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Nervous System:
The Central Nervous System
The Spinal Cord
Essential Information

Student Performance Objectives - for the lecture
1. State and identify, from diagrams, the 3 meninges covering the brain the spinal cord.
2. Compare the dural covering of the brain and spinal cord with regard to the meningeal and periosteal dural layers.
3. Describe the arrangement of ventricles and subarachnoid spaces in the brain through which cerebrospinal fluid flows.
4. Explain the origin, flow, and functions of cerebrospinal fluid.
1. Define each of the following terms as they relate to the nervous system:
    a. fascicle
    b. ganglion
    c. mixed nerve
    d. motor neuron
    e. nerve
    f.  nerve fiber
    g. neuroglia
    h. nucleus
    i.  plexus
    j.  sensory neuron
    k. synapse
    l.  tract.
2. Describe the number and placement of the cranial and spinal nerves.
3. Given a diagram of a transverse section of a spinal cord showing the spinal roots, the associated spinal nerve and peripheral connections of a single sensory and motor nerve fiber, identify and state the function of:     
     a. arachnoid mater
     b. association neuron
     c. cyton of motor neuron
     d. cyton of sensory neuron
     e. dorsal horns of gray matter
     f. dorsal roots
     g. dorsal root ganglion
     h. dura mater
     i.  gray matter
     j.  lateral horns of gray matter
     k. meninges
     l.  mixed nerves
    m. motor nerve fiber
     n. peripheral effector
     o. peripheral receptor
     p. pia mater
     q. sensory nerve fiber
     r. spinal nerves
     s. synaptic regions
     t. ventral horns of gray matter
     u. ventral roots
     v. white matter
4. Identify from a diagram, the following parts of the spinal cord: cervical region, thoracic region, lumbar region, sacral region, conus medullaris (medullary cone), cauda equina, and the filum terminale.
5. Explain why the spinal cord fills the spinal canal of the vertebral column during early fetal development, but ends between L1 and L2 as an adult.
6. Draw a diagram illustrating a transverse section through the spinal cord and a 3-neuron reflex arc.
7. Compare the functioning of the withdrawal (flexor) reflex and the crossed extensor reflex.
8. Explain the concept of synaptic inhibition observed in the central nervous system.
9. Explain the relationship of a spinal nerve with the dorsal and ventral nerve rootlets that actually penetrate the spinal cord.
10. Explain the relationship of a spinal nerve with its dosal, ventral and meningeal branches.
11. List the major plexuses arising from the ventral rami of the spinal nerves.
12. List one major nerve derived from each of the plexuses arising from the ventral rami of the spinal cord.

Lesson Outline
Overview of the Central Nervous System (CNS)
     1. In General - The CNS is composed of the brain and the spinal cord. The great mass of neural tissue known as the brain, located within the cranial cavity and protected by the flat bones of the cranium, has a neural continuation, the spinal cord, that occupies the spinal cavity and which is protected by the vertebral column. The foramen magnum, at the base of the skull, marks the anatomical (and artificial) division between the brain and spinal cord: the spinal cord's superior limit is the foramen magnum. The cord, in early fetal development, fills the spinal cavity. But, due to the vertebral column growing more rapidly than the spinal cord, by birth, the cord only extends as far as L3. In adults, the spinal cord extends to the border between L1 and L2.
    2. Meninges - The brain and spinal cord are both covered by 3 meninges that separate and protect the soft brain and spinal cord from the hard bones of the skull and vertebral column:
        a. Dura mater - an outer, tough, protective, fibrous connective tissue layer enclosing the brain and spinal cord. It consists of 2 layers - an outer periosteal layer and an inner meningeal layer.

            (1) The spinal cord is covered only by the inner meningeal dural layer. The epidural space is the region between the meningeal dura and the surrounding bone and is filled with a connective tissue matrix, adipose tissue, and blood vessels.
