own lymphocytesattack the myelin proteins. .2) The myelin sheath provides for faster conduction.3) The myelin sheath provides for the possibility of repair ofperipheral nerve fibers. Schwann cells help to maintain the micro-environments of the axons and their tunnel (the neurilemma tunnel)permits re-connection with an effector or receptor. (See below)CNS fibers, not having the same type of myelination accumulatescar tissue after damage, which prevents regeneration.
Regeneration of a peripheral nerve fiber (See Figure 13.3) depends upon several things.First the damage must be far from the cell body. Anterograde degeneration destroys theaxon distal to the point of damage. Retrograde degeneration causes the fiber todegenerate for a distance back toward the cell body. The amount of axoplasm lostdetermines whether the neuron can survive. Secondly the myelin sheath and itsneurilemma tunnel must be intact. Chemicals such as the myelin proteins tend to inhibitregrowth, but macrophages will enter the damaged area and phagocytize these proteinsand other debris. Schwann cells will proliferate and secrete growth stimulating factorsand provide the chemical and physical needs necessary for growth and re-innervation bythe axon.
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The Spinal Tracts: (See Figure 12.30)The white matter of the spinal cord contains tracts which travel upand down the cord. Many of these tracts travel to and from thebrain to provide sensory input to the brain, or bring motor stimulifrom the brain to control effectors. Ascending tracts, those whichtravel toward the brain are sensory, descending tracts are motor.Figure 12.30 shows the location of the major tracts in the spinalcord. For most the name will indicate if it is a motor or sensorytract. Most sensory tracts names begin with spino, indicating originin the spinal cord, and their name will end with the part of the brainwhere the tract leads. For example the spinothalamic tract travelsfrom the spinal cord to the thalamus. Tracts whose names beginwith a part of the brain are motor. For example the corticospinaltract begins with fibers leaving the cerebral cortex and travels downtoward motor neurons in the cord.
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