The main functional function of muscle tissue (whether it is striated or smooth, white or red) is its excitability. This irritability is most often manifested by shortening, spasm (contraction) of muscle fibers, which occurs under the action of external or internal stimuli.
Under normal conditions, the natural stimuli for the contraction of human muscles are impulses that come to each muscle through its motor nerve, and these impulses arise in the central nervous system, under the influence of changes in the internal and external environment, which are received through receptors. In other words, the stimulus generated in the receptors is transmitted through the sensitive nerves to the central nervous system, where it is transmitted to the motor nerve fiber and through it to the muscle, which, upon receiving a sufficiently strong impulse, immediately passes into a state of excitability, i.e. It contracts.
The contraction of skeletal muscle itself occurs in response to nerve impulses, which come to the muscle through special nerve cells of motoneurons. Muscles, together with the nerves that innervate them, make up the neuromuscular apparatus of man. The functional connection of motoneurons with muscles is carried out through the axon, so that each of the end branches of the axon ends on one muscle fiber, forming a neuromuscular synapse or the so-called terminal plate. Therefore, each motoneuron innervates as many muscle fibers as there are end branches. This physiological unit of the motoneuron, its axon and all the muscle fibers that it innervates, makes up the motor unit. It is the basic morphofunctional unit of the neuromuscular apparatus, and in the human body it differs in the size of motoneurons, as well as in the number of muscle fibers. Thus, small motor units have a relatively small motoneuron, with a small number of muscle fibers (up to a few dozen), and they are in the composition of all the small muscles of the face, fingers and toes, hand, and partly in the composition of the large muscles of the trunk and extremities. A large motor unit has a large, large motor neuron, which, with its axon branches, innervates up to several thousand muscle fibers. They are found in the composition of the large muscles of the trunk and extremities.
Each skeletal muscle is made up of a large number of muscle bundles, and a bundle is made up of thousands of muscle fibers. In humans, the number of these fibers is formed as early as the 4th to 5th month of life and practically does not change. However, their thickness changes considerably at birth, their diameter is 1/5 of the thickness of an adult’s fiber; Under the influence of training, this diameter can be significantly increased in adults.
