Bio 306 Action Potential Questions

What type of cell normally regulates levels of extracellular potassium in the CNS?

Astrocytes usually "absorb" excess potassium from the extracellular space by way of potassium channels in their membranes. This is a part of the "regulate extracellular fluid environment" function that is listed for astrocytes.

Dendrotoxin blocks voltage-gated potassium channels from functioning. What effect will the dendrotoxin have on the signaling capability of a neuron?

Because dendrotoxin voltage-gated potassium channels from opening, it prevents repolarization of the membrane during an action potential. The action potential is prolonged, allowing the neuron to release more neurotransmitter and produce a burst of signals.

How will preventing the closing of the voltage-gated sodium channels affect the signaling capability of a neuron?

Closing the voltage-gated sodium channels prevents additional sodium from entering the cell and allows repolarization to begin when potassium channels open. If the closing of the V.G. sodium channels is prevented, the sodium channels will remain open and allow sodium to continue to enter the cell. Initially, this will create a burst of action potentials and release of neurotransmitter. However, since the membrane potential is effectively destroyed and the cell cannot repolarize, neuronal signaling is eventually prevented.

How will the signaling of a neuron be affected if the voltage-gated sodium channels open at a more negative membrane potential?

If the sodium channels open at a more negative membrane potential, the membrane reaches threshold more easily; thus, the neuron is more likely to undergo an action potential. There is a toxin, called Batrachotoxin, that can lower threshold by 30-50mV, which means that many neurons will be firing at resting membrane potential.

How will non-functional voltage-gated sodium channels affect the signaling capabilities of a neuron?

If the voltage-gated sodium channels do not open, the neuron can undergo small depolarizations but not conduct action potentials. Even if the neuron reaches threshold, the voltage-gated sodium channels won't open so no action potential can occur. Therefore, the neuron will be unable to send signals.

How will increasing extracellular potassium affect the signaling capability of a neuron?

Increased extracellular potassium will depolarize the neuron and make it more likely to undergo an action potential. This occurs because the concentration gradient of potassium across the cell membrane is reduced. Less potassium flows out of the cell through the "leak" current channels and the intracellular concentration rises. This makes the inside of the cell more positive, which brings the membrane potential closer to threshold.

What effect will the destruction of myelin have on the signaling capability of a neuron?

Myelin functions as an insulator and allows the sodium ions to spread a greater distance once they enter the axon via the voltage-gated sodium channel during depolarization. The voltage-gated channels are only present at the nodes of Ranvier. If myelin is not present, the sodium ions will not be able to diffuse as far and the sodium ions would not reach the next voltage-gated sodium channel so the next region of the axon would not be able to conduct an action potential. Thus, the neuron would not be able to propagate action potentials and the neuron would not be able to communicate with a postsynaptic cell at the synapse.

What effect will opening more potassium leak channels have on the excitability of a neuron?

Opening more of the potassium leak channels allows more potassium to flow out of the cell down its concentration gradient, which makes the inside of the cell more negative. This hyperpolarizes the cell and it is less likely to fire.

How will the excitability of a neuron be affected by voltage-gated sodium channels that open at more positive membrane potentials?

V.G. sodium channels open at -60 (threshold). Increasing the threshold (to -50 for example) will require greater excitatory input to the cell before it will undergo an action potential, making it less likely that the cell will fire.