Animal Form, Nutrition, and Control Systems

This is Unit Twelve of Biology, Part Four: the senses. Part Three built the signalling machinery — resting potential, action potential, synapse. This part is where those signals start, and the through-line is that a sense is a translation. Every receptor converts some physical event, pressure, heat, a chemical, a photon, into the one currency the nervous system reads. What differs between the senses is the translation step, and that is also where each one can fail. By the end you will be able to say how a receptor potential differs from an action potential; name the two ways a system encodes stimulus intensity; say why counting five senses undercounts; say how the cochlea encodes pitch; and say why photoreceptors respond to light the opposite way to almost every other receptor.

Four entries. How a stimulus becomes a signal at all, where a receptor can be reached from, the difference between a receptor firing and a person experiencing something, and how strength gets carried.

Stimulus into signal.

Bio 2871 Sensory Transduction

Sensory transduction is the conversion of a stimulus into an electrical receptor potential, which can trigger an action potential if it reaches threshold. A mechanoreceptor's gated ion channels open when pressure bends its membrane, turning mechanical force into an electrical signal. A receptor potential is graded, scaling with stimulus strength, unlike the all-or-nothing action potential it triggers.

Where it can be reached.

Bio 2873 Receptive Field and Receptor Specificity

A receptor's receptive field is the region in space where a stimulus can activate it, and receptor specificity means each type responds mainly to one kind of stimulus. A touch receptor's field requires contact; a visual receptor's can reach light-years away. A free nerve ending shows little specificity, responding to touch, temperature and pain alike.

Firing, and feeling.

Bio 2875 Sensation versus Perception

Sensation is a receptor's activation by a stimulus; perception is the brain's interpretation of that activation into conscious experience, happening at higher levels rather than at the receptor. Two people with the same dental injury can perceive its pain differently. Assuming identical receptor input guarantees identical experience is the error.

How strong.

Bio 2877 Stimulus Intensity Encoding

Sensory systems encode stimulus intensity two ways at once: a stronger stimulus raises a receptor's firing rate, and it also recruits more nearby receptors. A light touch activates few receptors firing slowly; firm pressure activates many firing rapidly. Rate and recruitment work together — neither alone explains how strength is represented.

Four entries. The four mechanoreceptors in skin, the test that measures how densely they are packed, and the two systems — temperature and pain — that share a pathway to the brain.

Four kinds of touch.

Bio 2879 Somatosensory Mechanoreceptors

Human skin carries several mechanoreceptor types tuned to different touch qualities: Merkel's disks for light touch, Meissner's corpuscles for touch and low-frequency vibration, Ruffini endings for stretch, and Pacinian corpuscles for deep pressure. Merkel's disks, dense in the fingertips, give the fine edge-sensitivity typing needs. They are unevenly distributed — glabrous skin carries far more than hair-covered skin.

Measuring the packing.

Bio 2881 Two-Point Discrimination and Receptor Density

A two-point discrimination test estimates mechanoreceptor density by finding the closest spacing at which two simultaneous touches are still felt as separate. Fingertips distinguish two points much closer together than the back can. Feeling one point where two touched means both fell within a single receptor's receptive field, not that only one receptor is present.

Warm and cold.

Bio 2883 Thermoreception

Thermoreceptors, including cold-detecting Krause end bulbs and warmth-detecting Ruffini endings, detect temperature along pathways that also carry pain signals to the brain. A stimulus that starts as tolerable heat can rapidly be perceived as pain once intensity crosses a threshold. Cold detectors sit nearer the skin surface than warmth detectors, so cold registers first at the same intensity.

Injury, and the experience of it.

Bio 2885 Nociception and the Subjectivity of Pain

Nociception is the neural processing of injurious stimuli; pain is its perception, and tolerance varies among people receiving the same nociceptive stimulus. Capsaicin triggers pain receptors by opening the same channels warmth receptors use, without causing tissue damage. Nociception at the receptor and pain as conscious experience are distinct steps — the second does not automatically scale with the first.

