Synaptic transmissionAQA A-Level Biology: Revision notes

A nerve impulse is an electrical signal and cannot cross the gap between two cells. At a synapse the presynaptic neurone releases a neurotransmitter, a chemical messenger, directly onto its target cell. Because the transmitter is released onto the target and not into the blood, the response is specific to the target cell, rapid, short-lived and localised.

Contrast this with hormones, which travel in the blood to many target cells and give slower, longer-lasting, widespread responses.

A cholinergic synapse uses acetylcholine. Its parts are:

• Presynaptic knob: many mitochondria (ATP for synthesising and recycling transmitter), vesicles containing acetylcholine, and voltage-gated calcium ion channels in its membrane
• Synaptic cleft: a gap of about 20 nm containing acetylcholinesterase
• Postsynaptic membrane: receptors for acetylcholine linked to sodium ion channels

A neuromuscular junction is a synapse between a motor neurone and a muscle fibre. The postsynaptic (muscle) membrane is folded into clefts, giving a larger area with more receptors, and the clefts contain acetylcholinesterase.

1. An action potential arrives at the presynaptic knob, so voltage-gated calcium ion channels open.
2. Calcium ions diffuse into the knob.
3. They cause vesicles to fuse with the presynaptic membrane, releasing acetylcholine by exocytosis.
4. Acetylcholine diffuses across the cleft and binds to receptors on the postsynaptic membrane.
5. Sodium ion channels open, sodium ions enter and the membrane depolarises. If threshold is reached, an action potential is generated.
6. Acetylcholinesterase hydrolyses acetylcholine into acetate and choline, which are reabsorbed and used to resynthesise acetylcholine using ATP from mitochondria. This stops the response and prevents continuous stimulation.

Transmission is unidirectional because acetylcholine is released only from the presynaptic neurone and receptors are only on the postsynaptic membrane.

One vesicle release often causes a depolarisation that is below threshold. Two types of summation add these together:

• Temporal summation: one presynaptic neurone releases transmitter repeatedly in quick succession, so the depolarisations add up
• Spatial summation: several different presynaptic neurones release transmitter at once onto the same postsynaptic neurone

Example: resting −70 mV, threshold −55 mV, each release +5 mV. Four releases give −70 + 20 = −50 mV, above threshold, so an action potential starts. A single release gives only −65 mV.

At an inhibitory synapse the neurotransmitter binds to receptors and opens chloride ion channels (and often potassium ion channels). Chloride ions move in and potassium ions move out, so the postsynaptic membrane becomes more negative than its resting potential. This is hyperpolarisation.

A larger depolarisation is then needed to reach threshold, so an action potential is less likely. Inhibition lets the nervous system control and fine-tune responses by cancelling out excitatory input.

Similarities: both use acetylcholine, calcium-dependent exocytosis, diffusion across the cleft, receptors linked to sodium channels, and acetylcholinesterase.

Differences:

• Postsynaptic cell: a neurone at a synapse, a muscle fibre at a neuromuscular junction
• Neuromuscular junction has a folded membrane with more receptors
• A neuromuscular junction is always excitatory and ends in contraction; a synapse may be excitatory or inhibitory and may need summation

Predicting drug effects: work through the sequence and find the step affected. A drug that mimics acetylcholine and blocks receptors stops depolarisation. A drug that blocks calcium channels stops release. A drug that inhibits acetylcholinesterase leaves acetylcholine in the cleft, so the response is prolonged. You are not asked to recall drug names.