Synapses and Neuromuscular Junctions

An action potential arriving at a presynaptic terminal opens voltage-gated calcium channels. Calcium enters and promotes fusion of transmitter-containing vesicles with the presynaptic membrane. Acetylcholine is released by exocytosis, diffuses across the cleft and binds to receptors on the postsynaptic membrane.

At an excitatory cholinergic synapse, receptor activation increases suitable cation permeability, including sodium entry, producing depolarisation. If the combined response reaches threshold at the relevant region, a postsynaptic action potential is generated. Presynaptic calcium entry and postsynaptic sodium entry have different roles.

Acetylcholinesterase hydrolyses acetylcholine, ending continued receptor stimulation; components such as choline can be taken up and reused. The arrangement of vesicles and receptors makes ordinary chemical transmission functionally unidirectional at the junction.

Temporal summation combines closely spaced inputs from the same pathway, while spatial summation combines inputs from different presynaptic neurones. Inhibitory synapses can increase chloride entry or potassium exit, making threshold less easily reached. An excitatory postsynaptic potential is graded and need not trigger an action potential on its own.

At a neuromuscular junction, a motor neurone releases acetylcholine onto a muscle fibre’s specialised motor end plate. Normal transmission is excitatory and reliably initiates muscle electrical activity, leading through the muscle membrane system to calcium release and contraction. A neurone-to-neurone synapse may instead integrate many weaker excitatory and inhibitory inputs.

A drug could affect synthesis, vesicle release, receptors, transmitter breakdown or ion channels. Predict effects from the information provided rather than memorising every drug name. For example, inhibiting acetylcholinesterase can prolong transmitter action, but the eventual tissue response depends on dose, duration and the system being described.

Try each question before opening the worked response. Use precise biological terms, connect each cause to its effect, and check units and assumptions when interpreting data.