The spinal cord system is considered 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 during evolution, the ratio between the size of the brain and spinal cord changes in favor of the first. Next, we consider in more detail what constitutes a structure, 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 - 41-42 cm. Different parts of the spinal cord are distinguished. At each site, the body has different sizes. 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. Thickening begins where II-III segments (cervical) are located. In this section, the diameter reaches 13-14 mm. Moreover, the sagittal size is 9 mm. In the department, which is 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, we consider the constituent elements of the structure.
Spinal cord segments: picture, description
The body consists of similar (homomorphic) parts. Segments of the spinal cord are connected through nerve conductors to a specific site in the body. The length of a particular area of ββthe body is different. The total number of segments of the spinal cord is 31. The smallest number of elements are in the coccygeal zone. The structure contains:
- 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. Most (56.4%) occupy the thoracic segments. 7.3% of the length falls on the sacral zone. 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 same vertebrae. In this case, the difference between the one and the second is growing in the direction from top to bottom.
Location
The skeletotopy of sites is individually variable. For example, the lower region of the lumbar region in adults can range from the lower third of the XI thoracic vertebra to the disc between the first and second lumbar vertebrae. In this regard, a certain feature is visible. If the upper roots depart in the transverse direction, then the further down the canal, the higher the exit section will be relative to the inlet intervertebral foramen. The last elements vertically tend to areas located below the level at which the spinal cord ends. The end thread is surrounded by all this bundle. It is called the ponytail.
End thread
From the second lumbar element down the spinal cord passes into a special vestigial formation. It is called the "end thread". It is formed mainly by the pia mater. In its highest zone, nerve cells are present . The end thread is of two types. It may be internal. In this case, it passes in the meninges to the second vertebra in the sacrum. The end thread may be external. In this case, it stretches beyond the second vertebra of the coccyx. 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 is 8 cm.
Dissymmetry
Segments of the spinal cord are not completely symmetrical. Unequal lengths and different levels of root discharge are observed already at the stage of embryonic development. After birth, dissymmetry intensifies over time. It is more distinct in the chest area. In the posterior roots, dissymmetry is more pronounced than in the anterior ones. Apparently, this phenomenon is associated with differences in the cutaneous and muscle innervation of the left and right sides of the human body.
Internal features of elements
Consider briefly the structure of the spinal cord segment. Each element has a disk - a plate located horizontally. At the level of this site, neural connections pass. Their position is also horizontal. Between the discs there are vertical neural connections. So, the elements can be represented as a stack of plates. They, in turn, are united by interneuronal connections. Axons of the cells of the corresponding lateral horns of the spinal cord participate in the formation of the anterior roots. They have preganglionic sympathetic and efferent motor fibers; the posterior roots contain afferent structures. They are processes of ganglion neurons . The total number of fibers present in the posterior roots is about 1 million on each side; about 200,000 are detected in the anterior elements in the complex. This results in a 5: 1 ratio. Representatives
fauna, the predominance of the number of fibers of the posterior roots over those that are present in the anterior is not so pronounced. For example, in a mouse, rat, and dog, the ratio is 2.5: 1. In this, therefore, one of the evolutionary patterns of the development of the nervous system of all vertebrates is manifested. It lies in the fact that the formation of input channels is carried out more actively than weekend. Moreover, 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 a cross section, it is a figure similar to a butterfly that spread its wings, or the letter N. Distinguish between the rear, front and side horns. Their shape changes along the spinal cord. In the area bounded by the lateral and posterior horns, there is a reticular formation of a mesh type. 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 combined into three groups: insertion, fascicular, radicular. Gray matter constitutes a special apparatus of the structure. Here are some functions of the spinal cord. The irritation that comes through afferent fibers due to the presence of bonds can occur both in the downward and upward directions. They, in turn, provoke a common motor response.
