The somatic nervous system (from the Greek soma – body) is a part of the human nervous system, which is a combination of afferent (sensory) and efferent (motor) nerve fibers that innervate muscles (skeletal in vertebrates), skin, joints. The somatic system is a part of the peripheral nervous system that delivers motor (motor) and sensory (sensory) information to the central nervous system and back. This system consists of nerves attached to the skin, sensory organs and all the muscles of the skeleton.
It is responsible for almost all conscious muscle movements, as well as for processing sensory information coming through external stimuli: sight, hearing and touch. The name of the somatic nervous system comes from the Greek word “soma” (body). The somatic nervous system contains two main types of neurons: sensory (afferent) neurons, which supply information from nerve endings to the central nervous system, and motor (efferent) neurons, which deliver information throughout the body from the brain and spinal cord to muscle tissues.
The neurons of the somatic nervous system extend from the central nervous system directly to the muscles and receptors. The body of the neuron is located in the central nervous system, and the axons stretch further until they reach the skin, sensory organs, or muscles. Electrochemical impulses travel through axons from the brain to the spinal cord.
The somatic nervous system also includes reflex arcs responsible for unconscious actions (reflexes). With the help of reflex arcs, the muscles move without signals from the brain. This happens when nerve pathways connect directly to the spinal cord. Some examples of how reflex arcs work: you quickly lift your hand from a hot pot or unconsciously lift your foot when the doctor hits you on the knee.
The somatic nervous system consists of two parts:
- Spinal nerves are peripheral nerves that transmit sensory information to the spinal cord and motor commands from the spinal cord.
- Cranial nerves are nerve fibers that carry information to and from the brainstem. They innervate the nose, eyes, eye muscles, mouth, tongue, ears, face, neck, shoulders and transmit information from the organs of vision, hearing, smell, taste and vestibular apparatus
The somatic nervous system includes 12 pairs of cranial nerves extending from the brain, and 31 pairs of spinal nerves with numerous branches. These are nerves with predominantly somatic innervation.
Muscle Tissue – The Ability To Pronounced Contractions
The main morphological features of muscle tissue elements are: an elongated shape, the presence of longitudinally arranged myofibrils and myofilaments – special organelles that provide contractility, the location of mitochondria next to the contractile elements, the presence of inclusions of glycogen, lipids and myoglobin.
Special contractile organelles – myofilaments, or myofibrils – provide contraction, which occurs when the two main fibrillar proteins interact in them – actin and myosin, with the obligatory participation of calcium ions. Mitochondria provide energy for these processes. The supply of energy sources is formed by glycogen and lipids. Myoglobin is a protein that binds oxygen and creates its reserve at the time of muscle contraction, when blood vessels are compressed (oxygen supply drops sharply).
smooth muscle tissue
Consists of mononuclear cells – spindle-shaped myocytes 15-500 microns long. Their cytoplasm in a light microscope looks uniform, without transverse striation. This muscle tissue has special properties: it slowly contracts and relaxes, has automaticity, is involuntary (that is, its activity is not controlled by the will of a person). It is part of the walls of internal organs: blood and lymphatic vessels, urinary tract, digestive tract (contraction of the walls of the stomach and intestines). With the help of smooth muscles, the size of the pupil and the curvature of the lens of the eye change.
striated skeletal muscle tissue
Consists of myocytes, having a large length (up to several cm) and a diameter of 50-100 microns; these cells are multinucleated, containing up to 100 or more nuclei; Under a light microscope, the cytoplasm looks like alternating dark and light stripes. The properties of this muscle tissue are a high speed of contraction, relaxation and arbitrariness (that is, its activity is controlled by the will of a person). This muscle tissue is part of the skeletal muscles, as well as the walls of the pharynx, the upper part of the esophagus, it forms the tongue, oculomotor muscles . The fibers are 10 to 12 cm long.
striated cardiac muscle tissue
It consists of single- or double-nuclear cardiomyocytes with transverse striation of the cytoplasm (along the periphery of the cytolemma). Cardiomyocytes are branched and form connections between themselves – intercalated disks, in which their cytoplasm is combined. There is also another intercellular contact – anastomoses (the invagination of the cytolemma of one cell into the cytolemma of another). This type of muscle tissue is the main histological element of the myocardium of the heart.
It develops from the myoepicardial plate (visceral sheet of the splanchnotome of the neck of the embryo). A special property of this tissue is automatism – the ability to rhythmically contract and relax under the influence of excitation that occurs in the cells themselves (typical cardiomyocytes). This tissue is involuntary (atypical cardiomyocytes). There is a third type of cardiomyocytes – secretory cardiomyocytes (they do not have fibrils). They synthesize atrial natriuretic peptide (atriopeptin), a hormone that causes a decrease in circulating blood volume and systemic arterial pressure.
The Spinal Cord – An Organ Of The Central Nervous System Of Vertebrates
It is generally accepted that the border between the spinal cord and the brain passes at the level of the intersection of the pyramidal fibers (although this border is very arbitrary) or at the level of the occipital foramen of the occipital bone. Inside the spinal cord there is a cavity called the central canal (Latin canalis centralis) which is filled with cerebrospinal fluid.
The spinal cord is protected by the pia, arachnoid and dura mater. The spaces between the membranes and the spinal canal are filled with cerebrospinal fluid. The dura mater consists of the visceral and parietal sections. The space between the visceral and parietal dura maters is called the epidural space and is filled with adipose tissue and venous network.
The spinal cord (Latin medulla spinalis) has a clear segmental organization. It provides connections between the brain and the periphery and performs segmental reflex activity. The spinal cord lies in the spinal canal from the upper edge of the 1st cervical vertebra to the 1st or upper edge of the 2nd lumbar vertebra, repeating the direction of curvature of the corresponding parts of the spinal column. In a fetus at the age of 3 months, it ends at the level of the V lumbar vertebra, in a newborn, at the level of the III lumbar vertebra.
The spinal cord without a sharp border passes into the medulla oblongata at the exit of the first cervical spinal nerve. Skeletotopically, this border runs at the level between the lower edge of the foramen magnum and the upper edge of the first cervical vertebra.
At the bottom, the spinal cord passes into a conical point (lat. conus medullaris), continuing into the terminal (spinal) thread (lat. filum terminale (spinale)), which has a diameter of up to 1 mm and is a reduced part of the lower spinal cord. The terminal thread (with the exception of its upper sections, where there are elements of the nervous tissue) is a connective tissue formation.
Together with the dura mater, it penetrates the sacral canal and attaches at its end. That part of the terminal thread, which is located in the cavity of the dura mater and is not fused with it, is called the internal terminal thread (lat. filum terminale internum), the rest of it, fused with the dura mater, is the outer terminal thread (lat. filum terminale externum). The terminal thread is accompanied by the anterior spinal arteries and veins, as well as one or two roots of the coccygeal nerves.
The spinal cord does not occupy the entire cavity of the spinal canal: between the walls of the canal and the brain there is a space filled with adipose tissue, blood vessels, meninges and cerebrospinal fluid