IPSPs.
Any actual neuron receives input from dozens, hundreds, or thou-
sands of other neurons. Any given neuron will have many synapses
that are excitatory and many that are inhibitory. The excitatory and
inhibitory synapses will be located all over the cell, primarily among
the dendrites, and also on the body of the cell. Such a neuron then
adds up all the EPSPs and all the IPSPs generated as a result of sig-
nals received from other nerve cells. When the sum of all the EPSPs
and IPSPs is such that the membrane voltage at the location of the
axon hillock reaches the threshold voltage for opening voltage-gated
sodium channels (approximately —-50 mV), then an action potential
will be triggered (Fig. 6.4). It is at the axon hillock that voltage-gated
Na* channels occur in high density, and so it is at the hillock that the
action potential begins its propagation along the axon.For synapses located on dendrites, the voltage changes from any
EPSPs and IPSPs may have an appreciable distance to move through
the interior of the cell by diffusion of electric charge. The greater
the distance from the axon hillock, the more the voltage changes
will dampen out and decrease in magnitude as they move through
the cell. Thus, synapses that are closer to the axon hillock will have
greater impact on influencing whether or not the cell receiving the
input is triggered to generate action potentials. However, signals
in dendrites can propagate, too: voltage-gated Na* and K* chan-
nels and action potentials, discussed thus far only in the context
of signal propagation along axons, can also occur along parts of
dendrites. This maintains the effectiveness of signals received in
dendrites distant from the axon hillock, by boosting propagation
along dendrites to offset their dampening out with distance. Amechanism this good—that is, the action potential—is bound to be