Okay so we talked about the action potential, now let's talk about
the communication between two different neurons. I've drawn on the board here
two neurons. Up top you can see
the presynaptic neuron and below the postsynaptic neuron.
Now there are some really really interesting things that occur
in order for the communication between these two cells to happen
so let's take a look at those. So
as an action potential travels down to the length of a neuron,
it reaches this region right here.
That region is referred to as
the terminal button now within that terminal button
you have different little vesicles as you can see here
and within those vesicles are housed neurotransmitters.
They're in the red. So what are neurotransmitters?
Well they can be a multitude of different things.
They could be glutamate. They could be acetylcholine.
They could be epinephrine. In this case
we're going to be referring to them as acetylcholine. This is the most common
neurotransmitter that's found in both the sympathetic nervous system
and the parasympathetic nervous system. So the idea
for two cells communicate is to allow the neurotransmitters
from the post presynaptic cell to the postsynaptic cell.
So how do we do that? Well as I said
you have an action potential that comes down the length of the neuron
down the axon to the terminal button. What
that causes is a small influx
of extracellular calcium and I'm going to draw this in green.
So calcium
enters the cell and what that does
is it causes the movement of these vesicles
to the end of the synaptic button.
So at that point these vesicles move by a process called
exocytosis and what this does is it causes
the release of a neurotransmitter
from the presynaptic cell
into what's called the synapse. Now
these neurotransmitters, in this case acetylcholine
will cross the synapse and they will bind
to receptors on the postsynaptic cell.
Now these receptors that are located on the postsynaptic cell
are actually associated with sodium channels.
So what happens when the acetylcholine binds to these receptors
it causes a small change in conformational shape
of the sodium channels. This allows
a small amount of sodium to come into the cell. What this does
is it will bring the threshold of that cell
up to a certain level that will allow for
a ton of sodium channels to become opened and when
a lot of sodium channels are opened you have a lot of sodium influx
and if you can remember back to how an action potential works
this will cause action potentials to become
fired or activated and they will run down
the course of the postsynaptic cell.
And that's how two cells communicate
and that's how information is transmitted
in the body. So
to recap in the presynaptic cell, action potentials travel down the length
of the neuron's axon. When the action potential reaches the axon terminal
this causes a change in membrane potential which results in the entry
of extracellular calcium into the terminal button the arrival calcium into the cell
causes the neurotransmitter filled vesicles to move to
the ends at the terminal button. This process is termed exocytosis.
The fusion of vesicles with the membrane of the neuron
causes the release of neurotransmitters or acetylcholine in this case
into the synapse.
Acetylcholine then binds to receptors on the postsynaptic cell.
Remember that these receptors are associated with sodium channels
which undergo a conformational change allowing sodium ions to enter the
postsynaptic cell.
This in turn can cause the membrane potential the postsynaptic cell
to reach its threshold potential and start firing action potentials.
This in turn allows for the transmission of a signal down the postsynaptic cell.
The communication between two neuronal cells is extremely important
it allows signals to be passed throughout our bodies whether it is in
order to see
or hear, to think or even for reflex to occur.