Muscle cells are a unique type of cell in the human body due to their unique properties. They are the only cells in the body that are able to contract and relax. In addition, muscle cells along with neurons of the nervous system are the only type of cells which are excitable and are capable of communicating with other cells through an electrical impulse that passes from one cell to another.
In this article, I will discuss the function of muscle cells in addition to their general structure and ability to communicate with neurons through an action potential. Muscle cells differ than other cells in several ways. They have multi-nuclei in their cytoplasm but are unable to divide. Therefore, injury of the muscle tissue can be an irreversible process. Also direct cancer which affects muscle cells is not possible due to the inability of these cells to divide and reproduce.
Muscle cells are in this respect similar to neurons of the nervous system which are also unable to divide and are not liable to develop cancer. Muscle cells do not undergo hyperplasia or an increase in the number of their cells due to the fact that they are unable to divide. On the other hand, they can undergo a process of atrophy or hypertrophy. Atrophy of muscles refers to a process in which muscle cells shrink and become smaller in size mostly due to the lack of activity of the muscle tissue. In this process muscle cells become smaller in size.
The other possible process which muscle cells can undergo is called hypertrophy. This process involves enlargement of the muscle tissue in size but not in the number of cells. The muscle cells size become bigger. This is mostly due to hypertrophy of nuscle tissue such as in strenuous exercise. Muscle build up relies in this process of hypertrophy which occurs due to the strenuous muscle exercise that the individual does.
Muscle cells contract and relax based on a stimulus that they receive and which leads to the formation of action potential inside the cell. The most important process which involves the contraction and relaxation of muscle cells occurs in the skeletal muscles and in muscles that are innervated by the autonomic nervous system.
Skeletal muscles are under voluntary action and are important clinically for the reflexes that they exhibit. The state of muscle reflexes can give diagnostic information about the status of the nervous system. Low reflexes or hyporflexia can be indicative of dysfunction of the spinal cord or the motor cortex of the brain cerebrum. The damage of the spinal cord will be at various levels that will depend on the site of hyporeflexia.
High reflexes or hyperreflexia are usually indicative of upper motor neurons damage in the brain. Muscle reflexes can also be affected due to other diseases that have their origin outside the nervous system. Examples include neuropathy of diabetes mellitus in which case damage to neurons of the spinal cord can lead to hyporeflexia. Also late stages of syphilis can be manifested as hyporeflexia of the skeletal muscles.
Muscles of the internal organs contract and relax based on a stimulus from neurons of the autonomic nervous system. The neurotransmitter that usually leads to the contraction of muscle cells is called acetylcholine. The space between a neuron and a muscle cell is called the neuromuscular junction. Acetylcholine is released into the synaptic cleft between the neuron and the muscle cell based on a stimulus that the neuron receives and which leads to the flow of calcium ions into the neuron. The released acetylcholine binds to receptors on the muscle cells which are called the motor end plate.
This binding to the flow of sodium into the muscle cells in turn initiates an action potential that leads to the release of calcium ions into the cytoplasm of the muscle cells from the sarcoplasmic reticulum. This eventually leads to the muscle contraction.