Synapses are the invisible bridges between neurones, the site of synaptic transmission, vital for neurone communication within the nervous system. Understandably, synaptic transmission is complex, involving neurotransmitters, neurones, and action potentials. Let's explore the process of synaptic transmission over the billions of neurones in the brain.
- The explanation will review the process of synaptic transmission in psychology.
- First, we will define the process of synaptic transmission.
- Then, we will explore the overall process of synaptic transmission.
- Moving on from this, we will highlight the different synaptic transmission steps.
- Finally, we will close with the presentation of a diagram detailing the process of synaptic transmission.
Fig. 1: Synaptic transmission involves neurones and neurotransmitters.
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.
Around 80% of the total number of these neurones are located in the brain.
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.
Neurone A is the presynaptic neurone. Neurone B is the postsynaptic neurone.
Neurone A receives information from another neurone. Neurone A fires an action potential and sends the information down the axon to the axon terminal.
In this way, Neurone A sends information to Neurone B.
(If the information were to be transmitted to neurone C, then neurone B would become the presynaptic neurone, and C would become the postsynaptic one.) All neurones are both the pre- and the postsynaptic neurones when information travels within a network.
Synaptic Transmission Steps
Let's take a closer look at the process of synapse transmission. Synaptic transmission can be broken down into steps.
- 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.
- If the neurone is stimulated enough, it sends an action potential down its axon towards the axon terminals at the end of the neurone.
- 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.
- 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.
A receptor is a protein molecule in the cell membrane that reacts to a specific neurotransmitter. You can think of it as a gate or door that opens when unlocked by a particular neurotransmitter.
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.
| Excitatory Postsynaptic Potential | Inhibitory Postsynaptic Potential |
EPSPs occur when the neurotransmitter makes it likely that the postsynaptic neurone will fire an action potential down their axon. Noradrenaline is a neurotransmitter that usually evokes an excitatory potential in the postsynaptic neurone. | IPSPs make the postsynaptic neurone less likely to fire an action potential, which stops the information being sent down the axon of the postsynaptic neurone. Serotonin is an example of a neurotransmitter that usually decreases the likelihood of a postsynaptic neurone firing an action potential. |
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.
After this, the is a short period in which the neurone cannot fire another action potential (recovery phase).
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.
Process of Synaptic Transmission - Key takeaways
- Synaptic transmission is the process by which neurones exchange information. The presynaptic neurone, if stimulated enough, sends an action potential down its axon to release neurotransmitters into the synaptic cleft, which binds to receptors on the postsynaptic neurone.
- Two types of synaptic transmission are chemical synaptic transmission and electrical synaptic transmission.
- Electric transmission occurs when two neurones are not separated by a cleft but joined by the gap junction. Chemical transmission involves the release of neurotransmitters from the presynaptic neurones into the synaptic cleft (a small gap between the neurones) via vesicles, which then bind to receptors on the postsynaptic neurone.
- Neurotransmitters can have an excitatory or an inhibitory effect on the postsynaptic neurone.
- The neurotransmitters that remain in the synaptic cleft are reabsorbed or broken down in some form.
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