In this video, I want to provide
an overview of neuron function, which I think of sort of
like how a gun functions. And we'll go into
a lot more detail on how a neuron functions
in later videos. But in this video. I just want to give a
bird's eye overview of it. The function of neurons
is to process and transmit information. Without input, most neurons
have a stable electrical charge difference across
their cell membrane, where it's more negative
inside the cell membrane and more positive outside
the cell membrane. And we call this the resting
membrane potential or just resting potential for short. And this resting
potential is really how the neuron is
going to be able to be excitable and respond to input. And I think of this
as similar to loading a gun by putting a bullet in it. Neurons receive excitatory
or inhibitory input from other cells or
from physical stimuli like odorant
molecules in the nose. Input information usually
comes in through the dendrites. Although less often, it'll
come in through the soma or the axon. The information from the
inputs is transmitted through dendrites or
the soma to the axon with membrane potential changes
called graded potentials. These graded
potentials are changes to the membrane potential away
from the resting potential, which are small in size
and brief in duration, and which travel
fairly short distances. The size and the duration
of a graded potential is proportional to the size
and the duration of the input. Summation, or an adding
together of all the excitatory and inhibitory graded
potentials at any moment in time occurs at the trigger zone,
the axon initial segment right here. This summation of
graded potentials is the way neurons process
information from their inputs. If the membrane potential
at the trigger zone crosses a value called
the threshold potential, information will then
be fired down the axon. So I like to think of
this process of summation of the excitatory and
inhibitory graded potentials at the trigger zone as analogous
to the trigger of a gun. In fact, that's why it's
called the trigger zone. I think of the graded
potentials as being like the finger on the
gun, that may be squeezing a little harder or relaxing. But once the trigger of
the gun is pulled back past a certain
threshold distance, a bullet will be fired
down the barrel of the gun, just like if the membrane
potential of the trigger zone crosses a threshold
value, information will be fired down the axon. The way information
is fired down the axon is with a
different kind of change to the membrane potential
called an action potential. An action potential is usually
large in size and brief in duration. But it's usually conducted
the entire length of the axon, no matter how long it
is, so that it can travel a very long distance,
just like a bullet usually has no trouble making it
down the barrel of the gun. And like a bullet traveling
through the barrel of a gun, action potentials tend to
travel very quickly down the length of the axon. Action potentials are different
than graded potentials because they're usually
the same size and duration for any particular
neuron, as opposed to the graded potentials,
whose size and duration depends on the size and
the duration of the inputs. Action potentials are
conducted faster along larger axons, axons
with a larger diameter, and along axons that have
a myelin sheath, that I've drawn in yellow here. When an action potential
reaches the axon terminals at the end of the
axon, information will then cross,
usually a small gap, to the target cell
of the neuron. And the way this happens
for most synapses where an axon terminal makes
contact with the target cell is by release of molecules
called neurotransmitters that bind to receptors
on the target cell and which may
change its behavior. Neurotransmitter is then
removed from the synapse. So it's reset to transmit
more information. And I think of this part as
similar to the bullet leaving the gun, to hit the target. The input information
that was converted into the size and the
duration of graded potentials is then converted into the
temporal pattern of firing of action potentials
down the axon. And this information
is then converted to the amount and the temporal
pattern of neurotransmitter release at the synapse. These steps are how neurons
transmit information, often over long distances. This is the general way that
neurons usually function. But there are multiple
functional types of neurons. So let's take a look
at some of those. Here I've drawn a few
different neurons, with their somas in red,
their axons in green, and their dendrites in blue. And I've drawn a
line here to separate between the central nervous
system on this side-- so I'll just write
CNS for short-- and the peripheral nervous
system on this side-- so I'll just write
PNS for short. And there's some
different ways we can categorize functional
types of neurons. The first way is the
direction of information flow between the CNS and the PNS. If a neuron like this
pseudounipolar neuron right here brings information
from the periphery in toward the central
nervous system, we call that an afferent neuron. Afferent, meaning it's
bringing information into the central nervous system. We can also call
this type of neuron a sensory neuron
because the information it's bringing into the
central nervous system involves information
about a stimulus. And a stimulus is
anything that can be sensed in the internal or
external environment, which is to say anything
inside the body or anything outside the body. These neurons are
carrying information away from the central nervous
system out into the periphery. So instead of calling
them afferent neurons, we call them efferent neurons. And there are two main
kinds of efferent neurons. The first we call motor neurons. Motor, which means movement. These are efferent neurons
that control skeletal muscle, the main type of
muscle that's attached to our skeleton,
that moves us around. These motor neurons are also
called somatomotor neurons or neurons of the
somatic nervous system. The other type of
the efferent neurons are called autonomic neurons. And these neurons
control smooth muscle, like the muscle around
our blood vessels; cardiac muscle, the muscle of
our heart; and gland cells, the cells of our glands
that secrete hormones into the bloodstream. These autonomic neurons are
also called visceromotor neurons or neurons of the
autonomic nervous system. Most neurons of the
central nervous system aren't any of these types
of neurons, however. They're like this
neuron, in that they connect other neurons together. So these are called
interneurons, neurons between neurons. And there are many interneurons
in the central nervous system, forming very complex pathways
for information to travel. So that while an
individual neuron is processing and
transmitting information, these complex
networks of neurons in the central
nervous system are doing even more
complex processing and transmitting of information.