CB7 — Animal Coordination, Control and Homeostasis

The nervous system

The nervous system coordinates rapid responses to stimuli using electrical impulses.

Components:

• Central nervous system (CNS): brain + spinal cord.
• Peripheral nervous system: sensory neurones (receptor → CNS) + motor neurones (CNS → effector).
• Receptors: detect stimuli (light, sound, temperature, touch, pain, chemicals).
• Effectors: muscles (contract) or glands (secrete).

Reflex arc (fastest involuntary response): Stimulus → Receptor → Sensory neurone → Relay neurone (in spinal cord) → Motor neurone → Effector (muscle or gland)

Reflexes bypass the brain initially (relay neurone in spinal cord) — faster protection from harm. Example: knee-jerk reflex, pupil reflex, withdrawal reflex.

Synapses: gaps between neurones. Impulse arrives → vesicles release neurotransmitter (e.g. acetylcholine) into synaptic cleft → diffuses across → binds to receptors on post-synaptic membrane → triggers new impulse. Drugs can affect synapses (e.g., stimulants increase neurotransmitter release, sedatives inhibit).

The endocrine system

The endocrine system uses hormones (chemical messengers in blood) for slower, longer-lasting, widespread responses.

Blood glucose regulation (negative feedback)

Normal blood glucose ≈ 90 mg/100 cm³ (5 mmol/L).

After a meal (glucose rises): β cells of pancreas detect high glucose → secrete insulin → insulin binds to liver and muscle cells → glucose taken up from blood → converted to glycogen (glycogenesis) and stored → blood glucose falls.

Between meals or exercise (glucose falls): α cells of pancreas detect low glucose → secrete glucagon → binds to liver cells → glycogen broken down to glucose (glycogenolysis) + new glucose made from amino acids (gluconeogenesis) → glucose released into blood → blood glucose rises.

This is negative feedback: a change from the set point triggers a response that reverses the change.

Type 1 diabetes: autoimmune destruction of β cells → no insulin produced → blood glucose dangerously high (hyperglycaemia). Management: insulin injections + diet monitoring. Insulin cannot be taken orally (it's a protein — digested in gut).

Type 2 diabetes: body cells become resistant to insulin; β cells may also produce less insulin. Risk factors: obesity, inactivity, age, genetics. Management: diet, exercise, medication (metformin); sometimes insulin.

The menstrual cycle

28-day cycle (average):

• Day 1: menstruation begins (uterine lining shed).
• Days 1–5: FSH released by pituitary → stimulates follicle to develop → oestrogen secreted.
• Days 5–13: oestrogen rises → uterine lining thickens → LH surge suppressed initially.
• Day 13–14: high oestrogen → triggers LH surge from pituitary → ovulation (egg released from ovary).
• Days 15–28: empty follicle → corpus luteum → secretes progesterone → maintains uterine lining. If no fertilisation: corpus luteum degenerates, progesterone falls → menstruation.

Hormone interactions (positive and negative feedback):

• FSH stimulates oestrogen; oestrogen initially inhibits FSH (negative feedback) then triggers LH surge (positive feedback).
• Progesterone inhibits FSH and LH (negative feedback — prevents new follicle development).

Contraception: oral contraceptive pill contains oestrogen and/or progesterone → suppresses FSH (prevents follicle development) and LH (prevents ovulation).

Thermoregulation

Body temperature maintained at 37°C (optimal for enzymes).

When too hot:

• Vasodilation: blood vessels near skin surface widen → more blood flows near skin → more heat lost by radiation.
• Sweating: evaporation of sweat removes heat energy from skin.
• Hairs lie flat (less insulation in humans — minor).

When too cold:

• Vasoconstriction: blood vessels near skin narrow → less blood near surface → less heat lost.
• Shivering: rapid muscle contractions → generate heat by respiration.
• Hairs stand up (piloerection — traps air, better insulation).
• Increased metabolic rate.

All regulated by the hypothalamus in the brain — the thermoregulatory centre.

Osmoregulation and the kidneys

Kidneys regulate water and ion content of blood (osmoregulation) and remove urea (excretion).

Nephron (functional unit of kidney): glomerulus (filtration) → Bowman's capsule → proximal tubule (reabsorption) → loop of Henle (concentration) → distal tubule → collecting duct (ADH-controlled water reabsorption) → urine.

ADH (antidiuretic hormone) from pituitary: released when blood water potential is low (dehydrated). ADH increases collecting duct permeability → more water reabsorbed → concentrated urine. Low water potential inhibits ADH release → more dilute urine (negative feedback).