Two entries. The five tastes and the receptor mechanisms behind them, then the combinatorial trick that lets a limited set of olfactory receptors encode thousands of distinct smells.

Five tastes.

Bio 2887 Taste: Five Primary Tastants

Human taste buds detect five primary tastes — sweet, sour, bitter, salty and umami — each transduced by a mechanism matched to the tastant's chemistry. A salty tastant's sodium ions directly excite taste neurons, while sweet and bitter tastants trigger a G-protein-coupled receptor. Umami, the savoury taste of glutamate, was not widely accepted as primary until decades after its discovery.

Thousands, from a few.

Bio 2889 Olfaction: Receptor Diversity and Odor Coding

Olfactory receptors, each responding to specific odorant molecules, combine in overlapping patterns, so a limited set of receptor types can encode thousands of distinct smells. Coffee and chocolate activate overlapping but distinct combinations. Olfactory signals reach the cortex directly, unlike nearly every other sense, which is routed through the thalamus first.

Two entries, a misconception and its correction. The familiar count of five, and the receptor families that count quietly collapses into one.

The familiar count.

Bio 2891 "Humans Have Only Five Senses"

Counting exactly five human senses overstates a tidy number — somatosensation alone already splits into several distinct receptor systems beyond simple touch. Vestibular sense, proprioception and nociception are each physiologically distinct senses this chapter treats separately, and none is among the traditional five. The finer division comes next.

What "touch" contains.

Bio 2893 Somatosensation Splits Into Several Distinct Modalities

What is casually called touch splits into distinct modalities — light touch, pressure, stretch, temperature and pain — each with its own receptor type. A Pacinian corpuscle and a Merkel's disk both register as touch colloquially, yet detect different properties. The five-sense count undercounts by treating this whole receptor family as a single sense.

Five entries. What a sound wave is, the chain that amplifies it, where it becomes an electrical signal, how pitch is encoded by position, and the separate system sharing the same inner ear.

Frequency and amplitude.

Bio 2895 Sound Wave Properties: Frequency and Amplitude

A sound wave's frequency, measured in hertz, is perceived as pitch, and its amplitude, measured in decibels, as volume; humans typically hear between about twenty and twenty thousand hertz. Dogs detect up to around forty thousand, well above the human ceiling. Sound waves need a medium to travel through — unlike light, they cannot cross a vacuum.

Three small bones.

Bio 2897 Outer and Middle Ear: Ossicle Amplification

The outer ear channels sound waves to the tympanum, whose vibration is amplified by the middle ear's three ossicles — malleus, incus and stapes — before reaching the inner ear's oval window. Without the malleus and incus relaying vibration, the stapes alone is too short to reach the tympanum. The ossicles amplify force as well as transmit it.

Where sound becomes signal.

Bio 2899 Cochlea and the Organ of Corti

The fluid-filled cochlea houses the basilar membrane and the organ of Corti, where hair cells transduce fluid pressure waves into electrical signals through bending stereocilia. Bending stereocilia toward their tallest neighbour opens gated ion channels, depolarising the hair cell. Inner hair cells, not the more numerous outer ones, carry most auditory information to the brain.

Pitch, by position.

Bio 2901 Place Theory of Pitch Perception

Place theory holds that high-frequency sounds maximally vibrate the basilar membrane near the cochlea's entrance while lower frequencies vibrate it farther along, encoding pitch by location. Piano strings differ in frequency by about six percent between adjacent notes, while adjacent hair cells differ by two-tenths of a percent. Place theory is a first-order model, not a full account of fine resolution.

The other inner-ear sense.

Bio 2903 Vestibular System: Utricle, Saccule, Semicircular Canals

The vestibular system detects linear acceleration through the utricle and saccule, using gravity-pulled crystals to bend stereocilia, and angular acceleration through three semicircular canals. Fluid shifting in those canals as the head turns signals rotation direction. They detect changes in velocity, not velocity itself — steady motion produces no sensation once the fluid settles.

