Sensory Systems and Somatosensation

A graded, non-regenerative depolarisation produced by a stimulus in a sensory receptor — analogous to an EPSP; if it reaches threshold at the first node of Ranvier, action potentials fire.

Mechanosensitive cation channels of the Piezo2 family; membrane deformation opens them, allowing Na⁺/Ca²⁺ influx and depolarisation.

Capsaicin opens TRPV1, the same channel opened by temperatures >43 °C — identical signal on the same labelled line, so the brain cannot distinguish them. (TRPM8 = cooling and menthol.)

Light closes cGMP-gated cation channels, producing hyperpolarisation instead of depolarisation.

By labelled lines — each axon carries one kind of information based on where it starts and ends (Müller's law of specific nerve energies); pressing the eye yields phosphenes, not pressure.

By which fibres fire within a somatotopic map; receptive field size sets acuity (fingertip 1–2 mm vs back several cm), and lateral inhibition sharpens the activity peak (edge enhancement).

Frequency coding (more APs per second in a fibre) and population coding/recruitment (stronger stimuli recruit higher-threshold fibres).

Stevens: Ψ = k·S^n (perceived intensity is a power function of stimulus). Weber: ΔS/S = constant (just-noticeable difference is a fixed fraction).

Merkel discs, Ruffini endings, muscle spindle, nociceptors — report sustained stimuli such as pressure, posture and ongoing pain.

Meissner corpuscles, Pacinian corpuscles, hair follicle receptors — fire at onset/offset, signalling change and vibration; the Pacinian lamellated capsule redistributes sustained force.