Sensory receptors: transduction, receptive fields and thresholds
Classifying receptors, transduction and adaptation, the touch receptors and their frequencies, two-point thresholds and receptive fields, taste transduction, smell pathways and cochlear frequency mapping. With a two-point threshold investigation.
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The familiar five senses can be divided into as many as 17 modalities. Receptors are classified by structure (free nerve endings, encapsulated endings, specialised receptor cells), by location (exteroceptors, interoceptors, proprioceptors) and by the stimulus they respond to (chemo-, osmo-, noci-, mechano-, thermo- and photoreceptors).[1]
A receptor transduces mechanical, thermal or chemical energy into electrical signals that travel along axons to the central nervous system. Slowly adapting receptors keep firing during a constant stimulus; rapidly adapting receptors fire mainly when a stimulus starts or stops.[4]
Skin has several mechanoreceptors: Merkel discs respond to steady pressure and low-frequency vibration (about 5–15 Hz), Meissner's corpuscles to light touch and vibration below 50 Hz, Pacinian corpuscles to deep pressure and high-frequency vibration (around 250 Hz) and Ruffini corpuscles to stretch. Free nerve endings detect pain and temperature.[1][4]
Telling two points apart needs a high receptor density and small receptive fields, so that the two points activate different central neurons. Typical two-point thresholds are about 2–3 mm on a fingertip and 35–40 mm on the arm; fingertip skin has about 100 times more receptors per cm² than skin on the back.[4]
Salty and sour tastes are triggered directly by the ions Na⁺ and H⁺; sweet, bitter and umami (L-glutamate) work through G protein–coupled receptors. Some research suggests a possible sixth taste for fats.[1]
Each olfactory neuron carries one type of odour receptor, but there are more smells than receptor types, so a smell is coded as a pattern across many receptors. Smell is the only sense that reaches the cerebral cortex without first synapsing in the thalamus, and it connects to the limbic system.[3][1]
Hearing spans about 20 Hz to 20 kHz: low frequencies activate hair cells at the apex of the cochlea and high frequencies those at the base, so pitch is mapped along the cochlea. Taste and smell cells are the only sensory cells regularly replaced throughout life.[1][2]
Try it: Two-point thresholds and receptive fields
You need
- A paper clip bent into a U with blunt, rounded ends (or a two-point caliper from the lab)
- A ruler with mm markings
- Results table
- Calculator
Safety
Use blunt, rounded ends only and light pressure; never break or scratch the skin. Do not test near the eyes or on the face. Test only healthy, intact skin, and only on someone who has agreed; anyone may stop at any time.
What to do
- Work in pairs. The subject sits with eyes closed. Test four sites: fingertip, palm, back of the hand and forearm.
- Set the two ends 40 mm apart. Touch the skin gently, with both ends at the same moment and the same light pressure. Randomly mix in touches with only one end, so the subject cannot guess.
- The subject says 'one' or 'two'. Reduce the gap in 5 mm steps, then in 1 mm steps near the point where 'two' stops being reported.
- Define the threshold as the smallest gap at which 'two' is reported correctly in at least 3 of 4 two-point touches. Repeat the whole search 3 times per site.
- Calculate the mean threshold and the uncertainty (half the range) for each site. Calculate the ratio forearm ÷ fingertip and compare with the textbook values (about 2–3 mm and 35–40 mm).
What happens, and why?
Two touches feel separate only if they stimulate receptors whose signals reach different central neurons. Fingertips have a high receptor density and small receptive fields, so their threshold is a few millimetres; on the forearm the receptive fields are large, so two points several centimetres apart can still feel like one. One-point catch trials check that the subject is not just guessing 'two'.[4]
Play: Which touch receptor?
Each card describes how a skin receptor responds. Which receptor is it?
Your best score is kept only on this device. Nothing is sent anywhere.
Quiz
Other ages: Ages 6–7 · Ages 10–11