Synaptic transmission - AL only (3.6.2.2)
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A synapse is a junction where two neurones meet.
Synapses separate presynaptic and postsynaptic membranes, with a gap in between known as the synaptic cleft. Impulses cannot jump across this cleft.
The presynaptic membrane contains synaptic vesicles that contain neurotransmitters.
Cholinergic synapses are made up of several parts, including:
- Synaptic cleft – small gap (~20–30 ) between neurones.
- Presynaptic neurone – ends in a presynaptic bulb / knob that contains synaptic vesicles filled with neurotransmitter (e.g., acetylcholine, GABA).
- Postsynaptic neurone – with specific receptor proteins for neurotransmitters.
Impulse transmission at a synapse happens in the following way:
- Action potential arrives at the presynaptic bulb / knob.
- Voltage–gated channels open and calcium ions enter the presynaptic bulb / knob.
- Vesicles containing neurotransmitter fuse with the presynaptic bulb / knob membrane, releasing neurotransmitter into the synapse via exocytosis.
- Neurotransmitter moves across the synapse and fuses with receptors on the postsynaptic membrane.
- channels open, leading to depolarisation and generation of a new action potential in the postsynaptic neurone.
- If depolarisation exceeds the threshold at the postsynaptic membrane, then an action potential will be produced and propagated.
Synapses ensure the unidirectional transmission of impulses.
An impulse can be transmitted from one neurone to several, resulting in several responses arising from one stimulus.
An impulse can also be transmitted from several neurones to one, meaning that several stimuli can trigger a single coordinated response.
Excitatory synapses make the postsynaptic membrane more permeable to sodium ions, causing depolarisation.
At excitatory synapses, summation can occur:
- Spatial summation occurs when neurotransmitters released from multiple presynaptic neurones act on a single postsynaptic neurone at the same time. The combined depolarisations may reach the threshold and trigger an action potential.
- Temporal summation occurs when one presynaptic neurone releases neurotransmitter repeatedly in rapid succession. The depolarisations build up over time and, if the threshold is reached, an action potential is triggered.
Impulses arriving at inhibitory synapses open channels and channels; chloride ions will diffuse into the postsynaptic membrane and potassium ions will diffuse out.
The potential difference across the membrane is increased (hyperpolarisation), making it less likely for the membrane to reach the threshold required for depolarisation and generating an action potential.
A neuromuscular junction (NMJ) connects a neurone to a muscle fibre.
There are multiple neuromuscular junctions along a muscle to ensure rapid, strong and efficient contraction. A motor unit is all the muscle fibres associated with one motor neurone.
An action potential reaches the presynaptic bulb / knob and calcium ion channels open.
Vesicles fuse with the presynaptic membrane and release acetylcholine into the synapse by exocytosis. The acetylcholine binds to the sarcolemma causing sodium ion channels to open, leading to depolarisation.
Acetylcholine is broken down in the junction to give muscles a rest period.