Classification by selectivity of action
Pharmacology divides all medicinal substances that excite sensory receptors into two large groups:
- Selectively acting drugs. They stimulate strictly defined groups of receptors. These include:
- Bitters — act exclusively on taste receptors.
- Reflex-acting emetics and expectorants — stimulate the receptor apparatus of the stomach.
- Laxatives — act on intestinal receptors.
- Non-selectively acting drugs. It is this group that is referred to as irritants in classical pharmacology. They stimulate receptors of the skin and mucous membranes without strict specificity for a particular subtype of nerve endings.
Mechanisms of action: counter-irritant and trophic effects
The clinical use of irritants (for myositis, neuritis, arthritis, and respiratory tract diseases) is based on two physiological phenomena.
1. The "counter-irritant action" mechanism (analgesia) The drug is applied to healthy areas of skin that share a coupled (segmental) innervation with the affected organ or tissue. This generates two parallel streams of impulses traveling to the central nervous system along afferent fibers:
- Pathological pain stream from the diseased internal organ.
- Artificial stream from skin receptors excited by the drug.
In the segments of the spinal cord and higher divisions of the CNS, these streams interact. As a result, the perception of afferent pain impulses is suppressed, and the patient feels pain relief.
2. Trophic effect Receptor excitation reflexively improves blood supply and nutrition (trophism) in the affected organ. This effect is realized through several pathways:
- Sympathetic activation: excitation of skin receptors stimulates sympathetic innervation of internal organs.
- Axon reflex: nerve excitation spreads bypassing the CNS — antidromically (in the reverse direction) along the branches of postganglionic sympathetic fibers.
- Cutis-visceral reflex: classical transmission of an impulse through CNS centers to the effector organ.
- Humoral factor: biologically active substances (histamine, bradykinin) are released at the site of skin irritation, locally regulating vascular tone and tissue metabolism.
Characteristics of main drugs
Medical practice employs agents of both plant and synthetic origin. The main representatives of irritants include 10% ammonia solution, menthol (racementhol), and the following drugs:
- Mustard plasters (Sinapis). These are paper sheets coated with mustard seed powder. They contain the glycoside sinigrin and the enzyme myrosin. To activate the drug, it must be immersed in warm water at a temperature of about $38^\circ C$. At this temperature, myrosin triggers the hydrolysis of sinigrin, producing allyl isothiocyanate (volatile mustard oil) as the main irritating agent.
- Alcohol compresses. The active substance is ethanol (Spiritus aethylicus). A 40% concentration is used for adults, exhibiting a pronounced irritating effect. In pediatric practice, 20% alcohol is used to prevent chemical burns.
- Capsicum preparations (Capsicum). The main irritating agent is the glycoside capsaicin. It is available as a tincture, pepper plaster, and is also part of creams and ointments ("Nikoflex", "Efcamon").
- Rectified turpentine oil (Oleum Terebinthinae rectificatum). This is purified turpentine oil, a product of pine gum distillation. Lipophilic terpenes predominate in its chemical structure (the main component is $\alpha$-pinene). Due to high lipophilicity, turpentine easily penetrates the epidermis and effectively irritates nerve endings.
Briefly: answers to topical questions
In which diseases is the clinical use of irritants indicated?
The clinical use of irritants is indicated for various inflammatory and pain syndromes. Drugs of this group are used in the following pathologies:
- Respiratory tract diseases — used to achieve counter-irritant and trophic effects.
- Myositis — muscle pain.
- Neuritis and neuralgia — pain along nerve pathways.
- Arthritis — joint pain.
- Angina pectoris — use of mustard plasters (Sinapis) is indicated.
What side effects can occur with improper use of irritants?
Improper use of irritants can lead to local tissue damage. The main side effects include:
- Chemical skin burns — possible when using excessively concentrated solutions or treating the skin with iodine alcohol solutions.
- Tissue irritation — occurs when high concentrations of irritating substances are applied.
- Burns to delicate skin in children — to prevent them, pediatric practice uses 20% ethanol instead of 40%.
To easily recall the pathways of the trophic effect, use the acronym SAG: Sympathetic activation, Axon reflex, Gumoral (humoral) factor. If you add the Cutis-visceral reflex, you get the word SCAG.
Class and exam questions
Where are drugs applied to achieve a counter-irritant effect?
Irritants are applied to healthy areas of skin that share a coupled innervation with the affected organ (according to the segmental structure of the body). This is necessary for the convergence of two impulse streams in the CNS.
Why does turpentine penetrate the skin so easily?
Rectified turpentine contains alpha-pinene, a substance with high lipophilicity. Its lipophilic structure allows it to easily cross the epidermal barrier and reach sensory nerve endings.
What else to study on this topic
- Mechanisms of action of selective-type drugs: bitters, emetics, and laxatives.
- Detailed anatomy and physiology of coupled (segmental) body innervation.
- Neurophysiology of the axon reflex and antidromic conduction of nerve impulses.
- Effects of histamine and bradykinin on vascular tone within the humoral mechanism.
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