Emetics

Physiology of the vomiting act as the basis of therapy

Vomiting is not merely a symptom, but a crucial protective reflex action. Its physiological purpose is the emergency removal of various toxic or irritating substances from the stomach that could cause irreversible harm to the body. This process is a complex combination of motor and secretory reactions.

The central component coordinating this act is the vomiting center, which is anatomically located in the medulla oblongata. However, it does not work alone. A critical role is played by the starting or chemoreceptor trigger zone (chemoreceptor trigger zone). It is localized on the floor of the fourth ventricle of the brain.

Stimulation of the vomiting center can occur via three main pathways:

1. Signals from the vestibular apparatus. This specific mechanism is triggered during motion sickness, frequently referred to in medicine as kinetosis.
2. Peripheral stimulation. In this case, nerve impulses originate from sensory receptors located directly in the gastric mucosa and pharynx.
3. Activation of trigger zone chemoreceptors. This zone is unique in that it directly responds to the presence of chemical substances (toxins, poisons, drugs) in the systemic circulation or cerebrospinal fluid.

Receptor map of the vomiting reflex

To understand how pharmacological agents work, it is necessary to know the precise localization of the receptors that serve as drug targets. The receptor map is as follows:

• Vomiting center: its activity is regulated via $M_1$ muscarinic receptors.
• Trigger zone: $D_2$ dopamine and $5 ext{-HT}_3$ serotonin receptors are concentrated in this area. They detect chemical changes in the blood and cerebrospinal fluid.
• Vestibular nuclei: here, nerve impulse transmission is mediated by $M_1$ muscarinic receptors and $H_1$ histamine receptors.
• Visceral afferent system: neural pathways running from the pharynx and stomach to the brain transmit signals predominantly via $5 ext{-HT}_3$ receptors.

Pharmacological characteristics of emetics

The primary and most well-known representative of the emetic group is apomorphine. Structurally, it is a semisynthetic alkaloid derived from morphine.

The mechanism of action of apomorphine involves the direct and selective stimulation of $D_2$ dopamine receptors, which, as established earlier, are located in the trigger zone on the floor of the fourth ventricle.

The pharmacokinetic properties of the drug determine the speed of its clinical effect:

• Administration is performed strictly subcutaneously.
• The drug is characterized by very rapid penetration into the central nervous system.
• The latent period is minimal: a pronounced vomiting act occurs just 5–10 minutes after injection.
• Elimination from the body occurs in two stages: first, the substance undergoes hepatic metabolism, and then its metabolites are excreted by the kidneys.

Indications and absolute contraindications

In modern medical practice, the use of emetics is strictly limited to specific indications. Apomorphine is used in two main cases:

1. Acute poisoning. The drug is administered for the rapid removal of ingested poison from the stomach before it can be absorbed into the systemic circulation.
2. Treatment of chronic alcoholism. In addiction medicine, apomorphine is used to establish a persistent negative conditioned reflex to the taste and smell of alcohol.

Due to its potent systemic effects and the intensity of the vomiting act itself, apomorphine has a substantial list of contraindications. It is strictly contraindicated in:

• Severe heart disease.
• Active forms of tuberculosis and any pulmonary hemorrhage.
• Peptic ulcer disease of the stomach and duodenum.
• Presence of organic lesions of the central nervous system.

Class and exam questions

What else to study on this topic

• Detailed anatomy and physiology of the chemoreceptor trigger zone (chemoreceptor trigger zone)
• Pharmacokinetics of apomorphine: features of hepatic metabolism and renal elimination
• Mechanisms of signal transmission along the visceral afferent system from the pharynx and stomach
• Pathophysiology of motion sickness and the role of the vestibular apparatus in stimulating the vomiting center
• Principles of forming a negative conditioned reflex in the therapy of chronic alcoholism

Other topics in the section «Drugs Affecting Digestive System Functions»

Prepared by WhiteCoat Pro faculty in accordance with the medical school curriculum. For educational purposes only.