The spinal cord system is considered the most ancient area of the body. The mass of this part in an adult is about 34-38 g. In the course of progression of the central part of the nervous system in the process of evolution, the ratio between the size of the brain and spinal cord changed in favor of the first. Next, let's take a closer look at what the structure is, what tasks it performs.
General biology
The spinal cord is an irregular cylindrical body. Its length in men is about 45, in women it is 41-42 cm. There are different parts of the spinal cord. In each area, the body has a different size. So, the chest area has a sagittal size (in the plane from the back to the stomach) - about 8 mm. The diameter of this area is 10 mm. The thickening begins where the II-III segments (cervical) are located. In this area, the diameter reaches 13-14 mm. In this case, the sagittal size is 9 mm. In the section, 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 whole body is divided into certain areas (the number of segments of the spinal cord will be presented below). Next, considerconstituent elements of the structure.
Segments of the spinal cord: picture, description
The body consists of similar (homomorphic) parts. Segments of the spinal cord are connected by means of nerve conductors to a specific area in the body. The length of one or another area of the body is different. The total number of segments of the spinal cord is 31. The least elements are in the coccygeal zone. The structure contains:
- Lumbar segments (5).
- Sacrum (5).
- Breast (12).
- Coccygeal (1).
- Cervical spine segments (8).
The latter account for about 23.2% of the length of the entire structure. Most of all (56.4%) are occupied by the thoracic segments. 7.3% of the length falls on the sacral zone. The segments of the spinal cord externally 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 vertebrae of the same name. At the same time, the difference between one and the second increases from top to bottom.
Location
The skeletotopia of sites varies individually. For example, the lower region of the lumbar region in adults can be located from the lower third of the body of the XI thoracic vertebra to the disk between the first and second lumbar vertebrae. In this regard, a certain feature is visible. If the upper roots move in the transverse direction, then the farther down the channel, the higher it will beexit site relative to the inlet intervertebral foramen. The last elements vertically tend to areas located below the level at which the spinal cord ends. All this bundle is surrounded by a terminal thread. It's called a ponytail.
End thread
From the second lumbar element down, the spinal cord passes into a special rudimentary formation. It's called the "terminal thread". It is formed predominantly by the pia mater. In its uppermost zone there are nerve cells. The end thread is of two types. It may be internal. In this case, it runs in the meninges to the second vertebra in the sacrum. The terminal thread can be external. In this case, it extends beyond the second coccyx vertebra. The outer thread mainly consists of a continuation of connective tissue fibers. The inner end thread has a length of about 16, and the outer one - 8 cm.
Dissymmetry
Segments of the spinal cord are not completely symmetrical. The unequal length and different level of origin of the roots are noted already at the stage of embryonic development. After birth, dissymmetry increases over time. It is more distinct in the thoracic region. In the posterior roots, the dissymmetry is more pronounced than in the anterior ones. Apparently, this phenomenon is associated with differences in the skin and muscle innervation of the left and right sides of the human body.
Internal features of elements
Let's briefly consider the structure of a segment of the spinal cord. In each element there is a disk - a plate located horizontally. At the level of this area, neural connections pass. Their position is also horizontal. There are vertical neural connections between the discs. So, the elements can be represented as a stack of plates. They, in turn, are connected by interneuronal connections. The axons of the cells of the corresponding lateral horns of the spinal cord participate in the formation of the anterior roots. They contain preganglionic sympathetic and efferent motor fibers; the posterior roots contain afferent structures. They are outgrowths of ganglion neurons. The total number of fibers present in the posterior roots is about 1 million on each side; in the anterior elements, about 200,000 are detected in the complex. This results in a ratio of 5:1. Representatives
animal world, the predominance of the number of fibers of the posterior roots over those present in the anterior ones is not so pronounced. For example, mice, rats and dogs have a ratio of 2.5:1. Thus, one of the evolutionary patterns of development of the nervous system of all vertebrates is manifested in this. It lies in the fact that the formation of input channels is carried out more actively than output channels. Moreover, the latter are more stable. The number of nerve fibers in the posterior and anterior roots in one spinal segment is usually different. The difference can be up to 59% of the number of structures on the side where there are fewer.
Grey matter
On the cross section, it is a figure similar to a butterfly that has opened its wings, or the letter H. There are posterior, anterior and lateral horns. Their shape changes along the course of the spinal cord. In the area bounded by the lateral and posterior horns, there is a reticular formation of a reticulate type. Gray matter occupies about 5 cm3 (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 combined into three groups: intercalary, beam, radicular. Gray matter constitutes a special apparatus of structure. Here are some of the functions of the spinal cord. The stimuli that come along the afferent fibers due to the presence of connections can pass both in the descending and in the ascending direction. They, in turn, provoke a widespread motor response.
White matter
It contains projection, commissural and associative nerve pathways. The latter are bundles passing along the periphery of the gray structure and along all the cords of the spinal cord. Commissural tracts form a white commissure. It is located between the median anterior fissure and the gray matter (connecting its halves). Projection pathways (descending (efferent) and ascending (afferent)) provide communication with the brain.
