CHAPTER 3 PROPERTIES OF EXCITABLE MEMBRANES: THE SPIKE n the experiment shown in Figure 3-8, only very small amounts of current were Ipassed through the membrane, and these caused only small changes in membrane potential. If greater currents are used some new phenomena show up in the recordings. When current is passed through the membrane from outside to inside (the micropipette is the cathode), the voltages shown in Figure 3-15 can be recorded from the immediately adjacent membrane. The membrane responds as a simple passive resistance-capacitance circuit, i.e., the responses are predictable from Ohm’s law. Figure 3-15. Changes in membrane potential caused by inward The membrane potential changes more current flow. Five equal 4-msec steps of inward transmembrane slowly than the applied current as the current, with current magnitude increasing from a to e, resulted in membrane capacitor charges and discharges. five proportional hyperpolarizing changes in membrane potential. Even with the greatest currents the Responses reflect simple electrotonic potentials. membrane still behaves as a simple passive circuit. We speak of the resting membrane depolarization1. Inward currents, therefore, as being polarized (negative inside with hyperpolarize the resting membrane. respect to outside). The terminology applied When current is passed in the other in most textbooks to changes in the direction across the membrane, from inside membrane potential is often confusing and to outside, the membrane at first behaves as inaccurate. For example, many times the a simple resistance-capacitance circuit, membrane potential will be described as approximately symmetrical with its behavior "increasing or decreasing." But is a change that makes the membrane potential more negative inside with respect to outside an increase or a decrease? We will use the term hypopolarization to refer to a change in the membrane potential that makes the membrane less negative inside; a change that makes it more negative than V is called r 1 The term “depolarization” is used in an hyperpolarization. A change in the most literature and textbooks for any membrane potential to 0 mV is a hypopolarization. Though this is strictly incorrect, it is customary and widely accepted. Be warned! 3-1 for inward current. Responses in Figure 3- 16 were obtained for the same strength outward currents as were used for the correspondingly lettered responses to inward currents in Figure 3-15. Responses in Figure 3-16 a and b are symmetrical with responses in Figure 3-15 a and b2, but the change in the membrane potential is hypopolarizing instead of hyperpolarizing. Outward currents hypopolarize resting membrane3. Figure 3-16 c is, however, not symmetrical to its counterpart in Figure 3- 15. The response begins like its counterpart, but the response is larger in amplitude and longer in duration. Response d is even more Figure 3-16. Changes in membrane potential due to outward current flow. Five equal 4-msec steps of outward variant; the shape is no longer that expected transmembrane current, with current magnitude increasing for a capacitor being charged. An active from a to e (and the same magnitudes as for similarly lettered process has been initiated by the change in traces in Fig. 3-15), resulted in hypopolarizing electrotonic membrane potential that occurred in c and d; potentials in a and b, electrotonic potentials leading to local it was not initiated by the smaller changes in active responses (region between solid and dashed lines) in c and d, and an electrotonic potential leading to a spike in e. membrane potential in a and b. CFL=critical firing level. When a still stronger current is passed outward through the membrane, the membrane potential begins to change toward zero (the membrane is passively (Fig. 3-16 e); then the active hypopolarized), and it even becomes process begins; and finally the membrane positive inside with respect to outside. The polarization continues to diminish rapidly membrane polarity actually becomes reversed. After the peak of positivity is reached, the membrane rapidly returns to its 2 original polarity and potential and may Note: The amplification illustrated in proceed to a potential more negative than V . Fig. 3-16 is less than that in Fig. 3-15 so the r Finally, the membrane returns to Vr, the traces appear to be smaller. In fact, they whole event lasting 2-3 msec. Actually, the would not be. event, from its start at Vr to peak positivity and back to V (omitting the period when V 3 This should make sense if you recall r m is more negative than V ), requires only 0.5- that current entering a resistor makes that r 1.0 msec, depending upon the neuron. This end of the resistor positive with respect to event is called the action potential or the other end. Thus, current passing inward simply the spike. through the membrane will make the outside more positive with respect to the inside (that's hyperpolarization), whereas current passing outward through the membrane will make the inside more positive with respect to the outside (that's hypopolarization). 