This chapter discusses the drugs that selectively stimulate sensory nerve endings of the skin or mucous membranes and do not affect the surrounding tissues. Resultant reflexes may be useful in the management of a number of pathological conditions. The main drugs of this type belong to the following groups.
• Irritants
- Mustard paper
- Menthol
- Refined terpentine oil
- Ammonia solution
• Expectorants of reflex action (see Chapter 13.3).
• Bitter drugs, laxatives and cholagogue drugs with reflex action (see Chapter 15).
2.1. IRRITANTS
Drugs belonging to this group have stimulant action on the sensory nerve endings of the skin and mucous membranes.
Mustard paper (mustard plaster, poultice) is covered with a thin layer of fat-free mustard. Its irritant action is associated with mustard essential (volatile) oil (it is allyl isothiocyanate; Fig. 2.1). Mustard contains sinigrin glycoside and myrosin enzyme. When a mustard plaster is moistened with warm water (not higher that 40 °С) myrosin causes enzymatic degradation of sinigrin which leads to the formation of mustard essential oil that has a marked irritant action. Mustard plasters are mostly used to treat respiratory disorders, as well as neuralgia and myalgia.
Refined terpentine oil is an essential oil and is obtained by distillation of terpentine from pine-tree (Pinus sylvestris). The main active agent is α-pinene (from the group of terpenes - derivatives of partially or fully hydrated benzene hydrocarbon that is a part of most essential oils) that is quite lipophilic and thus it penetrates through the epidermis, irritating the sensory nerve endings. Drugs containing refined terpentine oil are mainly used locally as balms to treat neuralgias, myalgias and joint pain.