110

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6 Skin, The Body’s Largest Organ

Figure 6.2: Four channels for heat and two for cold were found in the nerve endings for temperature sens-

ing in the skin (adapted from [2]).

Table 6.1: Each receptor reacts to specific temperature ranges and specific chemicals. The channels that

react to the chemicals are the noci-receptors of the series, i. e., the pain receptors [2].

Channel

Temperature

Sensitivity

Common Nonthermal

Agonists

Tissue Distribution

TRPV1

≥42 °C

Capsaicin, acidic pH,

allicin, camphor

PNS, brain, spinal cord, skin, tongue, bladder

TRPV2

≥52 °C

Growth factors

PNS, brain, spinal cord

TRPV3

≥33 °C

camphor

PNS, skin

TRPV4

27 °C–42 °C

Hypotonic

PNS, brain, skin, kidney, inner ear, liver, trachea,

heart, hypothalamus, fat

TRPM8

≤25 °C

Menthol, eucalyptol

PNS, prostate

TRPA1

≤17 °C

Cinnamaldehyde,

mustard oil, allicin

PNS, hair cells

TREK-1

cold

Membrane stretch,

intracellular pH

PNS, brain

P2X3

warmth

ATP

PNS

BNCI,

ASIC

cold

PNS

sensors. The channels that react to the chemicals are the noci-receptors of the series, i. e.,

the pain receptors (Table 6.1). So “pain” is basically sensed as the extreme of a normal

sensation, such as heat or cold, and thus is sensed by the same receptors using the same

principles. Generally, the ion channel opens at a specific temperature. The mechanism

by which the channels open and close is still under debate, but the thermodynamics of