Control systems: receptors, centres and effectors
Homeostasis and response • Homeostasis
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Key concepts
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Receptors: definition and function
Receptors are specialised cells or groups of cells that detect specific changes (stimuli) in the internal or external environment. Receptors are often located in sense organs or in tissues that monitor internal conditions; each receptor type responds only to particular stimuli, such as light, sound, pressure, temperature or chemical concentration. Receptor stimulation generates an electrical signal or triggers the release of a chemical messenger that travels to a coordination centre.
Coordination centres: processing information
Coordination centres receive signals from receptors, integrate the information and select an appropriate response. Examples of coordination centres include the brain, spinal cord and endocrine glands (for example, the pancreas or pituitary). Coordinators convert incoming signals into outgoing commands: electrical impulses sent along nerves for immediate actions or hormones released into the bloodstream for systemic effects. The choice of pathway determines response speed and duration.
Effectors: muscles and glands
Effectors are muscles or glands that produce the response directed by the coordination centre. Muscle effectors change body position or organ function by contracting or relaxing; glandular effectors secrete substances such as hormones or sweat. The action of effectors changes the original condition (cause → effect), producing a response that moves the system toward a stable state. Both the nature of the effector and the size of its response determine how quickly and how completely the original change is corrected.
Signal types and timing
Nervous signals are electrical impulses that travel quickly along neurones and cause rapid, often short-lived responses. Hormonal signals are chemical messengers carried in the blood and cause slower, longer-lasting responses. Fast reflexes use the nervous route; systemic adjustments (for example, control of blood glucose or water balance) use hormonal routes. The difference in transport medium and mechanism explains variations in reaction time and duration.
Typical pathway (cause → effect)
Stimulus → receptor detects change → sensory neurone transmits impulse → coordination centre processes information → motor neurone carries command → effector produces response → condition changes toward normal. The pathway may simplify to stimulus → receptor → coordinator → effector → response for hormonal control where blood-borne messengers replace neurones.
Examples and limiting factors
Thermoregulation: temperature receptors in the skin and brain detect core and surface temperature; the thermoregulatory centre in the brain coordinates sweating, shivering and blood flow to the skin via muscles and glands to restore normal temperature. Hormonal water control: osmoreceptors influence the pituitary gland to change ADH release; ADH alters kidney reabsorption of water, changing urine concentration and blood water content. Response effectiveness depends on receptor sensitivity, signal speed, effector capacity and availability of required substrates (for example, fluid to sweat or hormones to release).
Key notes
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