10.6 Internal Anatomy of the Spinal Cord

10.6.1 Gray Matter of the Spinal Cord and Spinal Roots

• The gray matter of the spinal cord looks like a butterfly or the letter H in cross section (it consists of dendrites and cell bodies of neurons, unmyelinated axons, and neuroglia).
• In the center of the gray commissure is a small space called the central canal, it is filled with cerebrospinal fluid.
• Gray commissure, connects the gray matter of the right and left sides of the spinal cord.
• White commissure, connects the white matter of the right and left sides of the spinal cord.
• The gray matter on each side of the spinal cord is subdivided into regions called horns i.e. dorsal (posterior horns), ventral (anterior horns) & lateral gray horns.
  • The posterior (dorsal) gray horns contain cell bodies and axons of interneurons as well as axons of incoming sensory neurons.
  • The anterior (ventral) gray horns contain somatic motor nuclei, which are clusters of cell bodies of somatic motor neurons (that provide nerve impulses for contraction of skeletal muscles).
  • The lateral gray horns contain autonomic motor nuclei, which are clusters of cell bodies of autonomic motor neurons (regulate the activity of cardiac muscle, smooth muscle, and glands).
• The cell bodies of the sensory neurons, whose fibers enter the cord by the dorsal root, are found in an enlarged area called the dorsal root ganglion.
• The ventral horns of the gray matter contain cell bodies of motor neurons of the somatic (voluntary) nervous system, which send their axons out the ventral root of the cord.
• The dorsal and ventral roots fuse to form the spinal nerves.

10.6.2 White Matter of the Spinal Cord

• White matter of the spinal cord is composed of myelinated fiber tracts.
• Two grooves penetrate the white matter of the spinal cord and divide it into right and left sides.
  • The anterior median fissure is a wide groove on the anterior (ventral) side.
  • The posterior median sulcus is a narrow furrow on the posterior (dorsal) side.
• White matter on each side of the cord is divided into three regions—the dorsal, lateral, and ventral columns.
• Tracts conducting sensory impulses to the brain are sensory or afferent tracts.
• Those carrying impulses from the brain to skeletal muscles are motor or efferent tracts.
• All tracts in the dorsal columns are ascending tracts that carry sensory input to the brain. The lateral and ventral columns contain both ascending and descending (motor) tracts.

Tracts are bundles of axons in the CNS.

Nerves are bundles of axons in the PNS.

Meninges of the Spinal Cord

• From superficial to deep they are the

(1) Dura mater,

(2) Arachnoid mater

(3) Pia mater.

• The spinal meninges surround the spinal cord (Figure 13.1a) and are continuous with the cranial meninges, which encircle the brain.
• Dura mater
  • Outermost layer, thick, strong, composed of dense irregular connective tissue.
  • Starts from foramen magnum and ends at the second sacral vertebra.
• Arachnoid mater
  • Middle layer, thin, avascular, composed of loosely arranged collagen and elastic fibers.
  • Starts from foramen magnum and ends at the second sacral vertebra.
  • Between the dura mater and the arachnoid mater is a thin subdural space, which contains interstitial fluid.
• Pia mater
  • Innermost, thin transparent connective tissue layer, composed of squamous to cuboidal cells within interlacing bundles of collagen fibers and some fine elastic fibers.
  • Within the pia mater are many blood vessels that supply oxygen and nutrients to the spinal cord.
  • Between the arachnoid mater and pia mater is a space, the subarachnoid space, which contains shock-absorbing cerebrospinal fluid.

Figure 24- Internal Anatomy of the Spinal Cord