C2.2 Neural signalling. Practice questions with markscheme.

• A: radioactivity inside rises in step with the number of impulses, by about 0.2 counts per minute for every impulse (400 per 2 000 impulses) above the 120 of the resting axon, so each impulse carries a similar quantity of sodium ions across the membrane into the axon: the impulse is electrical because it is a movement of positively charged ions; A and C: the axon at rest gains far less radioactivity than the axons carrying impulses; B: the gain per impulse stays constant;

• D: Ca²⁺ influx triggers exocytosis of ACh; ACh diffuses across the cleft, opens ligand-gated (Na⁺) channels, and is then broken down by acetylcholinesterase;

• C: synapses are junctions between neurons and between neurons and effectors (muscle, gland);

• A: the presynaptic action potential is the same size in every solution, but less neurotransmitter is released as the Ca²⁺ concentration falls and none without it, so depolarization of the presynaptic membrane only leads to release when calcium ions can enter the neuron, where they act as a signal for exocytosis; the postsynaptic depolarization falls in step with the release; A and D are ruled out by the constant action potential, B by the zero release at 0 mmol dm⁻³;

• B: one synapse between the first and the second neuron, and one between the second neuron and the gland cells, which are the effector; synapses are junctions between neurons and also between neurons and effector cells, so 1 misses the synapse with the gland; the ganglion is where the first synapse is, not an extra one;

• D: clearing the transmitter ends each signal; without it the postsynaptic membrane stays depolarized/blocked (as with nerve agents and some insecticides);

• B: the pump uses ATP to move 3 Na⁺ out per 2 K⁺ in; with greater K⁺ permeability and impermeant anions inside, this keeps the inside negative, about −70 mV;

• C: neurons generate and conduct nerve impulses. Electrical signals conveyed along the (axon) membrane; nervous tissue also contains other cells (glia);

• sodium–potassium pumps move 3 Na⁺ out for every 2 K⁺ in, using ATP;
• (so) positive ions accumulate outside / concentration gradients are established;
• the membrane is more permeable to K⁺ (leak channels), which diffuses out, leaving the inside negative;
• (large) negatively charged proteins/anions remain inside, resting potential ≈ −70 mV, OWTTE;

• R² = 0.74 means that 74 % of the variation in conduction speed is accounted for by variation in axon diameter;
• the remaining 26 % of the variation is due to other factors, for example whether the fibre is myelinated, or temperature;
• the positive value of r shows the correlation is positive: axons of larger diameter conduct faster;
• R² measures how closely the points lie to the line of best fit, not how steep that line is;
• a high R² does not establish that a greater diameter causes the higher speed, since correlation alone cannot show causation;

The mark is for what the unexplained 26 % implies, not for restating the trend.

• the cell body contains the nucleus (and most organelles);
• dendrites (highly branched projections) receive signals from receptors / other neurons;
• the axon is an elongated (nerve) fibre that conducts the impulse over long distances / to other cells;
• impulses are electrical signals conveyed along the plasma membrane (as ion movements / depolarizations);

• cell body drawn as a distinct region containing a labelled nucleus (and cytoplasm);
• several short, branched fibres labelled as dendrites projecting from the cell body;
• a single long fibre labelled as the axon, clearly longer than the dendrites, ending in axon terminals / terminal branches;
• direction of impulse conduction shown by an arrow from the dendrites / cell body along the axon to the terminals;
• myelin sheath (Schwann cells) and nodes of Ranvier labelled along the axon;

Accept a diagram with or without a myelin sheath. Do not award the axon point if the axon is drawn as several fibres or is no longer than the dendrites.

• D: membrane potential is the charge inside relative to outside, so +30 mV means the polarity has reversed; only a tiny fraction of the ions cross, so Na⁺ stays more concentrated outside and most K⁺ stays inside; the pump never reverses;

• A: the pump uses ATP to move 3 Na⁺ out per 2 K⁺ in; with greater K⁺ permeability and impermeant anions inside, this keeps the inside negative, about −70 mV;

• B: one synapse between the first and the second neuron, and one between the second neuron and the gland cells, which are the effector; synapses are junctions between neurons and also between neurons and effector cells, so 1 misses the synapse with the gland; the ganglion is where the first synapse is, not an extra one;

• C: capsaicin opens the heat-sensitive (TRPV1) channels of nociceptor endings, letting positive ions in and depolarizing them to threshold, so the brain receives genuine "heat" signals without a temperature change;

• (a) [2]
  • X;
  • permeability to X rises first / at the same time as the rapid rise in membrane potential, and the entry of Na⁺ causes depolarization, OWTTE;

  Award the reason only if X is identified.

• (b) [2]
  • −55 mV is the threshold potential;
  • the sodium channels are voltage-gated and open only when the membrane is depolarized to the threshold;
  • sodium ions entering depolarize the membrane further, which opens more sodium channels (positive feedback);
• (c) [2]
  • voltage-gated potassium channels open (in response to the depolarization, after a delay);
  • K⁺ diffuses out of the axon down its concentration gradient;
  • the loss of positive charge makes the inside negative again / repolarizes the membrane (from +30 mV towards −70 mV);
  • permeability to Y is still raised when the potential reaches −70 mV / the potassium channels close slowly, so the potential falls below the resting potential (to about −78 mV) / the membrane is hyperpolarized;

• (a) [3]
  • 1: the inhibitory neurotransmitter binds to its own (specific) receptors, not to acetylcholine receptors / it does not act by blocking the entry of sodium ions;
  • 1: its receptors open channels that let chloride ions in / potassium ions out;
  • 2: the membrane is hyperpolarized / becomes more negative than the resting potential (an inhibitory postsynaptic potential);
  • 3: inhibitory and excitatory neurotransmitters are released at different synapses / by different presynaptic neurons, and the postsynaptic potentials from all of them are summed, so an inhibitory synapse reduces the effect of excitatory inputs at other synapses;

  Award [1] for each statement corrected, using any one of the points for that statement. Do not award a mark for saying a statement is wrong without the correct biology.

• (b) [3]
  • the excitatory inputs must total at least 20 mV (15 + 5) to outweigh the inhibition;
  • 4 (4 × 6 = 24 mV, and 24 − 5 = 19 mV reaches the threshold, whereas 3 give only 18 − 5 = 13 mV);
  • statement 4 is wrong: the neuron can still fire while it receives inhibitory input, if enough excitatory inputs arrive together / in quick succession;
  • whether the neuron fires depends on the net / summed effect of all its inputs, not on whether an inhibitory input has arrived, OWTTE;

  Award [1] for 4 with no working. Award [2 max] from the first two points and [2 max] from the last two.

• both are produced when a neurotransmitter binds to receptors on the postsynaptic membrane (and opens ion channels);
• both are summed in the postsynaptic neuron to determine whether threshold is reached;
• EPSP: channels open that let Na⁺ in, whereas IPSP: channels open that let Cl⁻ in / K⁺ out;
• EPSP: the membrane is depolarized / becomes less negative, whereas IPSP: it is hyperpolarized / becomes more negative than the resting potential;
• EPSP moves the potential closer to threshold / makes an action potential more likely, whereas IPSP moves it further from threshold / less likely, OWTTE;

Each point must be an explicitly paired statement. Do not accept 'depolarization' for an IPSP. Accept acetylcholine (EPSP) and GABA (IPSP) as examples, but not for a mark alone.

• action potentials are all-or-nothing / of fixed size, so amplitude carries no information;
• stronger stimuli produce a higher frequency of impulses (and recruit more neurons);
• the brain interprets frequency (and the number/identity of active neurons) as intensity, OWTTE;