Blood supply
The flow of blood is carried out through a network of numerous vessels. They depart in the upper part from the subclavian, thyroid and vertebral arteries. Also vesselsspread from the area where the second and third parts of the spinal cord are located. In this zone, the blood supply comes from the branches of the aorta. More than sixty paired radicular arteries, which form near the intervertebral foramina, are distinguished by a small (150-200 microns) diameter. They supply blood only to the roots and membranes adjacent to them. About 5-9 large (400-800 microns) caliber arteries participate in the nutrition of the spinal cord itself. All these vessels are of unpaired type. They enter the canal at different levels: sometimes through the right hole, sometimes through the left hole. These arteries are called main or radicular-medullary. The number of the largest of them is not constant. There are three vascular pools:
- Upper or cervico-dorsal. It nourishes the area where the segments of the spinal cord C1 - Th3 are located.
- Intermediate or medium. It includes sections Th4-Th8.
- Lower. It feeds the area below the level of the Th9 segment.
The spinal anterior artery extends to only a few fragments of the structure. Further, it is not presented in the form of 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 top to bottom, in the middle - from bottom to top, and in the lower sections - up and down.
Main Tasks
There are two main functions of the spinal cord. The first is reflex, the second is conductive. Each segment is associated with certain organs and provides themactivity and functionality. For example, the sacral elements relate to the legs and pelvic organs and are responsible for the activity of these areas of the body. One or another chest segment interacts with the corresponding organs and muscles. The upper elements are connected with the head and hands. The reflex functions of the spinal cord are simple reflexes inherent in nature. These, in particular, include a reaction to pain - a person pulls his hand, for example. The well-known knee jerk also belongs to this category. The brain may not be involved in the manifestation of these reactions. This theory has been proven by routine experiments with animals. In the absence of a head, the frog reacted to both strong and weak pain stimuli. The conduction functions of the spinal cord are in the transmission of impulses. First it goes up. On the ascending path, the impulse enters the brain, and from there it is sent as a return command to any organ. Due to this conductive connection, any mental activity is manifested: take, go, get up, pick up, cut off, run, throw, draw. Also, the conductive functions of the spinal cord ensure the implementation of actions that people, without noticing, perform daily at work or at home.
Side Horns
These elements have their own functions. In the lateral horns (intermediate zone in the gray matter) are the sympathetic cells of the autonomic nervous structure. 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 areasegments of the upper two sections of the spinal cord there is a reticular region - a bundle of a large number of nerves that are associated with areas of cortical activation in the brain and reflex activity. The activity of gray and white matter bundles, anterior and posterior roots is called a reflex reaction. The reflexes themselves are called, according to Pavlov's definition, unconditional.
Ascending paths
The anterior cords of the white matter have several paths, each of which performs certain tasks:
- Corticospinal (anterior pyramidal) is responsible for the transmission of motor impulses from the cortex in the brain to the anterior horns in the spinal cord.
- Spinothalamic anterior provides tactile sensitivity.
- Leventhal and Geld bundle - white matter fibers connect the vestibular nuclei of 8 pairs of cranial nerve endings with motor neurons in the anterior horns.
- The cerebrospinal tract forms a protective reflex, which is associated with visual or sound stimuli. This is done by connecting the visual centers under the cortex in the brain with the nuclei in the anterior horns.
- The longitudinal bundle provides coordination of the eye and other muscles by connecting the upper segments with the spinal cord.
- An impulse of deep sensitivity passes along the ascending paths. As a result, a person has a feeling of his body. Impulses travel through the spinothalamic, tectospinal, and cortical-spinal canals.
Descending paths
Transmission of an impulse from the cortex in the brain to the gray matter in the anterior horns is carried out through the lateral cortical-spinal canal. The red nuclear-spinal tract provides automatic adjustment of muscle tone and movements at a subconscious level. This channel is located in front of the lateral-pyramidal. The spinothalamic lateral and posterior spinal cerebellar tract adjoin the red nuclear-spinal tract.
Age features
Temporary changes affect both the structure of the spinal cord and its topography. In the second half of the period of intrauterine development, its growth is somewhat slowed down. In particular, it lags behind the development of the spinal column. And this goes on for quite a long time. In infants, the cerebral cone is located in the region of the third lumbar vertebra, and in an adult it ends at the level of the first or second. Over the entire period of growth, the length of the structure increases by 2.7 r. This is obtained mainly due to the thoracic segments. The mass of the structure increases by about 6-7 times. The growth of the white and gray matter of the spinal cord is quite uneven. The volume of the first increases by 14, and the second - by 5 times. This is due to the fact that development in the own segmental apparatus is completed earlier than in the projection nerve pathways.
In closing
A unique connection has been established between the spinal cord and the brain, the central nervous system, all organs and limbs of a person. She isconsidered the "dream of robotics". To date, not a single, even the most modern robot, can carry out all those possible actions and movements that are subject to a biological organism. These modern machines are programmed to perform highly specialized tasks. Most often, such robots are used in automatic conveyor production. The mass of the spinal cord as a percentage is different for different representatives of the animal world. For example, a frog has 45, a turtle has 120, a rat has 36, a macaque has 12, a dog has 18, and a human has 2. The structure of the spinal cord shows quite clearly the general design features and patterns of the central zone of the nervous system.
