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Sensory systems transduce physical energy (light, sound, pressure, heat, molecules) into neural electrical signals. The transduction process modifies the ionic conductances of the receptor membrane: a stimulus opens or closes specific ionic channels (mechanotransduction, phototransduction, chemotransduction), changing the membrane potential (receptor potential) which, if it exceeds the threshold, generates action potentials in the primary sensory neuron.

The Weber-Fechner and Stevens laws describe the compression of the sensory response: the sensory system assigns the greatest sensitivity (gain) to changes in the range of intensities frequent in the environment. Weber (I/I = constant) captures the proportionality of the differential threshold; Fechner integrates this relationship to obtain P = log(I/I). Stevens finds that the power law (P I^n) is more precise: the exponent n encodes the function of the modality (n < 1: beneficial compression in vision and hearing; n > 1: expansion in pain).

Phototransduction in photoreceptors (rods and cones) follows an amplification chain: 1 photon activates 1 rhodopsin 500 transducins 500 PDEs hydrolyzes 10 cGMP/s closes ~230 Na channels V 1 mV in rods. This process has amplification gain of ~10: hence the ability of the rods to detect individual photons with SNR > 1.

The basilar membrane of the cochlea performs passive mechanical Fourier analysis: each point along its length resonates at a characteristic frequency (tonotopic mapping), with f_CF decaying exponentially from base (20 kHz) to apex (20 Hz) over ~35 mm. The outer hair cells actively amplify (10100×) the vibration of the basilar membrane, sharpening frequency tuning and providing sensitivity up to the level of Brownian movement.

Sensory adaptation is the adjustment of the gain G_adapt to the average level of the stimulus (I_adapt): it allows the receiver to maintain high differential sensitivity in a dynamic range of ~120 dB without saturation. The visual system adapts in seconds (pupil) and minutes (photopigments); the auditory system, in milliseconds (stapedial reflex) and minutes (neural adaptation). Clinically, pure tone audiometry measures hearing thresholds to detect conductive (middle ear pathology) or sensorineural (inner hair cell damage) loss.

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