Lesson Explainer: Synapses Biology

In this explainer, we will learn how to describe the structure of a synapse and explain how information is transmitted across synapses.

One neuron may make thousands of contacts with other neurons through synapses. A Purkinje cell in the brain may even receive as many as two hundred thousand synapses, all coming from different nerves and converging on one neuron! Synapses are small gaps at the junction of two neurons that pass chemical messages to relay our nerve impulses from one neuron to the next. Synapses allow our nerves to link up to many, many other nerves to make up our incredibly complex nervous system. Our capacity to think, learn, and memorize depends crucially on the strength and number of our synapses.

Key Term: Synapse

A synapse is the junction between two neurons or a neuron and an effector.

Neurons are nerve cells, and they are specialized to transmit nerve impulses around our body to form an internal communication network. This allows us to respond to both our internal and external environments and helps us to survive.

Key Term: Neuron

A neuron is a specialized cell that transmits nerve impulses.

Let’s look at the structure of a neuron before looking in more detail at the synapses between them.

Figure 1 shows two neurons linked by a synapse. Each neuron consists of a cell body (soma), which contains its nucleus and branches into sections called dendrites. Also branching from the soma is a long, threadlike axon. At the end of the axon are the axon terminals, sometimes called axonal arborizations, which connect the neuron to other neurons via synapses.

After a stimulus triggers a nerve impulse in the dendrites of one neuron, it is transmitted along the axon. The electrical impulse then reaches the axon terminals of this neuron. As it comes before a synapse, this first neuron is called a presynaptic neuron. This signal is then carried across a synaptic gap by chemical messengers called neurotransmitters, which trigger a new nerve impulse in the dendrites of the next neuron. As this second neuron is after the synapse, it is called the postsynaptic neuron.

Neurons can not only form junctions with other neurons, but also with muscle fibers or gland cells. These circuits are all around our bodies, allowing information to be relayed between the tips of our toes all the way to our brains.

Key Term: Axon

An axon is the long threadlike part of a neuron along which nerve impulses are conducted.

Key Term: Neurotransmitter

A neurotransmitter is a chemical involved in communication across a synapse between adjacent neurons or a neuron and an effector.

Let’s look at the structure of a cholinergic synapse.

The word cholinergic refers to synapses where the neurotransmitter is acetylcholine. There are many different types of neurotransmitters. Acetylcholine is the major neurotransmitter in the parasympathetic nervous system and will be the focus of this explainer.

A cholinergic synapse consists of the axon terminal of a presynaptic neuron, a dendrite of a postsynaptic neuron, and the synaptic cleft forming the physical gap between them.

The structure of a cholinergic synapse can be seen in Figure 2.

Key Term: Cholinergic

The term cholinergic refers to nerve cells in which acetylcholine acts as a neurotransmitter.

Key Term: Acetylcholine

Acetylcholine is the main neurotransmitter found in synapses of the parasympathetic nervous system.

Key Term: Presynaptic Neuron

A presynaptic neuron transmits a signal toward a synapse by releasing vesicles containing neurotransmitters into the synapse.

Key Term: Postsynaptic Neuron

A postsynaptic neuron is one that receives the neurotransmitter after it has crossed the synaptic cleft and may experience an action potential if the neurotransmitter triggers enough sodium influx.

Key Term: Synaptic Cleft

The synaptic cleft is the physical space between two neurons.

The presynaptic neuron is the neuron in which the nerve impulse will arrive first. It is responsible for transmitting the signal toward and into the synaptic cleft. The end of the presynaptic neuron is called the synaptic knob.

Within the presynaptic neuron, there are synaptic vesicles that contain the neurotransmitter acetylcholine. Vesicles are small components of a cell filled with fluid and surrounded by a membrane. Vesicles are responsible for transporting materials around a cell’s cytoplasm and are involved in the processes of endocytosis and exocytosis. The prefix endo- means “into,” while exo- means “out of.” Cyto- refers to a cell, so endocytosis is the process by which a material enters a cell and exocytosis refers to it leaving the cell. Vesicles are surrounded by a plasma membrane consisting of the same phospholipid bilayer as the cell surface membrane. This means vesicles can be formed by the cell surface membrane pinching off in endocytosis or vesicles can fuse with the membrane in exocytosis.

Key Term: Vesicles

Vesicles are small membrane-bound and fluid-filled subcellular compartments responsible for transporting, importing, and exporting materials from the cell’s cytoplasm.

Example 1: Describing the Role of Synaptic Vesicles

What is the role of synaptic vesicles in the presynaptic neuron?

Answer

The presynaptic neuron is the neuron in which the nerve impulse will arrive first. It is responsible for transmitting the signal toward the synaptic cleft.

The end of the presynaptic neuron is called the synaptic knob. Within the presynaptic neuron, there are synaptic vesicles that contain the neurotransmitter. Vesicles are small components of a cell filled with fluid and surrounded by a membrane. Vesicles are responsible for transporting materials around a cell’s cytoplasm and the processes of endocytosis and exocytosis. The prefix endo- means “into,” while exo- means “out of.” Cyto- refers to a cell, so endocytosis is the process by which a material enters a cell and exocytosis refers to it leaving the cell. Vesicles are surrounded by a plasma membrane consisting of the same phospholipid bilayer as the cell surface membrane. This means vesicles can be formed by the cell surface membrane pinching off in endocytosis or vesicles can fuse with the membrane in exocytosis.