3-2 Figure 3-17 shows the action frequently absent. potential on an expanded time scale. The The action potential is initiated when various parts of the spike are labeled. The the membrane is hypopolarized beyond a rapid positive change in membrane potential certain value. This value, termed the is called the upstroke, the rising phase, or critical firing level (CFL), varies from cell the hypopolarization phase. The positive to cell; it is of the order of 10-20 mV, but portion of the spike is the overshoot and the constant for a given cell under its normal return to the resting potential is called the working conditions. The critical firing level falling phase or the repolarization phase. is a highly unstable condition. If the At the end of the falling phase, the membrane is hypopolarized just to but not repolarization (re-establishment of the beyond the critical firing level, it may either resting polarity) slows down and may pass discharge a spike or it may simply return to the resting potential to a value more Vr. negative than Vr, i.e., the membrane may Actually, a minimum rate of change become hyperpolarized. This is the in membrane potential is required to initiate the spike. If the membrane potential is changed very slowly, the critical firing level can be passed without an action potential being initiated. In Figure 3-18, the responses of an axon to stimuli with five different rates of rise are shown. As the rate is decreased (going from a to e), the apparent critical firing level becomes more positive, going from 21 mV of hypopolarization from Vr in a to 28 mV in d. In e, no spike is initiated at all, in spite of the fact that the membrane is hypopolarized Figure 3-17. Phases of the action potential. by more than 30 mV. Actually, during any The time course of the spike is shown with maintained hypopolarization that does not hypopolarization (depolarization) and cause a spike to occur, the critical firing repolarization phases, overshoot, and level becomes more positive. This hyperpolarizing and hypopolarizing phenomenon is called accommodation. after-potentials labeled. Also indicated are When the hypopolarization is terminated, levels of the resting membrane potential, Vr, and the critical firing level, CFL. both the membrane potential and the critical Transmembrane voltage is indicated on the firing level return to their original values. ordinate; time is indicated on the abscissa. If, however, the minimum rate of change of membrane potential is exceeded, the spike hyperpolarizing after-potential or after- will be initiated as the membrane potential hyperpolarization. In some cells, there becomes more positive than the critical may also be another phase of firing level. Most neural events are rapid, hypopolarization following this, the and it is doubtful the firing level is ever hypopolarizing after-potential or after- crossed in natural functioning of the healthy hypopolarization (not shown in Fig. 3-17). neuron without a spike occurring. There are This phase is usually small, a few mV, and certain pathological conditions where this 3-3 occurs, e.g., in certain kinds of epileptic when the spike is initiated starting with the seizures where there are extremely large membrane at or near Vr. The amplitude of changes in membrane potentials at synaptic the spike does not depend upon the size of junctions. the stimulus; larger stimuli do not give rise to larger spikes. Longer duration stimuli do not prolong spikes. Therefore, the spike is referred to as an all-or-none (often written all-or-nothing) event. The fixed size results from the fact that the stimulus only triggers events that lead to the spike; once they are triggered their time-course is independent of the stimulus. The consequences of slowly rising stimuli producing accommodation (Fig. 3-18) are an apparent contradiction to this all-or-none property; but, as pointed out above, most naturally-occurring changes in membrane potential occur rapidly at rates higher than 4 mV/msec, at which the spike amplitude is reduced by less than 2%. The voltage clamp. What are the events that lead to the action potential? Obviously, the change in membrane potential of the spike results from a membrane current, and that current must result from an increase in membrane conductance. If membrane conductance were unchanged, there would be no disturbance of the resting membrane equilibrium. Membrane currents can be Figure 3-18. Accommodation of the nerve membrane. measured directly using a device called Upper graph shows a single response and the nature of the the voltage clamp. The circuit of the ramp stimulus. In the lower graph, five superimposed voltage clamp is shown in Figure 3-19.