This is the lower portion of the CENTRAL NERVOUS SYSTEM which is situated within the SPINAL COLUMN. Above, it forms the direct continuation of the medulla oblongata, the part of the BRAIN directly above the foramen magnum, the large opening in the base of the skull. Below, the spinal cord extends to about the upper border of the second lumbar vertebra, where it tapers off into a fine thread, known as the filum terminale, that is attached to the coccyx at the lower end of the spine.
In its course from the base of the skull to the lumbar region, the cord gives off 31 nerves on each side, each of which arises by an anterior and a posterior root that join before the nerve emerges from the spinal canal. Below the point where the cord ends there is a sheaf of nerves, known as the cauda equina (horse's tail), running downwards.
The cord is a cylinder, about the thickness of the little finger. At the level of the lower part of the neck, the upper four cervical nerves unite to produce the cervical plexus. From this, the muscles and skin of the neck are mainly supplied, and the phrenic nerve, which runs down through the lower part of the neck and the chest to innervate the diaphragm, arises. The lower four cervical and first dorsal nerves unite to form the brachial plexus. This supplies nerves to some shoulder muscles and the skin around the shoulder and inner side of the arm; the plexus gives off large nerves that proceed down the arm. The other thickening of the cord is at the level of the last dorsal vertebra and supplies nerves to the lower limbs.
The thoracic or dorsal nerves, with the exception of the first, run around the chest along the lower margin of the rib to which it corresponds, whilst the lower six extend on to the abdomen.
The lumbar plexus is formed by the upper four lumbar nerves, and its branches are distributed to the lower part of the abdomen, and front and inner side of the thigh.
The sacral plexus is formed by parts of the fourth and fifth lumbar nerves, and the upper three and part of the fourth sacral nerves. Much of the plexus is collected into the sciatic nerves, the largest in the body, which go to the legs.
The SYMPATHETIC NERVOUS SYSTEM is joined by a pair of small branches given off from each spinal nerve, close to the spine. This system gives off nerve plexuses (cardiac, solar, diaphragmatic, renal, spermatic or ovarian etc) connected with various internal organs.
The spinal cord, like the brain, is surrounded by three membranes: the dura mater, arachnoid mater, and pia mater. The dura forms a wide tube which is separated from the cord by fluid and from the vertebral canal by blood vessels and fat, this arrangement protecting the cord from pressure in any ordinary movements of the spine.
the spinal cord consists of white matter (nerve fibres) surrounding a mass of grey matter (largely neurons – see NEURON(E)) arranged like the letter H. Each half of the cord possesses an anterior and a posterior horn, the masses of the two sides being joined by a wide posterior grey commissure with the central canal of the cord in the middle, containing cerebrospinal fluid. The roots of the nerves that leave the cord arise from the ends of the horns of grey matter.
The cord is, in part, a receiver and originator of nerve impulses, and in part a conductor of these impulses along fibres which pass through it to and from the brain. The cord contains centres able to receive sensory impressions and initiate motor instructions. These control blood-vessel diameters, eye-pupil size, sweating and breathing. The brain exerts an overall controlling influence and, before any incoming sensation can affect consciousness, it is usually ‘filtered’ through the brain.
Many of these centres act autonomously. Other cells of the cord are capable of originating movements in response to impulses brought direct to them through sensory nerves, such activity being known as REFLEX ACTION. (For a fuller description of the activities of the spinal cord, see NEURON(E) – Reflex action.)
The posterior column of white matter of the cord conveys sensory impressions upwards. The anterior column contains the pyramidal tract of motor fibres travelling towards the periphery. (See also NERVE; NERVOUS SYSTEM.)