Q1 (10 pts) When a neuron is voltage-clamped and the voltage is stepped from -70
ID: 202751 • Letter: Q
Question
Q1 (10 pts) When a neuron is voltage-clamped and the voltage is stepped from -70 mV to-40 mV, approximately 10% of the sodium channels open. When the same neuron is not voltage clamped and current is injected into the cell (current clamp) to raise the voltage from -70 mV to -40 mV, much more sodium channels open (but not all). Explain why this is so, making reference to the graph below. What is the physiological importance of most channels opening following current injection? more sodinum chanmel vhat is the physiological 70- 80 % ga- channtExplanation / Answer
Most of the neurons have a threshold potential of -50mV. For an action potential to be triggered by the nerve, there should be a certain level of depolarization happening before the nerve transmission takes place.
So, there should be a 10mV to 15mV of depolarization required to make the action potential to get transmitted all along the nerve.
In the resting phase of the nerve, the exterior of it has more Na+ ions while the inner cytoplasm has more number of K+ ions. In this state, K+ ion channels are open and Na+ ion channels will be closed.
In the depolarized state of the nerve when a stimulus hits the nerve, the Na+ ion channels open and allow the transfer of Na+ ions from exterior to interior. The K+ channels also open to allow the transfer of K+ ions from inside to outside.
This will allow the nerve membrane to increase the membrane potential to a certain extent that the action potential which is created by the stimulus will get transmitted along the nerve.
In the current question, when the membrane is attached with a voltage clamp, the depolarization that is not sufficient to transmit the impulse occurs and hence it can just take the voltage from -70mV to -40mV.
When the current is injected to increase the voltage from -70mV to -40mV, it allows the voltage to exceed the depolarizing threshold and pass on the voltage to other parts of the nerve. At this stage, more Na+ channels open so that Na+ ions move-in sending out K+ ions. This opening of Na+ channels happens till the membrane potential reached Nernst equilibrium for Na+. And then the opening of Na+ channels do not occur as the nerve will reach the stage of repolarization.
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