Coordination and Response: Nerves, Hormones, and Control
Reflex Mode: Sense It, React, Repeat ⚡🧠
Introduction
1. Introduction
Okay, let's talk coordination and response. Right now, without you even trying, your body is reading this sentence, keeping you at 37 °C, and holding 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 in balance, 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 in balance, 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 honestly half the marks in this chapter are just filling in that chain.
The nervous system runs on electrical impulses zipping along cells called neurones, and it's built for speed. Think of it like the group chat: instant, direct, done. Nothing physical travels along the neurone, no blood, no cell, just the electrical signal itself.
The nervous system runs on electrical impulses zipping along cells called neurones, and it's built for speed. Think of it like the group chat: instant, direct, done. Nothing physical travels along the neurone, no blood, no cell, just the electrical signal itself.
Key idea🔑 Key idea: stimulus → receptor → coordinator (CNS) → effector → response. Nerves carry electrical impulses, and they're fast.
Worked example
Worked Example: Who's In Charge?
Worked Example: The Coordinator 🎯
In a reflex, information passes from a receptor, through the central nervous system, to an effector. Why is the CNS described as a coordinator?
- 1"Coordinator" means it connects the message coming in to the correct reaction going out.
- 2So the CNS receives impulses from receptors and sends impulses to the correct effectors, making sure the right thing happens.
- 3That's the answer. Making hormones is a different system, and storing energy isn't "coordinating" at all.
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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. Little 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. Often the spinal cord handles it, which is why your hand moves before you feel the pain.
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. Often the spinal cord handles it, which is why your hand moves before you feel the pain.
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 🧭
P carries the impulse from the receptor into the spinal cord. Q is the short neurone lying within the spinal cord. R carries the impulse from the spinal cord out to the effector. What are P, Q and R?
- 1P goes in to the cord, so P is a sensory neurone.
- 2Q is short and inside the cord, so Q is a relay neurone.
- 3R goes out to the effector, so R is a motor neurone. Any answer that swaps sensory and motor fails the direction test.
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4. Crossing the Gap: Synapses
Neurones don't actually touch. The tiny gap where one meets the next is a synapse, and the electrical signal can't jump it. Instead: the impulse makes vesicles (little sacs in neurone 1) release a neurotransmitter chemical, which diffuses across the gap and binds to receptors on neurone 2, starting a fresh impulse.
Because the vesicles are only in neurone 1 and the receptors only on neurone 2, the signal can only travel one way. That's a favourite exam point.
Because the vesicles are only in neurone 1 and the receptors only on neurone 2, the signal can only travel one way. That's a favourite exam point.
Key idea🔑 Key idea: impulse → vesicles release neurotransmitter → diffuses across gap → binds receptors on next neurone → new impulse. One direction only.
Worked example
Worked Example: Put It In Order
Worked Example: The Synapse Sequence 🔢
Order these synapse events: (1) a new impulse starts in the next neurone; (2) an impulse makes vesicles release neurotransmitter; (3) neurotransmitter binds to receptors; (4) neurotransmitter diffuses across the gap.
- 1Release has to happen first, so 2 is first, then it diffuses across, so 4 is second.
- 2Once across, it binds to receptors, so 3 is third.
- 3Binding triggers a new impulse, so 1 is last. Final order: 2, 4, 3, 1. (Remember: it crosses by diffusion, not active transport.)
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5. The Eye: A Camera Made of Jelly
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 fovea is the spot with the sharpest detail, and 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: What Bends the Light?
Worked Example: Front vs Back 🔍
A student claims the part lining the back of the eye does most of the refracting (bending) of light. Is this right?
- 1The part at the back is the retina, and its job is to detect light, not bend it.
- 2The part that bends most of the light is the cornea at the front (the lens just fine-tunes).
- 3So the student is wrong: retina detects, cornea bends. Classic mix-up of a detecting part with a refracting part.
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6. Pupils and Focusing
Two different reflexes, two different muscle pairs, don't mix them up. The pupil reflex uses the iris muscles to control light: in bright light the circular muscles contract and radial muscles relax so the pupil gets smaller; in dim light it's the reverse. They're an antagonistic pair (one contracts as the other relaxes).
Accommodation is focusing, using the ciliary muscle and lens: for a near object the ciliary muscle contracts, the suspensory ligaments go slack, and the lens gets fatter (bends light more); for a distant object it's all reversed. Anchor line: "Near = ciliary contracts, ligaments slack, lens fat."
Accommodation is focusing, using the ciliary muscle and lens: for a near object the ciliary muscle contracts, the suspensory ligaments go slack, and the lens gets fatter (bends light more); for a distant object it's all reversed. Anchor line: "Near = ciliary contracts, ligaments slack, lens fat."
Key idea🔑 Key idea: iris muscles change PUPIL size for light (bright = circular contract, pupil small). Ciliary muscle changes LENS shape for distance (near = lens fat).
Worked example
Worked Example: Spotting the Fly
Worked Example: Focus Near 🪰
A fly comes nearer and the eye focuses on it. Q is the ciliary muscle, R is the lens, S is a suspensory ligament. What happens to Q, R and S?
- 1A near object needs the lens to bend light more, so the lens must get fatter.
- 2For that, Q (ciliary muscle) contracts, which lets S (ligament) go slacker.
