SN5
SHORT NOTE
5M
Describe the triple response of Lewis. Add a note on the axon reflex.
Definition
Triple response of Lewis = the characteristic sequence of three skin changes that follow firm stroking of the skin with a blunt point.
The Three Components
| Component | Timing | Appearance | Mechanism |
|---|---|---|---|
| 1. Red reaction | 3–15 s | Dull red line along the stroke | Direct effect of injury on capillaries → local capillary dilatation |
| 2. Flare (flush) | 15–30 s | Bright red irregular halo spreading a few cm around | Arteriolar dilatation via the axon reflex |
| 3. Wheal | 3–5 min | Pale, raised, localised swelling replacing the red line | ↑ capillary permeability → local oedema |
The Axon Reflex
Stimulus at skin → Impulse travels up a sensory nerve branch → At the branch point it turns antidromically → Passes down another branch to the arteriole → Releases substance P and CGRP → Arteriolar dilatation = flare
- It is not a true reflex — there is no synapse and no central connection; the whole event occurs within the branches of one axon
- Therefore it is present in a cord-transected limb but abolished if the peripheral nerve degenerates
Chemical Mediators
- Histamine — from mast cells; chiefly responsible for the red reaction and wheal
- Substance P and CGRP — released antidromically; produce the flare
- Also bradykinin, serotonin, prostaglandins
White Reaction
- Produced by very light stroking of the skin
- Appears in 15–20 s as a pale line
- Due to contraction of the precapillary sphincters and emptying of the capillaries
- Seen especially where vasomotor tone is high
Applied Aspects
- Test of sympathetic and sensory nerve integrity — the flare is lost in peripheral nerve degeneration, and its persistence indicates the lesion is proximal to the dorsal root ganglion
- Dermographism — an exaggerated triple response; writing on the skin raises visible weals
- The same mechanism underlies the urticarial weal of allergy, and the flare around an insect bite
- Antihistamines abolish the red reaction and wheal but only partly reduce the flare
All questions in Cardiovascular System
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- Q4Describe the conducting system of the heart. Explain the genesis of the pacemaker potential and the properties of cardiac muscle. ★
- Q5Describe the normal electrocardiogram. Explain the leads, waves and intervals with their normal values, and give the uses of the ECG. ★
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- SN3Describe the baroreceptor reflex and give its significance. ★
- SN4Describe the coronary circulation and the factors regulating coronary blood flow. ★
- SN5Describe the triple response of Lewis. Add a note on the axon reflex.
- SN6Describe the fetal circulation and the changes that occur at birth. ★
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