Abstract

A sensory receptor, in any organism anywhere, is sensitive through time to some distribution - energy, motion, molecular shape - indeed, anything that can produce an effect. The sensitivity is rarely direct: for example, it may track changes in relative variation rather than the absolute change of state (as when the skin responds to colder and hotter instead of to cold and hot as such); it may track differing variations under different conditions (the eyes' dark-adaptation; adaptation to sound frequencies can lower the difference threshold; the kinesthetic sense will shut down if a limb is held in a stationary position too long - the limb 'going to sleep'); it may be subject to distortion of the input from overloading (dazzle producing strong-after-images); it may not be confined to one channel of sensitivity (the retina is sensitive to pressure; the hands can feel some strong sound-vibrations, the tympanum of the ear records touch). Strictly speaking there is no limit as to what intensities and what ranges receptors could be sensitive. Sharks are sensitive to electrostatic fields, homing pigeons to magnetic fields; snakes to infra-red rays; bacteria to acid concentrations; perhaps there has even been a mutant organism sensitive to the passage of cosmic rays, even though that would hardly have bestowed any conceivable survival value. What is irrefutable is that individual receptors differ markedly from organism to organism, between different members of the species (one dog being better at tracing smells than another; one person being able to sense light-waves of 375 nanometres, another not; children able to hear 20,000 Hz, older persons not), and between receptors of the same kind within one organism (one eye being sensitive to 765 nm and the other not; one ear deaf to 15,000 Hz and over, the other not). There are also just-noticeable-differences (JND's), in that one person can see two shades of a colour where another sees only one; similarly with sounds