Discusión de resultados y mecanismos de la regla electoral

The interior of the cell also contains impermeable anions namely proteins, phosphates and sulphates. These anions are negatively charged and they also contribute to the electronegativity of the interior, which also contribute to the existence of resting membrane potential.

Equilibrium potentials and conductance of the membrane

When the membrane potential and the equili-brium potentials are same for an ion, then, there will be no movement of ion. That is, there is no driving force available for its movement. On the other hand, if the equilibrium potential and membrane potential for an ion are different, the difference between these two will be the driving force for the ion movement, provided, the permeability of the membrane for the ion is present.

The equilibrium potentials of K⁺, Na⁺, Cl^– ions across the membrane and conductance (g) of ions through the membrane can be combined to determine the resting membrane potential which is called Hodgkin- Huxley equivalent electrical circuit.

E = Equilibrium potential g = Conductance

Changes in ion concentration and Its effect on the membrane potential

Increase in ECF potassium concentration, will cause the membrane potential to decrease, as there is a concentration gradient for K⁺ to move into the cell.

Decrease in ECF concentration of K⁺ leads to a rise in the membrane potential (more

nega-tive). It is because of concentration gradient existing from the cell to the ECF.

Changes in ECF sodium concentration do not affect significantly the membrane potential due to the fact that the membrane is not freely permeable to it.

Changes in the ECF calcium level affect the membrane potential significantly. Decrease in plasma Ca⁺⁺ causes increased excitability of nerve and muscle. It is due to decreased calcium causing greater depolarization. Rise in plasma calcium causes stabilization of the membrane, which increases the threshold of excitation.

Action potential

The resting membrane potential develops into an action potential on application of an adequate strength of stimulus (Fig. 2.6). The sequence of events that occurs during the generation of action potential is described below.

Fig. 2.6: Action potential from an axon

Depolarization

The opening of voltage gated Na⁺ channels leads to a decrease in the membrane potential from the resting level. The fall in membrane potential opens more Na⁺ channels, as the conductance of the membrane for sodium is increased (Fig. 2.7). This is a positive feedback cycle, wherein, decrease in the membrane potential causes more Na⁺ entry, which in turn leads to further fall in the membrane potential. This is repeated until the inactivation of Na⁺channel

occurs.The result is, the potential reaches +45 mv and the inner side of the membrane becomes electropositive. At this point, the action potential is fully formed.

In the recording of action potential, the depolarization is marked as upward stroke. It can be learnt from the positive feedback cycle which occurs during depolarization, that increase in ion conductance reduces the mem-brane potential (Fig. 2.8). This leads to further ion entry and a greater fall in membrane

potential which leads to the development of action potential. Depolarization ends with the inactivation of Na⁺ channels.

Repolarization

It begins with the opening of K⁺ channels causing its exit. The membrane potential increases towards the negative side. The recor-ding shows repolarization as downward deflection. During repolarization, the membrane conductance for K⁺ is increased.

After potentials called after depolarization occurs at the end of repolarization.This is due to slow K⁺ exit. This phase is followed by after hyperpolarization, caused by the slow closure of K⁺ channels and the potential becomes more negative. With the closure of K⁺ channels, the membrane potential reaches the resting level.

Channel activity during action potential The channel activity is studied by patch clamp technique (See Fig. 1.18).

At the resting membrane potential level The Na⁺ channels show closure of activation gate and inactivation gates being in open state. These voltage gated channels are activated by the voltage stimulus. Now, activation gates open, allowing Na⁺ influx and depolarization. At the end of depolarization, the inactivation gates close, stopping the Na⁺ entry into the cell. Repolariz-ation begins with opening of gates of K⁺ channel, causing K⁺ exit. At the end of repolarization, gates of K⁺ channel close, which stop K⁺ exit. This leads to the membrane potential reaching the resting level.

Time dependence

The time dependence in the opening and closing of channels is important for the action potential to fully develop. For example, if inactivation gates of Na⁺ channels close, when activation gates open, depolarization will not reach the threshold level for the action potential to develop. In this situation, repolarization starts before the completion of depolarization.

Fig. 2.8:Relationship between action potential and conduc-tance of ions. Note the increased conducconduc-tance of Na⁺ during the upstroke of the action potential which corresponds to depolarization. The conductance of K⁺ is increased during the down stroke of the action potential which corresponds to repolarization

Fig. 2.7: Positive feedback mechanism in sodium conductance through the cell membrane during depolarization

Methods to study Ionic current during action potential

Voltage Clamp Technique

It has been said earlier that changes in voltage can bring about changes in membrane poten-tial, which inturn, alters the voltage. There is a mutual relationship between these two. To study the current flow and ionic flow independently without the influence of one on the other, an experiment was conducted by Hodgkin and Huxley in 1952 called voltage clamp technique.

In this the membrane potential is clamped at a desired level to record the movement of ions and current flow. It will be seen that, when voltage is clamped at 0 volts, (membrane is depolarised from –70 mv to 0 volts) there is an inward current flow, due to the influx of Na⁺ ions (Fig. 2.9).

However, the inward Na⁺ current declines over a period of time.It is because of clamping of membrane potential. If the clamp is fixed at a different level now, say, at +15 mv, again inward current of Na⁺ can be recorded. In the case of K⁺ current, it is the outward flow, due to the exit of K⁺ occurs. This current flow does not decline over a period of time and can be recorded, as long as the clamp is maintained (Fig. 2.10). Like this, the membrane potential can be clamped, at various levels and at each level, the flow of current and ion movement can be studied.

Properties of Action Potential All or None Law

Action potential developed in an excitable tissue obeys all or none law. It states that with a threshold strength of stimulus, the response obtained is maximum, or not at all if it is below threshold.

