Homeostasis and nervous coordination: negative feedback made simple

The negative feedback template

Every homeostasis answer fits the same frame: a change is detected by a receptor, information passes to a coordinator, an effector acts, and the change is reversed towards the set point. Naming the receptor and effector is where the marks are.

Negative feedback reverses a deviation; positive feedback amplifies it (as in the opening of sodium voltage-gated channels during depolarisation). Be explicit about which one you are describing.

Blood glucose

• High glucose: beta cells in the islets of Langerhans secrete insulin; more glucose channels are inserted into cell membranes; glycogenesis in liver and muscle.
• Low glucose: alpha cells secrete glucagon; glycogenolysis and gluconeogenesis in the liver raise blood glucose.
• Adrenaline also triggers glycogenolysis via the second-messenger model with cyclic AMP and protein kinase.

The nerve impulse in numbers

• Resting potential −70 mV, maintained by the sodium–potassium pump (3 Na⁺ out, 2 K⁺ in) and greater membrane permeability to K⁺.
• Threshold reached: voltage-gated sodium channels open, Na⁺ floods in, depolarisation to about +40 mV.
• Repolarisation: sodium channels close, potassium channels open, K⁺ leaves; brief hyperpolarisation follows.
• The refractory period ensures impulses are discrete and travel in one direction only.

Synapses

Depolarisation opens calcium channels; Ca²⁺ entry causes vesicles to fuse with the presynaptic membrane and release acetylcholine by exocytosis. It diffuses across the cleft and binds to receptors on the postsynaptic membrane, opening sodium channels. Acetylcholinesterase hydrolyses it so the response stops — mention this whenever a question involves an inhibitor drug.