Coordination and control

Multicellular animals coordinate their tissues and organs using two complementary systems: the nervous system (fast, electrical, short-lived) and the endocrine system (slow, chemical, long-lasting).

The reflex arc

A reflex is a fast, automatic protective response. It bypasses the conscious brain.

stimulus → receptor → sensory neurone → relay neurone (spinal cord) → motor neurone → effector → response

Synapses link the neurones — the electrical impulse causes vesicles to release neurotransmitter into the gap; this diffuses across and triggers an impulse in the next neurone.

Hormones (endocrine system)

Chemical messengers secreted by glands into the blood, acting on target organs.

Hormone	Gland	Target / effect
Insulin	Pancreas	Liver: convert glucose → glycogen (lowers blood glucose)
Glucagon	Pancreas	Liver: convert glycogen → glucose (raises blood glucose)
Adrenaline	Adrenal	Heart, lungs: fight-or-flight
Thyroxine	Thyroid	Sets metabolic rate

Homeostasis — blood glucose

After a meal, blood glucose rises. The pancreas detects this and releases insulin, causing the liver and muscles to take in glucose and store it as glycogen. If glucose falls too low, the pancreas releases glucagon, which converts glycogen back to glucose.

Type 1 diabetes: pancreas does not produce enough insulin → treated with insulin injections + carbohydrate counting. Type 2 diabetes: cells become resistant to insulin → treated with diet, exercise and (sometimes) medication.

Homeostasis — body temperature

Core temperature is held near 37 °C. Detected by the hypothalamus.

• Too hot → vasodilation of skin arterioles, sweating; hairs lie flat (erector muscles relax).
• Too cold → vasoconstriction, shivering, hairs stand up (trap insulating layer of air); thyroxine raises metabolic rate.

CCEA tip

For "compare nervous and hormonal control" structure your answer in pairs (speed, duration, transmission, target). Banking matched pairs scores faster than listing one system fully then the other.