Nervous System

Question 1 (12 marks)

(a) TTX effect on action potential (3)

• TTX blocks voltage-gated Na⁺ channels (1).
• The rising/depolarising phase of the action potential depends on Na⁺ rushing INTO the cell through these channels (1).
• With channels blocked, no depolarisation to threshold can occur → no action potential can be generated (1).

(b) Evaluating the claim (4)

• The claim is incorrect / only partly true (1).
• TTX acts immediately — as soon as Na⁺ channels are blocked, the next action potential fails (1).
• Ouabain-X blocks the Na⁺/K⁺ pump, which maintains the ion gradients but is NOT directly needed for a single action potential. Existing gradients persist for a while (1).
• So action potentials continue firing initially and only fade gradually as the gradients run down (Na⁺ builds inside, K⁺ leaks out) — a slow/delayed effect, unlike TTX's instant block (1).

(c) Partial TTX + slower conduction (3)

• In saltatory conduction the action potential regenerates only at nodes of Ranvier, "jumping" node to node (1).
• Partial TTX blocks some Na⁺ channels at nodes, so fewer are available; depolarisation to threshold at each node is harder/slower (1).
• Signals that still reach threshold propagate, but with reduced safety margin the conduction is delayed/slower overall (1).

(d) Rising phase vs resting potential (2)

• The rising phase is caused specifically by Na⁺ influx through voltage-gated channels — blocking them removes this (1).
• The resting membrane potential is set mainly by K⁺ leak channels and the Na⁺/K⁺ pump, not the voltage-gated Na⁺ channels, so resting potential is largely unaffected (1).

Question 2 (10 marks)

(a) Five components in order (5) — 1 mark each, correct order required:

1. Receptor (pain/nociceptor in skin)
2. Sensory (afferent) neuron
3. Interneuron (relay neuron, in spinal cord)
4. Motor (efferent) neuron
5. Effector (muscle in the leg)

(b) Withdrawal before pain (3)

• The reflex arc is processed at the level of the spinal cord (interneuron), giving a short, fast pathway to the effector (1).
• The conscious pain sensation requires signals to travel further up to the brain (cerebral cortex) (1).
• The shorter spinal pathway completes first, so withdrawal occurs before the brain registers pain (1).

(c) Damaged component (2)

• The sensory (afferent) neuron / receptor is most likely damaged (1).
• Justification: voluntary movement is intact → motor neuron and effector work; but the reflex fails → the incoming sensory signal is not reaching the cord, so the sensory limb is the fault (1). (Full marks also for "receptor or sensory neuron" with correct justification.)

Question 3 (10 marks)

(a) Bear — sympathetic (4)

• Division: Sympathetic nervous system (1).
• Any THREE valid survival changes (1 each): increased heart rate; dilation of pupils; bronchodilation/increased breathing; release of glucose / stimulation of adrenal medulla (adrenaline); dilation of blood vessels to skeletal muscle; inhibition of digestion; sweating.

(b) Lunch — parasympathetic (3)

• Division: Parasympathetic nervous system (1).
• Any TWO valid changes (1 each): decreased heart rate; increased digestive activity / gut motility; increased salivation; pupil constriction; increased secretion of digestive juices.

(c) Antagonism (3)

• They are antagonistic because they generally produce opposite effects on the same organs, maintaining balance/homeostasis (1).
• Example organ + opposite effects (2): e.g. the heart — sympathetic increases heart rate, parasympathetic decreases it. (Pupil, gut, lungs also acceptable.)

Question 4 (10 marks)

(a) Two speed features (4)

• Myelin sheath / insulation: reduces charge leakage and allows saltatory conduction, greatly speeding transmission (2).
• Larger axon diameter: lower internal resistance to current flow → faster propagation (2). (Either point fully explained = 2 marks each.)

(b) Gaps structure (3)

• The structure is the node of Ranvier (gaps in the myelin) (1).
• The action potential regenerates only at the nodes because that's where voltage-gated Na⁺ channels are concentrated (1).
• The signal effectively "jumps" from node to node (saltatory conduction), skipping the insulated regions and increasing speed (1).

(c) Chemical transmission sequence (3) — award 1 mark per correct stage (max 3):

• Action potential reaches axon terminal → voltage-gated Ca²⁺ channels open, Ca²⁺ enters (1).
• Synaptic vesicles fuse with membrane and release neurotransmitter into the synaptic cleft by exocytosis (1).
• Neurotransmitter diffuses across cleft and binds receptors on the postsynaptic membrane → opens ion channels → new potential in next neuron (1).

Question 5 (8 marks)

(a) Focusing structure (3)

• The lens is chiefly responsible (accommodation) (1).
• For a near object the ciliary muscles contract (1), suspensory ligaments slacken, and the lens becomes more rounded/thicker (more convex) to increase refraction (1).

(b) Photoreceptors (3)

• Rods: function in dim light / detect brightness (monochrome, high sensitivity) (1½).
• Cones: function in bright light and detect colour / high acuity (1½).

(c) Transducer role (2)

• A sensory receptor converts (transduces) a stimulus (light energy) into an electrical nerve impulse (1).
• This impulse travels along the sensory (optic) neuron to the brain for interpretation (1).

[
  {"claim":"Reflex arc has exactly 5 ordered components: receptor, sensory neuron, interneuron, motor neuron, effector","code":"components=['receptor','sensory neuron','interneuron','motor neuron','effector']; result = (len(components)==5)"},
  {"claim":"Q3(a) requires 3 survival changes worth 1 mark each plus 1 for naming division = 4 marks","code":"marks = 1 + 3*1; result = (marks==4)"},
  {"claim":"Q4 total marks sum to 10","code":"result = (4+3+3)==10"},
  {"claim":"Whole paper totals 50 marks","code":"result = (12+10+10+10+8)==50"}
]