Counterirritants

Classification by Selectivity of Action

Pharmacology divides all medicinal substances that stimulate sensory receptors into two major groups:

1. Selective agents. They stimulate strictly defined groups of receptors. These include:
2. Bitters — affect exclusively taste receptors.
3. Reflex-acting emetics and expectorants — stimulate the receptor apparatus of the stomach.
4. Laxatives — affect intestinal receptors.

1. Non-selective agents. This is the exact group referred to in classical pharmacology as counterirritants (irritants). They stimulate receptors of the skin and mucous membranes without strict specificity for a particular nerve ending subtype.

Mechanisms of Action: Counter-irritant and Trophic Effects

The clinical use of counterirritants (in myositis, neuritis, arthritis, and respiratory diseases) is based on two physiological phenomena.

1. Counter-irritant Mechanism (Analgesia) Topical application to healthy areas of skin that share a segmental innervation with the affected organ or tissue creates two parallel streams of impulses traveling to the central nervous system via afferent fibers:

• Pathological pain impulses from the diseased internal organ.
• Artificial impulses from skin receptors stimulated by the drug.

These streams interact in the spinal cord segments and higher centers of the CNS. As a result, the perception of afferent pain impulses is suppressed, and the patient experiences pain relief.

2. Trophic Effect Receptor excitation reflexively improves blood supply and trophism in the affected organ. This effect is achieved through several pathways:

• Sympathetic activation: skin receptor excitation stimulates sympathetic innervation of internal organs.
• Axon reflex: nerve excitation propagates bypassing the CNS — antidromically (in the opposite direction) along postganglionic sympathetic fiber branches.
• Cutovisceral reflex: classical impulse transmission via CNS centers to the effector organ.
• Humoral factor: biologically active substances (histamine, bradykinin) are released at the skin irritation site, locally regulating vascular tone and tissue metabolism.

Characteristics of Key Agents

Medical practice utilizes agents of both plant and synthetic origin. Key counterirritant representatives include 10% ammonia solution, menthol (racementhol), and the following preparations:

• Mustard plasters (Sinapis). Paper coated with a layer of ground mustard seeds. They contain the glycoside sinigrin and the enzyme myrosin. To activate the product, it must be immersed in warm water at about $38^\circ C$. At this temperature, myrosin initiates the hydrolysis of sinigrin, producing allyl isothiocyanate (volatile mustard oil) as the main irritating agent.
• Alcohol compresses. The active substance is ethanol (Spiritus aethylicus). For adults, a 40% concentration is used, possessing pronounced irritant action. Pediatric practice utilizes 20% alcohol to prevent chemical burns.
• Capsicum preparations (Capsicum). The main irritant agent is the alkaloid/vanilloid capsaicin. Available as a tincture, capsicum plaster, and included in various creams and ointments.
• Rectified turpentine oil (Oleum Terebinthinae rectificatum). Purified turpentine oil, a product of pine resin distillation. Lipophilic terpenes predominate in its chemical structure (main component — $\alpha$-pinene). Due to high lipophilicity, turpentine easily penetrates the epidermis and effectively irritates nerve endings.