            (2) In the brain the dura's two layers are observed: one is continuous with the dura that surrounds the spinal cord. This is the meningeal layer. The outer periosteal layer adheres to the inner surface of the skull bones with no epidural space separating it from overlying bone. The brain's meningeal and periosteal dural layers adhere to each other except in two regions where they separate forming a space (or sinus) between them: the superior sagittal sinus and the transverse sinus. These sinuses collect blood that circulated through the brain and pass it to veins that ultimately return the blood to the heart. The superior sagittal sinus also collects cerebrospinal fluid, returning it to and mixing it with blood that ultimately returns to the heart.
            (3) The brain's inner, meningeal dural layer also acts as a barrier separating selected brain regions: the tentorium cerebelli separates the cerebellum from the cerebrum; the falx cerebelli separates the 2 cerebellar hemispheres; and the falx cerebri is the dural barrier separating the 2 cerebral hemispheres.
        b. Arachnoid mater - this is the layer just beneath (deep to) the dura and consists of a simple epithelial layer adherent to the dura with a space beneath it, the subarachnoid space, that consists of connective tissue fibers arranged in a spider's web-like matrix in which is found cerebrospinal fluid (CSF). CSF supports the brain and spinal cord and acts as a fluid cushion protecting the brain and spinal cord (which have the consistency of soft custard) from damage from our own bony skull and vertebral column as we move through our daily life of movements and impacts that might potentially damage our CNS. CSF also acts as a nutritional fluid stabilizing the chemical environment of the underlying neural tissue; in this regard it works with the blood that circulates more intimately within the capillary beds of neural tissue.
        c. Pia mater - this thin membrane adheres to the surface of the brain and spinal cord providing the deepest and most delicate CNS covering. Lateral extensions of the pia along the spinal cord, called denticulate ligaments, pass through the arachnoid and attach to the dura thereby helping to stabilize the "floating" spinal cord preventing lateral movements.
    3. Ventricles and CSF circulation - the brain has spaces within it called ventricles that are filled with CSF. There are 2 large lateral ventricles within the cerebral hemispheres, the narrow third ventricle deep within the brain separating the right and left portions of the hypothalamus, and the triangular shaped fourth ventricle, just anterior to the cerebellum and posterior to the pons. Each ventricle possess a choroid plexus which forms some of the CSF found in the ventricles. Tiny canals (called interventricular foramina) connect the lateral ventricles with the third ventricle, the third ventricle connects with the fourth ventricle through the cerebral aqueduct (easily observed on a sheep's brain in the laboratory), and the fourth ventricle possesses 1 median and 2 lateral apertures through which CSF passes into the subarachnoid space surrounding the brain and the spinal cord.        
    4. Cerebrospinal Fluid
a. Chemical Composition - CSF has a chemical composition similar to interstitial fluid only with a slightly higher salt concentration and very little protein.
        b. Formation - CSF is formed in the choroid plexuses of the brain's 4 ventricles (30%), and also in vascular tissues other than choroid plexuses - from the ventricular lining (30%) and from the lining of the subarachnoid spaces (40%).
    5. Working Definitions of Common Neurological Terms
a. Neuron - the basic cell type of the nervous system.
        b. Cyton - the cell body of a neuron.
        c. Axon - the cellular extension of the cyton carrying nerve impulses away from the cyton.
        d. Dendrite - cellular extensions of the cyton that carry impulses into the cyton.
        e. Synapse - the region of communication between 2 neurons.
        f. Nucleus - the location of a group of cytons within the CNS (e.g., the red nucleus).
        g. Ganglion - the location of a group of cytons in the PNS (e.g., the dorsal root ganglion).
        h. Tract - a bundle of axons traveling from one location to another within the CNS.
        i. Nerve - a bundle of axons traveling from one location to another within the PNS.
        j. Nerve fiber - the general expression for an axon in either the CNS or the PNS. Note: dendrites are not long enough to qualify to be called nerve fibers, or tracts or nerves.
        k. Motor neuron - a neuron whose axon carries signals from the CNS to effectors.
        l. Effectors - organs that bring about effects - like muscles and glands.
      m. Sensory neuron - a neuron whose axon carries signals from receptors into the CNS.
       n. Mixed nerve - bundles of axons, some of which carry sensory signals and some of which carry motor signals. E.g., all 31 pairs of spinal nerves are mixed nerves.
       p. Plexus - a network of nerves or blood vessels.
       q. Translation - the conversion of bioelectrical signals (nerve impulses) into sensations. E.g., bioelectrical signals from the inner ear are translated into sounds.
        r. Interpretation - giving meaning to sensations. E.g., sounds are interpreted as speech.
        s. Integration - multiple interpretations occurring simultaneously are integrated into a common experience. E.g., the sights, sounds, tastes, odors, and cutaneous interpretations are understood as "having dinner with friends."

