Notes on the Web
Nervous System (In Part): Spinal Cord and Spinal Nerves

Bruce G. Stewart

Related Textbook Readings

Lecture Outlines

I. General Organization of Nerve Tissues Found in Either the CNS or PNS

A. Groupings of the neural tissues

1. white matter - aggregations of mylenated processes of neurons

2. gray matter - aggregations of neuron cell bodies and dendrites or unmyelinated axons and neuroglia

3. nerve

4. ganglia

5. tracts

6. nuclei (nucleus singlular)

7. horns

II. Spinal Cord

A. Protection and Coverings

1. Vertebral Canal- passageway formed by consecutive vertebral canals in the vertebral column

2. Meninges- coverings around spinal cord and brain (note: meninx is singular)

a. spinal meninges

  • * dura mater (“tough mother”)
    • tough, dense connective tissue that forms the outermost meninx
    • forms a tube from level of 2nd sacral vertebra up to brain (connects to filim terminale at the inferior point)
      • epidural space is outside dura mater-has fat and blood vessels (below 2nd lumber vertebra this is site of injection for anesthetics lick saddleblocks
  • * arachnoid - delicate connective tissue that forms middle meninx
    • # space between arachnoid and dura mater(subdural space)
    • contains serous fluid similar to that found in synovial joints
  • * pia mater- delicate, transparent fibrous membrane that adheres to spinal cord
    • # subarachnoid space above pia mater (and below the arachnoid) contains the cerebrospinal fluid which circulates around the spinal cord and brain (as will be discussed later)
    • # denticulate ligaments - extensions of the pia mater which attach to dura mater

3. meningitis - inflammation of the meniges

B. General Features

1. spinal cord is a slightly dorsoventrally flattened cylinder

a. continuous from medulla oblongata of brain to about the second lumber vertebra

b. about 2.6cm in diameter in the midthoracic area

  • * lumbar enlargement- 9th to 12th thoracic vertebra
    • # nerves supply lower extremities from here

c. conus medullaris - lower point at secound lumbar vertebra

d. filum terminale- non-nervous tissue (mostly pia mater) that attaches to coccyx on the inferior end

e. cauda equina - nerves that hang down from the inferior end of spinal cord; named for the appearance of a horse's tail

f. spinal segments - there are 31 segments which give rise to 31 pairs of spinal nerves; humans are segmentally designed creatures!

2. Spinal cord structure in cross section (turn to figure in your text book and find and memorize the following)

a. gray matter tissues of the spinal cord

  • * gray commissure which roughly forms an “H” pattern
    • this area allows the passage of axons from one side of the spinal cord to cross over to the other side in some neural pathways
  • * anterior (ventral) gray horns
    • one function is the sending of motor information out of the spinal cord by way of axons of motor neurons whose cell bodies are actually in the ventral gray horns
  • * posterior (dorsal) gray horns
    • one function is the receiving of sensory information from sensory neurons whose cell bodies lie outside the spinal cord in the dorsal root ganglia
  • * lateral gray horns

b. * central canal - small canal that runs longitudinally through the center of the spinal cord and is continuous with 4th ventricle (one of the larger spaces, but in the brain) of the medulla oblongata

c. white matter of the spinal cord arranged in columns (larger units) and tracts (subunits of columns)

  • * anterior (ventral) white columns
  • * posterior (dorsal) white columns
  • * lateral white columns
  • * tracts (or fasciculi) are bundles which are subdivisions of the columns and connect specific areas of the brain to their respective peripheral nerve origin areas
    • impulse conduction occurs via ascending and descending tracts of white matter
    • There are many important tracts and each has its own particular pattern and function; to gain an appreciation of the specificity of these patterns and functions, you will learn the details one ascending and one descending tract. Turn to your textbook illustrations and study the following two tracts:
      • tract lateral spinothalamic tract - an example of an ascending tract
      • lateral corticospinal tract - an example of a descending tract

d. reflex centers - areas of gray matter in spinal cord that serve for as a part of some reflex pathways; the following are components of these pathways both inside and outside of the spinal cord

