Learning Objective
By the end of this section, the learner should be able to describe the mechanism, effects, and clinical relevance of nicotinic ganglion-blocking agents, including their impact on autonomic tone and baroreceptor reflexes.
Ganglion-blocking agents inhibit nicotinic (NN) receptors located in autonomic ganglia of both the sympathetic and parasympathetic nervous systems.
Drugs
- Hexamethonium
- Mecamylamine
Mechanism of Action
- Block NN receptors in autonomic ganglia.
- Inhibit transmission from the preganglionic to the postganglionic neurons.
- Result → loss of predominant autonomic tone in each organ system.
Physiologic Effects – Loss of Predominant Autonomic Tone
| Organ System | Predominant Tone | Effect of Ganglionic Blocker |
|---|---|---|
| Heart (SA node) | Parasympathetic | Tachycardia |
| Arterioles | Sympathetic | Vasodilation → ↓ BP |
| Veins | Sympathetic | Venodilation |
| GI Tract | Parasympathetic | ↓ Motility → Constipation |
| Bladder | Parasympathetic | Urinary retention |
| Eyes (Iris) | Parasympathetic | Mydriasis |
| Sweat glands | Sympathetic (cholinergic) | ↓ Sweating |
Activity
Clinical Relevance
- Historically used for hypertension (no longer preferred).
- Prevent baroreceptor reflex changes in HR because:
- The reflex arc depends on intact autonomic ganglia.
- Without ganglionic transmission, → no compensatory tachycardia or bradycardia.
