Parasympathetic Nervous System Quiz

Pharmacology 1 (PPT301) - Lecture 6

• Parasympathetic Nervous System (Muscarinic Agonists): Lecture focusing on the parasympathetic nervous system and muscarinic agonists.

• Learning Objectives: Students will be able to describe the neuronal release and inactivation of acetylcholine; list the three types of cholinergic receptors and their locations; compare the pharmacologic actions of direct and indirect-acting cholinergic drugs; list clinical indications for indirect-acting anticholinesterase drugs; describe pharmacologic actions, uses, and adverse effects of anticholinergic drugs; and list preferred drug treatments for common parasympathetic conditions.

• Introduction: The autonomic nervous system controls internal organ function. The parasympathetic system is active during rest and is responsible for digestion and waste elimination, increasing activity in the gastrointestinal and genitourinary systems and decreasing the activity in the cardiovascular system. Drugs that increase parasympathetic activity (cholinergic) treat conditions like Alzheimer's disease, glaucoma, myasthenia gravis, and urinary/intestinal stasis. Conversely, drugs that decrease parasympathetic activity (anticholinergics) treat overactive bladder/bowel conditions, asthma, COPD, motion sickness, and aid in ophthalmic procedures.

Key Terms

• Acetylcholinesterase: An enzyme that inactivates acetylcholine.

• Anticholinergic: Drugs or effects that reduce parasympathetic activity.

• Cholinergic: Refers to nerves, receptors, and drugs that stimulate the parasympathetic nervous system.

• Muscarinic Receptor: Older term for the cholinergic receptor found in smooth and cardiac muscle.

• Nicotinic-Muscle (Nm) Receptor: Cholinergic receptor at the neuromuscular junction of skeletal muscle.

• Nicotinic-Neural (Nn) Receptor: Cholinergic receptor at parasympathetic and sympathetic ganglia.

• Parasympatholytic: Drugs that decrease parasympathetic activity (anticholinergics)

• Parasympathomimetic: Drugs that mimic parasympathetic stimulation (cholinergics).

Nervous System

• The central nervous system includes the brain and spinal cord; the peripheral nervous system further divides into sensory and motor divisions. The motor division contains the somatic and autonomic systems, the latter being divided into sympathetic and parasympathetic divisions.

Cholinergic Transmission

• Acetylcholine (ACh) is located at the neuromuscular junction (somatic nervous system), sympathetic ganglia, sympathetic postsynaptic fibers, all parasympathetic ganglia, and postganglionic parasympathetic nerves.

Two Major Classes of ACh Receptors

• Nicotinic receptors: A ligand-gated ion channel; located in skeletal muscle.
• Muscarinic receptors: G protein-coupled receptors; subtypes M₁, M₂, and M₃, with locations including the CNS, peripheral neurons, gastric parietal cells, cardiac tissue, and glandular/smooth muscle.

Action of ACh

• Muscarinic actions: Can be blocked by atropine; affect smooth muscle, cardiac muscle, and glands.
• Nicotinic actions: Can be blocked by hexamethonium or d-tubocurarine; affect autonomic ganglia and neuromuscular junctions.

Acetylcholine Synthesis

• Acetylcholine is synthesized in nerve terminals from choline using choline acetyltransferase (CAT) and acetyl-CoA.

Recycling of Choline

• Choline is reabsorbed into neurons using a sodium-coupled, high-affinity uptake system, then acetylated to form ACh, ready for subsequent release as needed.

Degradation of Acetylcholine

• Acetylcholine is rapidly degraded by acetylcholinesterase (AChE) within the synaptic cleft, limiting its duration of action. Two types: a specific AChE found primarily in nerve endings, neuromuscular junctions, and red blood cells (RBCs); a non-specific AChE present in plasma, intestines, liver, and skin.

Muscarinic Actions (Cardiovascular)

• Heart: Decreased force of contraction (primarily in atria), inhibition of AV conduction and decreased automaticity.
• Blood Pressure: Decreased blood pressure due to vasodilation and decreased cardiac output. Vasodilation is due to NO release by endothelium.

Smooth Muscles

• Gastrointestinal (GIT): Increased tone/motility and relaxation of sphincters.
• Urinary Bladder: Contraction of bladder muscles and relaxation of sphincters.
• Respiration: Increased bronchial secretions and contraction/spasm of bronchioles.
• Exocrine Glands: Stimulation of secretions in glands innervated by the parasympathetic nervous system.

Eye

• Pupil: Parasympathetic stimulation causes pupil constriction (miosis), while sympathetic stimulation causes dilation (mydriasis). Atropine paralyzes the ciliary muscle and leads to loss of accommodation (cycloplegia).

Nicotinic Actions (in Skeletal muscle, Autonomic ganglions)

• Skeletal muscles: Causes muscle twitching due to depolarization of the motor end plate.

• Autonomic ganglia: Increases blood pressure in the presence of atropine.

• Unwanted effects of ACh: Sweating, salivation, increased tear production(lacrimation), intestinal pain, cramps, diarrhea, flushing, difficult breathing, and hypotension can occur.

Test your knowledge on the parasympathetic nervous system with this quiz. It covers primary functions, drug classifications, and neurotransmitter details related to parasympathetic activity. Perfect for students studying anatomy or pharmacology.