Shells of the spinal cord: structural features, species and functions

The spinal cord enters the central nervous system. In the human body, he is responsible for motor reflexes and transmission of nerve impulses between organs and the brain. The shells of the spinal cord cover it, providing protection. What features and differences do they have?

Structure

The arches of the vertebrae form a cavity called the vertebral canal, and the spinal cord is located in it, along with the vessels and nerve roots. Its upper part is connected with the medulla oblongata (head part), and the lower part - with the periosteum of the second coccygeal vertebra.

The spinal cord looks like a thin white cord, whose length reaches 40-45 centimeters, and the thickness increases from the bottom up. Its surface is slightly concave. It consists of thirty-one segments, from which pairs of nerve roots leave.

The spinal cord is covered with shells from the outside. Inside it contains a gray and white substance, their ratio varies in different parts. Gray matter has the form of a butterfly, it contains the bodies of nerve cells, their outgrowths contain a white substance that is located at the edges.

In the center of gray matter is a canal. It is filled with cerebrospinal fluid (cerebrospinal fluid), which constantly circulates in the brain and spinal cord. In an adult, its volume is up to 270 milliliters. Likvor is produced in the ventricles of the brain and is updated 4 times a day.

Shells of the spinal cord

Three shells: hard, spidery and soft - cover both the head and spinal cord. They perform two main functions. Protective prevents the negative impact of mechanical stress on the brain. The trophic function is associated with the regulation of the cerebral blood flow, thanks to which the metabolism in the tissues is realized.

The shells of the spinal cord consist of cells of connective tissue. Outside, there is a hard shell, under it spider's and soft. They do not fit tightly together. Between them there is a subdural and subarachnoid space. To the spine they are attached by plates and ligaments, which prevent the stretching of the brain.

Shells are formed at the beginning of the second month of embryo development. The connective tissue is formed on the neural tube and spreads along it. Later, the tissue cells are divided to form the outer and inner membranes. After a while, the inner shell is divided into a soft and arachnoid.

Hard shell

The outer hard shell consists of the upper and lower layers. It has a rough surface, on which there are many vessels. Unlike a similar shell in the brain, it does not fit tightly to the walls of the spinal canal and is separated from them by the venous plexus, fatty tissue.

The hard shell of the spinal cord represents a dense shiny fibrous tissue. It envelops the brain in the form of an elongated bag of cylindrical shape. Covering cells (endothelium) constitute the lower layer of the membrane.

It envelops the nodes and nerves, forming cavities that expand, approaching the intervertebral openings. Near the head, the membrane connects to the occipital bone. To the knee it narrows and represents a thin thread that joins the coccyx.

Blood passes to the membrane through arteries connected to the abdominal and thoracic aorta. Venous blood enters the venous plexus. The shell is fixed in the vertebral canal with the help of processes in the intervertebral openings, as well as fibrous bundles.

Arachnoid

Slit-shaped space with a large number of connecting beams separates the hard and spider webs of the spinal cord. The latter has the appearance of a thin leaf, it is transparent and contains fibroblasts (connective tissue fibers that synthesize the extracellular matrix).

The spinal cord of the spinal cord is enveloped by neuroglia-cells, which provide the transmission of nerve impulses. It does not contain blood vessels. From the arachnoid shell, shoots, thread-like trabeculae leave, weaving into the next soft shell.

Under the shell is located subarachnoid space. Inside it is the liquor. It is widened in the lower part of the spinal cord, in the region of the sacrum and coccyx. In the neck area is a septum between the soft and arachnoid shells. The septum and the jagged ligaments between the nerve roots fix the brain in one position, not allowing it to shift.

Soft shell

The inner shell is soft. It envelops the spinal cord. Compared with a similar structure in the brain, it is considered more solid and thick. The soft shell of the spinal cord consists of a loose tissue that is covered by endothelial cells.

It has two thin layers, between which are numerous blood vessels. On the upper layer, represented by a thin plate or a sheet, there are jagged ligaments that fix the shell. The inner part is adhered by a membrane of glial cells that connects directly to the spinal cord. The shell forms the vagina for the artery and, along with it, penetrates the brain and its gray matter.

Soft shell is present only in mammals. Other terrestrial vertebrates (tetrapods) have only two - solid and inner. In the course of evolutionary development, the inner membrane in mammals was divided into arachnoid and soft.

Conclusion

The spinal cord belongs to the central nervous system of all vertebrate animals, including humans. It performs reflex and conductor functions. The first is responsible for the reflexes of the limbs - their flexion and extension, pulling, etc. The second function is the conductivity of nerve impulses between organs and the brain.

A hard, arachnoid and soft membranes envelop the spinal cord from the outside. They perform protective and trophic (nutritional) functions. The membranes are formed by cells of connective tissue. They are separated by spaces filled with cerebrospinal fluid, circulating in the spinal cord and brain. Between the shells are connected by thin fibers and processes.