The nervous system & reflex arc

The nervous system uses fast electrical impulses along neurones. It has the central nervous system (CNS) — the brain and spinal cord — and the peripheral nerves. A reflex is a fast, automatic response that protects the body. Its pathway is the reflex arc:

Synapse: where two neurones meet there is a tiny gap called a synapse. The impulse crosses it using a chemical (neurotransmitter), which then triggers a new impulse in the next neurone.

Put the arc in order

The eye

The eye is a sense organ that detects light. Learn these key parts and jobs:

• Cornea — transparent front; does most of the focusing (refraction).
• Iris — controls the size of the pupil, so it controls how much light enters.
• Lens — fine-focuses light onto the retina (changes shape for near/far — accommodation).
• Retina — contains light receptors that make impulses.
• Optic nerve — carries impulses to the brain.

Pupil reflex: in bright light the circular iris muscles contract and the pupil gets smaller, protecting the retina. In dim light the radial muscles contract and the pupil gets larger.

Hormones & endocrine glands

A hormone is a chemical made by an endocrine gland, carried in the blood to target organs. Hormonal responses are usually slower but longer-lasting than nervous ones.

• Adrenal glands → adrenaline — the "fight or flight" hormone (raises heart rate, breathing rate and blood glucose).
• Pancreas → insulin and glucagon — control blood glucose.
• Ovaries → oestrogen; testes → testosterone — control sexual development.

Nervous or hormonal?

Tap a feature, then tap which control system it describes. (Some are true of both.)

Homeostasis — glucose & temperature

Homeostasis is keeping a constant internal environment. It works by negative feedback: if a level rises too high, the body brings it down, and vice versa.

Blood glucose is controlled by the pancreas:

• Glucose too high → pancreas releases insulin → liver converts glucose to glycogen → glucose falls.
• Glucose too low → pancreas releases glucagon → liver converts glycogen back to glucose → glucose rises.

Temperature is controlled by the skin (about 37 °C):

• Too hot → sweating (evaporation cools) and vasodilation (more blood near the skin loses heat).
• Too cold → shivering (respiration releases heat) and vasoconstriction (less blood near the skin keeps heat in).

Watch out: in diabetes, the body cannot control blood glucose well — Type 1 is treated with insulin injections. Insulin lowers glucose; glucagon raises it.

After a sugary meal

Your turn — temperature change

Your turn — mean reaction time

Tropisms & auxin

Plants respond to stimuli by growing — this is a tropism. The response is controlled by a plant hormone called auxin.

• Phototropism — growth in response to light. Shoots grow towards light (positive phototropism).
• Gravitropism (geotropism) — growth in response to gravity. Roots grow downwards (positive gravitropism); shoots grow upwards.

How auxin works in a shoot: auxin gathers on the shaded side of the shoot. It makes those cells grow (elongate) more, so the shoot bends towards the light.

Why does the shoot bend?

Match each gland/hormone to its job

Tap a job on the left, then its matching hormone on the right.

Nervous vs hormonal

The big ideas to know

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