Function of the nervous system
The nervous system, as a part of the organism, serves to absorb, control and regulate stimuli and has a great influence on the body. It is “communicatively” connected with the body and the environment. The functioning of the nervous system can be simplified as follows: Via a stimulus receiver (sensor, receptor), stimuli from the sensory organs are perceived and directed via a sensitive nerve fiber to the central nervous system (CNS).
Here the supplied (afferent) information is processed. The information is usually encoded as an electrical signal (action potential). Various nerve cells are involved in the processing.
The transfer of information takes place via messenger substances (transmitters), among others. Finally, the information reaches a discharging motor (efferent) nerve fiber, which moves from the central nervous system towards the “far center” (periphery), to the successful organ, e.g. a muscle cell. There the processed information is passed on and a reaction occurs, e.g. the muscle is tensed.A nerve cell (neuron) has many dendrites, which are a kind of connecting cable to other nerve cells to communicate with them.
- Nerve cell
- Dendrite
Anatomy of the spinal cord
The spinal cord is strand-like and has a (ventral or anterior) furrow on its front side, which is called the ventral median fissure. The spinal cord artery (A. spinalis anterior) runs through this furrow. Directly opposite the anterior fissure is another notch, the so-called dorsal median sulcus posterior.
This continues inwards into a septum, the so-called septum medianum dorsale. The anterior notch, i.e. the fissure mediana ventralis/anterior and the posterior septum divide the spinal cord into two halves, which are mirror images of each other.
- Sulcus medianus posterior
- Hinterhorn grey substance
- White matter
- Anterior horn gray substance
- Fissure mediana anterior
A cross-section of the spinal cord shows the grey matter lying in the inner area and “butterfly-like” formed, which is divided into a front and a rear “horn”.
The gray matter is framed by the fibrous substantia alba, which is clearly distinguished by its white color. Depending on the localization, the “butterfly shape” of the gray matter may vary. In the spinal cord sections at the level of the chest and the loins, the gray substance contains a small lateral horn on each side in addition to the front and rear horns, which takes its place between the two horns.
In the middle of the gray matter there is the central canal (canalis centralis), in cross section it shows up as a small hole. The central canal is filled with liquor and represents the inner liquor space of the spinal cord. A longitudinal section shows that the spinal cord has thickenings called immittumescences in some places.
These can be found in the cervical and lumbar or sacral area and are due to an increased number of nerve bodies and nerve processes in this area, which are responsible for the nervous supply of the extremities, i.e. the arms and legs. The broad anterior horn (Cornu anterius) of the gray spinal cord substance contains the nerve cell bodies whose extensions (axons) move to various muscles (so-called motoneurons). The extensions of the nerve cell bodies of the anterior horn form the anterior motor (i.e. movement) part of the spinal nerve root, which emerges from the side of the spinal cord.
The posterior horn of the spinal cord is the point of entry for the posterior, sensitive part of the spinal nerve roots, which transmits the “felt” information generated in the periphery to the brain (e.g. pain, temperature, sense of touch). In contrast to the motor nerve cell bodies, the nerve cell bodies responsible for sensitivity are located in the so-called spinal ganglion, which is located outside the spinal cord (but still in the spinal canal). Nevertheless, cell bodies (strand cells) can also be found in the posterior horn, but these belong to the long anterior and lateral strands of the white matter.
The lateral horn includes the vegetative nerve cells (neurons) of the sympathetic nervous system (in the thoracic and lumbar marrow) and the parasympathetic nervous system (in the sacral marrow). The described 3 horns only appear as “horns” in cross section (“butterfly wings”). Seen three-dimensionally, they are actually columns in the context of which we also speak of columnae (groins).
The anterior column of the anterior horn is called columna anterior, the posterior column of the posterior horn and the lateral column of the lateral horn is called columna lateralis. The columnae should not be imagined as strands of equal thickness running through the entire spinal cord from top to bottom. The cell groups form small columns, which may extend over several segments (spinal cord layers).
These cell groups are called nuclei. The cells of such a grouping are then responsible for the innervation of certain muscles each time. If, for example, a cell group extends over several segments, its cell extensions (axons) also emerge from the spinal cord through several anterior roots.
After their exit, the axons rejoin to form a nerve, which then pulls into a muscle.In this case one speaks of a peripheral nerve. If a peripheral nerve is damaged, this leads to peripheral paralysis, which results in complete failure of the associated muscle. If, on the other hand, a nerve root of the nervous system is damaged, this leads to radicular paralysis (radix=root), i.e. certain functions of various muscles are lost.
In the area of the arms and legs, there is a peculiarity: here, the spinal nerves form nerve plexuses, the so-called plexus. The skin area that is supplied by the nerve fibers of a segment is called the dermatome. The muscle fibers that are supplied by the nerve processes of a segment are called myotoma.
It should be remembered that it is not one segment that supplies a muscle, but under certain circumstances many partial functions of several muscles. Around the central canal there are also nerve fibers that connect the two halves of the spinal cord, which are called commissure fibers (commissura grisea). These ensure that one half knows what the other half is doing.
This balance serves the balance process. The commissure fibers belong to the spinal cord’s own apparatus. This includes those nerve cells and their fibers, which communicate with each other at the spinal cord level and thus enable processes without having to use the central circuitry of the brain. This includes, for example, the spinal cord’s own reflexes.
