In this article we will discuss about the Structure and Functions of Human Spinal Cord.
Structure of Human Spinal Cord:
Location and Coverings (Meninges):
It is a posterior part of central nervous system which runs mid-dorsally within the vertebral column. It lies in the neural canal of the vertebral column. The spinal cord is surrounded by the same three protective membranes (meninges) as found in the brain, viz., a thin innermost piamater, the middle webby arachnoid membrane (arachnoid mater) and the outer tough duramater.
The subarachnoid space is filled with cerebrospinal fluid. There is an additional space, the epidural space above the Dura mater. The epidural space contains fatty and connective tissues and veins.
The spinal cord extends from the medulla oblongata. It is continuous, to the level of the second lumbar vertebra. In an adult the spinal cord is from 42 to 45 centimeters long. Its diameter varies at different levels, being enlarged in the cervical and lumbar regions.
The cord is also flattened. The cervical enlargement extends from the fourth cervical to the first thoracic vertebrae; it is the region from which nerves supplying the arms arise. It may seem strange that the lumbar enlargement should be in the thoracic region; this is the case because the spinal cord grows at a slower rate than the vertebral column.
By adulthood the area within the vertebral column below the second lumbar vertebra contains spinal nerves that branch from the spinal cord at higher levels.
These spinal nerves are collectively called, the cauda equina, or “horse’s tail”. The spinal cord ends as the conus medullaris. The conus medullaris ends at the level of the intervertebral disc between the first and second lumbar vertebral in adults. Actually the conus medullaris is a conical portion of lower spinal cord.
From the conus medullaris a fine connective tissue filament, the filum terminate, extends down to the coccygeal region. The filum terminate consists mostly of pia mater.
The spinal cord does not extend to the coccygeal region because during development the vertebral column elongates more rapidly than the spinal cord. The filum terminate anchors the spinal cord within the vertebral column. In-fact, the filum terminale is a long slender filament at the end of the spinal cord.
The internal anatomy of the spinal cord is best seen in cross section. Two indentations, the posterior median sulcus and the anterior median fissure, separate the spinal cord into left and right symmetrical halves. The inner butterfly-shaped area is the grey matter of the spinal cord.
Grey matter is so named because it lacks myelin and therefore, appears grey in an unstained preparation. Surrounding the grey matter are bundles of myelinated nerve fibres, called fasciculi or white columns, which together form the white matter of the spinal cord.
In each segment of the spinal cord a spinal nerve arises from each side of the cord. Each spinal nerve connects with the cord through two nerve roots.
The dorsal nerve root consists of a bundle of sensory axons (carrying incoming signals) whose cell bodies are located in the dorsal root ganglion. These axons extend into the posterior horn of the grey matter, where they often form synapses with other neurons, some of which are called intemeurons.
Interneurons, short neurons confined to the grey matter of the cord, form synapses with other intemeurons and with the motor neurons whose cell bodies are located in the anterior horn of the grey matter.
Aggregations of motor axons (carrying outgoing signals) from these cell bodies form the ventral nerve roots. The lateral horns lie between the anterior and posterior horns. The hollow central canal contains cerebrospinal fluid.
Along the white matter of the spinal cord there are two kinds of fasciculi, or bundles of axons, the ascending tracts, which carry sensory impulses to the brain, and the descending tracts, which carry motor impulses from the brain to the spinal nerves at various levels of the cord.
Functions of Spinal Cord:
Spinal cord performs two main functions:
(i) The stimuli are passed from and to the brain through the spinal cord.
(ii) It is the centre of spinal reflex action.