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Coordination & Response

Nervous system, eye, hormones, homeostasis

Central Nervous System (CNS)

Brain: Control centre. Processes sensory info. Coordinates response.

Spinal cord: Carries signals between brain & peripheral nerves. Coordinates reflex arcs.

Peripheral nervous system: Sensory nerves (in) + motor nerves (out)

Reflex Arc — Fastest Response

Stimulus → Receptor → Sensory neurone → Relay → Motor neurone → Effector → Response

Example: Touch hot plate → skin receptors → signal to spinal cord → motor neurone fires → arm pulls away (all in ~0.01 seconds)

Why so fast: No need to send signal to brain first. Spinal cord relays directly.

Effectors: muscles (movement) or glands (hormone release)

Synapses — Chemical Communication

Synapse: Gap between two neurones (20-30 nm). Electrical signal → chemical transmission

Process: Signal reaches axon terminal → vesicles release neurotransmitter → crosses synapse → binds to receptor on next neurone → new signal starts

Advantages: Can be amplified, modulated, allows one-way transmission

Some drugs block synapses (e.g., curare paralyses muscles)

Eye Structure & Focusing (Accommodation)

Cornea: Transparent front. Does most refraction (bending light).

Iris: Coloured muscle. Controls pupil size (more/less light).

Lens: Focuses fine details. Changes shape for near/far objects.

Retina: Light-sensitive layer. Contains photoreceptors (rods, cones). Sends signals to brain.

Ciliary muscle controls lens shape. Suspensory ligaments hold lens.

Accommodation — Focusing on Near Objects

Looking at near object: Ciliary muscles CONTRACT → reduce tension on suspensory ligaments → lens becomes THICKER (fatter) → refracts light MORE → focuses near object on retina

Looking at distant object: Ciliary muscles RELAX → suspensory ligaments PULL tight → lens becomes THIN (flattens) → refracts light LESS → focuses distant object

With age: Lens loses elasticity → can't focus on near objects (presbyopia). Solution: reading glasses.

Hormones — Endocrine Regulation

Insulin: Made by pancreas. Lowers blood glucose. Stimulates glucose uptake by cells, glycogen synthesis.

Glucagon: Made by pancreas. Raises blood glucose. Breaks down glycogen to glucose, promotes glucose synthesis.

Adrenaline: Made by adrenal gland. Fight-or-flight response. Increases heart rate, blood pressure, glucose release.

Note: Hormones slower than nerves but longer-lasting. Carried in blood.

Homeostasis — Maintaining Stability

Homeostasis: Body maintains constant internal environment (temp, glucose, water, pH)

Negative feedback: Change in environment → body corrects it back to normal

Example: High blood glucose → pancreas releases insulin → glucose uptake increases → glucose falls → insulin stops. Returns to normal.

Body temperature: 37°C is optimal for enzymes. Too high (fever) or too low → problems.

Tropisms — Plant Growth Responses

Phototropism: Growth towards light. Auxin (plant hormone) distributes unevenly → shaded side grows more → stem bends toward light.

Gravitropism: Roots grow down; shoots grow up. Auxin response: high auxin in roots slows growth (geotropism), low auxin in shoots slows growth.

Auxin: Plant hormone made in shoot tip. Promotes growth at low concentration, inhibits at high concentration.

Exam Tips

✓ Reflex arc: stimulus → receptor → sensory → relay → motor → effector → response

✓ Synapse: gap, neurotransmitters, one-way transmission

✓ Accommodation: near = thick lens (ciliary contract); far = thin lens (relax)

✓ Insulin ↓ glucose; glucagon ↑ glucose

✓ Negative feedback maintains homeostasis

Key Takeaways

✅ CNS: brain + spinal cord; controls body

✅ Reflex arc: fast response via spinal cord

✅ Synapse: chemical transmission between neurones

✅ Accommodation: ciliary muscles change lens shape

✅ Insulin lowers blood glucose; glucagon raises it

✅ Homeostasis: negative feedback maintains stability

✅ Auxin: plant hormone; phototropism & gravitropism

Coordination keeps everything in balance! 🧠✓