Coordination, Response and Homeostasis
AdvancedThe nervous system sends fast electrical impulses along reflex arcs, while the endocrine system coordinates slowly but lastingly through hormones in the blood; together they hold blood glucose, body temperature and water steady by negative feedback, with insulin and glucagon as the classic antagonistic pair.
- The reflex arc runs: receptor → sensory neurone → centre (intermediate neurones) → motor neurone → effector; reflexes bypass the brain, fast and brief.
- Nervous coordination uses electrical impulses — fast and short-lived; hormonal coordination rides the blood — slow but lasting; the two complement each other.
- Endocrine glands are ductless; hormones enter the blood and act only on target organs with matching receptors, while synapses keep nerve signals one-way.
文體小知識：科學知識說明文——比較神經與激素兩套協調系統，詳解反射弧五站、血糖拮抗調節與負反饋原理，再以體溫和尿量調節示範體內平衡的運作。
§Text Passage
1To respond to changes in the environment, an organism must first detect them with receptors. The human nervous system has a central part (brain and spinal cord) and a peripheral part, with the neurone as its basic unit: sensory neurones carry signals into the central nervous system, intermediate neurones relay them within it, and motor neurones deliver commands to effectors. Nerve signals travel along nerve fibres as electrical impulses and pass to the next cell across synapses via chemicals — fast and short-lived.
2A reflex is the fastest response of all, bypassing conscious thought; its route is the reflex arc of five stations: receptor → sensory neurone → centre (intermediate neurones in the spinal cord) → motor neurone → effector. For example, when a finger touches a pin, the hand withdraws before the pain is felt, because the pull-away command is issued directly by the spinal cord; the sensation of pain is produced by the brain a moment later. Voluntary actions, planned by the cerebrum, take a longer route and react more slowly.
3Hormonal coordination belongs to the endocrine glands: ductless glands that secrete hormones directly into the blood, which carries them to their target organs. Hormones act slowly but lastingly, complementing the fast, brief nerve signals. For example, the quickening heartbeat during exercise is controlled instantly by nerves, while the long regulation of blood glucose after a meal is handled by insulin and glucagon working in relay.
4Homeostasis is the maintenance of a stable internal environment, run by feedback mechanisms: when a parameter drifts from its normal level, the body detects the deviation and corrects it in the opposite direction — negative feedback. Blood glucose regulation is the classic case: when glucose rises, the pancreas secretes insulin, promoting glucose uptake by cells and conversion to glycogen in the liver; when glucose falls, glucagon makes the liver break glycogen back into glucose. The two hormones act in opposition, checking each other.
5Temperature regulation uses negative feedback too: when body temperature rises, skin vessels dilate and sweat glands secrete more to speed heat loss; when it falls, skin vessels constrict and skeletal muscles shiver to generate heat. The kidneys adjust urine volume under the control of the antidiuretic hormone, keeping the body's water balance. A stable internal environment depends on the nervous and endocrine systems working in concert — failure of any link throws the body off balance.
1Summary 重點摘要
The nervous system sends fast electrical impulses along reflex arcs, while the endocrine system coordinates slowly but lastingly through hormones in the blood; together they hold blood glucose, body temperature and water steady by negative feedback, with insulin and glucagon as the classic antagonistic pair.
2Vocabulary cards Vocabulary
A structure that detects a stimulus and converts it into nerve signals, such as the light-sensitive cells of the retina.
The basic unit of the nervous system — sensory, intermediate or motor — transmitting information as electrical impulses.
The signal route of a reflex: receptor → sensory neurone → centre → motor neurone → effector, five stations in all.
The junction between two neurones, where chemicals pass the signal one way to the next neurone.
A chemical messenger secreted by an endocrine gland directly into the blood, acting at its target organ.
A ductless gland that secretes hormones straight into the blood, such as the islets of the pancreas and the thyroid.
The maintenance of a relatively stable internal environment (blood glucose, body temperature, water and so on).
A mechanism that detects a deviation from the normal level and corrects it in the opposite direction.
A pancreatic hormone that promotes glucose uptake by cells and its conversion to glycogen in the liver, lowering blood glucose.
A pancreatic hormone that makes the liver break glycogen into glucose for release into the blood, raising blood glucose — the opposite of insulin.
3Step by step Steps
- Step 1: Stimulus, receptor and responseCoordination has three elements: stimulus, coordinating system and response. A change in the environment (the stimulus) is first detected by a receptor and converted into nerve signals; the coordinating system processes them; finally an effector — a muscle or gland — produces the response. Receptors are spread across the body, with the eye, ear, nose, tongue and skin each covering its own modalities. For example, light entering the eye is converted by the light-sensitive cells of the retina into nerve signals that race along the optic nerve to the brain, where the image appears — all in an instant.
- Step 2: Neurones and synapsesNeurones come in three functional kinds: sensory neurones carry signals from receptors into the central nervous system, intermediate neurones relay and integrate them within it, and motor neurones carry commands from the centre to the effectors. The junction between two neurones is a synapse: when a signal arrives, chemical transmitter is released and diffuses across the gap to excite the next neurone, which is why synaptic transmission is one-way. For example, picture nerve fibres as wires and synapses as switch-worked junctions: signals race along the wires but pause briefly at each junction, which is why responses crossing many synapses are slower.
