The postsynaptic potential (psp) is:
"a transient change in the electric polarization of the membrane of a
nerve cell (neuron). The result of chemical transmission of a nerve
impulse at the synapse (neuronal junction), it can lead to the firing
of a new impulse.
When an impulse arrives at the synapse from an activated neuron
(presynaptic neuron), a chemical substance called a neurotransmitter
is released and causes the opening of channel-shaped molecules in the
membrane of the resting neuron (postsynaptic neuron). Ions flowing
through the channels create a shift in the resting membrane
polarization, which usually has a slightly more negative charge inside
the neuron than outside. Hyperpolarization--that is, an increase in
negative charge on the inside--constitutes an inhibitory PSP, because
it draws the neuron away from the firing of an impulse.
Depolarization--a decrease in negative charge--constitutes an
excitatory PSP because, if it brings the neuron to the critical
threshold potential, it can excite the generation of a nerve impulse
The action potential is:
"the brief (about one-thousandth of a second) reversal of electric
polarization of the membrane of a nerve cell (neuron) or muscle cell.
In the neuron it constitutes the nerve impulse, and in the muscle cell
it produces the contraction required for all movement. Sometimes
called a propagated potential because a wave of excitation is actively
transmitted along the nerve or muscle fibre, an action potential is
conducted at speeds that range from 1 to 100 m (3 to 300 feet) per
second, depending on the properties of the fibre and its environment.
Before stimulation, a neuron or muscle cell has a slightly negative
electric polarization; that is, its interior has a negative charge
compared with the extracellular fluid. This polarized state is created
by a high concentration of positively charged sodium ions outside the
cell and a high concentration of negatively charged chloride (as well
as a lower concentration of positively charged potassium) inside. The
resulting resting potential usually measures about -75 millivolts
(mV), the minus sign indicating a negative charge inside.
In the classic generation of the action potential, stimulation of the
cell by neurotransmitter chemicals or by sensory receptor cells
partially opens channel-shaped protein molecules in the membrane.
Sodium diffuses into the cell, shifting that part of the membrane
toward a less negative polarization. If this so-called local potential
reaches a critical state (called the threshold potential and measuring
about -60 mV), then sodium channels open completely. Sodium floods
that part of the cell, which instantly depolarizes to an action
potential of about +55 mV (inside positive). Depolarization activates
sodium channels in adjacent parts of the membrane, so that the impulse
moves along the fibre.
If the entry of sodium into the fibre were not balanced by the exit of
another ion of positive charge, an action potential could not decline
from its peak value and return to the resting potential. The falling
phase of the action potential is caused by the closing of sodium
channels and the opening of potassium channels, which allows a charge
approximately equal to that brought into the cell to leave in the form
of potassium ions. Subsequently, sodium ions are pumped out of the
cell, and potassium ions pumped in, by protein transport molecules.
This restores the original ion concentrations and readies the cell for
a new action potential."
( The Encyclopedia Britannica Deluxe CD ROM Version 2001 )
Another, maybe simpler description of the difference:
"Communication of information between neurons is accomplished by
movement of chemicals across a small gap called the synapse.
Chemicals, called neurotransmitters, are released from one neuron at
the presynaptic nerve terminal. Neurotransmitters then cross the
synapse where they may be accepted by the next neuron at a specialized
site called a receptor. The action that follows activation of a
receptor site may be either depolarization (an excitatory postsynaptic
potential) or hyperpolarization (an inhibitory postsynaptic
potential). A depolarization makes it MORE likely that an action
potential will fire; a hyperpolarization makes it LESS likely that an
action potential will fire."
Neuroscience for Kids
( http://faculty.washington.edu/chudler/chnt1.html )
At the following page you will find a graph describing the mebrane potentials:
( http://www.tiem.utk.edu/~gross/bioed/webmodules/synapse.html )
I hope this answers your question. If anything should still be unclear
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