Every fact in the three senses lessons is listed here with where it comes from. The numbers in [brackets] on the lesson pages link to this list.
How we rate sources
High: space agencies (NASA), peer-reviewed open textbooks (OpenStax, Rice University) and professional scientific societies (American Physical Society).
Medium-high: reputable science news reporting directly on a primary document.
Medium: single-author essays and popular history sites. We only use these for history details, and never on their own: each one is paired with a second source.
Wikipedia was not used as a source. Numbers are rounded (for example 9.8 m/s² for Earth's gravity). The text was drafted with help from AI (Claude) and then checked against these sources. Corrections: hello@echoesof.science.
Simplifications and care points: this is general science education, not medical advice. 'Five senses' is the everyday picture; the textbook splits the senses into special and general senses and into as many as 17 modalities, and balance is a special sense too. Two-point thresholds (about 2–3 mm on a fingertip, 35–40 mm on the arm) are typical values and vary between people and with the method. A possible sixth taste for fats is still being researched. The food test uses only fruit the school allows; check allergies and school rules first. Questions about a pupil's own sight, hearing, smell or taste go to a parent, the school nurse or a doctor.
All sources
[1] Anatomy and Physiology, 14.1 Sensory Perception
OpenStax, Rice University · Reliability: High
Type: Peer-reviewed open textbook
Special senses with their own organs: taste, smell, hearing, balance and vision; general senses spread through the body, mainly somatosensation (touch, pain, temperature, proprioception). The major senses can be divided into as many as 17 modalities. Receptors are classed by structure (free nerve endings, encapsulated endings, specialised receptor cells), by location (exteroceptors, interoceptors, proprioceptors) and by function (chemoreceptors, osmoreceptors, nociceptors, mechanoreceptors, thermoreceptors, photoreceptors). Tastes: sweet, salty, sour, bitter and umami (L-glutamate), with a possible sixth taste for fats; salty and sour are triggered by Na⁺ and H⁺ ions, the others through G protein–coupled receptors. Smell is the only sense that does not synapse in the thalamus before the cerebral cortex and connects to the limbic system. Hearing spans 20 to 20,000 Hz: low frequencies activate hair cells at the apex of the cochlea, high frequencies at the base; hair cells respond when their stereocilia are bent. The utricle, saccule and semicircular canals sense head position and rotation. Merkel discs: low-frequency vibration (5–15 Hz); Meissner's corpuscles: light touch, vibration below 50 Hz; Pacinian corpuscles: deep pressure, high-frequency vibration (around 250 Hz); Ruffini corpuscles: stretch; free nerve endings: pain and temperature.
National Institute on Deafness and Other Communication Disorders (NIH, USA) · Reliability: High
Type: National health research agency
At birth you have about 10,000 taste buds, on the tongue, the roof of the mouth and the lining of the throat. Taste cells respond to five basic qualities: sweet, sour, bitter, salty and umami (from glutamate, found in broth and some cheeses), and the cells for different qualities are spread across the tongue. Flavour combines the tastes with the common chemical sense, heat, cold, texture and aroma; aromas released by chewing reach the smell receptors through a channel from the roof of the throat to the nose, so a blocked channel (for example from a cold) makes food taste bland. Taste cells and smell cells are the only sensory cells regularly replaced throughout life.
National Institute on Deafness and Other Communication Disorders (NIH, USA) · Reliability: High
Type: National health research agency
Smell depends on olfactory sensory neurons in a small patch of tissue high inside the nose, which connect directly to the brain; each neuron has one odour receptor. Molecules released by substances such as coffee or pine trees stimulate them. There are more smells than receptors, so one molecule can stimulate several receptors and the brain reads the pattern. Odours reach the receptors through the nostrils and through the channel from the roof of the throat to the nose; smell and taste work closely together, and without smell food often tastes bland. Smell warns of hazards such as gas leaks, spoiled food, smoke or fire.
Neuroscience for Kids, University of Washington · Reliability: Medium-high
Type: University neuroscience education site
Feeling two points rather than one depends on the density of receptors in the skin and on how sensory nerve cells connect in the brain (small receptive fields, so that neighbouring receptors connect to different central neurons). On the fingertips two points can be told apart when about 2–3 mm apart; on the arm they must be about 35–40 mm apart. Fingertip skin has about 100 times more receptors per square centimetre than skin on the back. Receptors convert 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 ones fire mainly when it starts or stops. Table: Merkel's disk (steady indentation, pressure, slow), Meissner's corpuscle (low-frequency vibration, gentle fluttering, rapid), Ruffini's corpuscle (stretch, slow), Pacinian corpuscle (vibration, rapid), hair receptor (brushing).
National Eye Institute (NIH, USA) · Reliability: High
Type: National health research agency
Light passes through the cornea, which is shaped like a dome and bends light; the iris, the coloured part of the eye, controls how much light the pupil lets in; the lens works with the cornea to focus light on the retina, where photoreceptors turn light into electrical signals; these travel along the optic nerve to the brain, which turns them into the images you see.