4) Na+ -dependent action potentials were investigated in a large axon of Sepia (

Question

4) Na+ -dependent action potentials were investigated in a large axon of Sepia (cuttlefish). Sepia axons behave similarly to the squid axon studied in class. Assume the resting potential = -75 mV, EK = -80 mV and ENa = +60 mV.

i)A threshold stimulus of 100 µA/cm 2 and duration 0.1 msec was able to generate an action potential in the Sepia axon. In order to generate an action potential approximately 6 msec later, a larger threshold stimulus of 200 µA/cm 2 stimulus (0.1 msec) was required. Explain the basis of this phenomenon.

ii) In a voltage clamp experiment on Sepia axons sketch the expected profile of the total membrane current during voltage steps from -75 mV holding potential to the following clamped potentials: (i) 0 mV and (ii) +70 mvV Assume potential is clamped for about 50 msec.

Explanation / Answer

The larger threshold stimulus is known as supra threshold stimulus. After one action potential, repolarization of the membrane occurs..Repolarization means K+ moves out of the cell. But at the same time, sodium channels do not open for the next action potential to occur... At this stage, if a suprathreshold stimulus is given, then depolarization of membrane will occur...

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