B. The Spinal Cord
     1. Functions
         a. Simple Behaviors - the spinal cord is capable of causing reflexes - these are involuntary, automatic, motor responses to stimuli. E.g., the knee jerks when the patellar tendon is tapped because the tap stretches the quadriceps muscle and the reflex response, mediated through the spinal cord, is contraction of the stretched muscle. The brain and consciousness play no role in this activity. Similarly, the spinal cord controls the reflex emptying of the urinary bladder or the bowel when stretch receptor sensory input to the spinal cord triggers motor output from the spinal cord to the smooth muscle of these organs.
        b. Communication Link - the spinal cord is the major link in the communication between the periphery (i.e., the part of the body outside the CNS) and the Brain. Sensory signals enter the cord from the body surface (skin), and internal sensory organs like muscle, tendon and joint stretch receptors, and are conducted to the brain for analysis and the generation of appropriate motor responses.
    2. Surface Anatomy -
The spinal cord begins at the foramen magnum and measures about 45 cm (about 1.5 ft) in length. It is about as thick as your pinky. Its shape varies along its length with a cervical enlargement, where nerves to the upper limbs originate, and a lumbar enlargement giving rise to nerves to the lower limbs and pelvis. The end of the spinal cord, between L1 and L2, is the conus medullaris (or medullary cone). Hanging from the cone like a horse's tail is the cauda equina - the mass of spinal nerves from the lower lumbar, sacral and coccygeal portions of the spinal cord. A thread of connective tissue, the filum terminale, originates from the pia mater around the conus medullaris. It extends inferiorly and attaches to the coccygeal vertebrae thus helping to anchor the spinal cord. Recall that the denticulate ligaments also originate from the pia mater and help to reduce spinal cord movements.
    3. Transverse Section Anatomy 
         a. Gray matter
(1) In general - the gray matter consists of neural tissue mostly lacking myelin - this consists of axon terminal branches, the cytons and dendrites of association neurons, the cytons of motor neurons, and synapses between these neural elements. As spinal nerves approach the cord they branch into anterior and posterior roots. Sensory signals enter the cord through posterior roots; motor signals exit from the cord through anterior roots. The cytons of sensory (unipolar) neurons, whose branched axons extend into the cord and all the way to peripheral receptors, are located in a swelling on each posterior root - each is called a posterior root ganglion (often called the dorsal root ganglion).
             (2) Shape - the shape of the spinal cord's gray matter observed in a transverse section of the cord is that of a butterfly or the letter H. The H consists of two posterior (dorsal) horns of gray matter and two anterior (ventral) horns of gray matter. They are linked together by the gray commissure. The tiny central canal is in the center. Lateral horns of gray matter are observed in the thoracic and lumbar areas of the cord.
                  (1) The posterior horns of gray matter receive the axons from sensory neurons. Some of these the axon terminals synapse with association neurons in this area. Some of these axons travel directly to the anterior horns of gray matter.
                  (2) The anterior horns of gray matter contain the cytons of motor neurons that receive signals from the association neurons or from the axons of the sensory neurons.
        b. White matter
             (1) In general - the white matter consists of myelinated nerve fibers traveling toward the brain (ascending tracts) and away from the brain to various levels of the spinal cord (descending tracts).
