Free nerve endings

Also known as: nociceptors, thermoreceptors, pruriceptors, unencapsulated nerve endings, peritrichial endings

Free nerve endings are unencapsulated terminals of thin Aδ and C fibres that branch through the dermis and into the epidermis to detect pain, temperature, itch and crude touch.

Overview

Aδ conduction velocity≈5–30 m/s (fast pain, cold)

C conduction velocity≈0.5–2 m/s (slow pain, warmth, itch)

TRPV1 threshold≈43 °C

Free nerve endings are the most numerous sensory receptors in the skin. They lose their Schwann cell covering as they cross the basement membrane and ramify between keratinocytes as far as the stratum granulosum. They belong to two fibre classes: thinly myelinated Aδ fibres (≈5–30 m/s), which convey sharp, well-localised "first" pain and cold, and unmyelinated C fibres (≈0.5–2 m/s), which convey dull, burning, poorly localised "second" pain, warmth and itch.

Their specificity comes from transducer ion channels. TRPV1 responds to noxious heat above ≈43 °C, protons and capsaicin; TRPM8 to cooling and menthol; TRPA1 to irritant chemicals and noxious cold; mechanosensitive channels to intense pressure. Polymodal C nociceptors respond to mechanical, thermal and chemical stimuli. During inflammation, mediators (bradykinin, prostaglandin E2, histamine, NGF, H⁺) lower their threshold (peripheral sensitisation, hyperalgesia), and the endings themselves release substance P and CGRP, causing the flare of neurogenic inflammation. Histaminergic and non-histaminergic C fibres mediate itch.

Modified free endings also wrap around hair follicles (peritrichial/lanceolate endings) and detect hair deflection. Central projections enter the dorsal horn (laminae I, II and V) and ascend in the anterolateral spinothalamic tract.

Location

Throughout the dermis and penetrating the epidermis; around hair follicles; also cornea, dental pulp and most tissues.

Function

Nociception (mechanical, thermal, chemical pain)

Thermoreception (warm and cold)

Itch (pruriception)

Crude touch and hair-movement detection (peritrichial endings)

Physiology

Transduction by TRP channels (TRPV1 heat/capsaicin, TRPM8 cold/menthol, TRPA1 irritants) and mechanosensitive channels; peripheral sensitisation during inflammation; neuropeptide release (substance P, CGRP).

Clinical significance

Small-fibre neuropathy

Diabetes and other conditions destroy intraepidermal C and Aδ fibres first, causing burning feet, loss of pain and temperature sensation and painless injuries (diabetic foot ulcers). Diagnosed by reduced intraepidermal nerve fibre density on skin biopsy.

Capsaicin and menthol

Topical capsaicin activates and then defunctionalises TRPV1 endings (used for post-herpetic neuralgia); menthol produces cooling via TRPM8.

Congenital insensitivity to pain

Loss-of-function mutations in the Nav1.7 sodium channel (SCN9A) abolish pain perception, leading to repeated unnoticed injuries — highlighting the protective role of nociceptors.