Six entries. How light is focused, the two photoreceptor classes and where each sits, the response that runs backwards compared with every other receptor, colour as a ratio, processing that happens before the signal leaves the eye, and the chapter's own walkthrough of a note from air to cortex.

Focusing light.

Bio 2905 Eye Anatomy: Cornea, Lens, Iris

Light entering the eye is bent by the cornea and lens to focus on the retina, while the iris adjusts pupil size to regulate how much light enters. A lens that loses flexibility with age causes presbyopia, a farsightedness from focus falling behind the retina. Myopia and hyperopia both concern where the image focuses relative to the retina, in opposite directions.

Two photoreceptors.

Bio 2907 Rods versus Cones

Rods are strongly photosensitive photoreceptors at the retina's edges, specialised for dim, achromatic peripheral vision; cones are weakly photosensitive, concentrated at the fovea, specialised for colour. Looking slightly away from a dim star lets rod-dense peripheral retina detect it better. Cones dominate the fovea — sharpest vision favours colour and detail over sensitivity.

Backwards, uniquely.

Bio 2909 Phototransduction: Rhodopsin and Hyperpolarization

Light striking rhodopsin isomerises its retinal component, triggering a cascade that closes sodium channels and hyperpolarises the photoreceptor — the reverse of how most sensory neurons respond. That hyperpolarisation removes the photoreceptor's steady inhibition of bipolar cells, exciting the ganglion cells that carry the signal onward. Visual receptors hyperpolarise where others depolarise.

Colour as a ratio.

Bio 2911 Trichromatic Color Coding

Human colour vision uses three cone types tuned to short, medium and long wavelengths, and perceived colour emerges from the relative activity ratio across all three rather than from any one cone alone. Equal stimulation of all three is perceived as white light. A two-cone dichromatic system is possible but distinguishes a narrower range of colours.

Sharpening, before it leaves.

Bio 2913 Retinal Processing: Lateral Inhibition

Horizontal cells inhibit neighbouring photoreceptors and bipolar cells when one is stimulated — lateral inhibition, which sharpens edges and boosts contrast before signals leave the retina. It makes a lit region appear brighter and its dark surroundings darker at their boundary. Significant visual processing happens within the retina itself, before the optic nerve.

A note, air to cortex.

Bio 2915 Tracing a Sound Wave From Air to Auditory Cortex, Worked

A mid-range note reaches the ear. The wave is collected by the outer ear and sets the tympanum vibrating; the three ossicles amplify that force through the oval window into the fluid-filled cochlea. The pressure wave travels the basilar membrane and, by place theory, maximally vibrates a specific middle region rather than the extremes reserved for very high or low pitches. There, hair cells have their stereocilia bent, opening channels and depolarising, and the signal travels the auditory nerve through medulla, pons and midbrain to the auditory cortex. Treating the ear as one transducer is the error — amplification, place coding and hair-cell transduction can each fail separately.

Five things to carry out of this part. A receptor potential is graded, scaling with how strong the stimulus is, which is what makes it different from the all-or-nothing action potential it may trigger once it reaches threshold. A sensory system encodes intensity two ways at once — a higher firing rate in each receptor, and the recruitment of more receptors nearby — and neither alone accounts for how strength is represented. Counting five senses undercounts, because what gets called touch is really several modalities, light touch, pressure, stretch, temperature and pain, each with its own receptor type, and because the vestibular sense, proprioception and nociception are distinct senses outside the traditional list. The cochlea encodes pitch by position: high frequencies maximally vibrate the basilar membrane near its entrance and lower frequencies farther along, with hair cells at each location transducing the movement, though place theory is a first-order model rather than a complete account. And photoreceptors run backwards compared with almost every other receptor — light closes sodium channels and hyperpolarises them, which lifts the steady inhibition they were holding on the bipolar cells downstream. Part Five takes up the endocrine system, the slower control network running alongside everything in Parts Three and Four.