Biology B13 of Cambridge IGCSE Co-ordinated Sciences 0654 joins rapid electrical coordination to slower hormonal control and negative-feedback homeostasis. The official boundary covers neurones and a simple reflex arc, sense-organ stimulus types, named endocrine glands, adrenaline, blood glucose and temperature control. Synapses, detailed eye optics and plant tropisms are outside this topic.
Nervous-system organisation
Electrical impulses travel along neurones. The mammalian nervous system coordinates and regulates body functions by detecting changes, processing information and producing responses.
The central nervous system, or CNS, consists of the brain and spinal cord. The peripheral nervous system, or PNS, consists of nerves outside the brain and spinal cord. Peripheral nerves connect receptors and effectors to the CNS.
Sensory neurones carry impulses from receptors toward the CNS. Relay neurones connect neurones within the CNS. Motor neurones carry impulses from the CNS to effectors. In diagrams, identify a neurone by the position of its cell body, direction of impulse and connections rather than by one memorised outline alone.
An effector is a muscle or gland. Muscles respond by contracting; glands respond by secreting. A receptor detects a stimulus, while an effector carries out the response.
The reflex arc
A reflex action automatically and rapidly integrates a stimulus with the response of an effector. A simple reflex arc follows this order:
For example, heat stimulates receptors in skin. A sensory neurone carries impulses to the spinal cord, a relay neurone transfers the signal to a motor neurone and the motor neurone stimulates an arm muscle to contract.
Rapid automatic action reduces delay and can protect the body. The brain may receive information about the stimulus, but conscious decision is not required before the initial reflex response. A reflex is therefore not accurately described as a pathway that completely excludes the CNS or brain awareness.
Sense organs and stimuli
Sense organs are groups of receptor cells that respond to specific stimuli. Cambridge names light, sound, touch, temperature and chemicals. Match the receptor group to the stimulus and then trace any response through the nervous system. Detailed eye structure and image formation are not required here.
Hormones and endocrine glands
A hormone is a chemical substance produced by a gland, carried by blood and altering the activity of one or more specific target organs.
The adrenal glands secrete adrenaline. The pancreas secretes insulin and, at Supplement level, glucagon. The testes secrete testosterone. The ovaries secrete oestrogen.
Hormones travel throughout the circulation, but only target organs with appropriate receptors respond. They do not travel along neurones.
Nervous control is generally faster and its effect shorter-lasting. Hormonal control is generally slower and its effect longer-lasting. Nervous impulses travel along specific neurones, while hormones travel in blood.
Adrenaline and fight or flight
Adrenaline is secreted in fight-or-flight situations. Its required effects are increased breathing rate, increased heart rate and increased pupil diameter.
Higher heart rate moves blood to tissues more rapidly. Higher breathing rate supports gas exchange, and wider pupils increase light entry.
Adrenaline prepares the body for rapid action; it does not directly supply energy as a material substance.
Homeostasis and negative feedback
Homeostasis is maintenance of a constant internal environment. "Constant" means kept within a suitable range around a set point, not absolutely unchanged every second.
Negative feedback reverses a departure from the set point. Receptors detect a change, a coordination centre processes it and effectors produce a response that reduces the original change. Once the variable returns toward the set point, the corrective response is reduced.
Negative feedback therefore opposes change. It is not "negative" because it is harmful.
Blood-glucose control
Insulin decreases blood glucose concentration. When concentration rises, the pancreas releases insulin. Insulin causes the liver to take up glucose and convert glucose to glycogen for storage, lowering blood glucose toward the set point.
When blood glucose concentration falls, the pancreas releases glucagon. Glucagon causes the liver to convert glycogen to glucose and release glucose into blood, raising concentration toward the set point.
Insulin and glucagon are antagonistic hormones. The liver is the target organ managing the glucose-glycogen store; the pancreas detects and secretes the controlling hormones.
Temperature homeostasis
The brain coordinates responses to deviations in internal body temperature. Temperature receptors provide information, and effectors alter heat loss or heat production.
When the body is too hot, sweat glands release sweat. Evaporation transfers heat from skin to the environment. Arterioles supplying surface capillaries dilate, increasing blood flow near the surface and increasing heat transfer to the surroundings. Hair erector muscles relax so hairs lie flatter, trapping less insulating air.
