Sensory receptors, coding and neuronal circuits !

Concept
Exam priority: medium

Sense organs turn touch, light, sound or chemicals into nerve impulses; the brain reads which nerve fires and how fast.

Updated 2026-11-01

Also called: Labeled-line principle, Receptor adaptation

See it in 3D: Skin · Dorsal root ganglia · Grey matter of the cord · Sensory cortex

Key points

Labeled-line principle: the sensation depends on which fibre and which brain area is activated, not on the stimulus (pressing the closed eye gives flashes of light).
Intensity is coded by firing frequency and by the number of receptors recruited (population coding); perceived intensity grows roughly with the logarithm of stimulus strength.
Slowly adapting (tonic): Merkel discs, Ruffini endings, muscle spindles, nociceptors. Rapidly adapting (phasic): Meissner and Pacinian corpuscles, hair follicle receptors.
Pacinian corpuscles sense vibration and adapt within a fraction of a second; Meissner corpuscles sense light touch and flutter; Merkel discs sense steady pressure and texture.
Two-point discrimination depends on receptor density, receptive field size and lateral inhibition: about 2–3 mm on the fingertip, several centimetres on the back.
Circuits: divergence amplifies, convergence integrates, lateral inhibition sharpens contrast, reverberation prolongs, synaptic fatigue damps.

What it is

Each receptor type responds best to one form of energy, its adequate stimulus, and converts it into a graded receptor potential. If that potential reaches threshold, the afferent fibre fires action potentials whose frequency encodes intensity, while the pathway it travels on (the labeled line) encodes modality and location. Receptors adapt: some keep firing to a steady stimulus (tonic), others signal only change (phasic). In the CNS, neuronal pools diverge, converge, inhibit their neighbours and reverberate to shape the signal.

Reference values

Parameter	Value	Note
Two-point discrimination, fingertip	≈2–3 mm
Two-point discrimination, back	≈4–7 cm
Aβ fibre conduction velocity	≈35–75 m/s	touch, vibration
C fibre conduction velocity	≈0.5–2 m/s	slow pain, temperature

Approximate values for a healthy adult at rest; each laboratory sets its own ranges.

Cutaneous and deep receptors

Receptor	Location	Adaptation	Sensation
Meissner corpuscle	Dermal papillae of hairless skin	Rapid	Light touch, flutter
Pacinian corpuscle	Deep dermis, periosteum, mesentery	Very rapid	Vibration
Merkel disc	Basal epidermis	Slow	Pressure, texture, shape
Ruffini ending	Dermis, joint capsules	Slow	Skin stretch, joint position
Free nerve endings	Almost every tissue	Slow (often sensitize)	Pain, temperature, itch
Muscle spindle, Golgi tendon organ	Muscle, tendon	Slow	Length and tension (proprioception)

Clinical notes

Diabetic polyneuropathy first removes vibration sense (128 Hz tuning fork) and protective sensation (monofilament) in the feet.
Phantom limb sensation: activity in the cut nerves or in reorganized cortex is felt in the missing limb, as the labeled-line principle predicts.
Leprosy, still reported in Colombia, destroys cutaneous sensory nerves and leaves numb patches and painless injuries.

Test yourself

Think of the answer, then tap the question to see it.

Why does a person stop feeling a watch on the wrist after a few minutes but notice at once when it slips?

Rapidly adapting receptors (Meissner, hair follicle, Pacinian) stop firing to a steady stimulus and respond only to change; the brain also filters steady input from slow receptors.

How can the same nerve fibre signal both a light and a strong touch?

By frequency: a stronger stimulus makes a larger receptor potential and a higher firing rate, and it also recruits more receptors with higher thresholds.

Why is two-point discrimination much finer on the fingertip than on the back?

The fingertip has many receptors with small receptive fields and a large cortical map, and lateral inhibition sharpens the gap between two points.

Why does pressing on the closed eye produce flashes of light?

The pressure activates retinal neurons whose pathway ends in the visual cortex, and the brain reads any activity on that line as light.

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See it in 3D

The structures of Durgical's 3D anatomy where it happens.

← All of physiology

Educational physiology content; not a diagnostic or treatment guide and no substitute for a health professional's advice. Values are approximate references. No doses are given.

Sources: 14.1 Sensory Perception (OpenStax (Rice University), 2022); 12.4 The Action Potential (OpenStax (Rice University), 2022); Hall JE. Guyton and Hall Textbook of Medical Physiology, 13th ed. (ch. 47). Reference book used to check facts; its text and figures are not reproduced (Elsevier, 2016).

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Representative model, for educational purposes only.