This week I started early and watched the fourth lesson of the “Synapses, neurons and brains” course from Idan Segev, at Coursera.org. It was highly related to the previous lesson, “Electrifying brains — passive electrical signals” and approached the axon side of the synaptic potential, the “Electrifying brains — active electrical spike”.
In summary, it discussed the excitable axon, the Hodgkin & Huxley experiments, the space clamp and voltage clamp, the membrane conductances and currents underlying the spike, the H&H model for spike initiation, and the spike propagation in axons.
Regarding Hodgkin & Huxley experiments, I was surprised to find their discoveries were found so early (1939) and base on the nervous system of a squid. Nonetheless, they demonstrated tremendous mathematical skills by writing the equations that allowed to measure the potential required to initiate a spike in the axon.
Their experiment based on the techniques of Space Clamp and Voltage Clamp “made the whole difference” in finding the sub-threshold and supra-threshold for the burst of a spike. These techniques (Space Clamp) granted that the axon kept an isopotential state via the insertion of an axial conductive wire, while (through Voltage Clamp) enabling the experimenter to dictate a specific voltage difference between the inside and the outside of the membrane, counterbalancing the membrane current.
In summary, it made possible to understand and measure what voltage deploys a spike response, and what are the equations that best describe the currents underlying the spike and how spikes propagate through the axon.
Furthermore, the experiments with pharmacological agents, allowed to understand how early and later stages of the spike work (by blocking them) and which particles are responsible for the inward and outward currents. It also clarified how the ion channels allow these function to let particles in and out, and the effects this has in the spike (the refractory period).