When the body is too cold, skeletal muscles shiver. Repeated contractions release more heat through respiration. Arterioles supplying skin capillaries constrict, reducing surface blood flow and heat loss. Hair erector muscles contract, raising hairs and helping trap an insulating air layer. Fatty tissue beneath skin also provides insulation.
Vasodilation and vasoconstriction refer to arterioles supplying surface capillaries, not the capillaries actively widening themselves. Vasoconstriction reduces but does not necessarily stop all skin blood flow.
The required skin structures are hairs, hair erector muscles, sweat glands, receptors, sensory neurones, blood vessels and fatty tissue. Link each named part to detection, coordination, insulation or heat-loss control.
Worked application: connect fast and slow coordination
A student touches a hot surface and withdraws a hand before consciously deciding. Skin receptors generate impulses that travel along a sensory neurone to the spinal cord. A relay neurone connects to a motor neurone, which carries impulses to an arm muscle. The muscle contracts as the effector response. Later, a fight-or-flight situation causes the adrenal glands to release adrenaline into the blood. Heart rate and breathing rate increase and pupil diameter becomes larger. The reflex uses electrical impulses along a specific nervous route; adrenaline is a chemical carried in blood to target organs. Both coordinate responses, but by different signal routes.
Common misconceptions and corrections
Saying impulses travel in blood. Electrical impulses travel along neurones.
Calling every nerve part of the CNS. Nerves outside brain and spinal cord form the PNS.
Sending sensory impulses from CNS to muscle. Motor neurones carry that direction.
Calling a relay neurone an effector. It connects neurones within the CNS.
Calling a receptor a muscle. Receptors detect; muscles respond.
Leaving the relay neurone out of the named simple reflex arc. It connects sensory and motor neurones.
Saying reflexes are always conscious decisions. They are rapid and automatic.
Saying a reflex bypasses the CNS. The simple arc is integrated through the CNS.
Saying hormones travel along nerves. They are carried in blood.
Saying every organ responds to every hormone. Specific target organs respond.
Pairing adrenaline with the pancreas. Adrenal glands secrete it.
Pairing insulin with the adrenal gland. The pancreas secretes it.
Saying hormonal control is always faster. Nervous control is generally faster.
Saying adrenaline is energy. It alters activity and increases substrate availability.
Defining homeostasis as no internal change. Variables fluctuate around set points.
Calling negative feedback harmful feedback. It reverses deviations.
Saying insulin raises blood glucose. It lowers it.
Saying glucagon lowers blood glucose. It raises it.
Saying the pancreas stores glycogen. The liver manages the named glucose-glycogen conversion.
Treating insulin and glucagon as the same hormone. They have antagonistic effects.
Saying sweat cools before it evaporates. Evaporation removes heat.
Saying vasodilation decreases skin blood flow. It increases surface flow.
Saying shivering reduces respiration. Muscle contraction increases energy transfer and heat production.
Saying fat increases heat loss. It insulates.
Assessment guidance
For nervous answers, preserve direction: receptor, sensory neurone, relay neurone, motor neurone and effector. Define a reflex as automatic and rapid, and identify effectors as muscles or glands. Define hormones through gland, blood and target organ, then match each named gland to its hormone. Homeostasis answers need a set point and a response that reverses deviation. Keep insulin and glucagon antagonistic, with the liver as the named target controlling glucose and glycogen. Explain thermoregulation through the brain, sweat evaporation, shivering, insulation and vasodilation or vasoconstriction of arterioles supplying surface capillaries.
Retrieval practice
Draw and annotate a reflex arc and a skin section. Match light, sound, touch, temperature and chemicals to sense-organ receptor groups. Build paired negative-feedback loops for glucose and temperature, compare nervous with hormonal control, list the three required adrenaline effects and reconstruct the hot-body and cold-body responses without confusing capillaries with the supplying arterioles.
Theory and practical ownership
This theory note owns nervous-system organisation, reflex arcs, sense-organ stimulus types, named hormones and homeostatic control of glucose and temperature. The dedicated Co-ordinated Sciences practical series owns planning, controls, safe observation, measurement, tables, graphs and evaluation.
