Structure and functions of spinal cord

  • It is a whitish, long, dorso-ventrally compressed tube of nervous tissues.
  • It is a part of the central nervous system that is posterior to the brain and enclosed within the vertebral column.
  • It runs through the neural canal of the vertebral column which protects it from injuries.
  • It shows two slight enlargements, the branchial and sciatic swellings at the level of fore and hind limbs respectively.
  • Nerves are supplied through these swellings to the two limbs.
  • It is 45cm in length, 1.25 cm in breadth and 30gm in weight.
  • It lies in the upper two-third of the vertebral canal.
  • It starts from the 1stCervical vertebrae (atlas) and terminates at the lower border of L1 which appears as a conical structure, Conus medullaris. In children it ends at 3rd Lumbar vertebrae.
  • It is also covered by meninges. The subarachnoid space extends upto 2nd Sacral vertebrae.
  • Filum terminale is the extension of piamater, which ends at the dorsal surface of the 1st coccyx vertebrae.
  • Cauda equina (similar to the horse tail) is a group of spinal nerves at the end of spinal cord.

Cut section of spinal cord

  • Cross section of spinal cord shows a tiny central canal, which contains CSF and a dark portion of H-shaped or butterfly shaped Gray matter surrounded by large white matter.
  • It is divided into more or less symmetrical left and right halves by a deep groove, called the anterior median fissure, and a median septum, called posterior median sulcus.
  • Ventral and dorsal roots of spinal nerves extends out from the spinal cord.

Gray matter

  • It consists of nerve cell bodies and dendrites of association and efferent neurons, unmyelinated axons of spinal neurons, sensory and motor neurons, and axons terminal of neurons.
  • It forms an H shape and is composed of 3 pairs of columns of neurons running up and down the length of spinal cord from the upper cervical level to the sacral level.
  • There are pairs of columns that form the two vertical bars of the H, called horns.
  • The two that run dorsally having function in afferent input are posterior horns.
  • Anterior horns run ventrally which have efferent somatic output.
  • The two small horns that extend laterally are the lateral horns.
  • It indicates the location of cell bodies of preganglionic efferent neurons of the sympathetic nervous system.
  • The lateral horns are located only in the thoracic and upper lumbar level of the spinal cord (T1-L2).
  • The nerve fibers that form the cross bar of the H is known as gray commissure.
  • It functions in cross reflexes.
  • The gray matter is actually a pinkish-gray colour because of rich network of blood vessels.

White matter

  • It is composed mainly of myelinated nerve fibers which has white colour.
  • Due to this reason it is called white matter.
  • It is divided into 3 pairs of columns, or funiculi, of myelinated fibers that run the entire length of the cord.
  • The funiculi consist of the anterior (ventral) column, the posterior (dorsal) column, the lateral column, and a commissure area.
  • The bundles of the fibers within the funiculus are subdivided into tracts called fasciculi.
  • Ascending tracts are made up of sensory fibers that carry impulses up the spinal cord to the brain.
  • The descending tracts of motor fibers transmit impulses from the brain down the spinal cord to the efferent neurons.
  • The nerve impulses are carried up-to the brain or down through the cord to neurons that innervate the muscles or glands by longer tracts.
  • The impulses are conveyed from one level of the cord to another by shorter tracts.

Ventral and dorsal roots

  • The spinal nerve divides into a ventral (anterior, motor) root and a dorsal (posterior, sensory) root in the vicinity of the cord.
  • Ventral roots convey motor information which contains efferent nerve fibers.
  • Dorsal roots enter the cord with sensory information which contains afferent nerve fibers.
  • There are ventral roots which emerge from the spinal cord in a groove called the ventrolateral sulcus and the dorsal roots enter the cord in the dorsolateral sulcus.
  • The axons of cell bodies of motor neurons located within the anterior horn (gray matter) emerge from the spinal cord to form the ventral (motor) roots.
  • In contrast, the group of cell bodies  lie outside the cord whose axons make up the dorsal roots in the dorsal root ganglia, or spinal ganglia.

Functions of spinal cord

  • It is the main center for reflex action.
  • It is the main pathway for all incoming and outgoing impulses from brain to periphery.

Structure and functions of spinal cord

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