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 Na⁺ influx
• 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 Cl⁻ influx through voltage-gated channels
• DAbout 0 mV, established by equal permeability to all ions

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 through 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
• CBinding of acetylcholine to presynaptic receptors
• DClosure of voltage-gated sodium channels

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 responses that amplify the original change
• BTwo factors change in the same direction simultaneously
• CA factor is kept constant by preventing any detection of change
• 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
• BBy increasing glucose uptake into cells and promoting glycogenesis in liver and muscle✓
• CBy converting glycogen to glucose in muscle cells
• DBy inhibiting the uptake of glucose into adipose tissue

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 skin arterioles and increased sweating
• BIncreased sweating and relaxation of erector pili muscles
• CVasoconstriction of skin arterioles, shivering and erection of hairs✓
• DVasodilation of skin arterioles and shivering

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.

In the human kidney, where does ultrafiltration of the blood occur, and what mainly prevents plasma proteins from entering the filtrate?

• AIn the loop of Henle; proteins are reabsorbed by active transport
• BIn the collecting duct; proteins are removed by the action of ADH
• CIn the proximal convoluted tubule; proteins are broken down by enzymes
• DIn the glomerulus and Bowman's capsule; proteins are too large to pass through the basement membrane✓

Answer: Ultrafiltration occurs at the glomerulus, driven by high hydrostatic blood pressure. The basement membrane acts as a molecular filter: small molecules pass into Bowman's capsule, but large plasma proteins are retained in the blood.