The events at the presynaptic membrane turn an electrical signal (the action potential) into a chemical one (the release of neurotransmitter). At a cholinergic synapse:
1. An action potential arrives at the synaptic knob and depolarises the presynaptic membrane.2. The depolarisation opens voltage-gated calcium ion channels in the presynaptic membrane.3. Calcium ions (Ca2+) are much more concentrated outside the knob, so they diffuse into the synaptic knob.4. The Ca2+ ions cause synaptic vesicles to move towards the presynaptic membrane and fuse with it.5. The vesicles release acetylcholine (ACh) into the synaptic cleft by exocytosis. Each vesicle contains thousands of ACh molecules.The Ca2+ is then actively pumped back out of the knob, so release stops when action potentials stop arriving. The more often action potentials arrive, the more Ca2+ enters and the more neurotransmitter is released.The synaptic knob contains many mitochondria. These provide ATP for:• resynthesising acetylcholine
• moving vesicles and exocytosis
• pumping Ca2+ back out of the knobAfter ACh has been broken down in the cleft, choline is taken back into the synaptic knob across the presynaptic membrane and combined with acetyl groups to make new ACh, which is packaged into vesicles. The vesicle membrane added to the presynaptic membrane during exocytosis is recycled to form new vesicles.If action potentials arrive very frequently for a long time, the knob can run out of vesicles ready for release, and the synapse becomes fatigued.
1. An action potential arrives at the synaptic knob and depolarises the presynaptic membrane.2. The depolarisation opens voltage-gated calcium ion channels in the presynaptic membrane.3. Calcium ions (Ca2+) are much more concentrated outside the knob, so they diffuse into the synaptic knob.4. The Ca2+ ions cause synaptic vesicles to move towards the presynaptic membrane and fuse with it.5. The vesicles release acetylcholine (ACh) into the synaptic cleft by exocytosis. Each vesicle contains thousands of ACh molecules.The Ca2+ is then actively pumped back out of the knob, so release stops when action potentials stop arriving. The more often action potentials arrive, the more Ca2+ enters and the more neurotransmitter is released.The synaptic knob contains many mitochondria. These provide ATP for:• resynthesising acetylcholine• moving vesicles and exocytosis
• pumping Ca2+ back out of the knobAfter ACh has been broken down in the cleft, choline is taken back into the synaptic knob across the presynaptic membrane and combined with acetyl groups to make new ACh, which is packaged into vesicles. The vesicle membrane added to the presynaptic membrane during exocytosis is recycled to form new vesicles.If action potentials arrive very frequently for a long time, the knob can run out of vesicles ready for release, and the synapse becomes fatigued.