  • *posterior or dorsal (sensory root) - fibers extend into spinal cord to dorsal gray horn
  • * posterior or dorsal (sensory) root ganglion has cell bodies from sensory neurons from the periphery
  • *anterior or ventral (motor) root contains motor neurons only which have cell bodies in the ventral gray matter of spinal cord
  • reflex arcs - functional units of the nervous system
    • * conduction pathways - circuits of neurons
    • includes two or more types of neurons which carry impulses from receptors to brain of spinal cord (by sensory neurons) and then to an effector (by way of motor neurons)
    • basic components - receptor, sensory ,neuron, center (usually with association neurons), motor neuron, effector (the organ which responds )
  • * reflexes - one type of reflex arc that provides exceptionally fast responses to stimuli
    • their are both spinal reflexes (carried out by the spinal cord and spinal nerves) and cranial reflexes (carried out by the brain and cranial nerves) and both help maintain homeostasis
      • spinal reflexes - somatic reflexes that affect skeletal muscles
      • cranial reflexes - visceral (autonomic) reflexes that involve brain centers and cranial nerves
        • affect smooth muscles, cardiac muscles and glandular secretion - examples of organs and processes affected include the heart (rate and force of contraction), respiration, digestion, urination, and defecation
    • example of a spinal reflex - the stretch reflex
      • * a monosynaptic reflex arc involving only one sensory neuron and one motor neuron (sometimes with an inhibitory motor neuron, too); the next bulleted items represent the reflex arc that begins with the sensory receptors in the skeletal muscle
      • muscle spindles - receptors in muscles that detect tension and can initiate an impulse
      • sensory neuron - tension causes the muscle spindle to initiate an action potential in sensory neuron which then travels along the axon to the dorsal root ganglia (where the cell body of the sensory neuron resides) and then on into the dorsal gray horn of spinal cord
      • motor neuron the end of the sensory neuron travels on to synapse with a motor neuron in the ventral gray horn of the spinal cord
        • the axon from the motor neuron travels out of the spinal cord through the ventral root of the spinal nerve and on to the antagonistic muscle (reciprocal innervation); this increases tension on the muscle that has the opposite function of the muscle originally stretched
      • inhibitory association neuron - to prevent tearing of the two muscles (the original stretched muscle and the reflex stimulated muscle, an inhibitory association neuron also receives the original sensory impulse (at the same time as the stimulatory motor neuron in the reflex arc) and it sends a simultaneous inhibitory impulse to the stretched muscle; this allows that muscle to relax when the other muscle is tightened
        • * patellar reflex is an example of an important spinal reflex
          • blocked by damaged afferent or efferent nerves to the muscle of reflex centers in the 2nd, 3rd , or 4th lumbar segments of the spinal cord
          • can also be eliminated by chronic diabetes and neurosyphilis.
          • an be exaggerated by injury to corticospinal tracts (descending ) and this can produce serious muscle spasms (Baclofen is used to reduce these debilitating spasms)
    • e. other spinal reflexes include the tendon reflex, flexor reflex (flexor stimulation and extensor inhibition), and crossed extensor reflex
      • examine the figures in your textbook, but you don't have to memorize the details

