Download Synaptic Potentials

Synaptic Potentials
Postsynaptic potentials develop in the postsynaptic cell’s membrane
when neurotransmitter binding to receptors leads to the opening of ion
channels. An excitatory postsynaptic potential (EPSP) occurs if
the ion movement depolarizes the membrane. If, on the other hand, the
membrane becomes hyperpolarized when the ions move, an inhibitory
postsynaptic potential (IPSP) is generated. EPSPs and IPSPs are
local potentials.
EPSP: Opening of sodium- or calcium channels leads to depolarization
of the membrane. If there is sufficient depolarization, the threshold
potential is reached and an action potential will be produced in the
postsynaptic membrane. Since an EPSP depolarizes the membrane, it
facilitates action potentials.
IPSP: Opening of potassium- or chloride channels leads to
hyperpolarization of the membrane. (Since the current is outward for
potassium ions, and inward for chloride ions, opening of either of these
two channels will cause the postsynaptic membrane to hyperpolarize.) A
hyperpolarized membrane has moved farther from the threshold
potential and has less probability of producing an action potential. Since
an IPSP hyperpolarizes the membrane, it inhibits action potentials.
Remember that a neuron synapses with many other neurons. So a
postsynaptic neuron can receive signals from many presynaptic neurons
simultaneously. Whether or not the postsynaptic cell has an action
potential depends on the summation (the additive effect) of all the
incoming signals. Each active synapse can result in a local potential
(either an EPSP or an IPSP). The net effect of all the local potentials on
the trigger zone determines whether or not there is an action potential
in the postsynaptic cell.
There are two different ways that local potentials can sum to excite the
postsynaptic cell to have an action potential. Temporal
summation occurs when successive EPSPs at a single synapse occur in
rapid succession. The successive potentials occur before the previous
ones die out producing an increasing membrane depolarization.
Summation can also occur when multiple presynaptic neurons stimulate
the postsynaptic neuron at the same time (spatial summation). Each
individual synapse lets in a limited number of ions and alters the
membrane potential a little. The collective effect of all the synapses
allows in enough ions to reach the threshold potential and an action
potential is triggered.
The interplay between IPSPs and EPSPs is important. Whether an
action potential is going to be produced depends not just on the
summation of the EPSPs, but on the summation of EPSPs and IPSPs.
The algebraic sum of all EPSPs and IPSPs has to be of sufficient
amplitude to raise the membrane potential to the threshold for an action
potential. What this means is that if the IPSPs prevail, then the postsynaptic cell will be “silent.” One can, therefore, visualize the process of
summation as a “tug-of-war” between excitatory and inhibitory currents
induced by the binding of neurotransmitters on excitatory or inhibitory
postsynaptic receptors, respectively.