The Nervous System - Synapses

Overview

Synapses are junctions at which two neurones meet, allowing the passage of signals from one neurone to another.
They play a vital role in the functioning of the nervous system by enabling complex interactions between neurones that coordinate behaviours, sensations, and emotions.

A synapse consists of the ending of a presynaptic neurone, the synaptic cleft, and the beginning of a postsynaptic neurone.
The terminus of the presynaptic neurone contains synaptic vesicles filled with neurotransmitters.
The synaptic cleft is the small gap between two neurones, which physiologically isolates them, ensuring that the electrical signal cannot go back to the presynaptic neurone.
The postsynaptic neurone has receptor sites on its surface which neurotransmitters can bind to.

The process by which the signal moves from one neurone to another at a synapse is called synaptic transmission.
When the action potential reaches the synaptic knob of the presynaptic neurone, this triggers the synaptic vesicles to move towards the synaptic membrane, undergo exocytosis, and release neurotransmitters into the synaptic cleft.
These neurotransmitters diffuse across the synaptic cleft and bind to their specific receptors on the postsynaptic neurone.
This binding induces a change in the permeability of the postsynaptic membrane to certain ions, creating a new action potential.
Once the communication is achieved, neurotransmitters in the synaptic cleft are quickly broken down by enzymes, or reabsorbed by the presynaptic neurone via reuptake mechanisms to reset the synapse for next transmission.

Synaptic transmission can be excitatory or inhibitory, depending on the type of neurotransmitters released and receptors involved.
Excitatory neurotransmitters, such as glutamate, increase the likelihood that an action potential will be initiated in the postsynaptic neurone.
Inhibitory neurotransmitters, such as gamma-aminobutyric acid (GABA), decrease this likelihood.

Synaptic plasticity, the ability of synapses to strengthen or weaken their connections, is believed to be the neurophysiological basis of learning and memory.
Long-term potentiation (LTP) and long-term depression (LTD) are two key mechanisms underlying synaptic plasticity.
LTP enhances synaptic strength following high-frequency stimulation, while LTD weakens synaptic connections with low-frequency stimulation.
These mechanisms are essential for memory formation, storage, and retrieval.

Certain neurotransmitters, such as dopamine, serotonin, and norepinephrine, are involved in controlling mood, attention, and behaviours.
Changes in the levels of these neurotransmitters have been linked to mental health disorders, such as depression and anxiety.
Many psychiatric drugs work by altering the neurotransmitter levels in the synaptic cleft, thereby affecting the transmission of signals through the synapses.