Central nervous system. Spinal cord

Central nervous system. Spinal cord.

The human nervous system consists of approximately 10 billion neurons; they are divided into two main categories: neurons belonging to the central nervous system, which form the brain and spinal cord, and neurons of the peripheral nervous system, which form the cranial and spinal nerves.

The spinal cord is a tube surrounded and protected by the neural arches of the vertebrae, and has two important functions: it transmits impulses to and from the brain, and it serves as a reflex center.

In cross-section, it can be seen that it consists of two types of tissue: an internal mass of gray matter, which has a butterfly shape in cross-section and consists of nerve cell bodies, and an outer mass of white matter formed by bundles of axons and dendrites. The white color of these bundles is due to the myelin sheaths of the nerve fibers; the ends of axons and dendrites located in the central gray matter do not have myelin sheaths. The “wings” of gray matter are divided into two posterior and two anterior horns.

The anterior horns contain the bodies of neurons, the axons of which are sent as part of the spinal nerves to the muscles; all other nerve cells of the spinal cord are interneurons. White matter axons and dendrites are divided into bundles with similar functions: ascending tracts, which carry impulses to the brain, and descending tracts, which carry impulses from the brain to effectors.

By carefully recording the symptoms observed in people with spinal cord injuries and comparing these data with the pattern of destruction of certain pathways found in postmortem brain examinations, neurologists were able to map the location and function of the various pathways.

For example, the posterior columns of white matter transmit impulses arising in the receptors of muscles, tendons and joints, thanks to which we sense the position of parts of our body. In an advanced syphilitic process, the posterior columns may be destroyed, so that the patient cannot tell where his arms and legs are if he does not see them, and when walking he must always look at his legs.

When studying the location and function of the pathways, one curious fact was discovered that has not yet received a satisfactory explanation. - All fibers of the spinal cord decussate, that is, they pass from one side of the body to the other somewhere along the way from the receptor to the brain or from the brain to the muscle. Thus, the right half of the brain controls the left half of the body and receives messages from the receptors on the left side.

In the center of the gray matter is a narrow canal that runs along the entire spinal cord and is filled with cerebrospinal fluid, similar to blood plasma.

The spinal cord and brain are covered by three connective tissue meninges. Meningitis is a disease in which these membranes become infected and inflamed. One of them (dura mater) is attached to the bony neural arches of the vertebrae, the other (pia mater) lies on the very surface of the spinal cord, and the third (arachnoid mater) is located between them.

The spaces between the membranes are also filled with cerebrospinal fluid, so that the spinal cord (as well as the brain) floats in this fluid and is protected from hitting the hard surface of the vertebrae (or skull) with each movement.