Our spinal cord is the oldest evolutionary formation of the nervous system. Appearing for the first time at the lancelet, in the process of evolution the spinal cord with its efferent (motor) and afferent (sensitive) neurons was improved. But at the same time, it retained its main functions - conducting and regulatory. It is thanks to the sensitive neurons of the spinal cord that we pull the hand away from the hot pan even before the onset of pain. 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 is hidden inside the spinal column. The spinal cord of a person 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, nevertheless, this is the control center of a complex network of nerve fibers that spreads throughout our body. Without it, the musculoskeletal system and all vital organs of our body will not be able to perform its functions . In addition to the vertebrae, the spinal cord protects its shell. The outer shell is solid, formed by a dense connective tissue. Blood vessels and nerves are located in this membrane. And, besides, it is in it that the highest concentration of pain receptors in the human body is observed. But in the brain itself there are no such receptors. The second membrane is arachnoid, filled with cerebrospinal fluid (cerebrospinal fluid). The last membrane - soft - fits snugly to the brain, penetrated by blood and lymph vessels.
A few words about neurons
The structural unit of the nervous tissue are neurons. Absolutely 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 one - an axon that conducts a nerve impulse in only one direction. Depending on the task and function, neurons are sensitive and motor. Intermediate or insertion neurons are a kind of “extension cords” that transmit momentum between other neurons.
Spinal cord structure
The spinal cord begins with the occipital foramen of the skull and ends in 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. Below in the spinal canal are extensions of the nerves, collected in a bundle and called the cauda equina (apparently due to the resemblance), which innervate the lower limbs and pelvic organs. Each segment has two pairs of roots that connect into 31 pairs of spinal nerves. Two back (dorsal) roots are formed by axons of sensitive neurons and have a thickening - the nerve node, where the bodies of these neurons are located. Two front (ventral) roots are formed by axons of motor neurons.
So different and so important
About 13 million nerve cells are in the human spinal cord. Functionally they are divided into 4 groups:
- Motor - form the front horns and front roots.
- Interneurons - form the rear horns. Here are sensitive neurons in which it arises for various stimuli (painful, tactile, vibrational, temperature).
- Sympathetic and parasympathetic neurons are located in the lateral horns and form the anterior roots.
- Associative - these are already 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 front, rear and side horns. These are the bodies of neurons. Sensitive neurons are located in the spinal ganglia, the long process of which is located on the periphery and ends with the receptor, and the short one is in the neurons of the posterior horns. The front horns are formed by motor neurons, the axons of which go to the skeletal muscles. In the lateral horns are located neurons of the autonomic system. Gray matter is surrounded by white - these are nerve fibers formed by axons of the ascending and descending pathways. The first sensitive 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 (sensitive) and anterior (motor) roots into one trunk. In addition, each pair of spinal nerves controls certain parts of the body.
How it works
Branched dendrites of the 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 the heart, gastrointestinal tract, liver and pancreas, kidneys and others. According to the dendrite, the impulse is transmitted to the body of the neuron. Further, a nerve impulse along the axons of afferent (sensitive) neurons enters the spinal cord, where they form synoptic connections with dendrites of efferent (motor) neurons. It is thanks to this direct contact that we pull our hand away from the hot pan or iron even before our main commander - the brain - analyzes the pain that has arisen.
To summarize
All our automatic and reflex actions take place under the supervision of the spinal cord. The only exceptions are those that the brain itself controls. For example, perceiving what we saw using an optic nerve that goes directly to the brain, we change the angle of view using the muscles of the eyeball, which are already controlled by the spinal cord. By the way, we also cry on the orders 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 gives the “reins of government” to his older brother in evolutionary terms.