Nervous & endocrine systems, homeostasis

Year 9 answers

Fluency

Dendrites: receive signals from other neurons. Cell body: contains the nucleus; integrates incoming signals. Axon: carries the nerve impulse away to the next cell (often insulated by myelin for speed).
A synapse is the gap between two neurons. When an impulse arrives, neurotransmitters are released from vesicles, cross the gap, and bind to receptors on the next neuron, triggering a new impulse.
E.g. pancreas - insulin (lowers blood glucose); thyroid - thyroxine (raises metabolic rate); adrenal - adrenaline (fight-or-flight); pituitary - growth hormone.
Homeostasis is the maintenance of a stable internal environment. Examples: body temperature, blood glucose, blood pH, water/salt balance.
Stimulus, receptor, control centre, effector (with response).
Nervous: fast (milliseconds), short-acting, electrical impulses along neurons. Endocrine: slow (seconds to days), long-acting, chemical hormones via blood.

Reasoning

Reflexes bypass the brain via the spinal cord, saving time. This minimises tissue damage in dangerous situations (hot surfaces, sharp objects).
Insulin is a protein. Digestive enzymes in the stomach and small intestine would break it down before it could act. Injection bypasses the digestive system and delivers insulin directly to the blood.
Possible effects: weight loss, rapid heart rate, feeling hot, anxiety, increased appetite, tremor — all from elevated metabolism.
Adrenaline, released by the adrenal glands (above the kidneys).
Rising glucose is detected by the pancreas; beta cells release insulin; liver and muscle cells take up glucose (stored as glycogen/fat); blood glucose returns to the set point.

Problem solving

Stimulus: body temperature rises above $37^{\circ}\text{C}$ . Receptor: thermoreceptors in skin/hypothalamus. Control centre: hypothalamus. Effectors: skin blood vessels (vasodilate), sweat glands (sweat), behaviour (seek shade). Response: heat lost by radiation and evaporation; temperature returns to set point.
Blood glucose rises and stays high (hyperglycaemia) because cells cannot take up glucose without insulin. Injected insulin replaces the missing hormone, allowing uptake and restoring normal levels.
Stimulus: core temperature falls below set point. Receptors and hypothalamus detect the fall. Effector: skeletal muscles contract rapidly (shivering), generating heat through respiration. Response: temperature rises back to set point.
Catching a ball $\approx 0.3$ s; puberty $\approx 2$ - $4$ years $\approx 10^{8}$ s. About 8-9 orders of magnitude apart.

Reasoning

Positive feedback — the response amplifies the stimulus. It stops when the baby is delivered, which removes the pressure on the cervix that triggered oxytocin release.
The adrenal glands sit on top of the kidneys, so adrenaline enters a major blood vessel near the heart and is pumped rapidly throughout the body. It also acts on receptors already present on many cell types, giving rapid widespread effects.
Stimulus: blood becomes too dilute (low solute concentration). Receptors: osmoreceptors in hypothalamus. Effector: pituitary reduces ADH release; kidneys reabsorb less water. Response: more dilute urine is produced, restoring normal blood water content.
Acetylcholine builds up in synapses, causing continued muscle stimulation — twitching, cramping, weakness. Overdose can cause paralysis of respiratory muscles and death. (This is how some nerve agents and pesticides kill.)