The spinal cord enters the central nervous system. In the human body, he is responsible for motor reflexes and the transmission of nerve impulses between organs and the brain. Shells of the spinal cord cover it, providing protection. What features and differences do they have?
Structure
Arches of the vertebrae form a cavity called the spinal canal, and the spinal cord is located in it along with the vessels and nerve roots. The upper part connects to the medulla oblongata (head section), and the lower part - to the periosteum of the second coccygeal vertebra.
The spinal cord looks like a thin white cord, the length of which a person 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 come out.
The spinal cord is sheathed outside. Inside it contains gray and white matter, their ratio varies in different parts. The gray matter has the shape of a butterfly, it contains the bodies of nerve cells, their processes contain a white matter that is located at the edges.
In the center of gray matter is a channel. 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. Liquor is produced in the ventricles of the brain and is updated 4 times a day.
Spinal cord membrane
Three shells: hard, arachnoid and soft - cover both the brain and spinal cord. They perform two main functions. Protective prevents the negative effects of mechanical stress on the brain. Trophic function is associated with the regulation of cerebral blood flow, due to which the metabolism in tissues is carried out.
Spinal cord membranes are made up of connective tissue cells. Outside is a hard shell, under it arachnoid and soft. They do not fit tightly together. Between them there is a subdural and subarachnoid space. They are attached to the spine by plates and ligaments that prevent the extension of the brain.
Shells form at the beginning of the second month of embryo development. Connective tissue forms on the neural tube and spreads through it. Later, tissue cells separate to form the outer and inner membranes. After some time, the inner shell is divided into soft and arachnoid.
Hard shell
The outer hard shell consists of upper and lower layers. It has a rough surface on which many vessels are located. Unlike a similar membrane in the brain, it does not fit snugly against the walls of the spinal canal and is separated from them by the venous plexus and adipose tissue.
The hard membrane of the spinal cord is a dense, shiny fibrous tissue. It envelops the brain in the form of an elongated bag of a cylindrical shape. Covering cells (endothelium) make up the lower layer of the membrane.
It envelops nodes and nerves, forming cavities that expand, approaching the intervertebral foramen. Near the head, the membrane connects to the occipital bone. It narrows down and is a thin thread that joins the tailbone.
Blood passes to the membrane through arteries connected to the abdominal and thoracic aorta. Venous blood enters the venous plexus. The membrane is fixed in the spinal canal with the help of processes in the intervertebral openings, as well as fibrous bundles.
Arachnoid
A slit-like space with a large number of connective bundles separates the hard and arachnoid membrane of the spinal cord. The latter has the form of a thin leaf, it is transparent and contains fibroblasts (connective tissue fibers that synthesize the extracellular matrix).
The arachnoid membrane of the spinal cord is enveloped in neuroglia - cells that transmit nerve impulses. It does not contain blood vessels. From the arachnoid membrane, processes, filiform trabeculae, weaving into the next soft membrane, depart.
Under the shell is located subarachnoid space. Inside it contains cerebrospinal fluid. It is expanded in the lower part of the spinal cord, in the region of the sacrum and tailbone. In the neck area there is a septum between the soft and arachnoid membranes. The septum and the serrated ligaments between the nerve roots fix the brain in one position, preventing it from moving.
Soft shell
The inner shell is soft. She envelops the spinal cord. Compared to a similar structure in the brain, it is considered more solid and thicker. The soft membrane of the spinal cord consists of loose tissue, which is covered by endothelial cells.
It has two thin layers, between which there are numerous blood vessels. On the upper layer, represented by a thin plate or leaf, there are serrated ligaments that fix the shell. A membrane of glial cells adjoins to the inner part , connecting directly to the spinal cord. The membrane forms the vagina for the artery and with it penetrates the brain and its gray matter.
The soft shell is present only in mammals. Other terrestrial vertebrates (tetrapods) have only two - solid and internal. During evolutionary development, the inner membrane of 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, twitching, etc. The second function is to conduct nerve impulses between the organs and the brain.
Hard, arachnoid and soft membranes envelop the spinal cord from the outside. They perform protective and trophic (nutritional) functions. The membranes are formed by connective tissue cells. They are divided among themselves by spaces that are filled with cerebrospinal fluid - the fluid circulating in the spinal cord and brain. Between themselves, the shells are connected by thin fibers and processes.