Impulses, synapses and receptors: exam questions

Answer: B (1) Habituation is a decrease in response to a repeated, harmless stimulus; D describes fatigue.

• (a) –70 mV (1) units required
• (b) voltage-gated Na⁺ channels open / permeability to Na⁺ increases (1)
• (b) Na⁺ diffuse into the axon down the electrochemical gradient, so the membrane depolarises / inside becomes positive (1)
• (c) Na⁺ channels close and voltage-gated K⁺ channels open / permeability to K⁺ increases (1)
• (c) K⁺ diffuse out of the axon (repolarisation); K⁺ channels slow to close so the potential falls below the resting level / hyperpolarisation (1)

• 6.0 ms = 6.0 × 10⁻³ s, speed = 0.60 ÷ 6.0 × 10⁻³ (1)
• 100 m s⁻¹ (1) (credit the second mark for the correct answer with units, or by error carried forward from a wrong conversion)

• myelin acts as an insulator / prevents ions crossing the membrane except at nodes of Ranvier (1)
• depolarisation (Na⁺ entry) only at nodes (1)
• local circuits carry the depolarisation from node to node / action potential jumps (saltatory conduction) (1)
• so fewer places have to depolarise and repolarise, which is quicker (than regenerating at every point) (1)
• Max 4. Accept less ATP is needed as fewer ions move (not a speed mark)

• action potential arrives at the presynaptic knob and opens voltage-gated Ca²⁺ channels / Ca²⁺ enter the knob (1)
• vesicles containing acetylcholine fuse with the presynaptic membrane and release ACh by exocytosis (1)
• ACh diffuses across the synaptic cleft (1)
• ACh binds to receptors on the postsynaptic membrane, opening Na⁺ channels (1)
• Na⁺ enter and depolarise the postsynaptic membrane, producing an action potential if threshold is reached (1)
• acetylcholinesterase breaks down ACh, so the response is not continuous (1)
• Max 5

• lidocaine blocks voltage-gated Na⁺ channels in the axon membrane (1)
• so no depolarisation / action potentials are not conducted to the brain, so pain is not felt (1)
• alpha toxin binds to the acetylcholine receptors on the postsynaptic membrane (1)
• so ACh cannot bind / Na⁺ channels do not open and the muscle is not depolarised, giving paralysis (1)

• receptors in the retina detect the light and send impulses (along the optic nerve) to the brain (1)
• impulses are sent along motor (parasympathetic) neurones to the iris (1)
• circular muscles contract and radial muscles relax / antagonistic muscles (1)
• pupil constricts, reducing the light that reaches the retina, protecting it from damage (1)
• Max 4

• light changes retinal from the cis to the trans form (1)
• rhodopsin splits / is bleached into opsin and retinal (1)
• cation channels / Na⁺ channels in the rod membrane close (1)
• less Na⁺ enters the rod, so the rod cell becomes hyperpolarised (1)
• less (inhibitory) transmitter released onto the bipolar neurone (1)
• bipolar neurone depolarised and releases transmitter, which depolarises the optic neurone to threshold, so action potentials form (1)
• Max 5