Transmission Across a Synapse

Transmission across the synapse or neurotransmission is when a neurone communicates with another neurone or cell by releasing neurotransmitters into the synaptic cleft. Transmission across a synapse is exclusive to chemical synapses.

What is a synaptic cleft?

There is usually a 20-30 nanometre wide gap between neurones called a synaptic cleft. The synaptic cleft is filled with a fluid called the interstitium. In the most common type of synapse, the chemical synapse, neurones do not touch (but can get very close). At the synapse, neurons and cells communicate with each other via chemical molecules called neurotransmitters, which wander across the synapse, like boats crossing a river.

The synapse converts electrical signals into chemical information through its unique mechanism, which is then again converted into electrical signals. The main communication of the nervous system is considered electrochemical because it combines elements of electrical and chemical information.

What happens in the process of transmission across a synapse?

When action potential (electrical charge firing along the axon) arrives in the axon terminal, neurotransmitters are released into the synaptic cleft. These then bind to receptors, which allow either only negatively charged or only positively charged ions to enter into the next cell and depolarise or hyperpolarise it.

What do neurotransmitters do in a chemical synapse?

Each synapse usually specialises in one type of neurotransmitter. These are specific messenger molecules produced in the cell body and transported along the cytoskeleton (a network of protein strings and tubes that act like scaffolding for the cell) to the end of the axon. This process is called axonal transport. Once they arrive in the axon terminal, they are wrapped in membrane sacs called vesicles and gather at the presynaptic end of the axon, ready to be released from the presynaptic membrane.

What are receptors?

A receptor is a protein molecule in the postsynaptic cell membrane that reacts to a specific neurotransmitter, hormone or other molecules. You can think of it as a gate or door that opens when unlocked by a particular molecule, called the lock-and-key principle. When a neurotransmitter binds to a receptor, the ion channel opens to let other specific molecules in, either ions with a positive or negative charge.

What is the excitatory and inhibitory synaptic transmission?

Depending on the neurotransmitter released, synaptic transmission that uses neurotransmitters that open ion channels can either be excitatory or inhibitory. The impulse that is received on the postsynaptic membrane is either called excitatory postsynaptic potential (EPSP) or inhibitory postsynaptic potential (IPSP), depending on whether the neurotransmitter has an excitatory or inhibitory effect.

Excitatory

Excitatory means that the gates opened by the neurotransmitters let positive ions such as Na+ (sodium) or K+ (potassium) flow into the cell, resulting in depolarisation of the cell membrane (the inside of the cell becoming positively charged). This process makes it more likely for action potential to be produced.

Inhibitory

Inhibitory means that the gates opened by these neurotransmitters let negative ions such as Cl- (chloride) into the cell, resulting in hyperpolarisation of the cell membrane (the inside of the cell becoming even more negatively charged than usual). Thus it is less likely for action potential to be produced.

What are non-channel synapses?

A third possibility is that the neurotransmitter released doesn’t open an ion channel but rather that sets off a protein chain reaction that has more long-term consequences involved in memory and learning processes. These are called g-protein cascades or second-messenger cascades. These chain reactions are often complex and involve many different molecules and cell mechanisms. Synapses that trigger these kinds of reactions are also called non-channel synapses.

Why is the process of synaptic transmission important?

Synaptic transmission is important because it allows for unidirectional travel, summation and integration.

Unidirectional travel

Synaptic transmission allows electrical impulses to travel unidirectionally. Neurotransmitters are released on one side and the receptors located on the other, making the direction of the impulse precise. This is important because it allows for reflexes and other automatic responses to be “programmed” in the hard-wiring of our nervous system.

Summation

Summation is the act of producing an action potential with input from multiple presynaptic cells.

The reason that summation is essential is that it allows the nervous system to filter out information that isn’t important. Suppose a stimulus is presented again and again, such as the feel of your mouse against your hand while you are scrolling through the internet. In that case, the time it takes to let your body replenish the neurotransmitters means that you get desensitised to repeated stimuli; this allows you to focus on the new information coming in, such as the following words you’re about to read.

