In this video, I want to
talk about action potential patterns. The information from
inputs to a neuron is converted to the size,
duration, and direction of graded membrane potentials
in the dendrites and the soma, so that a small excitatory
input to a dendrite, say, usually causes a small
excitatory graded potential, also called a depolarization. And a larger excitatory
input usually causes a larger
excitatory potential. And the same goes for
inhibitory inputs. A small inhibitory
input usually causes a small hyperpolarization
or inhibitory potential. And a larger inhibitory
input usually causes a larger
hyperpolarization or inhibitory potential. Neurons process that
information by summation of the graded potentials
at the trigger zone to determine if an action
potential will be fired down the axon. Action potentials,
however, are consistently the same size and duration
for any given neuron, so that the
information contained in the graded
potentials is, instead, converted into a temporal
pattern or a timing of action potentials
being fired down the axon. So here I've drawn some
lines to just represent time. And we'll look at the temporal
patterns or the timing of action potentials
that can happen to transmit different
kinds of information down the axons of
different types of neurons. Some neurons fire
no action potentials until there is sufficient
excitatory inputs. And then the size and
duration of depolarization over threshold is converted
into the frequency and duration of a series, which
is also called a train of action potentials. So let's say this is one of
these neurons that doesn't fire any action potentials at rest. But then if it gets
sufficient excitatory input to depolarize the trigger zone
over threshold right here, then we see a little train
of action potentials. And I'll just write
out one little line here that's often called a
spike to represent one action potential. And then this neuron will fire
a little train, a little series of action potentials for as
long as that depolarization is over the threshold potential. And then when the
depolarization ends or when it dips below the
threshold at the trigger zone, the train of action
potential stops, and then the neuron
is quiet again. It's not firing any
action potentials. So this is a very
common method used by lots of neurons in
the nervous system. For example, the
motor neurons that synapse on skeletal muscle,
they tend to fire very few or no action potentials
until they're excited enough. And then they'll fire a
train of action potentials, and then they're quiet again. Other neurons, however,
actually fire action potentials at a regular rate
in the absence of any input. And the reason they do this
is that they have differences in their leak channels and/or
their voltage-gated channels that actually
spontaneously depolarize the membrane to threshold
at a regular interval, which is very similar to how the
pacemaker cells in the heart function. And with these types of
neurons, excitatory input will cause them to fire action
potentials more frequently during the period of time
that they're excited. And then when that
excitation goes away, they go back to their
regular rate of firing. And inhibitory input will
have the opposite effect. That will slow down their
firing during the period of inhibition. And then when that
goes away, they go back to their regular
rate of firing again. And there are even more
complicated neurons that, in the absence of input,
fire little bursts of action potentials, followed
by a little space. And then they have another
regular little burst of action potentials. With these types of
neurons, excitatory input can cause the little bursts
to happen more frequently. It can cause changes
within the burst, and it can cause changes to
the spacing between the bursts. But then when the
input goes away, they go back to
their regular bursts. And the opposite happens
with inhibitory input. That can slow down the
frequency of these bursts. The advantage of these
sorts of systems, where the neurons fire at
regular rates spontaneously or in bursts, is that
information passed along to the target cells can be
fine-tuned in either direction, because with a neuron like
this that's quiet at rest, the information can only
go in one direction. It can only go from no
action potentials being fired to trains of
action potentials of different frequencies
and durations. But if there's more
inhibitory input to these types of
neurons, that information can't be passed along. But with these types
of neurons, information from both excitatory
and inhibitory inputs can be passed along in a
more fine-grained fashion. The different temporal
patterns of action potentials are then converted to the
amounts and temporal patterns of neurotransmitter
release at the synapse. And target cells can be set
up a lot of different ways to respond to these
temporal patterns and amounts of
neurotransmitter release.