Reflex action

Correct answer: D. Receptor → sensory nerve → spinal cord → motor nerve → effector

The governing concept is the reflex arc, a rapid pathway that produces a protective response without waiting for conscious brain processing. A receptor detects the stimulus and a sensory neuron carries the impulse to the spinal cord. A motor neuron then carries the command to an effector such as a muscle or gland. Therefore option D gives the correct complete sequence.

Correct answer: B. They may be affected or weakened

The governing concept is the role of the spinal cord in reflex pathways. Many reflexes are coordinated through the spinal cord, which receives sensory information and sends motor instructions to an effector. Severe injury can interrupt or impair these nerve pathways, so the reflex response may become weak, delayed, abnormal, or absent. Therefore option B is the appropriate general answer; the other choices are biologically unrelated.

Correct answer: A. Voluntary action occurs by choice and reflex action occurs quickly automatically

The governing concept is the distinction between conscious and protective responses. A voluntary action is initiated intentionally, such as writing or walking, and involves conscious control. A reflex action is a rapid, automatic response to a stimulus, often coordinated through a reflex arc to protect the body, such as withdrawing from a hot object. Plants do not perform human voluntary actions, reflexes are not always slow, and neither action depends only on hormones.

Correct answer: A. Because the quick withdrawal occurs through the spinal cord and pain information reaches the brain later

In a reflex arc, receptors detect injury and the spinal cord quickly sends a motor command to withdraw the limb, helping reduce further damage. At the same time, sensory information travels upward to the brain, where pain is consciously interpreted; this perception may occur slightly later. Hence option A is correct, while the other explanations are false.

Correct answer: A. They detect heat stimulus and start a nerve signal

The skin contains sensory receptors, including receptors that respond to temperature and pain. When a hand touches a hot object, these receptors are stimulated and convert the stimulus into nerve impulses. The impulses travel through sensory neurons to the central nervous system, where a suitable response can be organised. Thus option A is correct; the other options describe plant nutrition, hormonal regulation, or root growth.

Correct answer: A. Receptor to sensory nerve to spinal cord to motor nerve to muscle

Withdrawal of the hand from a hot vessel is a reflex action. Heat receptors in the skin detect the stimulus, and the impulse travels through a sensory neuron to the spinal cord. The spinal cord sends a command through a motor neuron to the arm muscle, which contracts and pulls the hand away. Thus option A gives the correct reflex arc sequence; the other sequences reverse or omit essential steps.

Correct answer: A. The message path is shorter

A reflex action is a rapid, automatic response to a potentially harmful stimulus. The sensory impulse is routed through a short reflex arc, usually involving the spinal cord, which can send instructions to the effector before conscious thinking by the brain is completed. Therefore option A is correct. The signal is not carried by blood, the brain is not permanently inactive, and muscles do not generate the initial nerve message.

Correct answer: B. Spinal cord

This is a reflex action, so the body responds through a reflex arc before conscious brain processing is completed. Heat receptors send an impulse through a sensory neuron to the spinal cord, where an immediate motor response is organised. The hand is then withdrawn through a motor neuron. The brain receives information too, but it does not act as the first centre; the heart and pancreas are not centres for this response.

Correct answer: A. To carry message from receptor to spinal cord

The governing concept is the pathway of a reflex arc. A receptor first detects a stimulus such as heat or pain and produces a nerve impulse. The sensory nerve, also called an afferent nerve, carries this impulse from the receptor to the spinal cord, where the response is coordinated. Therefore option A is correct. Motor nerves carry commands toward effectors; they do not carry the initial receptor message to the spinal cord.

Correct answer: A. From spinal cord to effector

The governing concept is the direction of impulses in a reflex arc. After the spinal cord processes the sensory information and forms a response, a motor or efferent nerve carries the command away from the spinal cord to an effector. The effector may be a muscle or gland and produces the required response. Thus option A is correct. The reverse direction belongs to sensory nerves, while the other choices describe unrelated pathways.

Correct answer: A. Muscle

The governing concept is the stimulus–response pathway: a receptor detects a stimulus, the nervous system processes the message, and an effector produces the response. A muscle is an effector because it contracts and can pull the hand away from a hot object. The retina and taste bud are receptors, while a dendrite receives signals in a neuron; therefore option A is correct.

Correct answer: A. The response occurs quickly through the spinal cord, but information may also reach the brain

A reflex action is a rapid, automatic response that is usually coordinated through a reflex arc in the spinal cord, allowing the body to react before conscious brain processing is complete. Sensory information can nevertheless travel to the brain afterward, where the event is felt and understood. Thus, option A corrects the absolute statement without denying the spinal cord’s role.

Correct answer: A. The spinal cord produces the quick response and the brain later gives the conscious feeling of pain

The governing concept is the reflex arc. A harmful stimulus is detected by receptors, and the sensory impulse reaches the spinal cord, which rapidly sends a motor command to the muscles before conscious interpretation is completed. A later impulse reaches the brain, producing the full awareness of pain. Therefore option A is correct. The other options deny the brain, receptors or muscle contraction, all of which are involved in normal response.

