Basic Structure of Synapses

• A synapse is a junction between two neurones, or between a neurone and an effector cell.
• The membrane of the presynaptic neurone, approaching the synapse, is referred to as the presynaptic membrane, while the subsequent neurone's membrane or effector's membrane is referred to as the postsynaptic membrane.
• The physical gap between these two membranes is known as the synaptic cleft.
• At a synapse, the signal is transmitted via chemical means, rather than electrical.

Synaptic Transmission Process

• Synaptic transmission is initiated when an incoming action potential reaches the axon terminal of the presynaptic neurone.
• This stimulates the opening of voltage-gated calcium channels, allowing calcium ions to flood into the presynaptic neurone.
• The increased concentration of calcium ions triggers the fusion of synaptic vesicles (which contain neurotransmitters) with the presynaptic membrane.
• Neurotransmitters are then released into the synaptic cleft via exocytosis.
• Neurotransmitters cross the cleft and bind to specific receptor molecules on the postsynaptic membrane, causing it to either activate (producing an excitatory postsynaptic potential, EPSP) or inhibit (producing an inhibitory postsynaptic potential, IPSP) the postsynaptic neurone.
• If the EPSP is strong enough, it may trigger or contribute to the generation of an action potential in the postsynaptic neurone.
• Unbound neurotransmitters are typically reabsorbed by the presynaptic neurone – a process known as reuptake – or broken down by enzymes in the cleft.

Types of Synapses and Their Effects

• Excitatory synapses lead to depolarisation of the postsynaptic membrane, increasing the likelihood that an action potential will be triggered.
• Inhibitory synapses lead to hyperpolarisation, decreasing the likelihood of an action potential.
• Furthermore, different postsynaptic cells will respond differently to the same neurotransmitter, influenced by the type of receptor it interacts with.
• Lastly, note that the influence of any single synapse is often not enough to trigger an action potential – summation must typically occur, where the combined influence of multiple excitatory synapses exceeds the threshold level.

Grasping the nature of synapses and synaptic transmission is paramount in understanding how neurones communicate, which directly feeds into the broader ideas around communication and homeostasis.