Overview of Muscle Cells

Muscle cells are polarizable like neurons.  Namely they possess a membrane electric potential that can be depolarized and hyperpolarized.  Muscle cells contain many nuclei inside their cytoplasm.  However, muscle cells do not divide but can atrophy and hypertrophy.

For the reason that muscle cells do not divide they are not likely to undergo mutagenesis or form cancer.  Muscle cells are unique among other cells in that they can contract and relax based on an external stimulus such as nerve impulse which can induce the muscle cells to contract. 

There are two types of muscles.  These are voluntary and involuntary muscles.  An example of a voluntary muscle is muscles of the hand and leg.  An example of an involuntary muscle is muscles of the gastrointestinal tract. 

The details of contraction and relaxation of  muscles is a very technical amount of information that would not add to the understanding of the whole picture of muscle physiology.  Therefore I will not bother the reader with these details but instead I will mention important and relevant information.  These are muscle cells depolarization and hyperpolarization and formation of the action potential.

Muscles of the extremeties are under motor control from the anterior cerebral cortex and the spinal cord.  In addition sensation of muscles location is also governed by the posterior part of the cerebral cortex. 

Involuntary muscles of the gastrointestinal tract are governed by the vagus nerve and the sacral part of the spinal cord.  All muscle function is based on the same principle of excitation of the cell membrane potential.  The basic information about depolarization and hyperpolarization in muscles and neurons is roughly the same.    

The mechanism that leads to the contraction and relaxation of muscle cells will be discussed from the point of view of excitation of the cells by an external stimulus such as nerve impulse or a chemical neurotransmitter.  The initial stage that leads to muscle contraction is a stimulus that leads to the excitation of the muscle electric potential for the cell membrane. 

Like neurons muscle cells possess a negative value for the cellular membrane electric potential.  The stimulus that stimulates the muscle cells can open ion channels of sodium ions. 

Normally sodium ions  are present in higher concentration outside the cell than inside the cell.  Opening of the sodium channel leads to influx of sodium ions toward the inner side of the cell.  This process increases the amount of positive charge inside the cell.  Thus increasing the value of the membrane electric potential. 

If the electric potential is sufficiently high to reach the value of a threshold an action potential occurs.  Once the threshold value is reached an action potential occurs which leads to depolarization of the muscle cell.  As a result a series of events occur which lead to muscle contraction. 

These details are not necessry for the understanding of the main ideas behind muscle function.  After the contraction of the muscle cell sodium channels close and subsequently potassium channels open.  By this process hyperpolarization occurs.  A relaxation of the muscle then occurs after restoring the membrane potential to its original value.

It is noteworthy that potassium ions inside the cell are higher in concentration than those outside the cell.  Sodium channels are more important than potassium channels in determining the electric potential across the cellular membrane.