Question

In your own words

Describe the events that take place during an action potential. Begin at the point where an EPSP reaches threshold in the trigger zone. Include all ion channels (8), ion movements (8) and potential changes (8) that take place during the various phases of the action potential.

Answer 1

Ans :

Action potential :

An action potential is a rapid change in membrane potential that is governed by the opening and closing of ion channels in the plasma membrane of the neuron. In this tutorial, we will review the phases of an action potential measured from a small area of a neuron's membrane. The action potential can be divided into five phases: the resting potential, threshold, the rising phase, the falling phase, and the recovery phase.

2 We begin with the resting potential, which is the membrane potential of a neuron at rest. At this point a small subset of potassium channels are open, permitting K⁺ ions to enter and exit the cell based on electrochemical forces. Note that there is no NET movement of K⁺ ions; for each K⁺ ion that leaves the cell, another returns, maintaining the membrane potential at a constant value.

3 As a depolarizing stimulus arrives at our segment of the membrane, a few Na⁺ channels open, permitting Na⁺ ions to enter the neuron. The increase in positive ions inside the cell depolarizes the membrane potential (making it less negative), and brings it closer to the threshold at which an action potential is generated.

4 If the depolarization reaches the threshold potential, additional voltage-gated sodium channels open. As positive Na⁺ ions rush into the cell, the voltage across the membrane rapidly reverses and reaches its most positive value.

5 At the peak of the action potential, two processes occur simultaneously. First, many of the voltage-gated sodium channels begin to close. Second, many more potassium channels open, allowing positive charges to leave the cell. This causes the membrane potential to begin to shift back towards the resting membrane potential.

6 As the membrane potential approaches the resting potential, voltage-gated potassium channels are maximally activated and open.

7 The membrane actually repolarizes beyond the resting membrane voltage. This undershoot occurs because more potassium channels are open at this point than during the membrane's resting state, allowing more positively charged K⁺ ions to leave the cell.

8 The return to steady state continues as the additional potassium channels that opened during the action potential now close. The membrane potential is now determined by the subset of potassium channels that are normally open during the membrane's resting state.

Events in EPSP :  An excitatory postsynaptic potential (EPSP) is the change in membrane voltage of a postsynaptic cell following the influx of positively charged ions into a cell (typically Na+) as a result of the activation of ligand-sensitive channels

inhibitory neurotransmitter binds to postsynaptic chemically gated chloride channels.

Chloride moves from outside the cell to the inside.

The inside of the postsynaptic cells becomes more negative.

The local chloride current becomes weaker as it moves towards the axon hillock.

An excitatory postsynaptic potentials (EPSP) is a temporary depolarization of postsynaptic membrane caused by the flow of positively charged ions into the postsynaptic cell as a result of opening of ligand-sensitive channels. An EPSP is received when an excitatory presynaptic cell, connected to the dendrite, fires an action potential. The EPSP signal is propagated down the dendrite and is summed with other inputs at the axon hilllock. The EPSP increases the neurons membrane potential. When the membrane potential reaches threshold the cell will produce an action potential and send the information down the axon to communicate with postsynaptic cells. The strength of the EPSP depends on the distance from the soma. The signal degrades across the dendrite such that the more proximal connections have more of an influence