Synapses

A synapse is a junction, not a fusion

At Core level, the required idea is simply that a synapse is a junction between two neurones. The two neurones do not physically touch at this point — there is always a gap between them — and a synapse is not some third type of neurone sitting between the other two. It is the meeting point itself, not a structure with its own separate identity. Describing the two neurones as fused into one continuous cell, or treating the synapse as an extra neurone in the chain, both miss what a synapse actually is.

Supplement: what a synapse is made of

Going one level deeper, a synapse has three required features: vesicles containing neurotransmitter molecules, sitting in the first neurone; a synaptic gap separating the two neurones; and receptor proteins on the surface of the next neurone. Each piece exists on a specific side of the junction, and that asymmetry — vesicles on one side, receptor proteins on the other — is exactly what gives the synapse its direction.

Supplement: how transmission crosses the gap

The impulse itself does not jump the synaptic gap electrically. It crosses chemically, in a fixed sequence: an impulse arriving at the end of the first neurone causes vesicles to release neurotransmitter molecules into the synaptic gap; the neurotransmitter diffuses across the gap; it binds to receptor proteins on the next neurone; and this binding stimulates a new electrical impulse in that neurone. The signal, in other words, is electrical along each neurone but chemical across the gap between them — a distinction worth stating explicitly if a question asks how the impulse actually crosses.

Because vesicles and neurotransmitter release only exist on one side of the gap, and receptor proteins only exist on the other, an impulse can only ever cross a synapse in one direction. That one-way structure is precisely why synapses keep impulses flowing correctly through a pathway like the reflex arcrather than bouncing back and forth between neurones. It’s also worth resisting the instinct to describe this as instantaneous — it’s a genuine chemical sequence, release, diffusion, binding, new impulse, and each step takes a real, if brief, amount of time.

The pattern examiners are actually checking for

Put the two levels together and the required logic reads: neurones meet at a junction rather than touching directly (Core), and that junction is crossed by a specific one-way chemical sequence involving vesicles, a gap, and receptor proteins (Supplement). A common way to lose marks is naming the components in the wrong order or vaguely — “a chemical crosses the gap” instead of naming the neurotransmitter’s release, diffusion and binding as three separate, ordered steps. Keep the sequence intact, and keep it moving in one direction, and the mark scheme has very little left to catch you on.