The synaptic vesicles in the presynaptic neuron are responsible for storing the neurotransmitter. They will fuse with the presynaptic membrane when stimulated by calcium ions and release the neurotransmitter into the synaptic cleft.

Therefore, the role of synaptic vesicles in the presynaptic neuron is to store neurotransmitters.

Example 2: Describing the Structure of a Synapse

The diagram provided shows a simple outline of a cholinergic synapse. What part of the synapse is indicated by the question mark?

Answer

A cholinergic synapse, such as the one in the diagram above, consists of a presynaptic neuron, a postsynaptic neuron, and the synaptic cleft forming the physical space of the synapse between them.

The presynaptic neuron is the neuron in which the nerve impulse will arrive first. It is responsible for transmitting the signal toward and into the synaptic cleft. The end of the presynaptic neuron is called the synaptic knob. Within the presynaptic neuron, there are synaptic vesicles that contain the neurotransmitter acetylcholine. The presynaptic neuron also has calcium ion channels embedded in its cell surface membrane.

Once the neurotransmitter has diffused across the synaptic cleft, it stimulates an action potential in the postsynaptic neuron. The postsynaptic neuron has sodium ion channels embedded in its cell surface membrane. These sodium ion channels have receptor sites specific to the neurotransmitter—in this case, acetylcholine.

Therefore, the structure marked by a question mark is the synaptic cleft.

Let’s look in more detail at the series of events that occur at a cholinergic synapse.

When an action potential arrives in the dendrites of a presynaptic neuron, it depolarizes it. This causes calcium ion channels that are sensitive to voltage changes to open, making the synaptic knob permeable to calcium ions (Ca2+). Ca2+ diffuses into the synaptic knob, as you can see in Figure 3.

The presence of Ca2+ triggers vesicles containing acetylcholine to fuse with the presynaptic membrane. One of the functions of vesicles is to carry out exocytosis, and in this case, the vesicles release acetylcholine out of the presynaptic neuron and into the synaptic cleft. Acetylcholine diffuses passively across the synaptic cleft, as you can see in Figure 4.

When acetylcholine reaches the postsynaptic membrane, acetylcholine binds to its receptor site, which opens the sodium ion channel of the acetylcholine receptor. This makes the postsynaptic neuron permeable to sodium ions (Na+), and Na+ diffuses into the postsynaptic neuron. This influx of sodium causes an action potential to be triggered in the dendrite of the postsynaptic neuron, as is visible in Figure 5. The action potential is propagated along to the postsynaptic neuron’s soma and, subsequently, its axon. The action potential will then reach the axon terminals of this neuron and will repeat this process to cross another synapse.

Example 3: Describing the Stages of Transmission of Information across a Synapse

The flowchart provided shows the stages of transmission of information across a synapse, with each stage assigned a number. State the correct order of stages.

Answer

When an action potential arrives in the dendrites of a presynaptic neuron, it depolarizes it. This causes calcium ion channels to open, making the synaptic knob permeable to calcium ions (Ca2+). Ca2+ diffuses into the synaptic knob, triggering the vesicles containing acetylcholine to fuse with the presynaptic membrane. One of the functions of vesicles is to carry out exocytosis, and in this case, the vesicles release acetylcholine out of the presynaptic neuron and into the synaptic cleft.

Acetylcholine diffuses passively across the synaptic cleft from an area of high to an area of low acetylcholine concentration. When it reaches the postsynaptic membrane, acetylcholine binds to its receptor sites on the sodium ion channels, causing them to open. This makes the postsynaptic neuron permeable to sodium ions (Na+), and Na+ diffuses into the postsynaptic neuron.

The influx of sodium causes an action potential to be triggered in the dendrite of the postsynaptic neuron. The action potential is propagated along to the postsynaptic neuron’s soma and, subsequently, its axon until it reaches the dendrites at the end of the neuron and crosses another synapse.

Therefore, the correct order of stages is 6, 2, 1, 4, 3, 5.

Once the neurotransmitter has diffused across the synaptic cleft, it stimulates an action potential in the postsynaptic neuron. The postsynaptic neuron has sodium ion channels embedded in its cell surface membrane. These sodium ion channels have receptor sites specific to the neurotransmitter—in this case, acetylcholine. This means that only this neurotransmitter can bind to them and cause the sodium ion channels to open. This is important, as these receptors are only present on the postsynaptic neuron and not the presynaptic neuron, so the nerve impulse can only travel in one direction. Synapses are therefore called unidirectional, and they function to ensure that nerve impulses do not travel in the wrong direction.

Key Term: Unidirectional

Synapses are unidirectional as they can only transmit information in one direction, from the presynaptic neuron to the postsynaptic neuron, due to the presence of neurotransmitter receptors on the postsynaptic neuron.

