Coordination and control
The nervous system
The nervous system allows rapid coordination of responses to stimuli.
Central nervous system (CNS): brain + spinal cord. Peripheral nervous system: all nerves connecting CNS to body.
Neurones (nerve cells)
- Sensory neurone: carries impulses from receptor to CNS.
- Relay neurone: connects sensory to motor neurone inside CNS.
- Motor neurone: carries impulses from CNS to effector (muscle or gland).
Structure of a motor neurone: cell body, dendrites (receive signals), long axon (carries impulse), myelin sheath (insulation; speeds up impulse), Schwann cells, axon terminals.
Synapse: gap between neurones. Impulse arrives → vesicles release neurotransmitter into gap → neurotransmitter diffuses across → binds to receptors on next neurone → new impulse generated.
Reflex arc
A reflex is a rapid, involuntary, protective response.
Pathway: stimulus → receptor → sensory neurone → relay neurone (spinal cord) → motor neurone → effector → response.
Example: touching a hot object → hand withdrawn before the conscious brain registers pain.
The endocrine system (hormones)
Hormones are chemical messengers produced by endocrine glands, transported in the blood plasma to target organs. Hormone responses are slower but longer-lasting than nervous responses.
| Hormone | Gland | Target | Effect |
|---|---|---|---|
| Insulin | Pancreas (β cells) | Liver, muscle | Lowers blood glucose → glycogen storage |
| Glucagon | Pancreas (α cells) | Liver | Raises blood glucose → glycogen → glucose |
| Adrenaline | Adrenal glands | Heart, liver, muscles | "Fight or flight"; increases heart rate, blood glucose |
| Oestrogen | Ovaries | Uterus, various | Female secondary sexual characteristics; menstrual cycle |
| Testosterone | Testes | Various | Male secondary sexual characteristics |
| ADH | Pituitary gland | Kidney tubules | Increases water reabsorption; more concentrated urine |
Homeostasis
Homeostasis is the maintenance of a constant internal environment. It uses negative feedback loops:
Stimulus detected by receptor → information sent to control centre → effector produces response → condition returns to set point → response switched off.
Blood glucose regulation
- After a meal (high blood glucose): β cells of pancreas release insulin → liver converts excess glucose to glycogen (glycogenesis) → blood glucose falls.
- Between meals (low blood glucose): α cells of pancreas release glucagon → liver converts glycogen back to glucose (glycogenolysis) → blood glucose rises.
- Type 1 diabetes: β cells destroyed (autoimmune) → no insulin produced → blood glucose uncontrolled. Treated with insulin injections.
- Type 2 diabetes: cells become resistant to insulin. Lifestyle-linked (obesity, poor diet). Managed by diet, exercise, medication.
Body temperature regulation (thermoregulation)
Regulated by the hypothalamus (the body's thermostat).
| Too hot | Too cold |
|---|---|
| Vasodilation (blood vessels near skin widen → more heat lost by radiation) | Vasoconstriction (blood vessels narrow → less heat lost) |
| Sweating (evaporation cools skin) | Shivering (muscle contractions generate heat) |
| Hairs lie flat (reduce insulating air layer) | Hairs stand up (traps air, insulates) |
Water balance (osmoregulation)
- Monitored by hypothalamus (detects blood water potential).
- Dehydrated/concentrated blood: pituitary releases more ADH → kidney tubules more permeable → more water reabsorbed → small volume of concentrated urine.
- Over-hydrated/dilute blood: less ADH released → less water reabsorbed → large volume of dilute urine.
⚠Common mistakes
- Confusing insulin and glucagon — insulin LOWERS glucose (stores it); glucagon RAISES glucose (releases it).
- Saying nerves "send messages" — they carry electrical impulses; signals between neurones are chemical (neurotransmitters at synapses).
- Forgetting that ADH increases water reabsorption — more ADH = more concentrated (not dilute) urine.
- Vasodilation vs vasoconstriction — dilation increases blood flow to skin (cooling); constriction reduces it (warming).
AI-generated · claude-opus-4-7 · v3-ccea-biology