Video Transcript
In this video, we’ll learn about
the structure of a synapse, explain how information is transmitted across the
synapse, and discuss the importance of acetylcholine in synaptic function. We’ll also learn why transmission
across the synapse is unidirectional. Then we’ll work through a practice
problem and finish up by reviewing some key points.
Neurons are specialized cells which
transmit nerve impulses around the body. To do this, they form connections
with other neurons, creating an internal communication network. These neuron-to-neuron connections
are unique because they meet at junctions called synapses, which include a small
space or gap. A synapse is the junction between
two neurons or a neuron and an effector cell. Synapses are used to pass chemical
or electrical messages from one neuron to the next. Neurons can form synapses with
other neurons, muscle fibers, or gland cells. Here we see a synapse formed
between two neurons. Our capacity to think, learn,
memorize, and move depends crucially on the strength and number of our synapses.
Let’s take a closer look at the
basic components of a synapse. There are three basic components to
a synapse. First is the neuron that precedes
the junction, which is called the presynaptic neuron. The prefix pre- in presynaptic
neuron helps to indicate that this neuron is before the synaptic junction. The presynaptic neuron ends in axon
terminals, which are also called the synaptic knob. At the synaptic knob, the nerve
impulse is converted from an electrical signal to a chemical signal.
After the presynaptic neuron is the
synaptic cleft, which is the space between two neurons. This gap is what the chemical
messengers will cross to transmit the signal to the next neuron. The neuron following the synaptic
cleft is called the postsynaptic neuron. Chemical messengers bind to the
receptors located on the dendrites of the postsynaptic neuron. This triggers a new nerve impulse
in the form of electrical energy in the postsynaptic neuron.
Now that we know where and what the
synapse is, we can discuss how the synapse functions. This is the nerve–muscle
synapse. To understand the basic function of
a synapse, the nerve–muscle synapse is the perfect example. This is because it was the first
synapse to be discovered, and so it is one of the most well-studied examples of a
synapse. So, by learning about the
nerve–muscle synapse, we can learn about the basic features found in all synapses,
including those found within the brain.
In transmission of a nerve impulse,
arrival of the action potential at the synaptic knob of the presynaptic neuron
causes it to depolarize. Depolarization causes calcium ion
channels that are sensitive to changes in voltage to open, making the synaptic knob
more permeable to calcium ions. With the calcium channels now open,
calcium can now diffuse into the synaptic knob. The influx of calcium ions triggers
the movement of synaptic vesicles within the synaptic knob.
Synaptic vesicles are small
subcellular components which contain chemical messengers called neurotransmitters
that are to be released into the synaptic cleft. They are the perfect container for
neurotransmitters as they are made easily within the synaptic knob. Synaptic vesicles are formed from
the cell membrane’s phospholipid bilayer. Typically, proteins help to pinch
off a small portion of the cell surface membrane, forming the synaptic vesicle. Most nerve muscle cell synapses are
cholinergic, which also describes the neurotransmitter that will be found within the
synaptic vesicles.
Cholinergic synapses use
acetylcholine as the primary neurotransmitter between the presynaptic neuron and the
postsynaptic muscle cell. If you’ve noticed the word choline
appears both within the word cholinergic and acetylcholine, well done! This is a great way to remember
that acetylcholine is the primary neurotransmitter in cholinergic synapses. You will usually see the word
acetylcholine abbreviated as Ach.
Following the influx of calcium
into the synaptic knob, the synaptic vesicles filled with acetylcholine fuse with
the presynaptic membrane closest to the synaptic cleft. When the synaptic vesicle fuses
with the presynaptic membrane, the process of exocytosis releases the acetylcholine
neurotransmitters into the synaptic cleft. Once in the synaptic cleft,
acetylcholine can passively diffuse to the receptors located on the postsynaptic
cell. The postsynaptic cell, be it neuron
or muscle fiber, has sodium ion channels, which are embedded into the cell surface
membrane. The sodium ion channels have a
receptor site specific to the neurotransmitter, in this case acetylcholine.
