Spinal Cord

The spinal cord is the cylindrical and elongated portion of the central nervous system that is located in the vertebral canal. Its anterior, upper end is continuous with the medulla oblongata, which lies above, while its posterior, lower end, with the terminal filaments in the coccygeal region. The spinal cord is rounded and it contains a central canal, which is a tube-like cavity that runs through its center along its whole length. The central canal is filled with cerebrospinal fluid. The spinal cord is snugly enveloped by three meninges: the pia mater (the innermost meninx), the arachnoid (the middle meninx), and the dura mater (the outermost meninx). It is held firmly in place by ligaments extending from the meninges to the inner wall of the bony canal of vertebral column. The space between the arachnoid, the pia mater, and the spinal cord itself is also filled with cerebrospinal fluid.

A deep anterior median fissure is present on the ventral (anterior) aspect of the spinal cord, which consists of two symmetrical halves joined by a thin transverse band of nervous substance which is called the commissure. Nerve cell bodies and their short processes (dendrites) form the gray matter that encircles the central canal. In cross section, the gray matter of the spinal cord resembles a butterfly with outstretched wings, with the commissure joining the two wings. The myelinated nerve fibers that comprise the descending and ascending tracts of the spinal cord form the white matter of the spinal cord which surrounds the gray matter lying at the center. Projections of the gray matter (the anterior, posterior, and lateral horns) divide the white matter into three bundles of nerve fibers: the anterior, posterior, and lateral funiculi (funiculus) of the spinal cord. The boundaries between the funiculi are the sites where the dorsal and ventral roots emerge.

Descending and ascending tracts

The descending tracts are motor myelinated nerve fibers which transmit efferent nerve impulses from the brain to skeletal muscles. They establish synapse with motor neurons in the anterior horns of the grey matter. They descending tracts comprise both pyramidal tracts (from Betz cells in the cerebral cortex) and extrapyramidal tracts (arise from the nerve cells in basal ganglia). The pyramidal tracts in turn are divided into lateral corticospinal and the anterior corticospinal tracts, which lie in the anterior aspect of the spinal cord. The ascending tracts of the white matter of spinal cord, on the other hand, transmit afferent nerve impulses from the sensory peripheral nerve endings to the brain; they comprise the lateral and anterior spinothalamic tracts as well as the fasciculus gracilis and fasciculus cuneatus, both lying on the posterior side of spinal cord.

Above, descending tracts of spinal cords. They are made up of motor nerve fibers.

The ascending tracts (afferent) of spinal cord.

The spinal cord is conventionally divided into segments according to the number of vertebrae present. In humans there are 31–33 segments: seven cervical, twelve thoracic, five lumbar, five sacral, and one to three coccygeal segments. A group of nerve fibers, which are the root filaments of the spinal nerves, extend out of the spinal cord from each segment and combine to form the roots of the spinal nerves. Each pair of roots corresponds to a vertebra and departs from the vertebral canal through an intervertebral foramina, or opening. In humans, the spinal cord is shorter than the vertebral canal, and as a result the roots of the lower segments extend downward in the form of a bundle and are transmitted from the vertebral canal through the intervertebral foramina.

The posterior (dorsal) roots of the spinal nerves contain sensory (afferent, or centripetal) nerve fibers along which impulses are transmitted to the spinal cord from receptors in the skin, muscles, tendons, joints, and internal organs. The ventral (anterior) roots of spinal nerves contain motor (efferent, or centrifugal) nerve fibers along which impulses are transmitted from the motor or sympathetic-nerve cells of the spinal cord to the periphery, to skeletal muscles to the smooth muscles of the blood vessels, and to the internal organs. The posterior and anterior roots join before entering the intervertebral foramina, forming mixed nerve roots upon emerging from the spine.

The neurons of the gray matter are usually found in groups (or nuclei) and arranged unevenly in such a way that each part of the gray matter is characterized by a specific type of nerve cell. Most important are the nuclei of the anterior horn, which contain motor neurons (motoneurons). Long motoneuronal processes (axons) depart through the ventral root and innervate all the skeletal muscles. The intermediate substance of the spinal cord contains a nucleus whose cells have short axons that form synaptic junctions with other neurons of the spinal cord. This nucleus contains intercalary cells (interneurons) that are joined in a chain of varying complexity.

A nucleus containing preganglionic cells of the sympathetic nervous system is situated in the intermediate substance between the thoracic and upper lumbar segments. The axons of these cells emerge from the spinal cord through the ventral roots and continue toward peripheral ganglia, where synaptic junctions are formed with postganglionic neurons; these neurons innervate the smooth muscles and secretory apparatus of the internal organs. The apex of the posterior horn contains an accumulation of nerve cells called the substantia gelatinosa, the processes of which interweave to form a network (the neuropil). The sensory fibers that enter the spinal cord through the dorsal roots pass through the substantia gelatinosa and form synaptic junctions primarily in interneuronal nuclei; very few of the fibers are in direct contact with motoneurons.

Function

The action of the spinal cord is reflexive in nature. Reflexes originate under the action of afferent signals that reach the spinal cord from receptors (the initial points of a reflex arc). They may also be effected by signals that proceed initially to the brain and then descend to the spinal cord along descending tracts. Specific muscular reflexes, defense reflexes, flexor and extensor reflexes, the contraction of blood vessels, and several other reflexes of the internal organs are maintained when the spinal cord is cut and the connection to the brain severed. The most complex reflex reactions of the spinal cord are controlled by various brain centers. In these reactions the spinal cord serves as a link in the transmission of signals from the brain to effectors.

Blood Supply

The spinal cord is supplied by two arteries: the anterior and posterior spinal artery, which arise from the vertebral arteries. Each vertebral artery arises in turn from the subclavian artery.

Cross-section of spinal cord showing the gross division, with its different parts

Above, cross-section of spinal cord showing a schematic picture of the motor reflex.