The nerves responsible for innervating muscle fibers are called motor neurons. A single motor neuron and the muscle fibers it innervates are collectively called the motor unit. The number of muscle fibers in a motor unit varies predictably with muscle function. For example, the motor units responsible for the facial expression muscles contain far fewer muscle fibers than the motor units responsible for the muscles involved in activities such as swimming. Skeletal muscle spasms are due to sudden and involuntary muscle contractions that last from a few seconds to a few minutes and lead to pain. Although they can be associated with diseases, muscle spasms most often occur in the absence of a clear pathology. They most often occur in older patients, who are pregnant or exercise vigorously. Since cramps are the product of muscle contractions, immediate pain relief can be achieved by stretching the affected muscle.  When muscle cells are seen under a microscope, they can be seen to contain a striped pattern (scratch). This model consists of a series of basic units called sarcomeres, arranged in a pattern stacked throughout the muscle tissue (Figure 1).
There can be thousands of sarcomeres in a single muscle cell. Sarcomas are very stereotypical and repeat in all muscle cells, and the proteins they contain can change in length, resulting in a change in the total length of a muscle. A single sarcoma contains many parallel filaments of actin (thin) and myosin (thick). The interaction of myosin and actin proteins is at the heart of our current understanding of sarcomere shortening. How does this shortening occur? This has something to do with a slippery interaction between actin and myosin. Figure 3.2. (A) A motor neuron is a neuronal cell that innervates a series of muscle fibers. It consists of a body, a tree of relatively short dendrites and a long axon with terminal branches at its end.
Each branch forms a synapse with a target muscle fiber. (B) The neuromuscular synapse is a complex anatomical structure composed of a presynaptic neuronal fiber, synaptic cleft and postsynaptic muscle fiber. An action potential entering along the nerve fiber always creates an action potential on the muscle fiber: this action is said to be mandatory. Invertebrates such as annelids, molluscs and nematodes have oblique striped muscles that contain thick, thin filament bands arranged spirally rather than transversely, as in the skeletal or cardiac muscles of vertebrates.  In mussels, obliquely striped muscles can maintain tension for long periods of time without consuming too much energy. Mussels use these muscles to keep their shells closed. Advanced insects such as wasps, flies, bees and beetles have asynchronous muscles that form the flight muscles of these animals.  These flight muscles are often called fibrillary muscles because they contain thick, noticeable myofibrils.
 A notable feature of these muscles is that they do not require stimulation for every muscle contraction. Therefore, they are called asynchronous muscles because the number of contractions in these muscles does not match (or synchronize) with the number of action potentials. For example, a muscle in the wing of an attached fly may receive action potentials at a frequency of 3 Hz, but it is able to beat at a frequency of 120 Hz.  High-frequency beats are made possible by the fact that the muscles are connected to a resonance system that is trained at a natural vibrational frequency. When a weak signal is sent by the central nervous system to contract a muscle, the smaller motor units, which are more excitable than the larger ones, are first stimulated. As the signal strength increases, more (and larger) drive units are excited. The largest motor units have up to 50 times the contractile strength of the smallest; Thus, as more and larger motor units are activated, the muscle contraction force becomes stronger and stronger. A concept known as the size principle allows a gradation of muscle strength during a weak contraction in small steps that become larger and larger with increasing strength. Coupling, depolarization conduction and Ca2+ release processes occur in the excitation and contraction of skeletal and cardiac (E-C) muscles. Although the proteins involved are similar, they differ in structure and regulation. Dihydropyridine receptors (DHPR) are encoded by different genes, and ryanodine receptors (RyRs) are different isoforms. In addition, DHPR touches with RyR1 (main isoform of RyR in skeletal muscle) to regulate the release of Ca2+ in skeletal muscle, while the L-type calcium channel (DHPR on cardiac myocytes) and RyR2 (main RyR isoform in heart muscle) in the heart muscle are not physically coupled, but face each other through compound coupling.
 During rapid and intense contraction, phosphagens can be used to quickly rebuild ATP and maintain its level as long as phosphating persists, which is only a few seconds in a human muscle that is functioning at its maximum. After contraction, ATP is used to form phophages from creatine; ADP is also formed. Clark, M. Milestone 3 (1954): Sliding filament model for muscle contraction. Filaments of muscle slippage. Nature Reviews Molecular Cell Biology 9, s6–s7 (2008) doi:10.1038/nrm2581. There are two types of heart muscle cells: autorythmic and contractile. Autorythmic cells do not contract, but determine the rate of contraction of other heart muscle cells that can be modulated by the autonomic nervous system.
In contrast, contractile muscle cells (cardiomyocytes) make up the majority of heart muscle and are capable of contracting. After systole, intracellular calcium is reabsorbed into the sarcoplasmic ATTiculum pump (SERCA) of the sarco/endoplasmic reticulum, ready for the next cycle. Calcium is also expelled from the cell, mainly through the sodium-calcium exchanger (NCX) and, to a lesser extent, through an ATPase calcium plasma membrane. .