Inter-system communication practice questions.

A footballer waiting for a penalty shoot-out has both direct sympathetic nerve supply to the heart and circulating adrenaline from the adrenal medulla. What does the hormonal route add that the nerve supply cannot?

• A faster onset at the sinoatrial node
• A larger change in the force of each beat
• An effect on every tissue carrying the receptor
• A signal that can be switched off within one beat

Answer: An effect on every tissue carrying the receptor

A nerve fibre delivers its message only where it ends, whereas a hormone is carried to every tissue and acts wherever a matching receptor exists, so liver, adipose tissue and airways respond alongside the heart. The trade-off is speed and precision: the hormonal signal is slower to start and slower to clear.

A weightlifter grinds out a maximal deadlift and the tension in the patellar tendon rises very sharply. Which receptors detect that tension?

• Baroreceptors in the wall of the nearby femoral artery
• Muscle spindles lying between the fibres
• Golgi tendon organs at the muscle-tendon junction
• Free nerve endings within the joint capsule

Answer: Golgi tendon organs at the muscle-tendon junction

Golgi tendon organs are in series with the fibres and respond to force, whereas muscle spindles lie in parallel with the fibres and respond to length and rate of stretch. Confusing the two reverses the reflex: one inhibits the muscle that is pulling, the other excites it.

A trampolinist completes a somersault with her eyes closed and still knows which way up she is. Which source of information makes this possible?

• The vestibular apparatus of the inner ear
• The muscle spindles of the trunk extensors
• The Golgi tendon organs of the calf muscles
• The pressure receptors in the soles of the feet

Answer: The vestibular apparatus of the inner ear

The semicircular canals detect rotation and the otolith organs detect linear acceleration and the direction of gravity, so orientation is available with no vision and no ground contact. Receptors in muscle and skin report where the limbs are relative to the body, which is a different question from where the body is in space.

Within the first two breaths of a rower starting a maximal 2000 m piece, ventilation has already risen sharply, before any change in arterial blood chemistry can be measured. Which input accounts for this immediate rise?

• Feed-forward drive from the motor cortex and input from limb mechanoreceptors
• A rise in arterial hydrogen ion concentration detected by the peripheral chemoreceptors of the carotid bodies
• A fall in cerebrospinal fluid pH detected by the central chemoreceptors in the medulla oblongata
• Adrenaline from the adrenal medulla acting on the smooth muscle of the airways

Answer: Feed-forward drive from the motor cortex and input from limb mechanoreceptors

The drive at the very onset of exercise is neural and anticipatory: the same cortical outflow that recruits the muscles also drives the respiratory centre, and joint and muscle mechanoreceptors add to it. Chemical drives follow later, because metabolites need time to reach the receptors, so blood chemistry is a consequence of the effort rather than its trigger.

A student writes: 'When a hockey player stands up quickly from the substitutes' bench, arterial blood pressure falls, the baroreceptors in the carotid sinus fire more frequently, and heart rate rises.' Identify the error in this account.

• Heart rate falls rather than rises when arterial blood pressure drops on standing up
• The baroreceptors fire less often, because a lower pressure stretches them less
• Baroreceptors are found only in the aortic arch and not in the wall of the carotid sinus
• Arterial blood pressure rises on standing, because gravity raises the pressure in the upper body

Answer: The baroreceptors fire less often, because a lower pressure stretches them less

Baroreceptors are stretch receptors, so their firing rate tracks the degree of stretch: less pressure means fewer impulses to the cardiovascular centre, which then withdraws vagal activity and raises heart rate. The rest of the student's chain is sound; only the direction of the receptor signal is inverted.

A student accounts for the extra force a rower produces during a maximal stroke by writing that 'the nervous system sends larger action potentials along each motor neuron'. Which correction is needed?

• Action potentials travel more slowly along the large motor neurons that supply the fast fibres
• The size of an action potential falls as it travels, so distant fibres receive a weaker signal
• Action potentials are all-or-none, so force is graded by recruitment and firing rate
• Force is set by the quantity of acetylcholine stored in the axon terminal rather than by the impulse

Answer: Action potentials are all-or-none, so force is graded by recruitment and firing rate

An action potential either occurs at full size or not at all, so the nervous system cannot make a single impulse bigger. Gradation comes from how many motor units are active and how rapidly each is fired, which is why an untrained lifter with the same muscle mass can produce less force.