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Chapter 21: Spinal Cord, Spinal Nerves, and the Autonomic Nervous System

Anatomy of the Spinal Cord

Spinal Cord – continuation of the brain stem.  Plays a major role in the spinal reflex activity and provides neural pathways to and from higher nervous centers.  Enclosed in the spinal column, the spinal cord extends from the foramen magnum of the skull to the first or second Lumbar vertebra where it

terminates in Conus Medullaris.  

The meninges of the spinal cord: dura mater, arachnoid mater, and pia mater are continuous with the Cranial meninges which encircle the brain.  All three meninges cover the spinal nerves up to the point of their exit from the intervertebral foramina.

Cauda Equina – Collection/ extension of spinal nerves exiting the base of the spinal cord; appearance resembles that of a horse’s tail.

Filum Terminale – membranous tissue, continuous with the pia mater of the spinal cord, that extends from the conus medullaris to the coccyx to anchor the spinal cord.

Organization of the gray matter

1.  Gray commissure – connects the posterior horns of the gray matter.

2.  Central canal – small opening in middle of the cord; contains CSF.

3.  Anterior horns – gray matter which contains (mostly) cell bodies of neurons for somatic motor control.

4.  Posterior horns – contains cell bodies of somatic and visceral sensory neurons.

5.  Lateral horns – occur only in certain regions of the spinal cord (thoracic, lumbar, sacral); contain cell bodies of visceral motor neurons.

6.  Dorsal root – enters the posterior cord, and carries afferent sensory information.

Dorsal root ganglion – cluster of sensory cell bodies outside the CNS.

7.  Ventral root – carries efferent motor information from the anterior surface of the cord.

After emerging each nerve divides into dorsal and ventral aspects:

1.  Dorsal rami (roots) – innervate the skin and back muscles.

2. Ventral rami (roots)– innervate most of the skeletal muscles of ventrolateral surface, body wall, and limbs.

Organization of the white matter

1.  Anterior median fissure – midline of spinal cord on anterior surface.

2.  Posterior median sulcus – less distinct groove in posterior midline; separates 2 posterior columns.

Posterior median septum  – Deep to Posterior median sulcus.  Filled with neuroglia.

Horns of gray matter divide white matter into regions:

1.  Anterior white columns (Funiculi)

2.  Posterior white columns (Funiculi)

3.  Lateral white columns (Funiculi)

Funiculi are composed of mylinated and unmylinated nerve fibers tracts that allow communication parts of the spinal cord and between the spinal cord and the brain.

A.  Ascending tracts – conduct sensory, info to brain.

B.  Descending tracts – conduct motor, signals from brain.

Spinal Nerves and Nerve Plexuses

The 31 pairs of spinal nerves are:
     8 Cervical (C1-C8) pairs  (even though only 7 cervical vertebrae)
   12 Thoracic (T1-T12) pairs
     5 Lumbar (L1-L5) pairs
     5 Sacral (S1-S5) pairs
     1 Coccygeal (Col) pair

These spinal nerves are:

· Motor to voluntary skeletal muscles

· Sensory from skin and musculoskeletal system

· Autonomic to and from smooth muscle, cardiac muscle and glands

These Mixed nerves contain both sensory and motor fibers.

Divides into plexuses (nerve networks):

A.  Cervical plexus  - C1-C5: supplies muscles of the neck and shoulders; the skin around the ear and the front of the neck; supplies the diaphragm with the phrenic nerve.

B.  Brachial plexus  - C5-T1:  provides the entire nerve supply to muscles of the upper extremities

Axillary nerve: innervates Deltoid.

Radial nerve: innervates Posterior muscles of arm and forearm.

Median nerve: innervates Lateral muscles of anterior forearm.

Ulnar nerve: innervates Medial muscles of anterior forearm.

C.  Lumbar plexus  - L1 to L4:  supplies nerves to the abdominal wall, external genitals and part of the lower extremities.

Femoral nerve: innervates Quadriceps femoris.

D.  Sacral plexus  - L4 to S4:  supplies nerves to the buttocks, perineum and lower limbs.

Sciatic nerve: 

Tibial nerve: innervates posterior thigh and leg muscles.

Common fibular nerve:  innervates biceps femoris head, lateral leg, and exterior toes.