PSM11I

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.

l. Thermal

2. Mechanical

3. Electrical

4. Chemical

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.

  1. Resting membrane potential This is a constant voltage measured across the cell membrane.
  2. Local (graded) potential A small change that occurs in the resting membrane potential in a small area of the cell membrane.
  3. Action potential A large change in the resting membrane potential that moves from its site of origin over the entire membrane.
  4. All of these electrical potentials are due to the movement of ions across the membrane. These ions move through protein channels. There are several types of these channels.
    1. Leak channels These are protein tubes through the membrane which are always open for ions to move through them. Leak channels are responsible for the resting membrane potential.
    2. Gated channels These channels have gates which may be opened or closed. When closed, no ions may move through the channels. When open, the ions may move through freely. There are three types of gated channels, based upon the control of the gate.
        1. Ligand (chemical) gates A molecule of some type attaches to the gate and causes it to open, permitting ions to move through the channel. Responsible for local potentials.
        2. Mechanical gates Physical pressure causes the gates to open. Responsible for local potentials.
        3. Electrical gates A changed in the membrane voltage causes the gates to open. Responsible for action potentials.

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.

  1. Local (graded) potentials These are small, graded changes in the resting potential which are brought about by various stimuli.
    1. Normally these changes result from chemical or mechanical stimuli (chemically gated, mechanically gated) which open up a limited number of sodium channels.
    2. Because of the high concentration of sodium on the outside of the cell, as these gated channels open, sodium diffuses inward bringing positive charges which alter thenegative resting membrane potential in the area.
    3. The local potential change is greatest in the immediate area where sodium diffuses inward and begins to decrease as the sodium spreads outward inside of the cell. This is similar to the ripples caused by a pebble dropped into still water. As the ripples move further away, their height becomes less.
    4. If the gates do not open again, the electrochemical equilibrium is reestablished and the membrane returns to the resting potential.
  2. Action potential - Excitable cells have the ability to momentarily alter or reverse their resting potential. This reversal follows an adequate stimulus and is conducted across the entire membrane. It constitutes the initial response of both muscle and nerve and is termed the action potential.
    1. It is important to note that following the stimulus the charges in the area of the stimulus momentarily reverse themselves. The inside of the cell becomes positive but very quickly reverse again to return to the resting condition. This reversal is conducted in a wave-like manner all over the membrane.

    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.

  3. Mechanism of the action potential - The initial response, the action potential, is so named because it is a rapid transient electrical change when compared to the steady resting potential characteristic of most cells. This electrical change is due to rapid diffusion of ions across the cell membrane.
    1. The initial resting potential is due to high concentrations of sodium outside of the cell and high concentrations of potassium inside of the cell. The concentrations of sodium inside of the cell are low and the concentrations of potassium outside of the cell are low. At rest, the bulk of both the sodium and potassium channels are closed.
    2. An adequate stimulus causes the cell membrane to change permeability, generating a local potential. If the local potential is of sufficient magnitude (threshold value 60 to 50) it will cause the electrically controlled sodium channels to open. This allows sodium to flood into the cell. The incoming sodium brings positive charges which destroy the resting membrane potential and depolarization is initiated.
    3. Sodium continues to diffuse into the cell until the inside of the cell becomes electrically positive. At this point the sodium gates become inactive but the electrically controlled potassium gates now open.
    4. Potassium diffuses out of the cell down its concentration gradient. This effectively removes the positive charges brought in by sodium and restores the resting potential (repolarization).
    5. Although the resting potential is restored, the ionic situation is different from the resting state. The original resting state is restored by the active exchange of sodium that diffused in for the potassium that diffused out.

  4. All or none principle When an excitable cell responds, it produces a maximum response, or it does not respond at all under any set of environmental changes.
  5.  

    1. In both muscle and nerve the action potential is complete or there is no action potential. Partial action potentials do not exist. Local potentials can have varying voltage levels, but they are not propagated along the membrane, and have no effect unless they reach the threshold level, at which time a full action potential moves across the membrane.
    2. In muscle, a given muscle cell contracts with maximum force or it does not contract at all.
    3. By changing the environmental conditions of the excitable cell (temperature, ions, etc.) the response may be altered, but will still be all-or-none for the altered conditions.

 

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.