3.7 Action potential 3.7 Action potential 3.7.2 The modern picture

3.7.1 The classic picture

Quotation: ”there is little hope of calculating the time course of the sodium and potassium conductances from first principles”
(surely, if one uses empirical functions) [9] In the classic picture, we assume that the membrane is equipotential. When the axonal input current charges it up to some threshold value, an intense charge-up process starts due to ion inflow. After the membrane’s voltage exceeds some other threshold, a spike begins. After some time, and for some reason, az outward “delayed rectifying current” starts from some hidden source and hyperpolarizes the membrane. Somewhat later, both currents stop, in a concerted way, for some reason. In the classic approach, a spike is sent and received instantly (an incoming spike “makes a hole” [81]), the charge it delivers is added to the membrane in a snap, and the neuron fires in a snap somewhat later. The process details are known (although some processes are only hypothesized, and others are misunderstood). However, the control mechanism of the process is unknown or mystic: the classic model answers the question ”what” but leaves the questions of type ”whyänd ”how” open. “Why action potentials are initiated in the axon is still unclear” [50].

Refer to caption — Figure 3.7: Errata in blue. (Fig. 7-1 in [41]). The sequential opening of voltage-gated $Na^{+}$ and $K^{+}$ channels generates the action potential.Wrong. The $Na^{+}$ channels are voltage controlled, and they open when AP is initiated. There are no $K^{+}$ channels in the game. Some resting $K^{+}$ channels exist and they contribute to the total current. Some $K^{+}$ flows out through the AIS, but it is by two order of magnitudes less then shown. Actually, these two currents are the condeser currents, before and after potential reversal on the condenser. The ’ $Na^{+}$ current’ is the current generated by the sharp ’switch-on’ potential gradient, with a distortion caused by approaxing the current’s time course by a polynomial. One of Hodgkin and Huxley’s great achievements was to dissect the change in conductance during an action potential into separate components attributable to the opening of $Na^{+}$ and $K^{+}$ channels. Mostly wrong. There is no change in conductance, it was a wong concept. They were tight in assuming the existence of ion channels and their opeing/closing, but it is not the case for $Na^{+}$ . The shape of the action potential and the underlying conduct- ance changes can be calculated from the properties of the voltage-gated $Na^{+}$ and $K^{+}$ . Wrong. They used empirical measured distributions, instead of propertios of ion channels.(Adapted, with permission, from [9].)