The detailed structure of a synapse and of a neuromuscular junction.

The idea

A synapse is the junction between the synaptic knob at the end of one neurone and the membrane of the next cell. The presynaptic knob contains many mitochondria, supplying ATP for making and packaging neurotransmitter, a large amount of smooth endoplasmic reticulum, and vesicles filled with acetylcholine. Its membrane holds voltage-gated calcium ion channels. The synaptic cleft is a gap of about 20 nm. The postsynaptic membrane carries specific receptor proteins whose binding sites are complementary in shape to acetylcholine and which form part of sodium ion channels, and acetylcholinesterase in the cleft hydrolyses acetylcholine into choline and ethanoic acid. A neuromuscular junction has the same basic components but its postsynaptic membrane is the folded sarcolemma of a muscle fibre, forming the motor end plate, and it is the muscle fibre that is excited to contract.

What earns the marks

• Vesicles containing acetylcholine and numerous mitochondria in the presynaptic knob.
• Calcium ion channels in the presynaptic membrane.
• The synaptic cleft as a gap of about 20 nm between the two membranes.
• Receptors on the postsynaptic membrane that are specific or complementary to the neurotransmitter and linked to sodium ion channels.
• The neuromuscular junction having a folded sarcolemma, the motor end plate, with acetylcholinesterase present in the cleft.

Common mistakes and how to put them right

• Drawing the two membranes in contact, leaving no cleft for the neurotransmitter to diffuse across.

• Putting receptors on the presynaptic membrane as well, which would destroy unidirectionality.

• Describing the neurotransmitter as crossing the cleft by active transport rather than by diffusion.

• Calling a synapse a gap between two nerves instead of between two neurones, or between a neurone and a muscle fibre.

• Omitting the mitochondria, and so being unable to explain where the ATP for resynthesising acetylcholine comes from.

Exam tips

• Learn the structure as a checklist that maps onto the transmission sequence; every structure you label has a job in the next question.
• Synaptic transmission takes time, so if a question asks why a measured response was slower than predicted, synaptic transmission or transmission at a neuromuscular junction earns a mark.
• State that receptors are only on the postsynaptic membrane; that single fact answers most unidirectionality questions.