Topic 10 - Nervous System 1

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The action potential

  1. A stimulus from a sensory cell or another neuron causes the target cell to depolarize toward the threshold potential. 
    • Stimulus results in the release of neurotransmitter molecules which bind to receptors located on a neuron’s dendrites, causing voltage-gated ion channels to open. At excitatory synapses, positive ions flood the interior of the neuron and depolarize the membrane, decreasing the difference in voltage between the inside and outside of the neuron. 
  2. If the threshold of excitation is reached, all Na+ channels open and the membrane depolarizes. 
  3. At the peak action potential, K+ channels open and K+ begins to leave the cell. At the same time, Na+ channels close. 
  4. The membrane becomes hyperpolarized as K+ ions continue to leave the cell. The hyperpolarized membrane is in a refractory period and cannot fire. 
  5. The K+ channels close and the Na+/K+ transporter restores the resting potential.

Q: Given that the cell membrane is hyperpolarised (i.e too negative) how would the Na+/K+ transporter help the cell membrane return to its resting potential?

A: At step 4, K+  channels begin to close but yes, the ongoing movement of K+ out of the cell results in the intracellular environment getting more negative.

So if i understand your question properly, you are asking how does it go from HYPERpolarised to simply polarised - this would imply some ions are moving into the cell https://www.cs.mcgill.ca/~rwest/wikispeedia/wpcd/wp/a/Action_potential.htm

This website notes 'Sodium and potassium ions diffuse through open ion channels under the influence of their electrochemical gradients.'

This would imply that when the cell is HYPERPOLARISED, Sodium and Potassium leak channels are still open - As the cell has taken in a load of Na+ and lost of K+, the electrochemical gradients will be directed such that there is a net +ve movement of charge INTO the cell - in fact, if the cell was dead (i.e. no NKA activity), you would expect that the leak channels would naturally result in the membrane potential to return to 0mV.

The NKA in step 5 therefore purely acts to maintain the -70mV depolarised state in light of the Na+ and K+ leak channels in the membrane.

I.e. the reason the voltage gradient exists is because there are Na+ and K+ leak channels trying to bring the membrane potential to 0mV, but the NKA is acting against this natural flow to maintain a -70mV voltage gradient.

Hyper and hypokalemia

Abnormal electrolyte concentrations in the blood can be caused by a wide variety of problems. For example, insulin shifts potassium from the extracellular to the intracellular fluid. Insulin overdoses can cause hypokalaemia (low blood potassium concentration).

Compare and contrast the effects of hypokalaemia and hyperkalaemia on nerve function and explain why they occur (100 words or fewer) (2 marks).

When approaching this question, think in two stages 1. What is the cell like IMMEDIATELY 2. How does the cell reach equilibrium?

e.g. Hypokalemia IMMEDIATE: - Normal cell milieu: Intracellularly high K+, low Na+, Cl-; Extracellularly high Na+, Cl-, low K+ - All of a sudden, the environment around the cell has EVEN LESS K+ (i.e. the electrochemical gradient directing K+ out of the cell is INCREASED)

EQUILIBRIUM: - K+ will leak out of the cell via K+-leak channels because of the increased electrochemical gradient - There will be less K+ intracellularly (and therefore the inside of the cell becomes more negative) - the cell is therefore HYPERPOLARISED (i.e. lower resting membrane potential) - the cell therefore requires GREATER stimulus to fire

The trick in this question is the not even bother thinking about changes in K+ in the ICF at the initial stage - the number of cells in the body taking in all the K+ is so large that the effect on any individual cell is minimal. Plus the question simply asks the effect of hypokalemia/hyperkalemia so just focus on the immediate question in front of you.