Accommodation

In excitable tissue, the intensity of stimulus that is applied should be of threshold value and it should be quickly rising. Threshold strength of current, if it is slowly rising, will not give any response, as the tissue is accommodated to this kind of current. With slowly rising current, the membrane potential will not reach the threshold level of firing to form the action potential. The number of Na⁺ channels activated are not many, due to the fact that the K⁺ channels also open, which allows repolarization to occur. Hence action potential cannot develop.

Relationship between excitability and action potential

There is a period during action potential, in which a second stimulus no matter how strong it may be, cannot produce a response.This period is known as absolute refractory period (Fig. 2.11). It corresponds to the entire depolari-zation and one third of repolaridepolari-zation. With the

Fig. 2.9: Voltage clamp technique showing sodium and potassium current

Fig. 2.10: Voltage clamp technique showing sodium current and potassium current separately Note the sodium current which is inward and its decline over time when the clamp is maintained at the 0 voltage.

The K⁺ current does not decline over time and exists as long as the voltage is maintained

beginning of repolarization, there will be only a few Na⁺ channels, which are in a state where activation gates close and inactivation gates open. These channels can be activated by a stronger intensity of stimulus and the excitation produced during this phase is called relative refractory period. This is followed by a period, which corresponds to the delayed K⁺ exit known as after depolarization. The excitability in this period is more and known as super normal phase. It leads to a period called after hyperpo-larization, in which the excitability becomes less (subnormal phase). This is caused by increased K⁺ exit due to slow closure of potassium channels. The membrane potential reaches the resting level and the excitability returns to the normal state after the subnormal phase.

Strength-duration relationship

It is known that threshold strength of current, if it is quickly rising will give a response. The minimum strength of stimulus that is required to elicit a response is called rheobase. The time required to produce this response is the utili-zation time. If the intensity of stimulus is kept twice the rheobase, the time taken to excite can be determined. This is chronaxie and it is defined as the duration at twice the rheobase strength (Fig. 2.12). Chronaxie values are useful in determining the excitability pattern of tissues.

Fig. 2.11: Relationship between action potential and excitability

Greater the chronaxie, lesser the excitability of the tissue.

Electrotonic potentials (Local response) They are the graded, nonpropagated potentials and help to develop the propagated action potential. When an action potential is developed at a point in the nerve fiber, there will be a current flow to the adjacent region. This is known as local response or electrotonic depola-rization. They are graded, that is, their size depends on the intensity of stimulus and it is nonpropagatory (Fig. 2.13) However, when the local response attains the threshold level, a propagated action potential is developed. The electrotonic potential, which spreads from the point of action potential, shows two features namely, space constant and time constant.

Space constant

It refers to the minimum distance over which the size of the electrotonic potential falls to 37%

of maximum value (Fig. 2.14). That is, when a point on the nerve fiber develops an action potential, the electrotonic depolarization along the length of membrane declines exponentially as the distance is increased. In mammalian nerve fibers, the space constant is 1 to 3 mm.

Fig. 2.12: Strength duration relationship in an excitable tissue

Space constant depends on

• Membrane resistance

• Axoplasmic resistance

If membrane resistance is increased, the space constant will become more. In otherwords, the electrotonic depolarization will spread to a greater distance.

There is presence of axoplasmic resistance for the spread of electrotonic potential and it is called longitudinal resistance. Increase in the axoplasmic resistance will cause decrease in the space constant. In larger size nerve fibers, the cross sectional area is increased and this lowers the longitudinal axoplasmic resistance. Hence, the speed of conduction in larger nerve fibers is greater.

Time constant

It is the time that is taken for the electrotonic depolarization to decline to 63% of the maximum (Fig. 2.15). More the time constant, lesser the excitation. This will also decrease the velocity of conduction. In diseases affecting the myelination of neuron, as in multiple sclerosis, there is increased capacitance and decreased resistance of the membrane. This increases the time constant and conduction becomes slower.

This is the reason for the occurrence of sensory and motor deficits in such disorders.

As mentioned earlier, subthreshold stimulus fails to reach the threshold level of firing. It gives rise to a nonpropagated, but graded potential

Fig. 2.13:Electrotonic potentials (depolarizing potentials ) Action potential develops when the local depolarizing potentials at the cathodal current reaches threshold level of firing. Note the potential at the anode, which shows linear relationship with the intensity of stimulation

Fig. 2.14: Space constant during electrotonic depolarization Space constant refers to the distance from the point of stimulus to the fall of 37% of potential. Greater the space constant, greater the spread of passive current along the axon

Fig. 2.15: Time constant during electrotonic depolarization

Time constant is the time taken for the electrotonic depolari-zation to fall at 63% of the maximum potential (Vmax).

Greater the time constant, more the time taken for the passive spread of current along the axon and the velocity of propagation of impulse will also be slow

called electrotonic potentials. When the intensity of stimulus is raised, the size of the potential rises to the threshold level and an action potential can be formed. Electrical potential developed at the cathodal current and at the anodal current is not similar.

Effect of cathodal current

Application of cathodal current causes depo-larizing potential called catelectrotonic potential.

At the cathode, the current flows outward and depolarizes the membrane. It is a nonpropa-gated, graded local response. However, depen-ding on the intensity of stimulus, the potential reaches the threshold level and an action potential can be formed.

Effect of anodal current

Anodal current causes hyperpolarization of the membrane, as the current flows inward.The potential recorded at the anodal current is called anelectrotonic potentials. There is a linear rise in the membrane potential, as the intensity of anodal current is increased. This is in compari-son to cathodal stimulation, where, the depola-rizing potential on reaching the threshold level, gives rise to an action potential.