- Step 3: The five-station reflex arcA reflex is an inborn, rapid, unconscious response, and its route is the reflex arc of five stations: receptor → sensory neurone → centre (intermediate neurones in the spinal cord) → motor neurone → effector. The spinal cord decides on the spot: the signal triggers the reaction without reaching the brain first, so it is fastest; sensory traffic climbs simultaneously and the pain arrives later. Voluntary actions are planned by the cerebrum, cross more synapses and react more slowly but can be controlled. For example, when a finger touches a pin the hand jerks back at once and only then hurts — proof that the spinal cord commanded first and the brain learned afterwards.
- Step 4: Hormonal coordination and the comparisonEndocrine glands are ductless: their hormones enter the blood directly and travel everywhere, yet act only on target organs carrying the matching receptors. The two coordinating systems differ sharply: nerves transmit electrical impulses along neurones, fast but brief, while hormones travel in the blood, slower but lasting — ideal for long-term regulation of metabolism and development. The two complement each other, and the brain can command endocrine glands, linking the systems. For example, within seconds of a fright, adrenaline quickens the heartbeat and raises blood glucose to prepare for action — hormones stepping in alongside fast nerve control.
- Step 5: Blood glucose and antagonismBlood glucose is monitored by the islet cells of the pancreas and regulated by two antagonistic hormones. When glucose rises (say, after a meal), insulin secretion increases: body cells take up more glucose and the liver stores the surplus as glycogen, so the level falls. When glucose runs low (say, while fasting), glucagon secretion rises, making the liver break glycogen back into glucose for release, and the level recovers. Acting in opposite directions, the hormones check each other — an antagonistic pair that holds glucose within a narrow healthy band. For example, after two slices of toast at breakfast insulin brings the rise back down within an hour; by noon, unfed and running low, glucagon takes over.
- Step 6: How negative feedback worksNegative feedback is the core mechanism of homeostasis, running in four steps: a normal level is set as the reference point; receptors detect the direction of any deviation; the coordinating system launches a correction in the opposite direction (lower it if too high, raise it if too low); and as the parameter returns to normal, the correction fades. Such self-correction keeps the fluctuations of the internal environment within tight bounds — blood glucose, body temperature and water balance all follow this pattern. For example, set an air-conditioner's thermostat to twenty-five degrees: cooling starts when the room gets warmer and stops when cooler, so the temperature wobbles gently around the setting — negative feedback is the body's built-in thermostat.
- Step 7: Regulating temperature and waterBody temperature is monitored by the brain. When it rises, skin vessels dilate to increase blood flow and sweat glands secrete more, speeding heat loss; when it falls, skin vessels constrict to conserve heat and skeletal muscles shiver to generate it, keeping the core near thirty-seven degrees Celsius. Water balance is guarded by the kidneys: when the body runs short of water, the pituitary gland releases more antidiuretic hormone, so the kidneys reabsorb more water and urine volume drops; with plenty of water, the hormone ebbs and urine flows. For example, after sweating heavily on a summer day without drinking enough, urine turns scanty and dark — the antidiuretic hormone working overtime.
4Diagrams Diagrams
5Worked example Example
After drinking a glass of glucose solution on an empty stomach, a student's blood glucose rises and then falls, returning to normal within about two hours. Explain how the pancreas's hormone secretion changes during this period and what role the liver plays, and name the feedback mechanism involved. If a person's insulin secretion stays low for a long time, what problem will his blood glucose show?
- After drinking the solution, glucose is absorbed into the blood and the blood glucose level rises; the pancreas detects the deviation and secretes more insulin.
- Insulin promotes glucose uptake by body cells and makes the liver convert the surplus glucose into stored glycogen, so the level falls.
- As the level returns to normal and drifts low, glucagon secretion increases, making the liver break glycogen back into glucose for release and preventing the level from dropping too far.
- Insulin and glucagon act in opposite directions and check each other — an antagonistic pair; the whole process pulls the deviation back to normal, so it is negative feedback.
- With long-term insulin deficiency, cells struggle to absorb glucose, the blood glucose stays high, and some glucose is lost in the urine — the hallmark of diabetes.
- Answer: Rising glucose triggers more insulin, which promotes uptake by cells and glycogen storage in the liver, lowering the level; when glucose runs low, glucagon rises and the liver releases glucose from glycogen, raising it. The liver acts as the buffer, storing and releasing glucose. The two hormones are antagonistic and the overall control is negative feedback. Insulin deficiency leaves blood glucose chronically high, with glucose appearing in the urine.
6Common mistakes Mistakes
7Quick check Quick check
Q1Name the five parts of a reflex arc in the order the signal travels.
Q2Give two main differences between nervous and hormonal coordination.
Q3After eating a lot of sugar, how does the body bring blood glucose back to normal?
Q4What is antagonism? Give a pair of examples.
Q5In cold weather, what do the skin and body do to conserve heat?
8Recap Recap
- The reflex arc runs: receptor → sensory neurone → centre (intermediate neurones) → motor neurone → effector; reflexes bypass the brain, fast and brief.
- Nervous coordination uses electrical impulses — fast and short-lived; hormonal coordination rides the blood — slow but lasting; the two complement each other.
- Endocrine glands are ductless; hormones enter the blood and act only on target organs with matching receptors, while synapses keep nerve signals one-way.
- Blood glucose is regulated antagonistically by insulin (lowering) and glucagon (raising), with the liver buffering glucose as glycogen — the model of negative feedback.
- Body temperature is held by skin vessel constriction and dilation, sweat and shivering; urine volume by the antidiuretic hormone — all following the negative-feedback pattern.