Spinal Cord and Associated Structures

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Spinal Cord and Associated Structures


Spinal Cord
  • Is cylindrical, occupies approximately the upper two thirds of the vertebral canal , and is enveloped by the three meninges.
  • Has cervical and lumbar enlargements for nerve supply of the upper and lower limbs, respectively.
  • Contains gray matter , which is located in the interior (in contrast to the cerebral hemispheres); the spinal cord is surrounded by white matter.
  • Has a conical end known as the conus medullaris , which terminates at the level of L2 vertebra or the intervertebral disk between L1 and L2 vertebrae.
  • Grows much more slowly than the bony vertebral column during fetal development; thus, its end gradually shifts to a higher level and ends at the level of L2 vertebra in the adult and at the level of L3 vertebra in the newborn.
  • Receives blood from the anterior spinal artery and two posterior spinal arteries and from branches of the vertebral, cervical, and posterior intercostal and lumbar arteries.
Spinal Nerves
  • Consist of 31 pairs of nerves (8 cervical, 12 thoracic, 5 lumbar, 5 sacral, 1 coccygeal).
  • Are formed within an intervertebral foramen by union of the ventral root and the dorsal root with ganglion, which contains cell bodies of sensory neurons.
  • Are divided into the dorsal primary rami , which innervate the skin and deep muscles of the back; the ventral primary rami , which form the plexuses (cervical C1 to C5; brachial C5 to T1; lumbar L1 to L4; and sacral S2 to S4); and the intercostal (T1 to T11) and subcostal (T12) nerves .
  • Are connected with the sympathetic chain ganglia by rami communicantes.
  • Are mixed nerves, containing all of the general functional components (i.e., general somatic afferent [GSA], general somatic efferent [GSE], general visceral afferent [GVA], and general visceral efferent [GVE]).
  • Contain sensory (GSA and GVA) fibers with cell bodies in the dorsal root ganglion.
  • Contain motor (GSE) fibers with cell bodies in the anterior horn of the spinal cord.
  • Contain preganglionic sympathetic (GVE) fibers with cell bodies in the intermediolateral cell column in the lateral horn of the spinal cord (segments between T1 and L2).
  • Contain preganglionic parasympathetic (GVE) fibers with cell bodies in the intermediolateral cell column of the spinal cord segments between S2 and S4. These GVE fibers leave the sacral nerves via the pelvic splanchnic nerves.
Meninges

Pia mater

  • Is the innermost meningeal layer; it is closely applied to the spinal cord and thus cannot be dissected from it. It also enmeshes blood vessels on the surfaces of the spinal cord.
  • Has lateral extensions (denticulate ligaments ) between dorsal and ventral roots of spinal nerves and an inferior extension known as the filum terminale .

Arachnoid mater

  • Is a filmy, transparent, spidery layer connected to the pia mater by web-like trabeculations.
  • Forms the subarachnoid space , the space between the arachnoid layer and the pia mater that is filled with cerebrospinal fluid (CSF) and that extends to the second sacral vertebral level. The enlarged subarachnoid space between vertebrae L1 and S2 is called the lumbar cistern.

Dura mater

  • Is the tough, fibrous, outermost layer of the meninges.
  • The subdural space is a potential space between the arachnoid and dura. It extends inferiorly to the second sacral vertebral level and contains only sufficient fluid to moisten the surfaces of two membranes.
  • The epidural space is external to it and contains the internal vertebral venous plexus and epidural fat.


Structures Associated with the Spinal Cord

Cauda equina

  • Is formed by a great lash of dorsal and ventral roots of the lumbar and sacral spinal nerves that surround the filum terminale .
  • Is located within the subarachnoid space (lumbar cistern) below the level of the conus medullaris .
  • Is free to float in the CSF within the lumbar cistern and therefore is not damaged during a spinal tap.

Denticulate ligaments

  • Are lateral extensions of the spinal pia mater , consisting of 21 pairs of toothpick-like processes.
  • Extend laterally from the pia through the arachnoid to the dura mater between dorsal and ventral roots of the spinal nerves.
  • Help hold the spinal cord in position within the subarachnoid space.

Filum terminale (internum)

  • Is a prolongation of the pia mater from the tip (conus medullaris) of the spinal cord at the level of L2.
  • Lies in the midst of the cauda equina and ends at the level of S2 by attaching to the apex of the dural sac.
  • Blends with the dura at the apex of the dural sac, and then the dura continues downward as the filum terminale externum (filum of the dura mater of the coccygeal ligament), which is attached to the dorsum of the coccyx.

Cerebrospinal fluid

  • Is contained in the subarachnoid space between the arachnoid and pia mater.
  • Is formed by vascular choroid plexuses in the ventricles of the brain.
  • Circulates through the ventricles, enters the subarachnoid space, and eventually filters into the venous system through arachnoid villi projecting into the dural venous sinuses, particularly the superior sagittal sinus.

Dermatome, Myotome, and Sclerotome

Dermatome

  • Is an area of skin innervated by sensory fibers derived from a particular spinal nerve or
  • segment of the spinal cord. Knowledge of the segmental innervation is useful clinically to produce a region of anesthesia or to determine which nerve has been damaged.

Myotome

  • Is a group of muscles innervated by motor fibers derived from a single spinal nerve segment.

Sclerotome

  • Is the area of a bone innervated from a single spinal segment.




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