Question

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 :

Events in action potential :

Hypopolarization is the initial increase of the membrane potential to the value of the threshold potential. The threshold potential opens voltage-gated sodium channels and causes a large influx of sodium ions. This phase is called the depolarization. During depolarization, the inside of the cell becomes more and more electropositive, until the potential gets closer the electrochemical equilibrium for sodium of +61 mV. This phase of extreme positivity is the overshoot phase.

After the overshoot, the sodium permeability suddenly decreases due to the closing of its channels. The overshoot value of the cell potential opens voltage-gated potassium channels, which causes a large potassium efflux, decreasing the cell’s electropositivity. This phase is the repolarization phase, whose purpose is to restore the resting membrane potential. Repolarization always leads first to hyperpolarization, a state in which the membrane potential is more negative than the default membrane potential. But soon after that, the membrane establishes again the values of membrane potential.

Epsp :

In neuroscience, an excitatory postsynaptic potential (EPSP) is a postsynaptic potential that makes the postsynaptic neuron more likely to fire an action potential. This temporary depolarization of postsynaptic membrane potential, caused by the flow of positively charged ions into the postsynaptic cell, is a result of opening ligand-gated ion channels. These are the opposite of inhibitory postsynaptic potentials (IPSPs), which usually result from the flow of negative ions into the cell or positive ions out of the cell. EPSPs can also result from a decrease in outgoing positive charges, while IPSPs are sometimes caused by an increase in positive charge outflow. The flow of ions that causes an EPSP is an excitatory postsynaptic current (EPSC).

EPSPs, like IPSPs, are graded (i.e. they have an additive effect). When multiple EPSPs occur on a single patch of postsynaptic membrane, their combined effect is the sum of the individual EPSPs. Larger EPSPs result in greater membrane depolarization and thus increase the likelihood that the postsynaptic cell reaches the threshold for firing an action potential.