In the intricate network of the human body, the Peripheral Nervous System (PNS) acts as a vital communication bridge between the central nervous system and various organs. Understanding how drugs influence this system has become a cornerstone of pharmacology, with therapeutic implications in treating disorders like myasthenia gravis, glaucoma, and hypertension.
This feature takes you inside Unit 3 of Pharmacology, where we explore the fascinating mechanisms of neurohumoral transmission, the classification of neurotransmitters, and the wide spectrum of drugs that modulate autonomic and neuromuscular functions.
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The Autonomic Nervous System: Organization and Function
The Autonomic Nervous System (ANS) is divided into two major branches — sympathetic and parasympathetic — each playing opposing roles to maintain physiological balance.
The sympathetic system prepares the body for “fight or flight” responses by increasing heart rate, dilating pupils, and mobilizing energy stores.
The parasympathetic system, on the other hand, promotes “rest and digest” functions such as slowing the heart rate, stimulating digestion, and conserving energy.
Together, these systems coordinate the automatic activities of the body — all without conscious control — making them central targets for a range of pharmacological agents.
Neurohumoral Transmission: The Chemical Language of Nerves
At the heart of neural communication lies the process of neurohumoral transmission, where nerve impulses are converted into chemical signals through neurotransmitters.
The sequence begins with the synthesis and storage of neurotransmitters within nerve terminals, followed by their release into the synaptic cleft upon stimulation. These chemicals then bind to specific receptors on the target cell, triggering physiological responses before being degraded or reabsorbed.
Co-Transmission and Classification of Neurotransmitters
In addition to the classic neurotransmitters like acetylcholine and noradrenaline, scientists have identified a phenomenon called co-transmission, where multiple chemical messengers are released simultaneously to fine-tune the body’s response.
Neurotransmitters are broadly classified into:
Cholinergic transmitters (acetylcholine)
Adrenergic transmitters (noradrenaline, adrenaline)
Peptidergic transmitters (neuropeptides)
Amino acid and purinergic transmitters (GABA, ATP)
This diversity allows for precise modulation of body functions — from heartbeat regulation to glandular secretion.
Parasympathomimetic and Parasympatholytic Drugs
Parasympathomimetic drugs mimic the actions of acetylcholine on the parasympathetic system, enhancing glandular secretion, slowing heart rate, and promoting digestion. Common examples include pilocarpine, used in the treatment of glaucoma, and bethanechol, prescribed for urinary retention.
Conversely, parasympatholytics (also known as anticholinergics) block the effects of acetylcholine, leading to effects such as pupil dilation and increased heart rate. Drugs like atropine and ipratropium are classic representatives of this group, widely used in anesthesia and respiratory disorders.
Sympathomimetic and Sympatholytic Agents
The sympathomimetic drugs activate adrenergic receptors, mimicking the effects of adrenaline and noradrenaline. They are crucial in emergency medicine — epinephrine, for instance, is life-saving in anaphylactic shock. Others like salbutamol are used as bronchodilators in asthma therapy.
Meanwhile, sympatholytics counteract these effects by blocking adrenergic receptors or inhibiting neurotransmitter release. Propranolol, a beta-blocker, is a well-known sympatholytic that helps control hypertension and cardiac arrhythmias.
Neuromuscular Blocking Agents and Skeletal Muscle Relaxants
In the realm of surgery and critical care, neuromuscular blocking agents play a crucial role. These drugs prevent acetylcholine from activating skeletal muscles, producing relaxation and facilitating procedures like intubation.
They are categorized as:
Depolarizing agents (e.g., succinylcholine)
Non-depolarizing agents (e.g., tubocurarine, pancuronium)
Additionally, skeletal muscle relaxants such as dantrolene act on muscle fibers directly to manage conditions like muscle spasticity.
Local Anesthetics: Blocking Pain at Its Source
Local anesthetic agents temporarily block nerve conduction, offering targeted pain relief during surgical and dental procedures. Drugs like lidocaine and bupivacaine work by inhibiting sodium channels in neuronal membranes, preventing the propagation of pain impulses.
Their clinical importance extends to minor operations and diagnostic interventions, where maintaining consciousness while ensuring analgesia is essential.
Drugs for Myasthenia Gravis and Glaucoma
Myasthenia gravis, an autoimmune disorder that weakens skeletal muscles, is managed with anticholinesterase agents such as neostigmine and pyridostigmine. These drugs enhance neuromuscular transmission by preventing acetylcholine breakdown.
In glaucoma, where increased intraocular pressure threatens vision, agents like pilocarpine (a parasympathomimetic) reduce pressure by improving aqueous humor drainage — a remarkable pharmacological achievement in eye care.