Nervous System & Homeostasis

What value of membrane potential is typical for the resting potential of a mammalian neurone, and which factor mainly establishes it?

• AAbout +40 mV, established mainly by a continuous influx of Na⁺ through open voltage-gated channels
• BAbout −70 mV, established mainly by the K⁺ concentration gradient and the membrane's relatively high permeability to K⁺✓
• CAbout −70 mV, established mainly by a steady influx of Cl⁻ through voltage-gated chloride channels
• DAbout 0 mV, established by equal permeability of the membrane to ions and water

Answer: The resting potential is about −70 mV (inside negative). The sodium–potassium pump sets up the gradients, but the resting value is dominated by the membrane being relatively permeable to K⁺, which leaks out down its concentration gradient.

During the depolarisation phase of an action potential, which ion movement is principally responsible for the rapid rise in membrane potential?

• ACa²⁺ moving out of the axon
• BK⁺ moving out of voltage-gated potassium channels
• CNa⁺ moving in through voltage-gated sodium channels✓
• DCl⁻ moving in through ligand-gated channels

Answer: When threshold is reached, voltage-gated sodium channels open and Na⁺ rushes into the axon down its electrochemical gradient, reversing the membrane potential to about +40 mV (depolarisation).

At a cholinergic synapse, what is the immediate trigger for the release of acetylcholine from the presynaptic neurone?

• AInflux of Ca²⁺ into the presynaptic knob through voltage-gated calcium channels✓
• BEfflux of K⁺ from the presynaptic knob through the voltage-gated potassium channels
• CBinding of acetylcholine to receptors on the presynaptic membrane
• DClosure of the voltage-gated sodium channels in the presynaptic membrane

Answer: Depolarisation of the presynaptic membrane opens voltage-gated calcium channels; the resulting Ca²⁺ influx causes synaptic vesicles to fuse with the membrane and release acetylcholine by exocytosis.

In a simple spinal reflex arc, in which order is the nerve impulse transmitted?

• ASensory neurone → motor neurone → relay (intermediate) neurone → effector
• BReceptor → motor neurone → relay neurone → sensory neurone → effector
• CReceptor → sensory neurone → relay neurone → motor neurone → effector✓
• DEffector → sensory neurone → relay neurone → motor neurone → receptor

Answer: A reflex arc runs: stimulus detected by a receptor → sensory neurone → relay (intermediate) neurone in the CNS → motor neurone → effector (muscle or gland), allowing a rapid, involuntary response.

Which statement best describes the principle of negative feedback in homeostasis?

• AA change in a factor triggers a series of responses that amplify the original change further
• BTwo separate factors change in the same direction at the same time, cancelling each other out
• CA factor is held constant because the receptors are prevented from detecting any change in it
• DA change in a factor triggers responses that reverse the change and restore the set point✓

Answer: In negative feedback, a deviation from the set point is detected by receptors and triggers corrective responses that oppose (reverse) the change, returning the factor towards its normal level.

How does insulin act to lower a raised blood glucose concentration?

• ABy stimulating glycogenolysis and gluconeogenesis in the liver, releasing glucose into the blood
• BBy increasing glucose uptake into cells and promoting glycogenesis in liver and muscle✓
• CBy converting stored glycogen back into glucose in the muscle cells
• DBy inhibiting the uptake of glucose into adipose tissue and muscle

Answer: Insulin (from beta cells of the islets of Langerhans) increases the number of glucose transporters in cell membranes, raising glucose uptake, and stimulates the conversion of glucose to glycogen (glycogenesis), lowering blood glucose.

When core body temperature falls below the set point in a mammal, which combination of responses is coordinated by the hypothalamus to conserve and generate heat?

• AVasodilation of the skin arterioles, increased sweating and flattening of the hairs
• BIncreased sweating and relaxation of the erector pili muscles to flatten the hairs
• CVasoconstriction of skin arterioles, shivering and erection of hairs✓
• DVasodilation of the skin arterioles together with shivering of the skeletal muscles

Answer: On cooling, the hypothalamus triggers vasoconstriction of skin arterioles (reducing heat loss), shivering (muscle contraction generating heat) and contraction of erector pili muscles to raise hairs and trap insulating air.

What maintains the resting potential of a neurone?

