Coordination and Response: Nerves, Hormones, and Reflexes
Reflex Mode: Sense It, React ⚡
Introduction
1. Introduction
Okay, let's talk coordination and response. Right now, without you even trying, your body is reading this sentence and keeping your blood sugar steady. That's a lot of background admin, and this chapter is how it all gets managed.
We're going to break down the two control systems your body runs on: the nervous system (fast, electrical, like a text) and the hormonal system (slower, chemical, carried in your blood like a letter in the post). We'll do reflexes, the eye, hormones, staying steady, and even how plants react without a brain. Let's get into it ⚡
We're going to break down the two control systems your body runs on: the nervous system (fast, electrical, like a text) and the hormonal system (slower, chemical, carried in your blood like a letter in the post). We'll do reflexes, the eye, hormones, staying steady, and even how plants react without a brain. Let's get into it ⚡
2. Detect, Coordinate, React
Everything your body does follows one chain: a stimulus (a change) is picked up by a receptor, sent to a coordinator (your brain/spinal cord = the CNS), which tells an effector (a muscle or gland) to produce a response. That's the whole framework, and half the marks in this chapter are just filling in that chain.
The nervous system runs on electrical impulses zipping along cells called neurones, built for speed. Nothing physical travels along the neurone, no blood, no cell, just the electrical signal itself. The CNS is your brain and spinal cord; the PNS is every other nerve.
The nervous system runs on electrical impulses zipping along cells called neurones, built for speed. Nothing physical travels along the neurone, no blood, no cell, just the electrical signal itself. The CNS is your brain and spinal cord; the PNS is every other nerve.
Key idea🔑 Key idea: stimulus → receptor → coordinator (CNS) → effector → response. Nerves carry electrical impulses, and they're fast.
Worked example
Worked Example: What's Actually Travelling?
Worked Example: The Signal 🔌
Along which type of cell do electrical impulses travel to carry information around the body?
- 1Information in the nervous system moves as an electrical impulse, not a physical substance.
- 2That impulse travels along a neurone.
- 3So the answer is neurones. Nothing to do with blood cells or gland cells, those carry hormones, a totally different system.
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3. Neurones and the Reflex Arc
Three neurones do the work, and the exam wants the direction each carries the signal: sensory neurone carries it in to the CNS, relay neurone sits inside the CNS, and motor neurone carries it out to an effector. Memory hack: Sensory senses (in), Motor moves (out).
A reflex action is a fast, automatic reaction you don't think about, like yanking your hand off a hot pan. The path is the reflex arc: stimulus → receptor → sensory → relay → motor → effector → response.
A reflex action is a fast, automatic reaction you don't think about, like yanking your hand off a hot pan. The path is the reflex arc: stimulus → receptor → sensory → relay → motor → effector → response.
Key idea🔑 Key formula: reflex arc = stimulus → receptor → sensory neurone → relay neurone → motor neurone → effector → response.
Worked example
Worked Example: Name That Neurone
Worked Example: In, Middle, Out 🧭
A reflex arc is started by a finger touching a cactus spine. S is a receptor, T is a sensory neurone, W is a relay neurone, X is a motor neurone, and R is the effector muscle. What's the correct order?
- 1The stimulus is detected by the receptor, S.
- 2The impulse travels in along the sensory neurone T, then to the relay neurone W inside the spinal cord.
- 3It then travels out along the motor neurone X, causing effector R to respond. Order: S, T, W, X, R.
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4. Reflexes and the Synapse Gap
A reflex action is a rapid, automatic response to a stimulus, no conscious thought needed. It's fast because it often skips the "thinking" parts of the brain, the spinal cord handles it, which is why your hand can move before you feel the pain.
Neurones don't actually touch. The tiny gap where one meets the next is a synapse, and that's the word to know at this level, junction between two neurones.
Neurones don't actually touch. The tiny gap where one meets the next is a synapse, and that's the word to know at this level, junction between two neurones.
Key idea🔑 Key idea: reflex action = fast + automatic + no conscious thought. Synapse = the junction between two neurones.
Worked example
Worked Example: Name the Junction
Worked Example: The Gap 🔍
What is the name of the junction between two neurones?
- 1Neurones don't touch each other directly, there's a tiny gap where one ends and the next begins.
- 2That junction is called a synapse.
- 3A receptor detects a stimulus and an effector produces a response, neither of those names the junction itself, so the answer is specifically synapse.
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5. The Eye: A Camera Made of Jelly
A sense organ is a group of receptor cells that detect one specific stimulus. The eye is a sense organ that detects light. Follow the light path: the cornea (clear front) bends most of the light, the iris controls how big the pupil is (how much light gets in), the lens fine-focuses it, and it lands on the retina at the back where the light detectors live. The optic nerve carries signals to the brain.
Big point people forget: making an image on the retina isn't the same as seeing it. You only "see" once the retina fires impulses along the optic nerve to your brain.
Big point people forget: making an image on the retina isn't the same as seeing it. You only "see" once the retina fires impulses along the optic nerve to your brain.
Key idea🔑 Key idea: cornea bends most light · iris/pupil control light in · lens focuses · retina detects · optic nerve to brain. Image on retina ≠ seeing until the brain gets the signal.
Worked example
Worked Example: Which Part Does What?
Worked Example: Match the Job 🎯
Part P lies just behind the pupil, part Q lines the inside of the back of the eye, and part R is the thick nerve leaving the back of the eye. Match P, Q and R to their functions.
