- 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.
- 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.
- 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.