Our spinal cord is the most ancient formation of the nervous system in evolutionary terms. Appearing for the first time in the lancelet, in the process of evolution, the spinal cord with its efferent (motor) and afferent (sensory) neurons improved. But at the same time, it retained its main functions - conductive and regulatory. It is thanks to the sensory neurons of the spinal cord that we withdraw our hand from the hot pot even before the pain appears. The structure of this organ of the central nervous system and the principles of its work are discussed in this article.
So vulnerable but very important
This soft organ hides inside the spinal column. The human spinal cord weighs only 40 grams, has a length of up to 45 centimeters, and its thickness is comparable to the little finger - only 8 millimeters in diameter. And yet, it is the control center of a complex network of nerve fibers,which is spread all over our body. Without it, the musculoskeletal system and all the vital organs of our body will not be able to perform their functions. In addition to the vertebrae, the spinal cord is protected by its membranes. The outer shell is hard, formed by dense connective tissue. This sheath contains blood vessels and nerves. And, besides, it is in it that the highest concentration of pain receptors in the human body is observed. But there are no such receptors in the brain itself. The second shell is arachnoid, filled with cerebrospinal fluid (cerebrospinal fluid). The last shell - soft - fits snugly to the brain, penetrated by blood and lymphatic vessels.
A few words about neurons
The structural unit of the nervous tissue are neurons. Very special cells, the main function of which is the formation and transmission of a nerve impulse. Each neuron has many short processes - dendrites that perceive irritation, and one long - an axon that conducts a nerve impulse in only one direction. Depending on the task and function, neurons are either sensory or motor. Intermediate or intercalary neurons are a kind of "extension" that transmit impulses between other neurons.
Structure of the spinal cord
The spinal cord starts at the foramen magnum of the skull and ends at the lumbar vertebrae. It consists of 31-33 segments that are not separated from each other: C1-C8 - cervical, Th1-Th12 - thoracic, L1-L5 - lumbar, S1-S5 - sacral, Co1-Co3 - coccygeal. Down in the channelof the spine are continuations of nerves, collected in a bundle and called the cauda equina (apparently for their external resemblance), which innervate the lower limbs and pelvic organs. Each segment has two pairs of roots that connect to form 31 pairs of spinal nerves. The two posterior (dorsal) roots are formed by the axons of sensory neurons and have a thickening - the ganglion, where the bodies of these neurons are located. The two anterior (ventral) roots are formed by the axons of motor neurons.
So different and important
There are about 13 million nerve cells in the human spinal cord. Functionally, they are divided into 4 groups:
- Motor - form the anterior horns and anterior roots.
- Interneurons - form the back horns. Here are sensitive neurons, in which it occurs to various stimuli (pain, tactile, vibration, temperature).
- Sympathetic and parasympathetic neurons - located in the lateral horns and form the anterior roots.
- Associative - these are brain cells that establish a connection between segments of the spinal cord.
Gray butterfly surrounded by white
In the center of the spinal cord is a gray matter that forms the anterior, posterior and lateral horns. These are the bodies of neurons. Sensory neurons are located in the spinal ganglia, the long process of which is located on the periphery and ends with a receptor, and the short process is in the neurons of the posterior horns. The anterior horns are formed by motor neurons, the axons of which goto skeletal muscles. In the lateral horns are the neurons of the autonomic system. The gray matter is surrounded by white - these are nerve fibers formed by the axons of the ascending and descending wire pathways. The first sensory neurons are located in the following segments: cervical C7, thoracic Th1-Th12, lumbar L1-L3, sacral S2-S4. In this case, the spinal nerve connects the posterior (sensory) and anterior (motor) roots into one trunk. In addition, each pair of spinal nerves controls certain parts of the body.
How it works
The branched dendrites of sensitive neurons of the spinal centers of the autonomic nervous system end with receptors, which are biological structures in which a nerve impulse is formed upon contact with a specific stimulus. Receptors provide vegetovisceral sensitivity - they perceive irritation from such parts of our body as blood vessels and heart, gastrointestinal tract, liver and pancreas, kidneys and others. The impulse is transmitted along the dendrite to the body of the neuron. Further, the nerve impulse along the axons of afferent (sensitive) neurons enters the spinal cord, where they form synoptic connections with the dendrites of efferent (motor) neurons. It is thanks to this direct contact that we withdraw our hand from a hot pot or iron even before our main commander - the brain - analyzes the pain that has arisen.
Bringingtotal
All our automatic and reflex actions take place under the supervision of the spinal cord. The only exceptions are those that are controlled by the brain itself. For example, when we perceive what we see using the optic nerve, which goes directly to the brain, we change the angle of vision with the help of the muscles of the eyeball, which are already controlled by the spinal cord. We cry, by the way, also by order of the spinal cord - it is he who “commands” the lacrimal glands. Our conscious actions begin in the brain, but as soon as they become automatic, their control passes to the spinal cord. We can say that our inquisitive brain likes to learn. And when he has already learned, he becomes bored and he gives the "reins of power" to his older brother in evolutionary terms.
