Nervous system

A receptor detects a stimulus, but it does not decide what the body should do. The brain or spinal cord acts as the control centre: it receives and interprets the nerve message, selects an appropriate response, and sends instructions through motor neurons to an effector such as a muscle or gland. Therefore, option A is correct. The other options describe functions unrelated to nervous coordination.

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 synaptic transmission. When an electrical impulse reaches the end of one neuron, chemicals called neurotransmitters cross the synaptic gap and stimulate the next neuron. If a drug blocks their release or action, the signal cannot pass normally, so the response may be delayed, weakened, or stopped. Therefore option A is correct. Options B, C, and D do not describe the specific effect of blocking a synapse.

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.

In a reflex pathway, a receptor detects the stimulus, a control centre processes the impulse, and an effector produces the response. Skin contains receptors that detect heat or touch; the spinal cord coordinates the rapid reflex; and a muscle contracts to move the hand. Therefore option A gives the correct order. The other combinations confuse receptors, effectors, glands, hormones, or plant structures and do not form a valid reflex pathway.

The nervous system coordinates responses through a sequence: receptors first detect a stimulus, sensory neurons carry information to the central nervous system, and motor pathways activate an effector. The effector then produces the response. Thus option A gives the correct order. The remaining choices list functions of plants, kidneys, bones, or imaginary processes rather than the three main nervous-system roles.

Bright light acts as a stimulus detected by the eye. Circular muscles in the iris contract automatically, reducing the pupil’s diameter and limiting the amount of light entering the eye. Because this adjustment occurs without conscious choice and is coordinated through the nervous system, option A is correct. It is not voluntary hormonal control, plant hydrotropism, or insulin production in muscle.

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.

The governing concept is nervous coordination of posture, movement, and balance. The cerebellum receives information about body position and helps coordinate the timing and strength of skeletal-muscle contractions. Damage can therefore produce unsteady walking and poor balance. Option A correctly states this relationship. The other choices wrongly assign the cerebellum roles in blood sugar production, thyroxine secretion, or transport of water in plants.

The governing concept is sensory transduction in vision. Light entering the eye is focused on the retina, where photoreceptors such as rods and cones absorb light and initiate electrical nerve signals. These signals are carried through retinal neurons and the optic nerve to the brain for interpretation. If photoreceptors fail, the first conversion step is disrupted, so later visual processing cannot receive accurate information. Thus option A is correct; the other choices concern unrelated body functions.

The cerebrum is the major part involved in conscious activities such as thinking, learning, memory, interpretation of sensations, and voluntary actions. The cerebellum does not perform these higher mental tasks; it coordinates muscular activity and helps maintain posture and balance. Therefore option A correctly separates their roles. Insulin and auxin are hormones, while digestion and plant roots are unrelated to these brain regions.

The governing concept is that the cerebellum coordinates muscular activity, posture, precision and body balance. Walking on a rope requires continuous adjustment of many muscles so that the body does not fall, making option A the clearest example. Lowering blood sugar is mainly associated with insulin, root growth towards water is hydrotropism, and thyroxine formation is an endocrine function of the thyroid gland.

The central nervous system, consisting mainly of the brain and spinal cord, receives, processes, and coordinates information. The peripheral nervous system consists of nerves outside the brain and spinal cord; these nerves carry sensory messages toward the central system and motor commands away from it to muscles or glands. Therefore, option A correctly states the two-way functional relationship. The other options wrongly assign digestion, hormone production, plant functions, or blood filtration to these systems.

The stapes is the smallest bone in the human body. It is one of the three auditory ossicles in the middle ear and helps transmit sound vibrations from the incus towards the inner ear. The incus and malleus are also middle-ear bones, but both are larger than the stapes. Exam tip: Remember: stapes is the smallest bone, while the femur is the largest.

The brain and spinal cord together form the central nervous system, which receives information and controls and coordinates body responses and activities. The peripheral nervous system consists of nerves that connect the CNS to the rest of the body; it does not include the brain and spinal cord. Exam tip: Remember CNS as Brain + Spinal cord.

A neuron is the structural and functional basic unit of the nervous system. It receives and transmits messages as electrical signals, enabling control and coordination in the body. The digestive system is involved in breaking down food, so it is not the correct answer. Exam tip: Remember that a neuron is a nerve cell of the nervous system.