In one breath
Itch is a modality of its own, defined as an unpleasant sensation that makes you want to scratch. Its receptors are endings of unmyelinated C fibers, and these are not the C fibers of pain; histamine is the chief chemical that provokes it, kinins do so too, and a high plasma bile salt level always itches. Temperature is sensed by two sets of naked nerve endings: cold receptors, firing over 10–40°C, and warmth receptors, over 30–50°C, with CMR1, VR1 and VRL1 as their molecular counterparts in the TRP family. Cold travels on Aδ and C fibers, heat on C fibers, and itch, tickle and temperature all ascend in the lateral spinothalamic tract.
Builds on: Sensory receptors and their classification · Pathways for specific sensations · Leads to: Pain: concepts, receptors and fibers · Sensory lesions and sensory function tests
Itch
Definition
- An unpleasant sensation whose hallmark is that it makes you want to scratch.
- It is a distinct sensory modality, not a weak form of pain.
- It can be brought on by mechanical or chemical stimuli.
- Tickle is its counterpart, and tickle is pleasurable.
Receptors
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Itch comes from itch receptors, which are the endings of unmyelinated C fibers.
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Those C fibers are not the same ones that carry pain — a favourite one-mark point.
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What produces itch:
- mechanical stimulation of the skin;
- histamine, the most important chemical agent, which is why antihistamines are prescribed for itching;
- kinins, which produce severe itching;
- a high plasma bile salt concentration, which itches without exception.
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Itch can still occur without histamine, so an antihistamine does not always work.
Pathway
- Itch and tickle are carried in the lateral spinothalamic tract (Ascending tracts: dorsal column and anterolateral system) — the same tract as pain and temperature.
Applied: the itch of obstructive jaundice
Your book’s point that a high plasma bile salt level itches without exception has a bedside use.
Worth knowing, though not in your pages: intense generalized itching is a standard feature of obstructive jaundice and of cholestasis in pregnancy, and antihistamines help it little — which fits the book’s other point, that itch can arise without histamine.
Temperature
Receptors
- Two kinds, each defined by the direction it responds to:
| Receptor | Responds maximally to | Range |
|---|---|---|
| Warmth receptors | temperature above body temperature | 30–50°C |
| Cold receptors | temperature below body temperature | 10–40°C |
- The sense organs themselves are naked (free) nerve endings.
- Three molecular sets have been described more recently:
| Channel | Senses |
|---|---|
| CMR1 | moderate cold |
| VR1 | very high temperature — a nociceptive thermoceptor |
| VRL1 | moderate to high temperature |
- All three are ion channels of the transient receptor potential (TRP) subfamily (Pain: concepts, receptors and fibers).
Menthol, not methanol
CMR1 stands for cold- and menthol-sensitive receptor 1. Menthol is the cooling compound of mint; your book prints “methanol”, a different substance altogether. Mint feels cold because menthol opens this channel.
Pathway
| Sensation | Afferent fibers |
|---|---|
| Cold | Aδ and C |
| Heat | C only |
- Temperature sensation ascends in the lateral spinothalamic tract (Ascending tracts: dorsal column and anterolateral system).
Draw it: the temperature ranges
- Draw a horizontal axis of skin temperature from 0 to 60°C, and a vertical axis for impulse frequency.
- Put four overlapping bell-shaped curves on it, left to right:
- cold nociceptors, peaking at the far left (below 5°C);
- cold receptors, spanning 10–40°C;
- warmth receptors, spanning 30–50°C;
- heat (warmth) nociceptors, at the far right (above 45°C).
- Mark 37°C with a vertical line and label it body temperature. Note that cold and warmth receptors overlap across it, which is how small changes either way are detected.
- Beside each curve write its afferent fiber: cold on Aδ and C, heat on C.
Both pain and temperature, one tract
Pain, temperature, itch and tickle all travel in the lateral spinothalamic tract. That is why a lesion of the anterolateral cord, or a syringomyelia, takes pain and temperature together while touch, in the dorsal column, is spared (Abnormalities of dorsal column sensations).
- Worth knowing, though not in your pages: thermoreceptors adapt briskly, which is why a hot bath stops feeling hot. Extreme cold, below about 5°C, can excite some warmth fibers, so very cold metal can feel scalding — paradoxical cold. These temperature afferents are also the input to the hypothalamic thermostat, not only to conscious sensation.
Exam-answer skeleton: "Receptors and pathway for itch and for temperature sensation" (short note)
- Itch: define it; a distinct modality; mechanical or chemical stimuli; tickle is the pleasurable counterpart.
- Its receptors: endings of unmyelinated C fibers, distinct from the pain C fibers.
- Its chemical causes: histamine first (and antihistamines), then kinins, and high plasma bile salts; itch can occur without histamine.
- Its pathway: lateral spinothalamic tract, with itch and tickle together.
- Temperature: the two receptor types, with their ranges of 10–40°C and 30–50°C, as naked nerve endings; draw the curves.
- The TRP channels: CMR1, VR1, VRL1, and what each senses.
- Its pathway: cold on Aδ and C fibers, heat on C fibers, both up the lateral spinothalamic tract.
- One clinical line: pain and temperature are lost together in an anterolateral cord lesion.