A comparison of transmission across a cholinergic synapse and across a…

The idea

Both use the same mechanism: an action potential opens calcium ion channels, vesicles release acetylcholine by exocytosis, the neurotransmitter diffuses across a cleft and binds to receptors that open sodium ion channels, and acetylcholinesterase hydrolyses it afterwards. The differences lie in what sits on the other side. A neuromuscular junction occurs only between a motor neurone and a muscle fibre and is always excitatory, with no inhibitory version, and a single action potential arriving is normally enough to make the muscle fibre contract because the folded sarcolemma of the motor end plate carries far more receptors. A cholinergic synapse between neurones may be excitatory or inhibitory, usually needs summation to reach threshold, and the postsynaptic neurone may receive input from many presynaptic neurones and pass impulses on to several others.

What earns the marks

• A shared feature, such as acetylcholine released by exocytosis and hydrolysed by acetylcholinesterase at both.
• The neuromuscular junction occurring only between a motor neurone and a muscle fibre, whereas a cholinergic synapse joins two neurones.
• The neuromuscular junction being only excitatory, whereas a cholinergic synapse may be excitatory or inhibitory.
• The motor end plate carrying more receptors, so one action potential is usually enough without summation.
• The different end points: contraction of a muscle fibre against an action potential in the next neurone.

Common mistakes and how to put them right

• Listing as differences features that are in fact the same at both, such as the use of acetylcholine.

• Saying an impulse passes into the muscle, when what crosses is neurotransmitter and what follows is depolarisation of the sarcolemma.

• Claiming a neuromuscular junction can be inhibitory.

• Comparing structures without naming the sarcolemma or the motor end plate.

• Answering a comparison question with two separate descriptions and no comparative words.

Exam tips

• Use comparative language in every line: whereas, but, compared with.
• If a table is provided, fill in both columns of every row; a row with one side blank scores nothing.
• Keep similarities and differences in separate paragraphs so the examiner can find each one.