Hey there,
Not sure how useful this will be to folks. I’ve gone ahead and tried to lay out the various “sensor agent” types of the human body. I figured this could be helpful in serving as a means of inspiration when developing your own sensors (especially if you’re involved in softbody robotics). Otherwise, if not helpful, hopefully you’ll at least find it interesting 
Now, before we start, we’ll briefly talk about about the transitory flow of information as it progresses from the external environment into the neocortex, and the transforms which happen along the way. Explained simply, the process is as follows:
- Environmental Data is picked up by the sensor agent
- A chemical translation step typically converts that signal into an action potential
- The action potentials are then structurally/topologically organized
- The now organized action potentials are then fed into the initial layers of the neocortex (eg. V1, A1, et cetera).
Today, we’re going to focus on that first step.
In humans, there are generally considered to be ~24 different sensor agents, depending on how you class them, grouped together then into eight different classifications.
Here is a (very) brief explanation of these classifications: Visual Input is concerned with the conversion of photonic energy into information. Auditory converts sound waves. Olfaction interprets aerosolized molecules. Gustation is similar to olfaction, though its molecules are generally heavier and thus interpreted by the tongue. Introception (also known as Interoception) relays information from lower-level embodied systems such as the heart, lungs, and other organs, and can generally be thought of as relaying ‘how one feels.’ The Vestibular System measures balance and spatial orientation and is used to help coordinate movement. Proprioception is the body’s awareness of its static positioning in space, whereas Kinesthesia (kinesthetic) is the body’s awareness of movement through that space.
**Proprioception, Kinesthesia, and Introspection are often grouped together into an umbrella category known as Somatosenation.
A Quick Note on Response Firings:
Response firings are important in defining how sensor agents encode information as well as how quickly that sensor can adapt to environmental change. That said, knowing this may not be applicable to your own project. Additionally, it’ll make an already pretty dense post quite a bit… well, denser. If you do want this info, however, let me know. Otherwise I’ll disinclude it for now.
How the tables are structured:
Below you’ll find hidden tables for each of the above-described classes. In those tables, I’ll list out the cell types most commonly associated with the respective class. Each of those types will then have the following information provided: approximate count (as to help inform sensor resolution), distribution pattern, the sensors function as well as an example of its use (just to help contextualize things).
Lastly, this will be a pretty involved post. I’ll likely be coming back to it throughout the week to include additional detail. If there’s anything else you would like to see, just comment. I’ll try to get it added.
Somatosensory System (Touch, Proprioception, Pain, Temperature)
Mechanoreceptors (Touch & Pressure)
Detect physical deformation such as pressure, vibration, stretch, and body movement. They provide feedback for tactile sensation, grip control, and spatial awareness of limbs.
| Receptor | Approximate Count | Distribution Pattern | Function | Example |
|---|---|---|---|---|
| Merkel Disks (SA1) | ~1,000 per cm² (fingertips), ~100 per cm² (general skin) | High density in fingertips, lips, face; lower density elsewhere else | Detects fine form, edges, and sustained pressure. High spatial resolution. | Holding a pen and recognizing its shape without looking. |
| Meissner Corpuscles (RA1) | ~150 per cm² (fingertips), ~20 per cm² (forearm) | Densely packed in fingertips, palms, lips, and tongue | Detect light touch, motion, and grip control. Rapidly adapting. | Feeling fabric texture when sliding fingers across it. |
| Pacinian Corpuscles (RA2) | ~250 per hand, ~1,000 total | Deep skin layers, highest in palms, soles, and joints | Detect vibration and deep pressure, particularly high-frequency changes. | Feeling vibrations from a phone in silent mode. |
| Ruffini Endings (SA2) | ~200 per cm² (hands), ~1,000 total | Distributed in fingers, palms, joints, deep dermis | Detect skin stretch and help monitor grip and hand posture. | Knowing your fingers are stretched while playing the piano. |
Proprioceptors (Body Position & Movement)
Detect limb position, muscle tension, and joint movement, providing the brain with continuous feedback on body posture, balance, and motion control without relying on vision.
| Receptor | Approximate Count | Distribution Pattern | Function | Example |
|---|---|---|---|---|
| Muscle Spindles | ~25,000 | Highest in fingers, neck, extraocular muscles | Detect muscle stretch and length for proprioception. | Knowing how much your arm is extended without looking. |
| Golgi Tendon Organs | ~4,000 | Located at muscle-tendon junctions | Monitor muscle tension to prevent excessive force. | Sensing when you’re gripping a dumbbell too hard and adjusting grip. |
| Joint Capsule Receptors | ~1,000 per major joint | Found in knees, elbows, shoulders | Detect joint position and movement. | Knowing if your knees are bent while standing in the dark. |
Nociceptors (Pain)
Respond to tissue damage, extreme temperatures, and chemical irritation, signaling pain to protect the body from harm and promote healing.