            (2) Decussation - many, but not all, spinal tracts physically cross over from the side of the body they originated from, to the other side of the body, before reaching their destination. This crossing over is called decussation. It occurs either in the medulla (a part of the brain just above the foramen magnum) or at some level of the spinal cord. This means that moving your right arm results from signals originating in the left side of your brain. Similarly, feeling an itch in your left palm is sensed by the right side of your brain. If a spinal tract decussates, we say its origin and destination are contralateral. If a tract does not decussate we say its origin and destination are ipsilateral.
            (3) Ascending tracts are sensory tracts and are named for their place of origin and their destination: e.g., the lateral spinothalamic tract is a sensory tract originating in the lateral portion of the spinal cord, and relaying sensory information to the thalamic portion of the brain. This particular tract relays to the brain typical sensory information from the skin - light touch, pressure, temperature and pain. Another example is the dorsal spinocerebellar tract- this tract originates in the dorsal region of the spinal cord and relays balance information (referred to as proprioceptive information) from muscles to the part of the brain known as the cerebellum.
            (4) Descending tracts are motor tracts and are also named for their place of origin and their destination; e.g., the lateral corticospinal tract is a motor tract originating in the cerebral cortex and relaying motor information to the spinal cord. "Lateral" in this instance indicates it travels along the lateral portion of the spinal cord. This particular tract relays information concerning skilled motor movements for the arms and legs. Another example is the lateral reticulospinal tract which relays balance and postural motor signals from the brain's reticular formation to the spinal cord. "Lateral" indicates that it travels along the lateral portion of the spinal cord.
     4. Reflex Arc - many relatively simple but important and protective human behaviors are mediated through the rapid activity of the spinal cord. These rapid, involuntary, automatic and unconscious behaviors are called reflexes. A reflex arc is the functional pathway for signal transmission that accomplishes the reflex. The following two examples of reflexes reinforces common elements in all reflexes and presents increasing levels of detail that clarify how some apparently simple behaviors are elegantly controlled.

       a. Stretch reflex
            (1) In general - when muscles are stretched they respond by contracting. This is a protective mechanism in that the maintenance of posture and balance depends on the unconscious adjustments of muscles to the force of gravity which might pull the body to one side or another. Once pulled by gravity, even ever so slightly, muscles on one side stretch and then reflexively contract, bringing the body back into correct balance. Continuous minor adjustments like this make such postural and balance maintenance smooth and graceful and unnoticed. Many receptors work to maintain balance and include muscle spindles, golgi tendon apparatuses, joint receptors, and gravity receptors and motion detectors in the inner ear. Clearly the ultimate coordination of all these balance (or proprioceptive) inputs is handled by the brain, in particular, the cerebellum. We will study the operation one of the balance receptors, a muscle spindle, such as the one found in the quadriceps muscle. When the patellar tendon, just below the knee, is lightly tapped with a rubber hammer, it transmits a stretch to the quadriceps muscle which responds with a slight contraction resulting in a knee jerk.
            (2) The reflex arc of the stretch reflex- tapping the patellar tendon stretches the quadriceps muscle which results in the stimulation of muscle spindles distributed within the muscle. Afferent signals travel along sensory neuronal axons, eventually enter the spinal cord through the posterior root of a spinal nerve and synapse directly with motor neuron cytons in the ventral horns of the cord's gray matter. Efferent signals travel along motor neuronal axons, leave the spinal cord through the anterior root of a spinal nerve, pass through the spinal nerve and eventually reach the muscle fibers of the quadriceps where they stimulate a contraction. This reflex arc involves only one synapse - it is called a monosynaptic reflex arc (or, sometimes, a 2-neuron reflex arc).
           (3) Reciprocal inhibition - When a muscle contracts and its antagonistic muscle relaxes, the process causing that is called reciprocal inhibition. In order for the quadriceps muscle to contract, antagonistic muscles (i.e., the hamstrings) must relax. So axons, carrying the afferent signal entering the spinal cord, in addition to stimulating the motor neurons connected to the quadriceps, also have a branch that stimulates an inhibitory association neuron in the spinal cord. This association neuron sends an inhibitory signal to the anterior horn motor neurons connected to the hamstrings preventing them from contracting. Now when the quadriceps receives the signal to contract, the hamstrings will not have received any signal at all and they remain relaxed.
