Show the model answerAttempt it first — that is what makes it stick
(a) Types of synapse 5 marks
What earns the marks5 marks
| The fundamental classification | Chemical (ionotropic or metabotropic) and electrical — state it first, in one line |
|---|---|
| Electrical | Gap junction, connexons, direct ionic current, no delay |
| Ionotropic chemical | Ligand-gated ion channel — receptor and channel are one molecule |
| Metabotropic chemical | G-protein-coupled receptor → second messenger |
| How the signal crosses | For each type: what generates the postsynaptic electrical impulse or chemical message |
| Examples | One or two per type |
Commonly lost: Many could not state the fundamental classification. Open with it before describing anything.
Commonly lost: “Interneurons” and “saltatory transmission through nodes of Ranvier” were commonly and incorrectly given as types of synapse. Neither is one.
A synapse is the junction at which a neuron communicates with its target cell — another neuron, a muscle fibre or a gland cell.
| Electrical | Chemical — ionotropic | Chemical — metabotropic | |
|---|---|---|---|
| Structure | Gap junction: six connexins form a connexon in each membrane, two connexons dock into a continuous aqueous pore | Presynaptic terminal separated from postsynaptic membrane by a cleft | Same as ionotropic |
| Cleft width | ~3 nm, with cytoplasmic continuity | 20-40 nm | 20-40 nm |
| How the signal crosses | Directly, as ionic current — no transmitter, no receptor | Ca²⁺ entry → vesicle exocytosis → transmitter binds a ligand-gated ion channel in which receptor and channel are the same molecule | Identical release, but transmitter binds a seven-transmembrane G-protein-coupled receptor |
| Postsynaptic event | Current flows straight into the next cell | Channel opens directly → ionic current | G protein acts on adenylyl cyclase, phospholipase C, or a channel at a distance → second messenger |
| Delay | Essentially none | ~1 ms | Hundreds of ms to seconds |
| Direction | Bidirectional | Unidirectional | Unidirectional |
| Examples | Cardiac muscle, single-unit smooth muscle, retina, between astrocytes; uncommon between mammalian CNS neurons | Nicotinic ACh at the neuromuscular junction; GABA-A and glycine (Cl⁻, inhibitory); AMPA and NMDA | Muscarinic ACh, adrenergic, opioid, GABA-B |
Commonly lost: Candidates comparing the neuromuscular junction with autonomic transmission often missed describing metabotropic receptors and G proteins specifically.
Synapses may also be named anatomically by site — axodendritic, axospinous, axosomatic, axo-axonic — but that is a supplementary description, not the fundamental classification.
Advantages of each type 2 marks
| Type | Advantages |
|---|---|
| Electrical | No synaptic delay — as fast as physically possible · bidirectional · synchronises populations of cells so they act as one unit · does not fatigue, since no transmitter has to be resynthesised |
| Ionotropic chemical | Fast, millisecond range · unidirectional · excitatory or inhibitory depending on the ion · can be modulated |
| Metabotropic chemical | Amplification — one occupied receptor activates many G proteins · long duration · can modulate other synapses · can alter gene expression, which is what makes plasticity and long-term potentiation possible |
Commonly lost: The advantages had to be physiological. Many discussed pharmacological ones instead.
(b) From the post-synaptic membrane to contraction 3 marks
What earns the marks3 marks
| Start at the membrane | The question begins at the postsynaptic membrane — do not re-describe ACh release |
|---|---|
| End-plate potential | Nicotinic channel opens → Na⁺ influx |
| Muscle action potential | Perijunctional membrane to threshold, propagates over the sarcolemma |
| T tubules | Conduct it into the depth of the fibre |
| DHPR → RyR | Mechanical coupling releases Ca²⁺ from the sarcoplasmic reticulum |
| Troponin C → cross-bridge | And the ATP requirement, twice |
| Step | Event | Note |
|---|---|---|
| 1 | ACh binds the two α subunits of the nicotinic receptor; cation channel opens; net Na⁺ influx gives an end-plate potential | Always suprathreshold, because of the large safety factor |
| 2 | The end-plate potential depolarises perijunctional membrane to threshold; voltage-gated Na⁺ channels open; muscle action potential propagates over the sarcolemma | — |
| 3 | Conducted into the fibre interior along the transverse (T) tubules | One of the two steps most often omitted |
| 4 | T-tubule depolarisation sensed by the dihydropyridine receptor, mechanically coupled to the ryanodine receptor of the sarcoplasmic reticulum, which opens and releases stored Ca²⁺ | Mechanical coupling — no extracellular calcium needed in skeletal muscle |
| 5 | Ca²⁺ binds troponin C, moving tropomyosin off the actin binding sites | — |
| 6 | Myosin heads bind actin and undergo the power stroke | ATP required for the power stroke and to detach the head so the cycle repeats |
| 7 | Relaxation: SERCA returns Ca²⁺ to the sarcoplasmic reticulum; acetylcholinesterase hydrolyses ACh, terminating the end-plate potential | ATP required again — which is why rigor mortis and the contracture of malignant hyperthermia occur |
Commonly lost: The two details most often omitted are the T-tubule step and the ATP requirement. Name both explicitly.