Synapses, neuromuscular transmission and drug effects

What you’ll learn

• Explain chemical transmission across a cholinergic synapse.
• Predict the consequences of modifying receptors or ion channels.
• Relate specified drug actions to nervous-system function.

Understand the idea

At a chemical synapse, an arriving action potential opens voltage-gated calcium channels in the presynaptic ending. Calcium entry triggers vesicles to fuse with the membrane and release neurotransmitter. Acetylcholine diffuses across the cleft and binds complementary receptors, opening channels that can depolarise the postsynaptic membrane. If threshold is reached, an action potential follows. Acetylcholinesterase breaks down acetylcholine, and components are recycled, allowing the signal to end.

Chemical synapses usually transmit in one direction because release machinery and receptors occupy different sides. Excitatory inputs can summate in time or space; inhibitory inputs reduce the likelihood of reaching threshold. Nicotine acts as an agonist at nicotinic acetylcholine receptors. Lidocaine blocks voltage-gated sodium channels, reducing action-potential propagation. Cobra alpha-neurotoxins can block nicotinic receptors at neuromuscular junctions, preventing normal muscle activation. These examples concern mechanisms, not advice for using any substance.

Parkinson’s disease involves loss of dopamine-producing neurones in relevant motor circuits. L-DOPA can cross the blood–brain barrier and be converted to dopamine, helping compensate for reduced signalling. MDMA alters monoamine signalling, notably increasing serotonin release and disrupting its reuptake. Serotonin pathways are implicated in mood and some medicines act on them, but depression is multifactorial and is not established by a simple measurement of “low serotonin”. Drug effects depend on site, dose and many other factors.

Worked example

Predict how blocking presynaptic voltage-gated calcium channels affects transmission even if action potentials still arrive.

1. The arriving action potential normally triggers calcium influx into the presynaptic ending.
2. With the channels blocked, the calcium signal for vesicle fusion is reduced.
3. Less neurotransmitter enters the cleft, so fewer postsynaptic receptors activate and threshold is less likely to be reached.

Answer: Chemical transmission is reduced because neurotransmitter release depends on calcium entry.