IX. Introduction to Excitable Tissues
When the basic tissue types were discussed earlier, it was pointed out that two of these tissue types, muscle and nerve, were known as excitable tissues because of the rapidity of their responses to external changes. Although muscle and nerve are different in both their form and function, they have a number of commonalities, and it is these common characteristics shared by both muscle and nerve that are the subject of this unit.
A. Excitability - This is the ability of a living system to respond to an adequate stimulus.
1. All cells are excitable to a certain extent. It is one of the five basic characteristics of life.
2. Muscle and nerve cells have specialized in this aspect and therefore have become known as excitable tissues.
B. Response of excitable tissues - There are two principal responses of all excitable tissues.
1. Primary response - This is a change in the electrical characteristics of the cell membrane. It is basically the same for both muscle and nerve.
2. Secondary response - This is caused by the primary response and differs in the two types of excitable tissues.
a. Muscle - The secondary response is shortening of the cell (contraction).
b. Nerve - The secondary response is the release of a chemical by the nerve cell (neuron).
C. Stimulus - All response in excitable tissues result from adequate stimuli. A stimulus is any change in the environment of the excitable cell. In order to cause a response a stimulus must meet three requirements.
a. The stimulus must have sufficient intensity or strength.
b. The stimulus must have a certain duration.
c. The rate of change of stimulus intensity must be sufficiently rapid.
D. Classes of stimuli - There are four kinds of stimuli which will cause response in excitable tissues.
The majority of stimuli which initiate responses in the body are chemical in nature.
E. Conduction (propagation)- This is the spreading of the initial response from the point of stimulus application over the entire membrane of the excitable cell. It is a basic property of excitable tissues and is analogous to a burning fuse.
F. Membrane Potentials – There are three different types of electrical potentials which are associated with cells.
G. The resting potential of the cell membrane - Every cell membrane has a resting membrane potential which resides across the cell membrane. Cell membranes have a negative charge on the inside and a positive charge on the outside. Therefore, a difference of electrical potential exists across the cell membrane from the inside to the outside. Electrical potential is measured in volts or millivolts.
1. Polarized membranes - The fact that the inside of the cell membrane is negative and outside is positive means that the membrane has a positive electrical pole and a negative electrical pole. Such a membrane is said to be polarized. A voltmeter placed between the poles will measure a voltage much in the same manner as it would if placed between the positive and negative poles of a battery.
2. Establishment of polarization - The polarization of the membrane and the resting membrane potential (voltage) that results is due to unequal concentrations of ions across the cell membrane. The normal situation is to have high concentrations of potassium inside of the cell and low concentrations in the tissue fluid outside of the cell. The situation for sodium is reversed, high concentrations in the tissue fluid, and low concentrations inside of the cell. This type of ionic arrangement yields the resting potential of all cells. This unequal or unbalanced concentration of ions is maintained by the following mechanisms.
a. Differential membrane permeability - The membrane normally does not permit the sodium and potassium ions to diffuse easily from one side to the other.
b. Electrochemical equilibrium - The ions diffuse across the membranes down their concentration gradients. For example, as potassium diffuses out of the cell it creates an increasing deficit of charge within the cell so that it grows more negative. Eventually the negative attraction for the positive potassium ions becomes so great that it equals the concentration gradient, and then ion movement stops. Similar situations exist for sodium (but in the opposite direction).
c. Active transport of ions - A sodium-potassium "pump" located in the cell membrane pumps sodium out of the cell and potassium into the cell. Three sodium out for each two potassium ions pumped inward. Thus, three positive charges out and two in, which contributes to the resting potential.
2. Although all cells possess a resting membrane potential, only muscle and nerve cells can alter it to form an action potential.
3. The alteration in charges across the membrane result from a movement of ions across the membrane. These ions rearrange themselves due to a momentary change in the permeability of the cell membrane which permits the ions to diffuse down their respective concentration gradients. As the ions carry electrical charges they alter the membrane potential.
4. The membrane permeability change is due to the stimulus. Stimuli initiate excitable tissue responses by altering the membrane potential.
5. The action potential is divided into two major phases.
a. Depolarization - Following the stimulus the membrane loses its polarization and we say that it has become "depolarized." This constitutes the first half of the action potential.
b. Repolarization - Following depolarization the membrane returns to the resting state which is again polarized. Repolarization constitutes the second portion of the action potential.
L. Refractory periods - These are periods of times in which additional action potentials cannot be initiated by stimuli. The refractory period generally corresponds to the length of the action potential as it is not possible to depolarize the membrane while it is in a state of depolarization. Representative refractory periods are as follows.
1. Neurons - 0.5 to 4.0 milliseconds
2. Skeletal muscle - 5 milliseconds
3. Cardiac muscle - 250 milliseconds
Refractory periods limit the number of action potentials possible in a given period of time.