PHYSICS · Multi-part · Fermi University Physics · Biophysics & Medical Physics · Bioelectricity, the senses and the physics of the body · Bioelectricity, the senses and the physics of the body
Forces, Machines and Signals, Ch The nerve impulse
Problem
Two pieces of raw data are given below. Translate each into the physical statement it supports.
((a)) A squid giant axon is voltage-clamped to a fixed depolarised level. The membrane current (outward positive) is recorded first with normal seawater outside ("control") and then with the external sodium replaced by the impermeant ion choline ("Na-free"), the standard ion-substitution trick: t(ms)Icontrol(μAcm−2)INa-free(μAcm−2)0.5−30001−380−102−120−30480758220215 Compute the sodium current INa(t)=Icontrol(t)−INa-free(t) at each time and describe its time course in one sentence, naming which Hodgkin-Huxley gating process is responsible for its disappearance by 8ms.
((b)) At a frog neuromuscular junction bathed in low calcium, 300 nerve stimuli produce 47 failures (no end-plate potential) and end-plate potentials with amplitudes clustering at multiples of a unit voltage. Counting the responses by number of quanta released gives 90 one-quantum, 78 two-quantum and 47 three-quantum events (the rest, 38, involve four or more quanta). Use the failure method to estimate the mean quantal content m, then predict the number of one-, two- and three-quantum events out of 300 trials expected under the Poisson model, and say whether the prediction is consistent with the observed counts.