Nervous system

Receptors are responsible for detecting a stimulus and transmitting information; they do not normally select the final action. The control centre must analyse the incoming message, compare its significance, and send an appropriate command to the effector. If that interpretation is absent or incorrect, the command may not match the stimulus, so the response can be delayed, unsuitable, or wrong. Thus option A is correct.

The governing concept is division of function in the visual pathway. Photoreceptors in the retina detect light and convert it into nerve impulses, but seeing requires the brain to receive, process, and interpret those impulses. If the visual area is damaged, normal retinal signals may not produce a meaningful image. Therefore option A is correct; B and C assign unrelated functions, and D is false because the eye is connected to the brain by the optic nerve.

The governing concept is nervous control of the pupillary light reflex. Photoreceptors in the retina detect intense light and generate nerve impulses. These signals travel through the optic pathway to the brain, which sends motor instructions to the iris muscles; the circular muscles contract and the pupil becomes smaller. Hence option A is correct. The other pathways use unrelated organs or an unsuitable hormonal route.

The cerebellum coordinates the timing, precision and smoothness of voluntary muscle movements, and it also contributes to posture and balance. Writing requires carefully graded movements of the fingers, wrist and hand, so disturbed fine control points toward cerebellar involvement even if walking remains possible. Option A is therefore best. The medulla controls vital involuntary functions, while an olfactory receptor and the pancreas do not coordinate hand movement.

The governing concept is the integrated control pathway: receptor, sensory pathway, control centre, motor pathway and effector must work together. If receptors detect a stimulus normally but the motor nerve is damaged, the command cannot reach the effector, so the expected response fails. Therefore A demonstrates why the whole pathway matters. B denies communication, C reduces understanding to memorisation, and D concerns plant growth rather than pathway integrity.

Nervous control depends on electrical impulses travelling through specific neurons and nerve pathways. This makes it rapid and relatively precise, but its direct action is limited to cells or organs connected to the nervous system. Therefore option A correctly states a limitation. Option B describes a feature more typical of hormonal distribution, option C incorrectly assigns long duration to all nerve effects, and option D is not a valid description of plant coordination.