Synaptic transmission

The big idea: A nerve impulse travels along a neuron as an electrical signal — but neurons do not actually touch.

Where one neuron meets the next there is a tiny gap called a synapse. The signal cannot jump the gap electrically, so it is carried across by a chemical instead.

That chemical is a neurotransmitter: it is released by the first neuron, drifts across the gap, and triggers a brand-new impulse in the second neuron.

Synapse

The junction (a tiny gap) between two neurons, where a signal is passed from one to the next using a chemical.

Synaptic cleft

The narrow gap between the two neurons that the neurotransmitter diffuses across.

Presynaptic neuron

The neuron BEFORE the synapse — the one that releases the neurotransmitter (the sender).

Postsynaptic neuron

The neuron AFTER the synapse — the one that receives the neurotransmitter (the receiver).

Neurotransmitter

The chemical messenger released into the synaptic cleft; it carries the signal across the gap.

Synaptic vesicle

A small membrane-bound sac inside the presynaptic neuron that stores neurotransmitter ready for release.

Receptor

A protein on the postsynaptic membrane that the neurotransmitter binds to, passing the signal on.

Why a chemical, not just electricity: The impulse is electrical INSIDE each neuron, but the two neurons are separated by the synaptic cleft.

Electricity cannot cross that gap, so the signal is converted into a chemical (the neurotransmitter) to bridge it — then turned back into an electrical impulse on the other side.

Synaptic transmission is a chain of cause and effect. Each step makes the next one happen, so it is easiest to learn as an ordered sequence.

Split it into two halves: first the presynaptic neuron releases the neurotransmitter, then the postsynaptic neuron responds to it.

Step 1 — release at the presynaptic membrane: When the impulse reaches the presynaptic neuron's terminal:

1. Calcium ions (Ca²⁺) enter the presynaptic neuron through channels that open.

2. This makes synaptic vesicles fuse with the presynaptic membrane.

3. The vesicles release neurotransmitter into the synaptic cleft by exocytosis.

Step 2 — response at the postsynaptic membrane: The neurotransmitter then diffuses across the cleft and:

1. Binds to receptors on the postsynaptic membrane.

2. Binding opens ion channels, so sodium ions (Na⁺) enter the postsynaptic neuron.

3. The postsynaptic membrane depolarises — this is an excitatory postsynaptic potential (EPSP).

4. If the depolarisation is big enough to reach threshold, a new nerve impulse fires in the postsynaptic neuron.

Exocytosis

The process in which vesicles fuse with the cell membrane and empty their contents to the outside — how neurotransmitter is released.

Excitatory postsynaptic potential (EPSP)

A depolarisation of the postsynaptic membrane caused by neurotransmitter binding — it makes a new impulse MORE likely.

Threshold

The level of depolarisation that must be reached for a new nerve impulse (action potential) to fire.

The whole sequence in order: Impulse arrives → Ca²⁺ enters → vesicles fuse → neurotransmitter released (exocytosis) → diffuses across cleft → binds receptors → Na⁺ enters → membrane depolarises (EPSP) → new impulse fires.

Writing the steps in this order is exactly how a 'Describe' mark scheme awards its marks.

Why the gap is so narrow: The synaptic cleft is extremely thin.

That matters because the neurotransmitter crosses it by diffusion, and diffusion is fast only over a short distance.

A narrow gap therefore means a short diffusion distance, so the signal is passed across quickly — a favourite 'Suggest an advantage' answer.

How this is tested: On Paper 2 a 3-mark Describe question often asks how a neurotransmitter is released at the presynaptic membrane — score it as an ordered chain (Ca²⁺ in → vesicles fuse → exocytosis into the cleft).

A second 3-mark Describe can ask how an excitatory postsynaptic potential (EPSP) is generated (neurotransmitter binds receptors → ion channels open → Na⁺ in → depolarisation).

A favourite data-question format shows a synapse micrograph: you may have to deduce what the vesicles hold, distinguish the presynaptic from the postsynaptic membrane, or suggest why a narrow cleft is an advantage.

✓ Why this scores full marks: Each sentence is a separate, ordered event — calcium in, vesicles fuse, exocytosis.

A 3-mark 'Describe' of a process wants the cause→effect chain in order, not one idea written three ways. Naming Ca²⁺ and exocytosis secures the marks.