- 3So R (lens) becomes fatter. Answer: Q contracts, R fatter, S slacker. (Watch for the trap: a contracted ciliary muscle gives slack ligaments, not tight ones.)
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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 and raises your blood glucose (by making the liver break glycogen into glucose), so your muscles get more fuel delivered faster.
Quick compare: nerves = fast but short-lived; hormones = slow but long-lasting. The body picks the right tool for the job.
Quick compare: nerves = fast but short-lived; hormones = slow but long-lasting. The body picks the right tool for the job.
Key idea🔑 Key idea: hormone = chemical from a gland, in the blood, acting on target organs. Adrenaline raises heart rate + blood glucose for fight-or-flight.
Worked example
Worked Example: Adrenaline Rush
Worked Example: Fight or Flight 🏃
What is the effect of adrenaline on blood glucose concentration and on pulse rate?
- 1Adrenaline gets your body ready for sudden action.
- 2Ready for action means more fuel and faster delivery, so blood glucose goes up and pulse rate goes up.
- 3So both increase. Any option where either one drops makes no sense, you don't prep for a sprint by lowering your fuel or slowing your heart.
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8. Staying Balanced: Homeostasis
Homeostasis = keeping your internal environment constant. It works by negative feedback: if something drifts off its normal set point, the body reacts in the opposite direction to push it back, then stops. Your temperature is held near 37 °C mostly by the skin.
Too hot? You sweat (sweat evaporates and carries heat away) and do vasodilation (skin blood vessels widen, more heat escapes). Too cold? You shiver (makes heat) and do vasoconstriction (vessels narrow, less heat lost). Key detail: sweat only cools you when it evaporates.
Too hot? You sweat (sweat evaporates and carries heat away) and do vasodilation (skin blood vessels widen, more heat escapes). Too cold? You shiver (makes heat) and do vasoconstriction (vessels narrow, less heat lost). Key detail: sweat only cools you when it evaporates.
Key idea🔑 Key idea: homeostasis = constant internal environment via negative feedback (react opposite to the change). Too hot → sweat + vasodilation; too cold → shiver + vasoconstriction.
Worked example
Worked Example: Cooling Down
Worked Example: Too Hot to Handle 🥵
What is a response of the human body to overheating?
- 1Too hot means the body needs to lose heat.
- 2Vasodilation widens the arterioles supplying the skin, so more warm blood flows near the surface.
- 3More blood near the surface = more heat lost to the air. Answer: vasodilation of the skin arterioles. (Vasoconstriction is the opposite, that's for when you're cold.)
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9. Blood Sugar Control and Diabetes
Blood glucose is kept steady by two hormones from the pancreas, both acting on the liver. Blood sugar too high (after a meal)? The pancreas releases insulin, and the liver turns glucose into glycogen for storage, sugar drops. Blood sugar too low? The pancreas releases glucagon, and the liver turns glycogen back into glucose, sugar rises. Memory hack: INsulin puts sugar INto store.
In Type 1 diabetes, the pancreas doesn't make enough insulin, so blood sugar stays high. It's treated by injecting insulin and controlling the diet. Why inject? Insulin is a protein, so if you swallowed it, it'd get digested before it worked.
In Type 1 diabetes, the pancreas doesn't make enough insulin, so blood sugar stays high. It's treated by injecting insulin and controlling the diet. Why inject? Insulin is a protein, so if you swallowed it, it'd get digested before it worked.
Key idea🔑 Key formula: insulin lowers blood glucose (glucose → glycogen); glucagon raises it (glycogen → glucose). Both from the pancreas, acting on the liver.
Worked example
Worked Example: After the Meal
Worked Example: Sugar Comes Down 📉
After a carbohydrate meal, blood glucose rises then returns to normal. Fill the gaps: the ___ releases insulin, which makes the ___ turn glucose into ___; this is an example of ___.
- 1The pancreas releases insulin.
- 2Insulin acts on the liver, which turns glucose into glycogen to store it.
- 3Bringing the level back down is negative feedback. So: pancreas, liver, glycogen, negative feedback.
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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 direction): shoots grow towards light (positive), so leaves catch more light for photosynthesis.
The magic chemical is auxin, made in the shoot tip, which makes cells grow longer. When light comes from one side, auxin moves to the shaded side and builds up, so those cells grow longer and the shoot bends towards the light. Two things to always say: auxin makes cells grow longer (it does not kill them), and it gathers on the shaded side.
The magic chemical is auxin, made in the shoot tip, which makes cells grow longer. When light comes from one side, auxin moves to the shaded side and builds up, so those cells grow longer and the shoot bends towards the light. Two things to always say: auxin makes cells grow longer (it does not kill them), and it gathers on the shaded side.
Key idea🔑 Key idea: auxin is made in the shoot tip, builds up on the shaded side, and makes those cells grow longer, so the shoot bends towards the light.
Worked example
Worked Example: Where's the Auxin Made?
Worked Example: Tip Test 🔬
Shoot X keeps its tip; shoot Y has its tip cut off. Both get even light from all sides for two days. X grows taller; Y hardly grows. What does this show?
- 1The only difference between the two shoots is the tip.
- 2The shoot with a tip grew, and the one without one barely grew.
- 3So the growth substance (auxin) must be made in the shoot tip. (Notice: the light was even, so this says nothing about light direction.)
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