Overview of Excitable Cells in Humans

Cells in the body that are excitable and which communicate with each other through an action potential are cells of the nervous system that are called neurons.  This is in addition to muscle cells which are also excitable and contract based on signaling using an action potential.  Other manners of cellular communication is through the secretion of hormones that travel in the blood stream and communicate with other cells by binding to their cellular receptors.  This type of communication does not involve excitation nor inhibition of traget cells directly. 

In this article, I will discuss how neurons communicate with each other in addition to the manner in which neurons comunicate with muscle cells at the neuromuscular junction. Neurons as well as muscle cells are excitable due to the presence of an electric potential on their celullar membrane.  This electric potential is usually of negative value.  The electric potential across the cellular membrane is the result of concentration gradient of two main ions in the body. 

These ions are sodium and potassium.  The concentration gradient is the result of ions pumping across the cellular membrane.  This ion pumping happens across ion channels and is catalyzed by enzymes that are called ATPases that use energy in the form of ATP molecules.  Cellular communication between two adjacent neurons can happen in one of two manners.  These are:  Communication through a neurotransmitter or through the so called electrical synapses.

In Electrical synapses there is a connection between adjacent cells and which the ions flow from one neuron to the other by way of canals between the cells that are called tubular connexons.  They connect between the cytosol of one neuron and the cytosol of the neigbouring cell.  This type of signaling is common in smooth muscles of various internal organs in the body in addition to its presence in muscles of the heart.

The neuron in this type of connection receives a stimulus and depolarization occurs in which sodium ions begin to flow inside the cell.  Once action potential occurs sodium ions begin to flow to the adjacent neurons next to it through the tubular connexons, thus the action potential is spread from one neuron to the other.  This type of signaling is very fast in comparison with that which requires a neurotransmitter. 

The second type of signaling between neurons is through the release of a neurotransmitter to the space between neurons or the synaptic cleft.  In this type of communication there is no direct contact between the cells.  Once a neuron receives a stimulus sodium ion channels open and depolarization process occurs in which the cellular electric potential starts to be more positive.  Once an action potential is reached vesicles that contain the neurotransmitters in the neurons start to fuse with the cellular membrane releasing the neurotransmitter to the synaptic cleft by exocytosis. 

The neurotransmitter can be excitatory or inhibitory.  Namely it can open ion channels that can increase or decrease the cellular electric potential.  An inhibitory neurotransmitter usually opens chloride or potassium ion channels in which case there is a process of hyperpolarization.  In this case the value of the cellular electric potential becomes more negative in value.  If the neurotransmitter is excitatory it opens either sodium or calcium ion channels in which case there is depolarization of the cellular membrane electric potential.

In the case of communication between a neuron and a muscle cell there is the so called neuromuscular junction between the neuron and the muscle cell.  The neurotransmitter that usually functions in this case is acetylcholine.  In this case the neuron which contains the acetylcholine neurotransmitter receives a stimulus that initiates the depolarization process in the neuron.  This depolarization occurs in response to inflow of calcium ions inside the neuronal cell. 

Once an action potential occurs due to the inflow of the calcium ions into the neuron, there is a process of exocytosis of vesicles which contain acetylcholine.  This neurotrasnsmitter then binds to receptors on the cellular membrane of muscles cells which are called the motor end plates.  This binding is excitatory and leads to the inflow of sodium ions into the muscle cells.  As a result an action potential occurs.  Subsequently the action potential that is triggered in the muscle cells travels along the sarcolemma in the T tubules which leads to the release of calcium ions into the cytosol from the sarcoplasmic reticulum.  Evetually, this leads to muscle contraction.