Spinal cord


The spinal cord is primarily made up of gray matter and is housed within the spinal column. Its purpose it to communicate from various parts of the body to the brain. The peripherally located tracts of white matter are responsible for sending the various impulses to and from the brain. The spinal cord runs the length of the thorax and is continuous with the brain via the skull’s foramen magnum. The spinal cord perform two basic functions.


The spinal cord conducts impulses as a means of neural communication back and forth between the brain and the rest of the body via the white matter tracts. Peripheral sensory receptors throughout the human body send the information back to the brain to be interpreted. The descending tracts then send motor impulses to either the muscles or the glands in order to respond to the stimuli.

The spinal cord is also involved in spinal reflexes. Dedicated nerve pathways determine the reflex directly from the spinal cord thus eliminating the extra time and distance required for voluntary actions to receive their information. This means that reflexes are often faster than voluntary action. Reflexive action can contribute to various body needs including the heart, breathing, coughing, sneezing, motor reflexes, blood pressure, vomiting, hiccupping, and digestion.


Extending up the inferior side, near the foramen magnum that belongs to the occipital bone, the spinal cord runs up to the same level of the first vertebrate. It looks like an ovular cross section due to its mildly flattened posteroventral shape. From a posterior viewpoint, the two rather obvious enlargements can be determined. Between the third cervical vertebrate and the second thoracic vertebrate the cervical enlargement can be detected. The upper extremities receive the nerve supply from this area of the spinal cord. In between the thoracic vertebrates number 9 and 12, the lumbar enlargement can be detected. The lower extremities are served via this area. In an adult human body, the spinal cord does not protrude beyond the first vertebrate, L1, due to the slower fetal development of the spinal cord. The vertebral column extends a bit further. The conus medullaris refers to the end of the spinal cord, where it tapers into its termination. A strand of mostly fiber material which is comprised nearly entirely out of pia mater is referred to as the filum terminale. The filum terminale runs along the inferior side from the conus medullaris even with L1 to the coccyx.

Radiating inferiorly from the conus medullaris, nerve roots also run through the vertebral canal. Resembling the tail of a horse, these nerve roots are referred to as cauda equine.


Spinal cord
Image: Spinal Cord

There are 31 segments that create the spinal cord. Each segment radiates one pair of spinal nerves which protrude from the spinal cord via the intervertebral foramina.There are two distinct grooves that partially segregate the spinal cord into a general right and left side. These grooves are referred to as the anterior median fissure and the posterior median sulcus. The three diverse meninges that help to protect the brain also have their place in protecting the spinal cord, as does the cerebrospinal fluid. The highly vascular pia mater helps to supply the cells with much needed blood nutrients and oxygen. The majority of the spinal cord is created by the gray matter, which is centrally located and aptly surrounded by white matter. The gray matter is composed of a combination of neuroglia, nerve cell bodies, and unmyelinated association neurons.


Bundles of myelinated fibers consisting of motor and sensory neurons make up the white matter. The amount of white matter or gray matter varies throughout the spinal cord, with one matter being more present than the other at various locations. The closer the spinal cord gets to the brain, the thicker the nerve tracts need to be and thus there is a higher percentage of white matter. The area where the lower and upper extremities make their innervation connection with the spinal cord, near the lumbar and cervical enlargements, the gray matter takes over nearly all of the spinal cord.

When looking at the core of gray matter, it appears to create the letter H. Horns, which are projections of gray matter throughout the spinal cord, are appropriately determined by the direction of their projection. Thus, the paired horns which extend posteriorly are known as the posterior horns, and the anterior horns are those which project anteriorly. The shorter pair of lateral horns protrude to the sides. In the thoracic and lumbar areas, the lateral horns can be discerned, but typically not in other regions. The paired lateral horns are conjoined across the middle of the spinal cord by a transverse rod of gray matter. This is known as the gray commissure. The central canal lies withing the gray commissure. The gray commissure is also continuous with the brain’s ventricles and is protected by cerebrospinal fluid. Within the columns of white matter, there are ascending and descending tracts for the nerve impulses to travel along, delivering their messages either to the brain or to the body structure. There are six of these white matter columns within the spinal cord. They are called funiculli, and are named appropriately for their relationship with the relative position. Between the anterior gray horns, the anterior funiculi are positioned on either side of the anterior median fissure. Likewise, the posterior funiculi are positioned between the posterior horns of gray matter. These are on either side of the posterior median sulcus. In between the gray matter that creates the anterior and posterior horns, the posterior funiculi are positioned.

Ascending and descending tracts line the individual funiculi. The nerve fibers that line the tracts are myelinated and have been named by they original location or their location of termination, as the fibers within these tracts either remain on the same side they began or they cross over either within the medulla oblongata or the spinal cord. Decussation is the term deemed for the crossing over of the nerves. The descending tracts are also named for their location of origin, corticospinal tracts descend directly and continuously without any synaptic interruption. The cell bodies that create the fibers for these tracts are derived from precentral gyrus of the frontal lobe. At least 85% of the fibers from the corticospinal tracts decussitate. The crossing fibers then create the lateral corticospinal tracts. The remaining 15% of the fibers which do not decussitate create the anterior corticospinal tracts.

The process of nerve fibers which cross over contributes to the right brain, left brain notion. Many of the skeletal and other voluntary actions that happen on the right side of the body are directed by the left side of the brain, and vice verse.

Originating in the brain stem area, the remaining nerve tracts are the extrapyramidal tracts. Due to a variety of synaptic connections, the basal nuclei, cerebellum, and cerebral cortex all indirectly produce motion. The extrapyramidal system shoots off branches of the reticulospinal tracts, which are actually significant descending pathways from the extrapyramidal tracts. These tracts are named for the origin of the impulses, the reticular formation.

Neurostimulation of the reticular formation by the cerebrum or cerebellum either assists or impedes the motion of lower motor neurons (depending on the region which has been stimulated). The cerebellum is devoid of descending tracts. The cerebellum can only indirectly influence motor activity, through the vestibular nuclei, red nucleus, and basal nuclei. These structures, however, affect lower motor neurons via the vestibulospinal tracts, rubrospinal tracts, and reticulospinal tracts. Harm to the cerebellum interrupts the synchronization of movements with spatial judgment. Under-reaching or overreaching for an object may occur, enhanced by an unintention tremor, in which the limb moves back and forth in a pendulum-like action. The basal nuclei, acting through synapses in the reticular formation in particular, materialize normally to exert an inhibitory influence on the movement of lower motor neurons. Injury to the basal nuclei results in diminished muscle tone. A human body with this type of damage display akinesia (complete or partial loss of muscle control) and chorea (sudden and uncontrolled random motions).
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