Segments of the spinal cord. Functions of the spinal cord

The spinal cord system is considered to be the most ancient zone of the body. The mass of this part in an adult is about 34-38 g. As the central part of the nervous system progresses in the course of evolution, the ratio between the size of the brain and spinal cord changes in favor of the former. Next, let us consider in more detail what the structure is, what tasks it performs.

General Biology

The spinal cord is a body of irregular cylindrical shape. Its length in men is about 45, in women it is 41-42 cm. Different parts of the spinal cord stand out. On each site the body has different sizes. Thus, the thoracic region has a sagittal size (in the plane from the back to the abdomen) - about 8 mm. The diameter of this area is 10 mm. Thickening begins where the II-III segments (cervical) are located. In this area, the diameter reaches 13-14 mm. The sagittal size is 9 mm. In the department, which is located from the first lumbar to the second sacral fragment, the diameter is about 12 mm. Its sagittal size is 9 mm. The entire body is divided into specific areas (the number of segments of the spinal cord will be presented below). Next, consider the constituent elements of the structure.

Segments of the spinal cord: picture, description

The body consists of similar to each other (homomorphic) parts. Segments of the spinal cord are connected by means of nerve conductors with a specific site in the body. The length of this or that zone of the body is different. The total number of segments of the spinal cord is 31. The fewest elements in the coccygeal zone. In the structure there are:

• Lumbar segments (5).
• Sacral (5).
• Thoracic (12).
• Coccygeal (1).
• Segments of the cervical spine (8).

The latter account for about 23.2% of the length of the entire structure. The thoracic segments occupy the most (56.4%). 7.3% of the length is in the sacral area. Segments of the spinal cord outwardly represent the posterior and anterior correctly alternating outgoing roots - nerve processes. It should be noted that the structure does not fill the entire channel. In this regard, the spinal segments are located higher than the same vertebrae. In this case, the difference between one and the second increases from top to bottom.

Location:

Individually variable skeleotopia of sites. For example, the lower region of the lumbar region can be located in adults from the lower third of the body of the XI thoracic vertebra to the disc between the first and second lumbar vertebrae. In this regard, a certain feature is seen. If the upper roots move in the transverse direction, the further down the channel, the higher will be the exit site relative to the entrance intervertebral opening. The last elements are vertically tending to areas below the level at which the spinal cord ends. All this bundle is surrounded by an end thread. It is called the "ponytail".

End thread

From the second lumbar element down the spinal cord turns into a special rudimentary formation. It is called the "end thread". It is formed mainly by a soft cerebral membrane. In its upper zone there are nerve cells. The end thread is of two kinds. It can be internal. In this case, it passes in the meninges to the second vertebra in the sacrum. The end thread can be external. In this case, it extends beyond the second vertebra of the coccyx. The outer filament preferably consists of the continuation of the connective tissue fibers. The inner end thread has a length of about 16, and the outer end thread has a length of 8 cm.

Dissymmetry

Segments of the spinal cord are not completely symmetrical. The unequal length and different level of the departures of the roots are already observed at the stage of embryonic development. After birth, dissymmetry increases with time. It is more distinct in the thoracic area. At the posterior root dissymmetry is more pronounced than in the anterior ones. Apparently, this phenomenon is associated with differences in cutaneous and muscular innervation of the left and right sides of the human body.

Internal features of the elements

Consider briefly the structure of the segment of the spinal cord. In each element there is a disk - a plate, located horizontally. At the level of this site pass neural connections. Their position is also horizontal. Between the disks there are vertical neural connections. So, the elements can be represented as a stack of plates. They, in turn, are united by inter-neural connections. In the formation of the anterior roots, the axons of the cells of the corresponding lateral horns of the spinal cord participate. Preganglionic sympathetic and efferent motor fibers are present in them; The posterior roots contain afferent structures. They are the processes of the ganglion neurons. The total number of fibers present in the posterior roots is about 1 million on each side; In the front elements, about 200,000 occur in the complex. This is how the ratio of 5: 1 is obtained. Representatives

The prevalence of the number of fibers of the posterior roots above those present in the anterior region is less pronounced. For example, in mice, rats and dogs the ratio is 2.5: 1. This, then, manifests one of the evolutionary patterns of development of the nervous system of all vertebrates. It consists in the fact that the formation of input channels is more active than output. The latter are more stable. The number of nerve fibers in the posterior and anterior roots in one spinal segment, as a rule, is different. The difference can be up to 59% of the number of structures on the side where there are fewer.

Gray matter

On the cross-section, it is a figure resembling a butterfly, revealing wings, or the letter H. The rear, front and side horns are distinguished. Their shape changes in the course of the spinal cord. On the site bounded by the lateral and posterior horns, the reticular formation of the reticular species is located. The gray matter occupies about 5 cm ³ (about 17.8%) of the total volume of the spinal cord. The number of neurons present in it is approximately 13.5 million. They are united into three groups: intercalary, bundle, radicular. Gray matter constitutes a special apparatus of structure. Here some functions of the spinal cord are laid. Irritations that come through afferent fibers due to the presence of bonds can take place in both the descending and ascending directions. They, in their own way, provoke a common motor response.

White matter

It contains projection, commissural and associative neural pathways. The latter are bundles that run along the periphery of the gray structure and along all the cords of the spinal cord. The commissural pathways form a white spike. It is located between the middle front slit and the gray matter (connecting its halves). Projection pathways (descending (efferent) and ascending (afferent)) provide a link to the brain.

