Answer
When a neurotransmitter binds onto post-synaptic receptors, ions pass through the post-synaptic membranes into the cytosol, and graded potentials are generated. The receptors in the post-synaptic membrane are combinations of receptor proteins and chemically gated channel proteins. If the ion in the receptor-channel protein combination allows potassium ions and sodium ions through, at the same time, then the neurotransmitter binding will generate an excitatory post-synaptic potential (EPSP). On the other hand, if the receptor-bound ion channel will permit only chloride ions(Cl-) to pass in, or only potassium ions to pass out of the cell, then the potential generated at the post-synaptic memmbrane will be an inhibitory one, an IPSP.
Work Step by Step
The kind of potential generated when a neurotransmitter binds receptors on a post-synaptic membrane is determined by the kind of chemically-gated ion channel that is combined with he post-synaptic membrane receptor proteins. In some cases, both sodium ion channels and potassium channels are combined with the post-synaptic receptor proteins. In such a case, when the neurotransmitter binds on the receptors, both types of channels open and the cations( Na+ and K+) move down their respective electrochemical gradients. Sodium ions move into the cell, and potassium ions move out. The net result is that the inside of the cell membrane is made less. negative because more sodium ions enter than the number of potassium ions than pass out of the cell . This change in the polarity of the cell generates a local graded excitatory potential--an EPSP.
The ion channel proteins combined with the post-synaptic receptors may be chloride channels or exclusive potassium channels; then transmitter binding will open these chemically gated ion channels and allow potassium cations into the cell or permit chloride anions to move out of the cell. In either case, this will increase the polarity of the of the cell ( hyperpolarization) and make it more difficult for the membrane to fire.