• ASodium-potassium pumps and equal membrane permeability
• BCalcium pumps and differential membrane permeability
• CDiffusion alone, without any active transport involved
• DSodium-potassium pumps and differential membrane permeability✓

Answer: The pump moves three sodium ions out for every two potassium ions in, and the membrane is far more permeable to potassium leaking back out. Together they leave the inside about −70 mV relative to the outside.

What causes depolarisation during an action potential?

• AVoltage-gated potassium channels open and potassium enters
• BVoltage-gated sodium channels open and sodium leaves
• CVoltage-gated potassium channels open and potassium leaves
• DVoltage-gated sodium channels open and sodium enters✓

Answer: Reaching threshold opens sodium channels, and sodium rushes in down its electrochemical gradient, reversing the potential to about +40 mV. Potassium leaving afterwards causes repolarisation.

What is the function of the refractory period?

• AEnsuring impulses travel in one direction only✓
• BEnsuring impulses travel in both directions equally
• CIncreasing the speed at which the impulse travels
• DIncreasing the size of the action potential produced

Answer: Sodium channels are inactivated immediately after an action potential, so the region behind cannot fire again straight away. That enforces one-way travel and also sets an upper limit on impulse frequency.

Why does myelination increase conduction speed?

• ADepolarisation occurs continuously along the whole axon
• BDepolarisation jumps between the nodes of Ranvier✓
• CMyelin increases the diameter of the axon considerably
• DMyelin allows sodium ions to cross the sheath rapidly

Answer: Myelin insulates the axon so ion movement occurs only at the nodes, and the impulse jumps node to node — saltatory conduction. Continuous conduction along an unmyelinated axon is much slower.

What happens at a cholinergic synapse when an impulse arrives?

• ACalcium leaves and vesicles release acetylcholine
• BCalcium enters and vesicles release acetylcholine✓
• CSodium enters and vesicles release acetylcholinesterase
• DPotassium enters and vesicles absorb acetylcholine

Answer: Depolarisation opens voltage-gated calcium channels; calcium influx makes vesicles fuse with the presynaptic membrane and release acetylcholine into the cleft. Acetylcholinesterase then breaks it down to stop continuous stimulation.

Which of these is an example of positive rather than negative feedback?

• AThe surge in oxytocin that intensifies contractions during labour✓
• BThe release of sweat when core body temperature rises above normal
• CThe release of glucagon when blood glucose concentration falls low
• DThe narrowing of arterioles in the skin when the body becomes cold

Answer: Positive feedback amplifies the original change rather than reversing it: oxytocin strengthens contractions, which trigger the release of more oxytocin, until birth ends the cycle. The other three all act to restore a set point, which is negative feedback.

Why does an impulse travel in one direction only across a cholinergic synapse?

• AOnly the presynaptic neurone holds vesicles of neurotransmitter✓
• BOnly the postsynaptic membrane depolarises during an impulse
• CAcetylcholinesterase is found solely in the presynaptic neurone
• DThe synaptic cleft is far too wide for diffusion to run backwards

Answer: Vesicles of acetylcholine are confined to the presynaptic knob and the receptors to the postsynaptic membrane, so transmission can only run one way. Acetylcholinesterase sits in the cleft and hydrolyses acetylcholine so the response does not continue indefinitely.

Which hormone raises blood glucose when it falls too low?

• AInsulin, released from the alpha cells
• BGlucagon, released from the alpha cells✓
• CGlucagon, released from the beta cells
• DInsulin, released from the beta cells

Answer: Alpha cells in the islets of Langerhans release glucagon, which promotes glycogenolysis and gluconeogenesis in the liver. Beta cells release insulin, which lowers blood glucose — the two cell types and two hormones are easy to swap.

Why is the second messenger model relevant to glucagon?

• AGlucagon binds a receptor and cyclic AMP relays the signal inside✓
• BGlucagon binds a receptor and enters the cell to act directly
• CGlucagon crosses the membrane and binds DNA in the nucleus
• DGlucagon is broken down into cyclic AMP inside the cell

Answer: Being protein-based, glucagon cannot cross the membrane. It binds a surface receptor, activating adenylate cyclase which makes cyclic AMP; that activates enzymes inside the cell. Steroid hormones such as oestrogen do enter and act on DNA.