White matter
It contains projection, commissural and associative neural pathways. The latter are bundles passing along the periphery of the gray structure and along all the cords of the spinal cord. Commissural paths form a white commissure. It is located between the median anterior slit and the gray matter (connecting its halves). Projection paths (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 extend in the upper part from the subclavian, thyroid and vertebral arteries. The vessels also spread from the area where the second and third parts of the spinal cord are located. In this zone, blood supply comes from the branches of the aorta. More than sixty paired radicular arteries that form near the intervertebral foramen are small (150-200 microns) in diameter. They supply only the roots and membranes adjacent to them with blood. About 5β9 large (400β800 ΞΌm) caliber arteries are involved in the nutrition of the spinal cord itself. All these vessels are unpaired. They enter the channel at different levels: either through the right or through the left hole. These arteries are called trunk or radicular-medullary. The number of the largest of them is inconsistent. Three vascular pools are distinguished:
- Upper or cervico-dorsal. It feeds the area where the segments of the spinal cord C1 - Th3 are located.
- Intermediate or intermediate. This includes sections of 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 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 top to bottom, in the middle - from bottom to top, and in the lower - up and down.
Main goals
There are two main functions of the spinal cord. The first is reflex, the second is conductor. Each segment is associated with specific organs and ensures their activity and functionality. For example, sacral elements relate to the legs and organs of the pelvis and are responsible for the activity of these zones of the body. This or that thoracic segment interacts with the corresponding organs and muscles. The upper elements are connected with the head and hands. Reflex functions of the spinal cord are simple reflexes laid down by nature. They include, in particular, a reaction to pain - a person pulls his hand, for example. Also in this category are known knee jerks. The brain may not participate in the manifestation of these reactions. This theory is proved by ordinary 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 impulse transmission. First he goes up. On an ascending path, an impulse enters the brain, and from there it is sent as an inverse command to any organ. Due to this conductor connection, any mental activity is manifested: to take, go, stand, raise, cut, run, throw, draw. Also, the conduction functions of the spinal cord provide the implementation of actions that people, without noticing, perform daily at work or at home.
Side 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, 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, front and rear roots is called a reflex reaction. The reflexes themselves are called, by Pavlov's definition, unconditioned.
Ascending paths
The front cords of the white matter have several paths, each of which performs certain tasks:
- Cortical-spinal (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.
- The beam of Leventhal and Geld - the fibers of the white matter bind the vestibular nuclei of 8 pairs of cranial nerve endings with motor neurons in the front horns.
- The pit of the spinal cord forms a protective reflex that is associated with visual or sound stimuli. This is due to the connection of the visual centers under the cortex in the brain with the nuclei in the front horns.
- The longitudinal bundle provides coordination of the eye and other muscles, due to the binding of the upper segments to the spinal cord.
- An impulse of deep sensitivity passes through the ascending paths. As a result, a person has a sensation of his body. Impulses pass through the spinal thalamic, tympanic-spinal and cortical-spinal canals.
Descending paths
An impulse is transmitted from the cortex in the brain to the gray matter in the front horns via the lateral cortical-spinal canal. The spinal cord pathway provides automatic adjustment of muscle tone and movements at a subconscious level. This channel is located in front of the lateral pyramidal. Spinal thalamic lateral and spinal cerebellar adjacent to the rednuclear-spinal path.
Age features
Temporary changes relate to 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 continues for a rather long period. 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 growth period, the length of the structure increases by 2.7 p. This is mainly due to the thoracic segments. The mass of the structure increases by about 6-7 times. The growth of 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 one's own segmental apparatus is completed earlier than in projection nerve pathways.
Finally
A unique connection has been established between the spinal cord and brain, central nervous system, all organs and limbs of a person. She is considered the "dream of robotics." Today, not one, even the most modern robot, can 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, such robots are used in automatic conveyor production. The mass of the spinal cord in percent is different for different representatives of the animal world. For example, a frog has 45, turtles 120, rats 36, macaques 12, dogs 18, and humans 2. In the structure of the spinal cord, the general structural features and patterns of the central zone of the nervous system are clearly manifested.