Arrival of Action Potential: When an impulse reaches the presynaptic membrane, it causes depolarization, which triggers the opening of voltage-gated calcium () channels.
Calcium Influx: ions diffuse into the synaptic knob down their electrochemical gradient. This influx is the signal that causes synaptic vesicles to move toward and fuse with the presynaptic membrane.
Exocytosis and Diffusion: Neurotransmitters like Acetylcholine (ACh) are released into the cleft. They diffuse across the gap, a process that creates a short delay in nerve transmission.
Receptor Binding: ACh molecules bind to specific receptor sites on sodium ion channels in the postsynaptic membrane. This binding causes the channels to open, allowing ions to diffuse into the postsynaptic cell.
Depolarization and Hydrolysis: The influx of causes a generator potential. If this reaches the threshold, a new action potential is initiated. To prevent continuous stimulation, the enzyme Acetylcholinesterase (AChE) breaks down ACh into choline and ethanoic acid.
Mechanism of Inhibition: Unlike excitatory synapses that depolarize the membrane, inhibitory synapses make the postsynaptic membrane more negative, a state known as hyperpolarization.
Ion Movement: Inhibitory neurotransmitters bind to receptors that open chloride () channels (allowing negative ions in) or potassium () channels (allowing positive ions out). Both actions move the membrane potential further away from the threshold.
Signal Cancellation: Inhibitory inputs can cancel out excitatory inputs. This allows the nervous system to coordinate complex behaviors, such as relaxing one muscle while contracting another (antagonistic pairs).
Structural Differences: While both use chemical transmission, a neuromuscular junction (NMJ) connects a motor neurone to a muscle fiber (sarcolemma), whereas a cholinergic synapse connects two neurones.
Functional Comparison Table:
| Feature | Cholinergic Synapse | Neuromuscular Junction |
|---|---|---|
| Postsynaptic Cell | Another neurone | Muscle fiber |
| Membrane Structure | Smooth postsynaptic membrane | Folded membrane (sarcolemma) |
| T-tubules | Absent | Present to carry signal deep into muscle |
| Response Type | Can be excitatory or inhibitory | Always excitatory in skeletal muscle |
| Outcome | New action potential initiated | Muscle contraction occurs |
Precise Terminology: Always use the term "diffuse" when describing the movement of ions or neurotransmitters across the cleft. Never say the impulse "jumps" across the gap; the electrical signal stops and a chemical signal takes over.
Sequence of Events: When describing the mechanism, ensure you mention channels opening before vesicle fusion, and channels opening after neurotransmitter binding. Mixing these up is a common error.
Unidirectionality: If asked why impulses only travel one way, explain that vesicles are only found in the presynaptic neurone and receptors are only found on the postsynaptic membrane.
Enzyme Importance: Always mention the role of Acetylcholinesterase. If it were inhibited, the postsynaptic membrane would remain permanently depolarized, leading to continuous firing or muscle spasms.