            (4) Groups of muscles - the stretch reflex, like many reflexes, does not involve just one muscle. It usually involves groups of muscles all of which operate to bend a joint in a similar way. In the example given, tapping the patellar tendon stretches fibers and stimulates muscle spindles in the rectus femoris, vastus lateralis, vastus medialis, and vastus intermedius muscles, all of which make up the "muscle" we call the quadriceps. So the reflex contraction observed after the quadriceps tendon is tapped is actually the simultaneous contraction of fibers in all the muscles composing the quadriceps. Similarly, inhibition of the "hamstrings" means inhibiting the contraction of the biceps femoris, the semitendinosus and the semimembranosus muscles as these three muscles compose the hamstrings. So the typical "simple" reflex involves much coordination of many muscles working synergistically and antagonistically.
        b. Withdrawal reflex and crossed extensor reflex
         (1) In general - these two reflexes are discussed together because they generally act together to protect us from a painful stimulus.
            (2) The withdrawal reflex (flexor reflex) is initiated if you walk barefoot on a wooden floor and an exposed nail pierces the bottom of your foot. Afferent signals enter the spinal cord and, working through association neurons in the cord, simultaneously stimulate motor neurons connected to the leg flexors - (hamstrings) and, inhibit motor neurons connected to the leg extensors (quadriceps). The result is that you flex your knee (raise your leg) away from the painful stimulus. This reflex arc involves 2 synapses (between the sensory and association neurons, and between the association and the motor neurons). It is called a polysynaptic reflex arc (or, sometimes, a 3-neuron reflex arc.
            (3) The crossed extensor reflex works in this particular situation because it extends the knee of the opposite leg, keeping you standing. When the afferent signal enters the cord bringing about the withdrawal reflex as just described, that signal also crosses to the other side of the spinal cord and stimulates association neurons there. Some of these association neurons stimulate leg extensors so you remain standing. Other association neurons inhibit the action of the leg flexors (hamstrings) since they are antagonists to the extensors that must contract to keep you upright. So in this reflex we see reciprocal inhibition, as we did for the stretch reflex, and we also see coordination of muscle groups on both sides of the body permitting an efficient response to a painful stimulus.
        c. More complex reflexes occur at the level of the brainstem such as crying, laughing, yawning, coughing and sneezing. These reflexes utilize the principles of polysynaptic reflex arcs and reciprocal inhibition but involve the coordination of many more muscles and muscle groups.

C. Nerves - As a review - there are 12 pairs of cranial nerves and 31 pairs of spinal nerves that make up the PNS (peripheral nervous system).
   1. Each nerve is an organ composed of:
        a. nervous tissue itself: bundles of axons which are sometimes just referred to as nerve fibers, and
b. specialized connective tissue, neuroglia, that are mainly Schwann cells forming the myelin sheaths around the axons that connect to skeletal muscle, and
        c. fibrous connective tissue that encloses the entire nerve (epineurium), that encloses bundles of nerve fibers (fascicles enclosed in perineurium), and also connective tissues enclosing individual fibers (endoneurium). The nerve also possesses its own
        d. blood vessels that exchange nutrients and wastes with the other tissues of the nerve.
     2. All cranial and spinal nerves are mixed nerves (carrying both sensory and motor nerve fibers) except for cranial nerve I (olfactory nerve for sense of smell) and cranial nerve II (optic nerve for sense of sight). These 2 nerves are purely sensory.
    3. The cytons of most of the nerve fibers whose axons make up the nerves are located either within the spinal cord or brain. Those cytons that are located outside the brain or spinal cord occur in clusters called ganglia, like the dorsal root ganglion discussed previously.
    4. Spinal nerves have dorsal and ventral roots that enter the spinal cord. Afferent signals enter the cord through the dorsal roots and motor signals leave the cord through the ventral roots. The dorsal root has a swelling, the dorsal root ganglion (DRG) that contains the cytons of the sensory (afferent) nerve fibers. Very close to the spinal cord the dorsal root subdivides into 6-8 smaller roots (rootlets) that actually enter the cord. Similarly, 6-8 ventral rootlets exit the cord. The dorsal and ventral rootlets fuse to form the dorsal and ventral roots. Dorsal and ventral roots pass through the dura mater surrounding the cord, then fuse together, just past the DRG, as they pass through the intervertebral foramina (the openings between adjacent vertebrae) to form the spinal nerve itself.