Once an action potential has been generated in the postsynaptic neuron, acetylcholine needs to detach from its receptor sites on the sodium ion channels. If this did not occur, the postsynaptic neuron would continue to generate action potentials, as the sodium ion channels would remain open. The enzyme acetylcholinesterase is also embedded in the postsynaptic membrane. Acetylcholinesterase hydrolyzes (breaks down with water) acetylcholine into choline and ethanoic acid. This allows the sodium ion channels of the acetycholine receptors to close and to come back to their initial state. These products are then reabsorbed into the presynaptic neuron to be recycled back into acetylcholine so that the process can occur again.

Key Term: Acetylcholinesterase

Acetylcholinesterase is an enzyme that breaks down acetylcholine to stop excitation of a neuron after transmission of an impulse.

Example 4: Describing the Outcome for Acetylcholine in a Synapse

What happens to the acetylcholine in the synaptic cleft once an action potential has been triggered in the postsynaptic neuron?

  1. It binds to acetylcholine receptors on the presynaptic membrane.
  2. It dissolves into the cytoplasm of the neuron.
  3. It diffuses out of the cleft and into the bloodstream.
  4. It is broken down by enzymes.

Answer

Once an action potential has been generated in the postsynaptic neuron, acetylcholine needs to detach from its receptor sites on the sodium ion channels. If this did not occur, the postsynaptic neuron would continue to generate action potentials, as the sodium ion channels would remain open.

The enzyme acetylcholinesterase is also embedded in the postsynaptic membrane. Acetylcholinesterase hydrolyzes (breaks down with water) acetylcholine into choline and ethanoic acid. This allows the sodium ion channels of the acetycholine receptors to close and to come back to their initial state. These products are then reabsorbed into the presynaptic neuron to be recycled back into acetylcholine so that the process can occur again.

Receptors for acetylcholine are only present on the sodium ion channels on the postsynaptic membrane. This is to ensure that the nerve impulses do not travel in the wrong direction, so nerves that use synapses are unidirectional. There are no receptors for acetylcholine on the presynaptic membrane.

Acetylcholine will not dissolve in the cytoplasm of the neuron, as whenever it is present in the neuron, it is found within a vesicle to transport it around the cell.

Synapses are not directly in contact with the bloodstream, so acetylcholine does not diffuse into the blood. Acetylcholine is made in the neurons themselves and can be recycled to be used again.

Therefore, once an action potential has been triggered in the postsynaptic neuron, acetylcholine is broken down by enzymes.

A real-life example of what can happen if acetylcholinesterase does not function effectively is when a person is exposed to the nerve agent VX. VX blocks the action of acetylcholinesterase, which means that the neurotransmitter is not hydrolyzed and the synaptic cleft becomes flooded with acetylcholine. This means that the nerve is constantly in an excited state. As nerves also form synapses with muscle cells, such as those that control ventilation of the lungs, this means that these muscles surrounding the lungs are receiving repeated signals to contract. If this occurs, the person will be unable to exhale and can die from suffocation. Just 10 mg (1100th of a gram) of VX can cause death.

Other nerve agents and drugs can mimic the shape of a neurotransmitter. For example, nicotine found in cigarettes mimics the shape of acetylcholine and binds to acetylcholine receptors in the postsynaptic membrane to trigger action potentials. Other chemicals can stimulate more neurotransmitters to be released from the presynaptic membrane or even block receptors on the postsynaptic membrane so that no action potentials can be triggered.

Example 5: Defining Features of Synapses

Which of the following is a defining feature of synapses?

  1. Synapses can only pass information in one direction.
  2. Synapses transmit information as electrical signals only.
  3. Synapses only form between two neurons.
  4. Acetylcholine receptors are only located on the presynaptic neuron.

Answer

Synapses are small gaps between neurons or between a neuron and a muscle or gland cell. They transmit information through chemical messengers called neurotransmitters. Though this is slower than electrical transmission, synapses ensure that a nerve impulse only travels in one direction.

Synapses consist of a presynaptic neuron, a postsynaptic neuron, and a synaptic cleft forming the space between the two. When an action potential arrives, a neurotransmitter is released by the presynaptic neuron into the synaptic cleft. The neurotransmitter then binds to receptors that are only present on the postsynaptic membrane. This binding triggers another action potential to be generated in the postsynaptic neuron. It is also what ensures that the signal is unidirectional, as no receptors are present on the presynaptic membrane so the information cannot be sent back to the presynaptic neuron.

Therefore, the defining feature of synapses is that synapses can only pass information in one direction.

Let’s recap some of the key points we have covered in this explainer.

Key Points

  • Neurons form synapses with other neurons, muscle fibers, or gland cells.
  • Cholinergic synapses are those that use acetylcholine as a neurotransmitter.
  • Synapses consist of a presynaptic neuron to which a nerve impulse arrives first and triggers a neurotransmitter to be released into a synaptic cleft.
  • The neurotransmitter binding to receptors on the postsynaptic neuron’s membrane causes an action potential to be generated in this neuron.
  • Acetylcholinesterase hydrolyzes acetylcholine so it can be recycled by the synaptic knob.

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