4. Spinal nerves

a. 31 pairs - 8 pairs of cervical, 12 thoracic, 5 lumbar, 5 sacral, 1 coccygeal

b. composition and coverings

  • two points of attachment - mixed nerves with posterior (dorsal) root of sensory fibers (axons) and anterior (ventral) root of motor fibers (axons)
  • coverings
    • endoneurium - connective tissue around fiber (axon)
    • perineurium - connective tissue around a group of fibers (axons) that each have their individual covering of endoneurium; these groups or bundles of fibers are called fascicles
    • epineurium - outermost connective tissue covering of the whole nerve; the epineurium of both spinal nerves and cranial nerves fuses with the meninges of the spinal cord and brain, respectively

c. distribution of fibers in spinal nerves

  • rami - branches of the spinal nerve which diverge after exit from intervertebral foramina (or converge depending on your perspective)
    • dorsal (deep muscles of back)
    • ventral (extremities and lateral and ventral trunk)
    • meningeal (vertebrae, vertebral ligaments, etc.)
    • rami communicantes - part of autonomic nervous system
  • plexes
    • * complex interconnections of various branches of spinal nerves which serve particular areas of the body
    • # cervical plexus
    • # brachial plexus
      • learn the five major nerves produced by this plexus and know what spinal nerve pairs contribute to the plexus
    • #(note: there is not a thoracic plexus)
    • # lumbar plexus
    • #sacral plexus

III. Some Clinical Applications

A. Injuries to the brachial plexus

1. crutch palsy - injury to brachial plexus such as to the radial nerve which serves extensors

2. radial nerve damage - causes wrist drop

a. deltoid intramuscular injections can cause damage if not given properly

b. cast pressure

3. medial nerve damage - causes numbness tingling, pain in palm and fingers; weak thumb movements; inability to pronate the forearm and difficulty in flexing the wrist

a . carpal tunnel syndrome - compression of the median nerve inside the carpal tunnel (between the flexor retinaculum ligament and the carpals )

  • *caused by trauma, edema, overuse (e.g. needlepoint, driving, cutting hair, piano playing.)

B. Injury to sciatic nerve

1. Caused by slipped disc, dislocated hips, osteoarthritis of the lumbosacral spine, or improperly administered gluteal intramuscular injection

2. Causes pain down the back of the leg, foot drop, loss of sensation over the leg and foot, etc.

C. Spinal cord injury - the complicated nature of the the tracts and functions of the gray mater make the specifics of spinal cord injury very unique depending on exact nature of damage; however here are some general terms that apply

1. Paralysis - lose of motor control of muscles (and generally with some lose of sensory perception depending on the cause of injury)

a. monoplegia - one extremity

b. diplegia - two

c. paraplegia - paralysis of both lower extremities

d. hemiplegia - two on one side of body

e. quadriplegia - all four extremities-

2. Transections

a. complete transections - cuts all ascending and descending tracts

b. hemisections - partial cuts of spinal cord; again the specific nature of an injury will produce its own set of specific symptoms

3.  Spinal shock - areflexia (period of loss of reflexes)

a. some may return within several days (e.g stretch reflex) while others take months

b . loss of control of urinary bladder and bowel functions, etc. also occurs

4 . Hope for future - stem cell research provides the greatest potential for spinal cord injury treatment

D. Peripheral nerve damage and disease

1. recall the ability of axons and dendrites with neurolemmocytes to regenerate; makes possible the reattachment of limbs with some return of motor control and sensory perception

2. neuritis - inflammation of a single nerve

a. caused by irritations due to blows, fractures, etc.

b. thiamine difficiency

c poisons : CO, CH4 , heavy metals

d. many illegal drugs of abuse (e.g. methamphetimine which has many extremely dangerous contaminants)

  • note my personal story on lose of sensory perception in my hand during the end of my active addiction

3. shingles - acute and recurring infection of the peripheral nervous system caused by herpes zoster ( the chickenpox virus)

a. virus retreats to posterior root ganglia & travels down sensory neurons causing pain and invading skin.

Reminder about Textbook Study

As with other topics, your textbook has excellent presentations of the materials on the spinal cord and spinal nerves of the nervous system. While you should focus on the specific material in the Notes on the Web, you should always use your textbook as a resource for illustrations and for understanding content that your notes cover.

As with all materials throughout the semester, you will have opportunities to ask questions or ask that any relevant material from your assignments be discussed in class.

Related Internet Resources

Back to A&P Page