- 1P (just behind the pupil) is the lens, its job is to focus light onto the retina.
- 2Q (lining the back of the eye) is the retina, its job is to contain the light receptor cells.
- 3R (the thick nerve leaving the eye) is the optic nerve, its job is to carry impulses to the brain. So P focuses, Q detects, R carries the signal.
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6. Pupils in Action
The pupil reflex adjusts pupil size automatically, based on how bright it is. In bright light, the pupil narrows, so less light gets in and the retina isn't damaged. In dim light, the pupil widens, so more light gets in and you can still see. It's a reflex, meaning it's automatic, you can't just decide to override it.
Key idea🔑 Key idea: bright light → pupil narrows. Dim light → pupil widens. Automatic, not something you can control on purpose.
Worked example
Worked Example: Walking Into a Dark Room
Worked Example: Eyes Adjusting 🌒
Surroundings become dimmer. What happens to the pupil, and why?
- 1Less light is available in dim conditions.
- 2To keep seeing properly, the eye needs to let more of that light in.
- 3So the pupil widens, to let more light in. (Narrowing would do the opposite, that's the bright-light response.)
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7. Hormones: The Slow-Burn System
A hormone is a chemical made by a gland, carried in the blood, that changes the activity of target organs. Slower than nerves to kick in, but the effect lasts longer. The one to know: adrenaline, made by the adrenal glands (on top of your kidneys), released when you're scared or hyped, the "fight or flight" hormone. It raises your heart rate, raises your breathing rate, and widens your pupils, getting your muscles more fuel, faster, plus more light in to help you react.
Other glands to know: the pancreas makes insulin, the testes (males) make testosterone, the ovary (females) makes oestrogen.
Other glands to know: the pancreas makes insulin, the testes (males) make testosterone, the ovary (females) makes oestrogen.
Key idea🔑 Key idea: hormone = chemical from a gland, in the blood, acting on target organs. Adrenaline (adrenal glands) raises heart rate, breathing rate, and pupil size for fight-or-flight.
Worked example
Worked Example: Adrenaline Rush
Worked Example: Fight or Flight 🏃
A person is being chased, which releases a hormone into their blood. How does this affect breathing rate, heart rate, and pupil diameter?
- 1Adrenaline gets your body ready for sudden action.
- 2Ready for action means more fuel and faster delivery, so breathing rate, heart rate, and pupil diameter all increase.
- 3Any option with one of these decreasing makes no sense, you don't prep for a sprint by slowing your heart or narrowing your pupils.
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8. Nerves vs Hormones
Quick compare: nerves = fast but short-lived; hormones = slow but long-lasting. Nerves send an electrical impulse along neurones. Hormones send a chemical in the blood. The body picks the right tool for the job, nerves for a reflex, hormones for something that needs to last, like keeping blood sugar steady.
Key idea🔑 Key idea: nervous = electrical impulse, neurones, fast, short-lived. Hormonal = chemical, blood, slower, longer-lasting. A "compare" question wants both systems mentioned.
Worked example
Worked Example: Two True Things About Hormones
Worked Example: Pick the Pair ✅
Which two statements about hormones are correct when compared with the nervous system?
- 1Hormones travel in the blood, which is slower than an electrical impulse racing along a neurone, so hormones act more slowly than the nervous system.
- 2Hormones like insulin genuinely help keep things steady in the body, so hormones may be involved in homeostasis.
- 3Both of those are correct. Anything saying hormones are impulses, travel along neurones, or act faster/shorter than nerves has it backwards.
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9. Staying Balanced: Homeostasis
Homeostasis = keeping your internal environment constant. Blood glucose concentration is one thing your body keeps steady, unlike a fixed feature like eye colour, which isn't regulated at all.
The pancreas releases insulin, and insulin decreases blood glucose concentration. After a sugary meal, glucose rises, insulin gets released, and glucose comes back down toward normal.
The pancreas releases insulin, and insulin decreases blood glucose concentration. After a sugary meal, glucose rises, insulin gets released, and glucose comes back down toward normal.
Key idea🔑 Key idea: homeostasis = constant internal environment. Insulin (from the pancreas) decreases blood glucose concentration.
Worked example
Worked Example: Sugar Spike, Sorted
Worked Example: Insulin to the Rescue 🍬
Blood glucose rises too high after a meal. Which hormone is released to bring it back down?
- 1The hormone that lowers blood glucose is insulin.
- 2It's released by the pancreas.
- 3Adrenaline is a trap answer here, it's also a hormone, but it raises blood glucose, the opposite of what's needed. Oestrogen has no role in blood glucose at all. Answer: insulin.
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10. Plants React Too: Tropisms
Plants have no nerves or muscles, so they respond by growing in a direction. A tropism is growth towards or away from a stimulus. Gravitropism (gravity): roots grow down (positive), shoots grow up (negative). Phototropism (light): shoots grow towards light (positive), roots grow away from light (negative).
Key idea🔑 Key idea: shoots grow up and towards light (negative gravitropism, positive phototropism). Roots grow down and away from light (positive gravitropism, negative phototropism).
Worked example
Worked Example: Bending Toward the Window
Worked Example: Chasing the Light 🪟
A plant on the floor of a room, near a bright window, grows its shoot towards the window. Which growth responses explain this?
- 1Shoots always grow upwards, away from gravity, that's negative gravitropism.
- 2Shoots also grow towards a light source, that's positive phototropism.
- 3Both happen together here: the shoot grows away from gravity and towards the light, so it ends up growing up and across, toward the window.
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