How does the nervous system detect and respond to changes, and how is the internal environment kept steady?
The structure of the nervous system, the reflex arc and the role of synapses, receptors and effectors, the principle of homeostasis, and the control of body temperature, blood glucose and water as examples of negative feedback.
A focused answer to the OCR Gateway GCSE Combined Science A topic B3 on the nervous system and homeostasis, covering the central and peripheral nervous systems, the reflex arc and synapses, receptors and effectors, and homeostasis as negative feedback controlling temperature, glucose and water.
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What this topic is asking
OCR wants you to describe the parts of the nervous system, explain the reflex arc and the role of synapses, define homeostasis, and use temperature, blood glucose and water control as examples of negative feedback.
The nervous system
Information travels along nerve cells called neurones. A receptor detects a stimulus (such as light, sound, temperature or a chemical) and starts an electrical impulse. The impulse travels along a sensory neurone to the CNS, which processes it and sends an impulse along a motor neurone to an effector, which is a muscle that contracts or a gland that secretes a substance. Where two neurones meet there is a tiny gap called a synapse: the impulse cannot jump the gap directly, so the first neurone releases a chemical (a neurotransmitter) that diffuses across and triggers an impulse in the next neurone.
The reflex arc
A reflex action is a fast, automatic response that protects the body and does not involve conscious thought. The pathway, the reflex arc, is: stimulus produces an impulse in a receptor, the impulse passes along a sensory neurone to the spinal cord, crosses a synapse to a relay neurone, crosses another synapse to a motor neurone, which carries the impulse to an effector that produces the response. Because the impulse goes through the spinal cord rather than the conscious brain, the response is much faster, which is why you pull your hand off a hot object before you feel the pain.
Homeostasis and negative feedback
Homeostasis is the regulation of the internal conditions of the body to keep them within narrow limits, so that cells and enzymes work properly. It relies on negative feedback: a change away from the normal level is detected by receptors, the body responds in a way that opposes (reverses) the change, and the condition returns towards its set point. Three examples OCR uses are:
- Body temperature. If too hot, you sweat (evaporation cools the skin) and skin blood vessels dilate (more heat lost); if too cold, you shiver (respiration releases heat), skin blood vessels constrict (less heat lost), and hairs stand up to trap air.
- Blood glucose. Controlled by the hormones insulin and glucagon (covered in hormonal coordination).
- Water and ions. Controlled by the kidneys, which adjust how much water is reabsorbed.
In every case the response opposes the original change, which is the defining feature of negative feedback.
Exam-style practice questions
Practice questions written in the style of OCR exam questions on this dot point, with worked answer explainers. The year tag is the paper they imitate, not the source.
OCR 20194 marksDescribe the pathway of a reflex action from a painful stimulus to the response, naming the parts involved in the correct order.Show worked answer →
A Biology Paper 1 structured question on the reflex arc. Reward the correct sequence: a stimulus (such as a pin) is detected by a receptor in the skin, an electrical impulse passes along a sensory neurone to the central nervous system (spinal cord), where it crosses a synapse to a relay neurone, then crosses another synapse to a motor neurone, which carries the impulse to an effector (a muscle) that contracts to move the hand away. Markers award the order receptor, sensory neurone, relay neurone, motor neurone, effector, and credit naming the synapses between neurones. A reflex is fast and automatic because it does not involve the conscious brain.
OCR 20216 marksExplain how the body responds when its core temperature rises above normal, and why this is described as negative feedback.Show worked answer →
A B3 six-mark extended response, marked on levels. Reward: receptors in the skin and the brain (the thermoregulatory centre) detect that the temperature is too high. The body responds to increase heat loss: sweat glands release more sweat, which evaporates and takes heat from the skin; and the blood vessels supplying the skin capillaries dilate (vasodilation), so more blood flows near the surface and more heat is lost by radiation. These responses lower the temperature back towards normal. It is negative feedback because the response opposes the original change (a rise) and returns the condition towards the set point. Top answers name sweating and vasodilation, link them to heat loss, and define negative feedback as a response that reverses the change.
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