Summation explains how humans can focus attention and filter important information from the mass of stimuli at any given moment.

Integration of information

Action potential from one presynaptic neurone can generate postsynaptic potential in multiple cells, making dispersal and creation of set patterns of neural firings possible. Vice versa, it also allows for integrating information from various sources and stimuli on one neurone.

Integration of information is essential because it gives a biological explanation for learning. It can also explain our subconscious and instincts because convergent information may be integrated into our biology before it is made conscious in our thoughts.

When does synaptic transmission lead to action potential?

Action potentials or the electrical impulse that travels along the axon can only be initiated if a certain voltage threshold is reached (usually -60mV). You will find out more in our action potential article.

Action potentials follow the all-or-nothing principle and only travel in one direction. But to initiate the action potential that starts transmission to the next cell via the axon, one incoming impulse is usually not enough. The addition of a few incoming signals is needed. This process is called summation.

Two types of summation can lead to depolarisation/action potential:

• Spatial summation is when enough excitatory impulses arrive on one cell from different locations.
• Temporal summation is when enough excitatory impulses arrive on one cell from one other cell in quick succession.

What are the steps of transmission across a synapse?

In synaptic transmission, electrical charge is converted to chemicals that bridge a gap between the two cells. These chemicals react with the cell membrane to create an electrical charge in the receiving cell.

Transmission across a cholinergic synapse

Let’s go step by step to see how the process of synaptic transmission across a cholinergic synapse works (remember - cholinergic synapses release the neurotransmitter acetylcholine):

1. Action potential (electrical current) arrives at the axon terminal from the cell body.
2. The electrical charge opens ${\mathrm{Ca}}^{2+}$(calcium) channels in the axon terminal. These calcium channels are voltage-gated, meaning they open up in response to electrical current. Calcium is more abundant outside the cell and is attracted to the negative charge in the cell, so as soon as the gates open, calcium rushes into the cell.
3. ${\mathrm{Ca}}^{2+}$enters the axon terminal, enabling exocytosis. This means that the vesicles’ membrane that holds the neurotransmitters fuse with the presynaptic membrane.
5. Vesicles open up, and the acetylcholine neurotransmitter molecules are released into the synaptic cleft.
6. The acetylcholine molecules diffuse across the synaptic cleft and bind with cholinergic receptors on the postsynaptic membrane.
7. The ion channels open up, and positive ions enter the cell if it’s a nicotinic receptor (nAChR) (a receptor that responds to acetylcholine).
8. The remaining neurotransmitters are broken down by the enzyme acetylcholinesterase and the parts recycled into the presynaptic cell.

Drug effects on synapses

Many drugs can affect the central nervous system (CNS) in different ways. They can alter the way people behave, feel, or think by influencing transmission across synapses. Examples of drugs include anaesthetics, muscle relaxants and CNS stimulants.

Drugs that mimic neurotransmitters

Drugs can mimic neurotransmitters and bind to the receptors instead. They are referred to as antagonistic. Painkillers (more specifically, opiates) such as morphine and codeine will attach to the receptors, which cause the person to feel pain relief. They mimic natural endorphins (are also known as chemical messengers).

Another example, marijuana, which will mimic cannabinoid (involved in appetite, mood, memory and pain sensation) neurotransmitters called anandamide.

Drugs that interact with molecular components

These types of drugs will interact with different molecular components in the body. For example, cocaine attaches to the molecule that transports dopamine. This does not allow the dopamine transporter to enter the neuron. It will start to build up in the synapse, and the receptors that receive dopamine will be stimulated to a larger degree.

Drugs that alter the stimulation of receptors

Some drugs can change the amount of receptors that are stimulated. For example, Valium (also known as diazepam) will make you feel relaxed by enhancing the response from the neuron when GABA (a neurotransmitter that blocks certain brain signals and lowers the nervous system’s activity by attaching to inhibiting GABA receptors). GABA produces a calming effect on its own by decreasing anxiety and stress; with Valium, this feeling will be induced.