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Sensory receptors transduce specific stimuli (mechanical, thermal, chemical, electromagnetic) into receptor potentials and ultimately action potentials that travel to the CNS. General senses include touch, pressure, vibration, pain (nociception), temperature, and proprioception; special senses are vision, hearing, balance (vestibular), smell (olfaction), and taste (gustation). In the eye, photoreceptors (rods for dim light/achromatic, cones for color) convert light via phototransduction; the retinal image is processed in the visual cortex. In the ear, the cochlea converts sound waves via the basilar membrane into hair cell deflection. Sensory information is projected to the somatosensory cortex via the thalamus, with body regions mapped in the somatosensory homunculus.
Trace the visual pathway from photoreceptor to visual cortex, noting the optic chiasm and the implications for visual field defects. For hearing, follow sound energy from air vibration to cochlear fluid wave to neural signal.
You already know from your study of neurons and action potentials that the nervous system speaks a single language: voltage spikes traveling along axons. The entire sensory system is therefore built around a fundamental translation problem — how do you convert light, sound, pressure, chemicals, and heat into that one electrical language? The process is called sensory transduction, and it is the defining task of every sensory receptor. A sensory receptor is a specialized structure (or a specialized neuron) that detects a particular form of energy and converts it into a receptor potential — a graded change in membrane voltage that, when large enough, triggers action potentials in the afferent sensory neuron. Once the signal is in action potential form, it travels the same way all nerve signals do, following the pathways you already understand.
The general senses are distributed throughout the body: touch receptors (Meissner's corpuscles, Merkel's discs), vibration detectors (Pacinian corpuscles), pain fibers (nociceptors), temperature receptors (thermoreceptors), and proprioceptors in muscles and joints that report body position. Each type uses a different receptor structure adapted to its stimulus modality, but all eventually produce action potentials that travel via spinal nerves to the dorsal horn of the spinal cord, ascend through the spinothalamic tract or dorsal columns, relay through the thalamus, and terminate in the somatosensory cortex. The body is mapped onto this cortex as the familiar somatosensory homunculus — a distorted body map where regions with dense receptor populations (fingertips, lips) occupy disproportionately large cortical territory.
The special senses have dedicated organs that perform highly specialized transduction. In vision, light enters the eye and is focused on the retina, where photoreceptors perform phototransduction. Rods use the pigment rhodopsin and respond to low light intensities but cannot discriminate wavelength — they give you black-and-white sensitivity in dim conditions. Cones come in three types (L, M, S — roughly red, green, blue) and compare wavelength through differential activation, enabling color vision. The optic nerves from both eyes meet at the optic chiasm, where fibers from the nasal half of each retina cross to the opposite side. The result is that your left visual field (detected by the right half of each retina) is processed by your right visual cortex, and vice versa. In hearing, sound waves enter the ear canal, vibrate the tympanic membrane, and are amplified through the ossicles before reaching the fluid-filled cochlea. The basilar membrane inside the cochlea vibrates at different positions depending on sound frequency (high-frequency sounds near the base, low-frequency near the apex), and hair cells detect this vibration mechanically, converting it into neural signals carried by the auditory nerve.
What ties all sensory systems together is the concept of the sensory pathway: receptor → afferent neuron → relay in CNS (often the thalamus) → sensory cortex. The pathway maintains topographic organization — neighboring body regions, neighboring cochlear positions, or neighboring retinal positions map to neighboring cortical positions. This spatial precision is what lets the brain know not just that a stimulus happened, but where it happened, how intense it was, and what kind of stimulus it was. The specificity of receptor types (only responding to their appropriate stimulus modality) is called the labeled-line principle: when a pain fiber fires, the brain interprets it as pain regardless of what actually triggered it — which is why pressing on your eyeball in the dark produces flashes of light rather than pressure sensation.