Level 3 — ProductionNervous System

Nervous System

⏱ 45 minutes60 marksprintable — key stays hidden on paper

Level 3 Examination Paper (Production)

Time Limit: 45 minutes Total Marks: 60 Instructions: Answer ALL questions. Produce full explanations and derivations from memory. Diagrams may be drawn where helpful. Show reasoning for full credit.

Question 1 — Resting Membrane Potential (12 marks)

(a) Explain from first principles how a resting membrane potential of approximately $-70\text{ mV}$ is established and maintained in a neuron. In your answer address ion distributions, membrane permeability, and the role of the Na⁺/K⁺ ATPase. (7)

(b) The equilibrium potential for a single ion is given by the Nernst equation: $E_{ion} = \frac{RT}{zF}\ln\frac{[ion]_{out}}{[ion]_{in}}$ Using $\frac{RT}{F} = 26.7\text{ mV}$ at body temperature and natural log, calculate $E_K$ for a neuron where $[K^+]_{out} = 5\text{ mM}$ and $[K^+]_{in} = 140\text{ mM}$ ( $z = +1$ ). Show all steps. (3)

Question 2 — Action Potential (11 marks)

From memory, produce a labelled account of the action potential. Your answer must:

(a) Sketch/describe the voltage trace naming the phases: resting, depolarisation, repolarisation, hyperpolarisation. (4)

(b) Explain the sequence of voltage-gated channel behaviour that drives each phase. (5)

Question 3 — Saltatory Conduction (9 marks)

(a) Explain, from scratch, what saltatory conduction is and how myelin enables it. Include the role of the nodes of Ranvier. (5)

(b) A myelinated axon conducts at $100\text{ m/s}$ ; an unmyelinated axon of the same diameter conducts at $2\text{ m/s}$ . Calculate how many times faster the myelinated axon is, and the time each takes to conduct a signal over $1\text{ m}$ . (4)

Question 4 — Synaptic Transmission (10 marks)

Explain out loud (in writing) the full sequence of chemical synaptic transmission, from arrival of an action potential at the presynaptic terminal to generation of a postsynaptic response. Your answer must include: the role of Ca²⁺, vesicle fusion, neurotransmitter diffusion, receptor binding, and at least two mechanisms of neurotransmitter removal. Distinguish an EPSP from an IPSP. (10)

Question 5 — Reflex Arc & Nervous System Organisation (10 marks)

(a) Draw and label the five components of a spinal reflex arc, using the withdrawal (pain) reflex as your example. Trace the signal in order. (5)

(b) Explain why a reflex is faster than a voluntary response, referencing neuron types and the CNS pathway involved. (3)

Question 6 — Autonomic Nervous System (8 marks)

(a) Distinguish the somatic from the autonomic nervous system in terms of targets and control. (3)

(b) Compare the sympathetic and parasympathetic divisions by producing a table of at least four contrasting effects (e.g. heart rate, pupils, digestion, airways). (5)

Answer keyMark scheme & solutions

Question 1 (12 marks)

(a) (7 marks)

High $K^+$ inside, high $Na^+$ (and $Cl^-$ ) outside the cell — concentration gradients established (1)
Membrane at rest is much more permeable to $K^+$ than $Na^+$ due to open K⁺ leak channels (2)
$K^+$ diffuses out down its gradient, leaving the inside negative (1)
Build-up of negative charge inside opposes further K⁺ exit → electrochemical equilibrium near $E_K$ (1)
Na⁺/K⁺ ATPase pumps 3 Na⁺ out and 2 K⁺ in per ATP (1) — why: maintains the gradients long-term and adds slight net negativity (electrogenic) (1)

(b) (3 marks) $E_K = 26.7 \times \ln\frac{5}{140}\text{ mV}$ (1 setup) $= 26.7 \times \ln(0.03571) = 26.7 \times (-3.332)$ (1) $= -88.9\text{ mV} \approx -89\text{ mV}$ (1)

(c) (2 marks)

Resting potential is set by ALL permeant ions, not K⁺ alone (1)
Small resting Na⁺ permeability leaks positive charge in, making resting V (–70 mV) less negative than $E_K$ (–89 mV) (1)

Question 2 (11 marks)

(a) (4 marks) — 1 mark each phase correctly named on rising/falling trace: resting (~–70 mV), depolarisation (rise to ~+30 mV), repolarisation (fall), hyperpolarisation (undershoot below –70 then recovery).