Correct answer: A. Walking and withdrawing hand from hot object

A voluntary action is initiated or consciously controlled by a person; ordinary walking can be started, stopped or changed deliberately. Withdrawing the hand from a hot object is a rapid protective reflex, usually coordinated through a reflex arc before conscious awareness is complete. Heartbeat and digestion are involuntary, the hormones are chemical messengers, and the last pair describes plant tropisms. Hence option A is correct.

Correct answer: A. The sensory nerve carries the message to the spinal cord and the motor nerve carries the command to the effector

A reflex arc follows a definite direction. A receptor detects the stimulus, and the sensory neuron carries the resulting impulse from the receptor to the spinal cord, where the response is coordinated. The motor neuron then carries the outgoing command to an effector, such as a muscle, which produces the action. Hence option A is correct; the remaining choices assign unrelated plant or body functions to nerves.

Correct answer: A. They detect the heat stimulus and initiate a nerve signal

A reflex action begins when a receptor detects a potentially harmful stimulus. Heat or pain receptors in the skin sense excessive temperature and convert that stimulus into a nerve impulse. The impulse travels through a sensory neuron to the spinal cord, which coordinates a rapid response through a motor neuron. Therefore option A correctly describes the first step; the other options are unrelated functions.

Correct answer: A. It gives a very quick protective response to dangerous stimuli

The governing concept is rapid protective action through a reflex arc. A reflex arc provides a short pathway from receptor to sensory neuron, relay centre, motor neuron, and effector, allowing the body to respond before conscious thinking is complete. This reduces injury from heat, pain, or sudden danger. Hence option A is correct. The other options describe unrelated or impossible effects and do not explain the survival value of reflexes.

Correct answer: A. So that the situation is known later and further caution can be taken

A reflex action is designed for rapid protection, so the initial response can be coordinated through the spinal cord without waiting for conscious brain processing. At the same time, information about the stimulus is transmitted to the brain, where the person becomes aware of what happened and can make a later voluntary decision or take precautions. Hence option A is correct; the other choices are unrelated to the purpose of brain awareness.

Correct answer: A. Stimulus → receptor → sensory nerve → spinal cord → motor nerve → effector → response

A reflex arc is the rapid neural pathway that produces an automatic response to a harmful or sudden stimulus. First, a receptor detects the stimulus; a sensory neuron carries the impulse to the spinal cord, where it is processed. A motor neuron then carries the command to an effector such as a muscle, producing the response. Therefore option A gives the correct sequence; the others mix unrelated organs and pathways.

Correct answer: A. A message going from the spinal cord to the muscle to pull the hand away

A motor nerve carries commands from a central nervous-system centre to an effector such as a muscle or gland. In a withdrawal reflex, the spinal cord sends a response through a motor nerve to the arm muscle, causing contraction and movement of the hand away from the hot object. Heat detection by skin and light detection by the retina involve receptors and sensory pathways, while thyroxine production is an endocrine function.

Correct answer: A. Voluntary action is by will and reflex action is a very fast automatic response

A voluntary action is consciously initiated and generally involves decision-making by the brain, such as choosing to raise a hand. A reflex action is a rapid, automatic response to a stimulus, often coordinated through a reflex arc in the spinal cord to reduce delay. Therefore option A correctly contrasts intention and speed. The other choices reverse the definitions or wrongly restrict both actions to hormones or plants.

Correct answer: A. Walking is controlled by will while hand withdrawal is a quick reflex action

The governing concept is the distinction between voluntary actions and reflex actions. Walking normally involves conscious planning and control, although it can become partly automatic with practice. Pulling the hand away from a hot object is a rapid protective response organized through a reflex arc, before conscious processing is complete. Thus option A is correct. The other options confuse reflex action with digestion, hormones, or plant growth.

Correct answer: A. They detect the heat stimulus and initiate a nerve signal

A reflex response begins when a receptor detects a potentially harmful stimulus. Heat receptors in the skin sense an unusually high temperature and convert that information into a nerve impulse. The impulse then travels through a sensory neuron toward the central nervous system, allowing a rapid response. Thus option A describes the first step; the other choices refer to unrelated functions of digestion, endocrine activity, or plants.

Correct answer: A. It provides a rapid response to a dangerous stimulus without unnecessary delay

A reflex arc is a rapid neural pathway involving a receptor, sensory neuron, relay neuron in the central nervous system, and motor neuron. In many protective reflexes, the response is organised before conscious thought is completed, allowing an organism to withdraw from heat, sharp objects, or other threats quickly. Therefore option A explains the survival advantage. The other options have no relationship to the function of a reflex arc.

Correct answer: A. Because its rapid coordination often occurs at the spinal-cord level

The governing concept is the reflex arc. In a protective reflex, a receptor detects a stimulus, a sensory neuron carries the impulse to the spinal cord, and a motor neuron quickly activates an effector. The response can begin before the brain completes conscious analysis, so option A is correct. Hormonal transport is generally slower, and the other choices involve unrelated systems.