Blood supply

The flow of blood is carried out through a network of numerous vessels. They depart at the top of the subclavian, thyroid and vertebral arteries. Also, the vessels spread from the site where the second and third parts of the spinal cord are located. In this zone, the blood supply goes from the branches of the aorta. More than sixty pairs of radicular arteries, which form near the intervertebral openings, are distinguished by a small (150-200 μm) diameter. They supply blood only the roots and shells adjacent to them. In the diet, actually, the spinal cord itself is involved in about 5-9 arteries of large (400-800 microns) caliber. All these vessels are unpaired. They enter the channel at different levels: then through the right, then through the left hole. These arteries are called trunk or radicular-medullary. The number of the largest of them is not constant. There are three vascular pools:

• Upper or cervico-dorsal. It feeds the zone where the segments of the spinal cord C1-Th3 are located.
• Intermediate or intermediate. To it carry sites Th4-Th8.
• Lower. It feeds the area below the level of the Th9 segment.

Spinal anterior artery extends only to several fragments of the structure. Further, it is not represented as a single vessel. It is a chain of anastomoses of several radicular-medullary large arteries. The blood flow in the spinal anterior artery goes in different directions. In the upper sections - from the top down, in the middle - from the bottom up, and in the lower - up and down.

Main goals

There are two main functions of the spinal cord. The first is a reflex, the second is a conductor one. Each segment is associated with specific bodies and ensures their activity and functionality. For example, sacral elements refer to the legs and pelvic organs and are responsible for the activity of these areas of the trunk. This or that thoracic segment interacts with the appropriate organs and muscles. The top elements are connected with the head and hands. The reflex functions of the spinal cord are simple reflexes inherent in nature. These include, in particular, the reaction to pain - a person pulls his hand, for example. Also, this category includes the known knee-jerk reflex. In the manifestation of these reactions, the brain may not participate. This theory is proved by the usual experiments with animals. In the absence of a head, the frog reacted to both strong and weak pain stimuli. The conductor functions of the spinal cord are enclosed in the transfer of momentum. First he goes up. On the ascending path, the impulse enters the brain, and from there it is sent as an inverse command to an organ. Due to this conductor connection, any mental activity manifests itself: take, go, get up, lift, cut, run, throw, draw. Also, the conductor functions of the spinal cord provide the implementation of actions that people, without noticing, commit daily at work or at home.

Lateral horns

These elements have their own functions. Sympathetic cells of the autonomic nervous structure are located in the lateral horns (an intermediate zone in the gray matter). It is with their help that interaction with internal organs is carried out. These cells have processes that are connected to the anterior roots. A path is formed in this zone: in the region of segments of the upper two sections of the spinal cord is the reticular region - a bundle of a large number of nerves that are associated with regions of cortical activation in the brain and reflex activity. The activity of gray and white matter beams, front and back roots is called a reflex reaction. The reflexes themselves are called unconditional, as Pavlov defined.

Ascending paths

The front cables of the white substance have several paths, each of which performs certain tasks:

• Cortico-spinal (anterior pyramidal) is responsible for the transmission of motor impulses from the cortex in the brain to the anterior horns in the dorsal.
• The spinal front provides tactile sensitivity.
• A bunch of Levental and Geld - white matter fibers connect the vestibular nuclei of 8 pairs of cranial-brain nerve endings with motor neurons in the anterior horns.
• The cover-spinal path forms a protective reflex, which is associated with visual or auditory stimuli. This is done by linking the visual centers under the cortex in the brain with the nuclei in the anterior horns.
• The longitudinal beam provides coordination of the ocular and other muscles, due to the binding of the upper segments to the trunk of the spinal cord.
• A pulse of deep sensitivity passes along the ascending paths. As a result, you get a feeling of your body in a person. Impulses pass along spinalotal, spinal cord and cortico-spinal canal.

Descending ways

Transmission of the impulse from the cortex in the brain to the gray matter in the anterior horns is carried out along the lateral cortical-spinal canal. The cerebro-spinal cord provides automatic adjustment of the muscle tone and movements at a subconscious level. This channel is located in front of the lateral-pyramidal one. The spinal column and spinal cord are adjacent to the red-nuclear-spinal path.

Age features

Temporal changes concern both the structure of the spinal cord and its topography. In the second half of the intrauterine growth period, its growth is somewhat slower. In particular, it lags behind the development of the spinal column. And this continues a fairly long period. The infants have a conical cone in the region of the third lumbar vertebra, and the adult ends at the level of the first or second vertebra. Over the entire period of growth, the length of the structure increases in 2.7 r. This is mainly due to the thoracic segments. The mass of the structure is increased approximately 6-7 times. The growth of white and gray matter of the spinal cord is rather uneven. The volume of the first increases in 14, and the second - in 5 times. This is due to the fact that in its own segmental apparatus development is completed earlier than in the projection neural pathways.

Finally

Between the dorsal and cerebrum, the central nervous system, all organs and limbs of man, a unique connection is established. It is considered a "dream of robotics". To date, none, even the most modern robot, can not carry out all those possible actions and movements that are subject to the biological organism. Such modern machines are programmed to perform highly specialized tasks. Most often these robots are used in automatic conveyor production. The mass of the spinal cord in percent varies among the representatives of the animal world. For example, in a frog - 45, turtles - 120, rats - 36, macaques - 12, dogs - 18, and for a man - 2. In the structure of the spinal cord, the general structural features and regularities of the central zone of the nervous system are clearly manifested.