So now let's talk a little bit about the synapses.
A synapse is the junction between neurons or between a neuron and an effector.
Recall from the muscular system when we talked about the neuromuscular junction
that there is a synapse between the axon terminal of the neuron
and the motor endplate of a muscle fiber. There are two different types of synapses.
You can have an electrical synapse where the gap junctions between cells
are going to allow for transfer of information in order to synchronize the activity of a group of cells.
We see this in the heart. Or we have chemical synapses.
In a chemical synapse, this is a one-way transfer of information from a presynaptic neuron
at its axon terminal and a postsynaptic neuron usually at its dendrites.
There are several different types of neuron to neuron synapses.
We can have axoaxonic synapses which is a synapse between one axon and another axon.
We can also have axodendritic.
This one is usually more common and in this case,
the axon terminal of the presynaptic neuron is going to synapse with the dendrites of the postsynaptic neuron.
Finally, we can have axosomatic and in axosomatic synapse,
the axon terminals are going to synapse directly with the cell body of the neuron.
When we look at the axon terminal of a presynaptic neuron
and we compare it to the dendrites or the receiving end of the postsynaptic neuron,
we can see very important structures.
First, in the axon terminal you have synaptic vesicles
which are going to contain the neurotransmitter that is going to elicit
some type of response on the postsynaptic neuron.
These are usually exocytosed or the contents is usually exocytosed across a synaptic cleft.
On the postsynaptic neuron side, there's usually ligand-gated channels
in which the ligand is the neurotransmitter and is going to receive these neurotransmitters
and lead to some type of response on the postsynaptic side of the synapses.
The postsynaptic neuron can receive an excitatory or inhibitory signal.
In an excitatory postsynaptic potential,
a depolarization is going to happen on the postsynaptic side or in the postsynaptic neuron.
In an inhibitory postsynaptic potential, hyperpolarization is going to happen on the postsynaptic side.
A postsynaptic neuron may receive multiple signals at the same time.
The neurotransmitters at the chemical synapse can cause either an excitatory or an inhibitory graded potential.
There are two different types of neurotransmitter receptors that can be found in the postsynaptic neuron.
First, we have ionotropic receptors. Ionotropic receptors are ligand-gated channels
where the ligand binds directly to the channel that will cause an influx
or an exit of the cations that will lead to either depolarization or repolarization.
A metabotropic receptor is a little different.
In this case, the ligand is going to bind to the metabotropic receptor
and this is going to usually lead to a series of reactions
that will lead to the opening of a channel further down in the membrane.
Most metabotropic receptors are G proteins.
So how do we get rid of the neurotransmitter once it's in the synaptic cleft?
Remember, what goes up must come down.
This can happen by diffusion in which the neurotransmitter diffuses out of this area between the two neurons.
Enzymatic degradation where enzymes found in the synaptic cleft break down the neurotransmitter.
For example, when we talked about the neuromuscular junction, we had acetylcholinesterase
which breaks down the acetylcholine in the snyaptic cleft.
And then finally, the neurotransmitter itself can be taken up into cells
and remove from the synaptic cleft in that manner.
If several presynaptic end bulbs release their neurotransmitter at about the same time,
the combined effect may generate a nerve impulse due to something called summation.
There are two types of summation. Summation can be spatial or temporal.
In spatial summation, the postsynaptic neuron will receive a signal from multiple sources.
Imagine you are trying to compete in a contest on social media and people need to vote for you.
This would like -- this would be like if multiple people from all over the world
or if you had 500 friends voting for you from different computers.
The opposite of this is temporal summation. In temporal summation, you receive --
or the postsynaptic neuron receives multiple signals but from the same source.
This would be like if you are participating in a social media contest where people need to vote for you
and you don't have that many friends so instead of getting 500 friends to vote for you like in spatial summation,
you get one friend to vote for you 500 times over and over and over.
A combination of signals received by the postsynaptic neuron will determine the postsynaptic potential.
A postsynaptic neuron can receive either excitatory signals from excitatory neurotransmitters
which will result in an excitatory potential or an EPSP.
It can also receive an inhibitory signal from an inhibitory neurotransmitters
which will result in an inhibitory potential on the postsynaptic neuron or an IPSP.
Whichever type of signal it receives the most of will determine the postsynaptic potential.
So if it receives more EPSPs than IPSPs, then the postsynaptic potential will be excitatory.
However, if it receives more IPSPs than EPSPs, then the postsynaptic potential will be inhibitory.