The specificity of the receptor
site means only acetylcholine can bind to the sodium ion channels, causing them to
open. You may notice that these receptors
are only present on the postsynaptic cell surface membrane. This is so the nerve impulse can
travel in only one direction. This arrangement means that
synapses are unidirectional, which helps to ensure that the nerve impulses do not
travel in the wrong direction. So the acetylcholine released into
the synaptic cleft travels in one direction towards its receptor binding site
located on the postsynaptic cell’s membrane.
When acetylcholine binds to the
receptor site on the sodium ion channels, it causes the sodium ion channels to
open. This makes the postsynaptic cell
more permeable to sodium ions and allows them to freely diffuse into the
postsynaptic cell. The influx of sodium causes an
action potential to be triggered in the dendrite of the postsynaptic neuron, or in
this case the muscle fiber. When the postsynaptic cell is a
muscle fiber, the transmission of an action potential causes the muscle cell to
contract.
After the action potential has been
transmitted to the postsynaptic cell, the sodium channels must be closed, which can
only happen if acetylcholine is broken down. The enzyme acetylcholinesterase,
which is also embedded in the postsynaptic membrane, hydrolyzes acetylcholine into
choline and acetate. The breakdown of acetylcholine
allows the sodium ion channels to close and come back to their initial state. Once the broken-down products of
acetylcholine are reabsorbed by the presynaptic neuron, the synapse is ready to
transmit another action potential.
Now that we’re familiar with the
location, structure, and function of the synapse, we can try our hand at a practice
problem.
What is the role of synaptic
vesicles in the presynaptic neuron? (A) To store neurotransmitters. (B) To store enzymes. (C) To attach to receptor sites on
the postsynaptic membrane. (D) To stimulate the influx of
sodium ions.
To answer this question, let’s
start by reviewing key components of the synapse, which comprises the presynaptic
neuron. The junction of two neurons in
communication is called the synapse. It is usually composed of three
parts: the axon terminals of the presynaptic neuron; the synaptic cleft; and the
postsynaptic cell, which can be the dendrites of a postsynaptic neuron, a gland, or
a muscle fiber. Our capacity to think, learn,
memorize, and move depends crucially on the strength and number of our synapses. This is because synapses play a
major role in the transmission of nerve impulses.
To transmit the nerve impulse from
one neuron to the next, the presynaptic neuron must release specialized chemical
messengers called neurotransmitters, which are shown here in green. These neurotransmitters are able to
cross the synaptic cleft and carry the message of the nerve impulse to the next
neuron. To make sure that neurotransmitters
are ready to transmit on demand, they’re stored in special packages called synaptic
vesicles, which are concentrated at the ends of presynaptic neurons.
As the nerve impulse reaches the
end of the presynaptic neuron, it stimulates an influx of calcium ions into the
presynaptic neuron. The influx of calcium ions causes
the synaptic vesicles to fuse with the membrane of the presynaptic neuron and
release neurotransmitters into the synaptic cleft. The neurotransmitters will travel
unidirectionally to the receptors on the postsynaptic membrane, which open sodium
ion channels and generate the next nerve impulse.
With these key facts in mind, let’s
take a look at our possible answer choices. The role of synaptic vesicles in
the presynaptic neurons is to store neurotransmitters.
Let’s wrap up by taking a moment to
review some of the key points we’ve learned in this video. Neurons form synapses with other
neurons, muscle fibers, or gland cells. Synapses consist of the presynaptic
neuron, the synaptic cleft, and the postsynaptic cell. Cholinergic synapses use
acetylcholine as a neurotransmitter. Neurotransmitters binding to the
receptors on the postsynaptic cell starts the generation of a new nerve impulse. Acetylcholinesterase breaks down
acetylcholine so it can be recycled by the synaptic knob.
