Process of Synaptic Transmission

Process of Synaptic Transmission Psychology

Humans have around 100 billion neurones. Neurones are specialised cells specific to the nervous system and are found in the brain and spinal cord.

Initially, researchers thought that neurones were physically connected. However, with the development of the investigation techniques, researchers proved this hypothesis partly wrong. Although some cells are indeed physically connected, some others are not.

With such a discovery, a new research question emerged; how do neurones send information between them if they are not physically connected?

Despite the high number of neurones in the human brain, neurones are arranged in a way that they usually do not touch each other. When neurones are close enough, they can communicate over the small gap between them, known as the synaptic cleft. So, what is the process of synaptic transmission?

Define the Process of Synaptic Transmission

Synaptic transmission is the process by which two neurones exchange information. An action potential can send information from a neurone's dendrites down the axon towards the axon terminal. Synaptic transmission is the process that takes place afterwards.

Synaptic transmission can be chemical or electrical.

Fig. 2: Anatomy of the presynaptic neurone as well as postsynaptic neurone's dendrites.

Synaptic Transmission Steps

Let's take a closer look at the process of synapse transmission. Synaptic transmission can be broken down into steps.

1. To summarise the process of synaptic transmission in chemical transmission, the presynaptic neurone receives information via its dendrites, which contain many synapses, from other nerves.
2. If the neurone is stimulated enough, it sends an action potential down its axon towards the axon terminals at the end of the neurone.
3. The neurone will then release neurotransmitters (stored in vesicles, tiny sacs containing neurotransmitters) into the synaptic cleft, a small gap between neurones, to diffuse across the synaptic cleft and then bind to receptors on the dendrites of the postsynaptic neurone.
4. Depending on the type of neurotransmitter, they can be excitatory or inhibitory. Excitatory neurotransmitters increase the likelihood of an action potential firing in the postsynaptic neurone, and inhibitory do the opposite.

The neurotransmitters that remain in the synaptic cleft are reabsorbed (reuptake) or broken down in some form.

Types of Synaptic Transmission

As it was mentioned earlier, some neurones are physically connected, while some other neurones are not. This distinction sets two types of synapses; electrical and chemical synaptic transmissions.

Electrical Synapse Transmission

Electrical synapses are more common in squid or zebrafish but can also be found in humans. Electric synapses occur when two neurones are not separated by a cleft but are joined by the gap junction (paired channels in the pre- and postsynaptic neurone membrane, where each channel is known as a pore), as they are very close together.

Electrical synapses do not require neurotransmitters because the information can travel electrically between neurones through the gap junction. The action potential is not transformed into chemical information.

Electric transmission is faster than chemical transmission (flow can occur almost instantaneously) and is bidirectional (this means that the ionic current flow can travel back and forth between the cells.)

Chemical Synapse Transmission

Chemical synapses are the most common type of synapses in the human brain. The chemical synapse takes place between neurones that are not directly connected.

Chemical transmission involves the release of neurotransmitters from the presynaptic neurones into the synaptic cleft (a small gap between the neurones) via vesicles. These neurotransmitters then diffuse across the synaptic gap.

The neurotransmitters bind to specific receptors on the postsynaptic neurone. The impulse received on the postsynaptic membrane is either called excitatory postsynaptic potential (EPSP) or inhibitory postsynaptic potential (IPSP), depending on the neurotransmitter's effect on the postsynaptic neurone.

Action Potentials

An action potential is a change in the electric voltage of a neurone. Neurones have an electrical potential difference across their membrane, so the inside of the neurone is negative with respect to the outside.

First comes the resting potential (60-70 mV). This is the potential at which the neurone rests, as it is not stimulated.

When the neurone is stimulated, the polarity is reversed (depolarisation). At this point, the neurone's potential reaches the threshold of excitation (-50mV), and an action potential begins, as sodium (Na+) ions enter the membrane due to sodium voltage-gated channels opening.

The cell becomes more positive on the inside and reaches a threshold of +40mV. At this point, the neurone repolarises, which means it becomes more negative again as potassium voltage-gated channels open.

Process of Synaptic Transmission Diagram

The following diagram represents the two different types of synaptic transmission. On the left, the chemical synapse is represented. The electrical synapse can be seen on the right hand of the diagram.