| Receptor | Approximate Count | Distribution Pattern | Function | Example |
|---|---|---|---|---|
| Thermal Nociceptors | ~200,000 total | Highest in fingertips, lips, face | Detect sharp pain from cuts or punctures. | Feeling pain when stepping on a sharp rock. |
| Mechanical Nociceptors | ~1,000 per cm² | Spread evenly across skin, viscera, muscles | Detect burning heat (>45°C) or extreme cold (<10°C). | The stinging pain from touching a hot stove. |
| Polymodal Nociceptors | ~1,000 per cm² | Found in skin, cornea, internal organs | Detect chemical, thermal, and mechanical pain. | The ache from inflammation after an injury. |
Thermoreceptors (Temperature)
Detect hot and cold stimuli, helping regulate body temperature and respond to environmental thermal changes.
| Receptor | Approximate Count | Distribution Pattern | Function | Example |
|---|---|---|---|---|
| Cold Receptors (Aδ Fibers) | ~1,000 per cm² (fingertips), ~50 per cm² (torso) | Highest in face, hands, lips | Detect cooling (10-35°C). | Feeling a cold breeze on your skin. |
| Warm Receptors (C Fibers) | ~30 per cm² | Scattered, concentrated in face and extremities | Detect warming (30-45°C). | The slow warmth of sunlight on your skin. |
Visual System (Photoreceptors)
Convert light into neural signals, enabling vision, color perception, and motion detection under different lighting conditions.
| Receptor | Approximate Count | Distribution Pattern | Function | Example |
|---|---|---|---|---|
| Rods | ~120 million | High density in peripheral retina, absent in fovea | Detect low-light (scotopic) vision and motion. Very sensitive to light but lacks color detection. | Seeing a silhouette move in the dark. |
| Cones | ~6 million | Highest in fovea (~200,000 per mm²), fewer in periphery | Detect color and high-resolution details (three types: red, green, blue). | Recognizing a red apple versus a green one. |
Auditory System (Cochlear Hair Cells)
Translate sound waves into neural signals, allowing the perception of pitch, volume, and spatial localization of sounds.
| Receptor | Approximate Count | Distribution Pattern | Function | Example |
|---|---|---|---|---|
| Inner Hair Cells | ~3,500 per ear | Arranged in a single row along basilar membrane | Convert sound vibrations into neural signals. Primary auditory processing. | Distinguishing between someone saying “cat” and “bat.” |
| Outer Hair Cells | ~12,000 per ear | Arranged in three rows along cochlea | Amplify sound vibrations and improve auditory sensitivity. | Hearing a whisper more clearly in a quiet room. |
Vestibular System (Balance & Orientation)
Detect head movement, orientation, and acceleration, enabling balance, posture control, and coordination.
| Receptor | Approximate Count | Distribution Pattern | Function | Example |
|---|---|---|---|---|
| Otolith Organs (Utricle & Saccule Hair Cells) | ~30,000 per utricle, ~16,000 per saccule | Found in inner ear’s macula | Detect linear acceleration and head tilt. | Feeling motion when an elevator starts moving |
| Semicircular Canal Hair Cells | ~3,000 per canal (total ~9,000) | Distributed across three semicircular canals | Detect rotational motion and angular acceleration. | Feeling dizzy after spinning around. |
Olfactory System (Smell)
Sense airborne chemicals, allowing the detection of odors and contributing to flavor perception.
| Receptor | Approximate Count | Distribution Pattern | Function | Example |
|---|---|---|---|---|
| Olfactory Receptors | ~5-6 million (in humans) | Located in olfactory epithelium (~5 cm² in nasal cavity) | Detect airborne chemicals and transmit smell information. | Smelling fresh coffee and identifying it instantly. |
Gustatory System (Taste)
Detect chemical composition of food, distinguishing sweet, salty, sour, bitter, and umami flavors to assess edibility and nutrition.
| Receptor | Approximate Count | Distribution Pattern | Function | Example |
|---|---|---|---|---|
| Sweet, Bitter, Umami Receptors | ~5,000-10,000 total (taste buds) | Concentrated on tongue, soft palate, throat | Detects sugars and carbohydrates, potential toxins (alkaloids, quinine), and glutamate (savory, protein-rich flavors). | Tasting honey as sweet, the bitterness of black coffee, or the The richness of soy sauce. |
| Sour, Salty Receptors | Same as above | Located within taste buds on tongue papillae | Detect acidity (H⁺ ions), and sodium (Na⁺) ions. | Tasting lemon juice as sour, or recognizing the saltiness of potato chips. |
Interoceptive System (Internal Monitoring)
Monitor internal body conditions such as blood pressure, hydration, oxygen levels, and pH, helping regulate homeostasis and autonomic functions.
| Receptor | Approximate Count | Distribution Pattern | Function | Example |
|---|---|---|---|---|
| Baroreceptors | ~1,000 per carotid artery, ~1,000 per aortic arch | Found in major arteries (carotid sinus, aortic arch) | Detects blood pressure changes. | Feeling dizzy when standing up too quickly (low BP). |
| Chemoreceptors | ~100,000 | Scattered in arterial blood, brainstem, lungs | Detect pH, O₂, and CO₂ levels in the blood. | Gasping for air after holding your breath. |
| Osmoreceptors | ~100,000 | Hypothalamus, kidneys | Detect water and ion balance (hydration levels). | Feeling thirsty when dehydrated. |
