Overview of Physiology of Neurons

Neurons are the functional cells of the nervous system.  The nervous system contains in addition supporting cells that are called neuroglia.  Neurons do not divide although they possess nuclei.  On the other hand, neuroglia do divide.  Because neurons do not divide they are not suceptible to mutagenesis or cancer. 

Neuroglia because they do divide are suceptible to mutagenesis and cancer.  Therefore, they are the source of malignancy in tumours of the nervous system.  

Neurons contain nuclei and are divided histologically into a dendrite and a body and an axon.  The body of the neuron is connected to the dendrite and the axon of the same neuron. 

Neurons transform information to other neurons by contact of their axon with dendrite of a neigboring neuron.  The receive information on the other hand by a contact between their dendrite and axon of another neuron. 

A neuron has a resting electrical membrane potential which makes it electrically charged.  This is so due to the presence of ion channels and active transport across the cellular membrane.  In addition to neurons, muscle cells are also polarizable. 

Normally active transport of ions, mainly sodium and potassium renders a resting electrical membrane potential.  Na / K pumps usually transport sodium outside the cell in exchange for potassium which is transported inside the cell. 

Normally an equilibrium state occurs in which the concentration of sodium is higher outside the cell than its concentration inside the cell.  On the other hand, potassium ions prevail inside the cell more than its concentration outside the cell. 

This equilibrium state is disrupted if the neuron receives a signal that can depolarize the cell.  Usually, the neuron resting electric potential is negative.  If it receives an electric signal sodium channels open and sodium ions begin to flow inside the neuron.   

This process increases the positive charge of the neuron and renders a depolarization of the cell.  This increases the resting electric potential of the neuron and an action potential takes place. 

The action potential can let the cell do several things.  It can excite another neuron and create a cascade in which in this way many neigboring neurons are excited sequentially.  The action potential can alternatively excite a muscle cell causing its contraction. 

The action potential predominates for a short period of time after which sodium channels close and potassium channels open.  When the potassium channels open potassium ions begin to flow in the direction outside the cell. 

This process causes hyperpolarization and as a result a normal state of electric membrane potential is restored.  The sodium ions that entered the cell return to the outside by diffusion and the potassium ions that left the cell return now to the inside by diffusion also. 

This process of action potential generation and the whole physiology that is associated with it is used clinically and pharmacologically in the design of drugs that have the potential to affect nervous system in general and the neurons in particular. 

Anesthetic drugs use the fact that sodium channels blockers induce a state of anesthesia due to their effect on blocking the depolarization of the neuron.  Thus blocking the initiation of action potential and consequently blocking the impulse along the pathway of the nervous system.  This can be used to kill pain for example.