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A-level Biology Membrane Potential Question

Hi,
I know that voltage-gated sodium ion channels open when the membrane potential reaches the threshold, but what causes the few sodium ion channels to open when a stimulus is applied? Are these sodium ion channels voltage gated? Thanks
Original post by Har6547
Hi,
I know that voltage-gated sodium ion channels open when the membrane potential reaches the threshold, but what causes the few sodium ion channels to open when a stimulus is applied? Are these sodium ion channels voltage gated? Thanks

Can you be clearer - this question is muddled. Are you talking about ligand-gated sodium channels as the 'other ones' - in which case the stimulus should be obvious from the name.
Reply 2
Original post by Reality Check
Can you be clearer - this question is muddled. Are you talking about ligand-gated sodium channels as the 'other ones' - in which case the stimulus should be obvious from the name.

I have included a screen-shot from my textbook. At first it says that 'some' sodium ion channels open after the stimulus. Then I assume it is voltage gated sodium channels opening when it says 'lots' as the membrane potential has changed. So what causes the few to open first? Is it just different stimuli? I know some are stretch mediated? Thanks.
(edited 1 year ago)
Original post by Har6547
I have included a screen-shot from my textbook. At first it says that 'some' sodium ion channels open after the stimulus. Then I assume it is voltage gated sodium channels opening when it says 'lots' as the membrane potential has changed. So what causes the few to open first? Is it just different stimuli? I know some are stretch mediated? Thanks.

Both of these are the same voltage-gated channels. Action potentials originate from the axon hillock, and propagation of the action potential along the axon causes local depolarisation as the channels begin to open and sodium rushes in. More channels open, the membrane depolarises further and the AP is propagated. Does that make sense?

Stretch mediated channels are different and have mechanical sensitivity. If you're doing AQA, you learn about these and specialised cells in the retina.
(edited 1 year ago)
Original post by Har6547
Hi,
I know that voltage-gated sodium ion channels open when the membrane potential reaches the threshold, but what causes the few sodium ion channels to open when a stimulus is applied? Are these sodium ion channels voltage gated? Thanks


It depends on the stimulus, however, most stimuli will cause ion channels to open. As you have already said, some are stretch mediated, when the cell membrane is stretched, this causes stretch-activated ion channels to open, causing the membrane to depolarise, causing voltage-gated Na channels to open. In cases of ligand-gated ion channels, the binding of a particular ligand to its receptor will cause ion channels to open, causing cations to enter the cell and depolarise the membrane. An example of this would be the nAChR (nicotinic acetylcholine receptor) on skeletal muscle, which is a ligand-gated ion channel. Acetylcholine diffuses across the synapse and binds to the nAChR causing the ion channel to open, allowing Na and Ca ions to move into the cell, causing depolarisation, and so on.
Original post by Har6547
Hi,
I know that voltage-gated sodium ion channels open when the membrane potential reaches the threshold, but what causes the few sodium ion channels to open when a stimulus is applied? Are these sodium ion channels voltage gated? Thanks


The opening of sodium ion channels in response to a stimulus is caused by the change in the local membrane potential, which can be generated by a variety of mechanisms such as a mechanical, chemical, or electrical stimulus. These sodium ion channels are indeed voltage-gated, meaning that they are activated by changes in membrane potential. When the membrane potential reaches the threshold for sodium ion channel activation, the channels open and allow sodium ions to enter the cell, leading to depolarization and the generation of an action potential.

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