Mechanism of Synaptic Transmission — Definition

Definition

Imagine your brain and nervous system as a vast, intricate network of wires, but instead of continuous wires, these 'wires' (neurons) have tiny gaps between them. These gaps are called synapses, and the process of sending a message across these gaps is called synaptic transmission. It's like a relay race where one runner (the presynaptic neuron) passes a baton (a chemical messenger called a neurotransmitter) to the next runner (the postsynaptic neuron).

Here's how it generally works: When an electrical signal, known as an action potential, reaches the end of the first neuron (the presynaptic terminal), it can't just jump across the gap. Instead, this electrical signal triggers a series of events. First, tiny channels on the presynaptic terminal open up, allowing calcium ions to rush in. This influx of calcium is the crucial signal that tells the neuron to release its chemical messengers.

These chemical messengers, or neurotransmitters, are stored in small sacs called synaptic vesicles. When calcium enters, these vesicles fuse with the neuron's outer membrane and release their neurotransmitters into the synaptic cleft – that tiny gap between the neurons.

Once in the cleft, these neurotransmitters float across and bind to specific 'docking stations' or receptors on the surface of the second neuron (the postsynaptic membrane). Think of it like a key fitting into a lock. When the neurotransmitter (key) binds to its receptor (lock), it causes tiny gates (ion channels) on the postsynaptic neuron to open.

Depending on which ions enter or leave the postsynaptic neuron, this can either excite the neuron, making it more likely to fire its own electrical signal (action potential), or inhibit it, making it less likely to fire.

This change in electrical potential in the postsynaptic neuron is called a postsynaptic potential. Finally, to ensure the signal is precise and doesn't linger, the neurotransmitters are quickly removed from the synaptic cleft, either by enzymes breaking them down, or by being reabsorbed back into the presynaptic neuron, or by glial cells.

This entire sequence, from electrical signal to chemical signal and back to electrical signal, is what we call synaptic transmission, and it's how all communication happens in our nervous system.