    5. There are 7 cervical vertebrae but 8 cervical spinal nerves because the 1st cervical nerve exits between the base of the cranium and the atlas. From C2 through L1, each spinal nerve exits from the cord laterally through its adjacent intervertebral foramen. From L2 downward the spinal nerves form the cauda equina and then exit from adjacent and lower intervertebral foramina and the sacral foramina.
    6. Each spinal nerve forms 3 branches right after emerging from the intervertebral or sacral foramina: a small meningeal branch innervates the local vertebra and meninges, a dorsal branch (or dorsal ramus) innervates the muscles, joints and skin of the back in that region, and a ventral branch (ventral ramus) innervates muscles and skin of ventral and lateral areas in that region. In addition, the ventral rami form the nerve plexuses that give rise to the nerves that pass into the arms and legs.
    7. Nerve plexuses - there are no nerve plexuses formed in the thoracic region. There are, however, nerve plexuses formed from the ventral rami of the spinal nerves in the area of the neck - cervical plexus, the shoulder - brachial plexus, the lower back - lumbar plexus, and the sacrum - the sacral and coccygeal plexuses. Only selected nerves will be indicated as arising from each major plexus. More extensive coverage of nerves may be required in the laboratory portion of this course, depending on your specific lab instructor.
        a. Cervical plexus - arises from the ventral rami of spinal nerves C1-C5 and gives rise to several nerves of which the 2 phrenic nerves that innervate the diaphragm - easily seen in a cat or pig dissection running inferiorly past the heart - are the most significant.
        b. Brachial plexus - arises from the ventral rami of spinal nerves C4-T2 and gives rise to the axillary nerve of the shoulder and the radial, median and ulnar nerves of the arms.
        c. Lumbar plexus - arises from the ventral rami of T12-L4 and gives rise to the ilioinguinal, femoral, saphenous and obturator nerves.
 d. Sacral plexus - the sacral plexus arises from the ventral rami of spinal nerves L4-S4 and gives rise to the tibial and common fibular nerves that travel united and are referred to as a unit - the sciatic nerve of the buttock and thigh - and which separate as individual tibial and common fibular nerves at the back of the knee (popliteal fossa).
 e. Coccygeal plexus - arises from the ventral rami of spinal nerves S4-Co1 (Co1 stands for the first coccygeal vertebra) and gives rise to the pudendal nerve that is sensory to the genitals and motor to the muscles of the perineum.
    8. Dermatomes
- a dermatome is a specific area of the skin that is innervated by specific spinal nerves. They are used diagnostically to ascertain damage to or compression of spinal nerves that supply sensory fibers to these areas. Observe the dermatome maps. Dermatomes overlap meaning that more than one spinal nerve provides sensation to any given skin region. Therefore there is not a perfect one-to-one correspondence of sensation (or loss of sensation) in an area of skin with a specific spinal nerve; partial loss of sensation (numbness) in a specific region approximates the area of the vertebral column where some compression may be occurring.
For a nice powerpoint review of the nervous system, including spinal tracts, see:

Biomedical terminology:   Define each term.

arachnoid mater
association neuron
brachial plexus
cervical plexus
coccygeal plexus
cyton of motor neuron
cyton of sensory neuron

dorsal horns of gray matter
dorsal ramus
dorsal roots
dorsal root ganglion
dura mater
gray matter
lateral horns of gray matter
lumbar plexus
meningeal ramus
mixed nerves
motor nerve fiber
motor neuron
nerve fiber
peripheral effector
peripheral receptor
pia mater
sacral plexus
sensory nerve fiber
sensory neuron
spinal nerves
synaptic regions
ventral horns of gray matter
ventral ramus
ventral roots
white matter

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