Contrast the postsynaptic mechanisms of excitatory and inhibitory synapses.

EXCITATORY CHEMICAL SYNAPSES:

1. At an excitatory synapse, the postsynaptic response to the neurotransmitter is a depolarization, bringing the membrane potential closer to threshold.
2. The usual effect of the activated receptor on the post synaptic membrane at such synapses is to open non-selective channels that are permeable to Na+ and K+.
3. Both electrical and concentration gradients drive Na+ into the cell, whereas for K+, the electrical gradient opposes the concentration gradient.
4. The net movement of positive ions is into the postsynaptic cell, causing a slight depolarization. This membrane potential change is called an excitatory post synaptic potential (EPSP).
5. EPSP is a graded potential that decreases in magnitude as it spreads away from the synapse by local currents.

INHIBITORY CHEMICAL SYNAPSES:

1. At inhibitory synapses, the potential change in the postsynaptic neuron is generally a hyperpolarizing graded potential called an inhibitory postsynaptic potential (IPSP).
2. Activation of an inhibitory synapse lessens the likelihood that the postsynaptic cell will depolarize to threshold and generates an action potential.
3. At an inhibitory synapse, the activated receptors on the postsynaptic membrane open Cl- or K+; Na+ permeability is not affected. As Cl- channels open, Cl- enter the cell, producing a hyperpolarization.
4. An increase in Cl- permeability does not change the membrane potential but is able to increase chloride's influence on the membrane potential. This makes it more difficult for excitatory inputs from other synapses to change the potential when these Cl- channels are simultaneously open.
5. Increased K+ permeability, when it occurs in postsynaptic cell, also produces an IPSP. Thus, with increased K+ permeability more potassium ions leave the cell and the membrane moves closer to the K+ equilibrium potential, causing a hyperpolarization.