(b) (5 marks)

Stimulus depolarises membrane to threshold (~–55 mV) (1)
Voltage-gated Na⁺ channels open → rapid Na⁺ influx → depolarisation (positive feedback) (1)
Na⁺ channels inactivate at peak (~+30 mV) (1)
Voltage-gated K⁺ channels open → K⁺ efflux → repolarisation (1)
K⁺ channels slow to close → hyperpolarisation; Na⁺/K⁺ pump + leak restore resting state (1)

(c) (2 marks)

Absolute refractory period: time during which Na⁺ channels are inactivated and no second AP can be fired regardless of stimulus (1)
Ensures AP propagates in one direction only (cannot travel back into just-fired region) and limits firing frequency (1)

Question 3 (9 marks)

(a) (5 marks)

Myelin (Schwann cells/oligodendrocytes) is an insulating sheath around the axon (1)
It prevents ion flow across membrane except at gaps = nodes of Ranvier (1)
Voltage-gated Na⁺ channels concentrated at nodes (1)
AP "jumps" from node to node — regenerated only at nodes (saltatory = "leaping") (1)
This speeds conduction and saves metabolic energy (fewer ions to pump) (1)

(b) (4 marks)

Speed ratio $= 100/2 = 50$ times faster (1)
Time myelinated $= 1\text{ m} / 100\text{ m/s} = 0.01\text{ s} = 10\text{ ms}$ (1.5)
Time unmyelinated $= 1\text{ m} / 2\text{ m/s} = 0.5\text{ s} = 500\text{ ms}$ (1.5)

Question 4 (10 marks)

Award up to 10, ~1 mark per correct step:

AP reaches presynaptic terminal, depolarises it (1)
Voltage-gated Ca²⁺ channels open; Ca²⁺ enters (1)
Ca²⁺ triggers synaptic vesicles to fuse with presynaptic membrane (1)
Neurotransmitter released by exocytosis into synaptic cleft (1)
Neurotransmitter diffuses across cleft (1)
Binds to receptors on postsynaptic membrane, opening ligand-gated channels (1)
EPSP: excitatory (e.g. Na⁺ in) depolarises postsynaptic cell toward threshold; IPSP: inhibitory (e.g. Cl⁻ in/K⁺ out) hyperpolarises (2)
Removal mechanisms (any two): enzymatic breakdown (e.g. acetylcholinesterase), reuptake by transporters, diffusion away (2)

Question 5 (10 marks)

(a) (5 marks) — 1 mark each in order:

Receptor (pain receptor in skin)
Sensory neuron → dorsal root into spinal cord
Interneuron (relay) in spinal cord grey matter
Motor neuron → out via ventral root
Effector (muscle) contracts → withdrawal

(b) (3 marks)

Reflex is processed in the spinal cord, not requiring the brain (1)
Fewer synapses / shorter pathway = less delay (1)
Automatic and involuntary, so no conscious processing time (1)

(c) (2 marks)

Neuron 1 = sensory; Neuron 2 = interneuron; Neuron 3 = motor (2, all correct)

Question 6 (8 marks)

(a) (3 marks)

Somatic: controls skeletal (voluntary) muscle, conscious control, single motor neuron (2)
Autonomic: controls smooth/cardiac muscle and glands, involuntary, two-neuron pathway (1)

(b) (5 marks) — table, ~1 mark per correct contrasting pair (max 5):

Organ	Sympathetic	Parasympathetic
Heart rate	Increases	Decreases
Pupils	Dilate	Constrict
Digestion	Inhibits	Stimulates
Airways	Dilate	Constrict
Saliva	Decreases (thick)	Increases (watery)

("Fight or flight" vs "rest and digest" correctly stated = worth reinforcing.)

[
  {"claim":"E_K ≈ -88.9 mV using Nernst 26.7*ln(5/140)","code":"import sympy as sp\nval=26.7*sp.log(sp.Rational(5,140))\nresult=abs(float(val)-(-88.9))<0.5"},
  {"claim":"Speed ratio myelinated/unmyelinated = 50","code":"result=(100/2)==50"},
  {"claim":"Myelinated conduction time over 1 m = 10 ms","code":"result=abs((1/100)*1000-10)<1e-9"},
  {"claim":"Unmyelinated conduction time over 1 m = 500 ms","code":"result=abs((1/2)*1000-500)<1e-9"}
]