Question

Describe the events of an action potential is conducted down the membrane of an axon and how it is transmitted to another cell across a synapse causing an IPSP or an EPSP.

Answer 1

When a neurotransmitter binds to the receptor on a receiving cell, it causes ion channels to open or close. This can produce a localized change in the membrane potential—voltage across the membrane—of the receiving cell.

In some cases, the change makes the target cell more likely to fire its own action potential. In this case, the shift in membrane potential is called an excitatory postsynaptic potential, or EPSP.
In other cases, the change makes the target cell less likely to fire an action potential and is called an inhibitory post-synaptic potential, or IPSP.

An EPSP is depolarizing: it makes the inside of the cell more positive, bringing the membrane potential closer to its threshold for firing an action potential. Sometimes, a single EPSP isn't large enough bring the neuron to threshold, but it can sum together with other EPSPs to trigger an action potential.

IPSPs have the opposite effect. That is, they tend to keep the membrane potential of the postsynaptic neuron below threshold for firing an action potential. IPSPs are important because they can counteract, or cancel out, the excitatory effect of EPSPs.

A single axon can have multiple branches, allowing it to make synapses on various postsynaptic cells. Similarly, a single neuron can receive thousands of synaptic inputs from many different presynaptic—sending—neurons.

Inside the axon terminal of a sending cell are many synaptic vesicles. These are membrane-bound spheres filled with neurotransmitter molecules. There is a small gap between the axon terminal of the presynaptic neuron and the membrane of the postsynaptic cell, and this gap is called the synaptic cleft.

When an action potential, or nerve impulse, arrives at the axon terminal, it activates voltage-gated calcium channels in the cell membrane. Ca^2+ which is present at a much higher concentration outside the neuron than inside, rushes into the cell. The Ca2+ allows synaptic vesicles to fuse with the axon terminal membrane, releasing neurotransmitter into the synaptic cleft.

The molecules of neurotransmitter diffuse across the synaptic cleft and bind to receptor proteins on the postsynaptic cell. Activation of postsynaptic receptors leads to the opening or closing of ion channels in the cell membrane. This may be depolarizing—make the inside of the cell more positive—or hyperpolarizing—make the inside of the cell more negative—depending on the ions involved.