Postsynaptic response

Once the neurotransmitter binds to the receptors, the chemical signs that traversed the synaptic cleft once again becomes and electrical signal. The binding causes a graded potential in the postsynaptic membrane. An incoming impulse may be either excitatory or inhibitory. An excitatory impulse causes depolarization, known as an excitatory postsynaptic potential(EPSP). An inhibitory impulse causes a hyperpolarization, known as an inhibitory postsynaptic potential(IPSP).

The discharge of a single presynaptic terminal generally changes the postsynaptic potential less than 1mV. Clearly this is not sufficient to generate an action potential, because reaching the threshold requires a change of at least 15 to 20 mV. But when a neuron transmits an impulse, several presynaptic terminals typically release their neurotransmitters so that they can diffuse to the postsynaptic receptors. Also, presynaptic terminals from numerous axons can converge on the dendrites and cell body of a single neuron. When multiple presynaptic terminals discharge at the same time, or when only a few fire in rapid succession, more neurotransmitter is released. With an excitatory neurotransmitter, the more that is bound, the greater the EPSP will be.

Triggering an action potential at the postsynaptic neuron depends on the combined effects of all incoming impulses from these various presynaptic terminals. A number of impulses are needed to cause sufficient depolarization to generate an action potential. Specifically, the sum of all changes in the membrane potential must equal or exceed the threshold. This summing of the individual impulses’ effect is called summation.

For summation, the postsynaptic cell must keep a running total of the neuron’s responses, both EPSPs and IPSPs, to all incoming impulses. This task is done at the axon hillock, which lies on the axon just past the cell body. Only when the sum of all individual graded potentials meets or exceeds the threshold can an action potential occur.

Individual neurons are grouped together into bundles. In the CNS(brain and spinal cord), these bundles are referred to as tracts, or pathways. Neuron bundles in the PNS are reffered to as nerves.