Parasympathetic Nervous System

Overview:

Cranial Outflow:

  • Oculomotor (III) - smooth eye muscles.

  • Facial (VII) - lacrimal/salivary glands.

  • Glossopharyngeal (IX) - salivary glands (specifically parotid gland).

  • Vagus (X) - postganglionic fibres usually in a target organ, such as the heart.

Sacral Outflow:

  • Sacral nerves form pelvic plexuses containing scattered ganglia.

    • They innervate distal parts of the intestine, bladder, ureter and reproductive organs.

Neuroeffector Pathway:

Acetylcholine is the neurotransmitter involved in this pathway.


Cholinergic Synapses and Receptors:

Receptors:

  • Nicotinic - at ganglia; ionotropic (ion channel receptors regulate Na+ and K+ transport) - 2 ACh bind to the a-subunit proteins.

  • Muscarinic - at autonomic target tissues; metabotropic - G-protein coupled to secondary messenger alters K+ and Ca2+ ions.

Synapses:

  • ACh acts postsynaptically on nicotinic receptors at ganglionic synapses.

  • ACh acts postjunctionally on muscarinic receptors at the effector cell.

Parasympathetic Drug Action:

  • Parasympathomimetic Drugs - cholinomimetic agonists, which increase parasympathetic action.

  • Parasympatholytic Drugs - muscarinic antagonists, which decrease parasympathetic action.

  • Cholinesterase Inhibitors (ACHEIs) increase parasympathetic action.

  • Ganglionic Blocking Drugs - nicotinic antagonists, which decrease both sympathetic and parasympathetic action.

Co-transmission:

  • During low frequency stimulation, ACh is released.

  • During high frequency stimulation, both ACh and VIP (vasoactive intestinal polypeptide) are released.

This is co-transmission because multiple neurotransmitters (ACh and VIP) are released from the same nerve terminal, but in an activity-dependent manner.

Control of the Heart:

Postganglionic nerves release ACh which acts at M2 muscarinic receptors to decrease heart function.

  • Sinoatrial Node - decreases heart rate.

  • Atrial Muscle - decreases contractility.

  • Atrioventricular Node - decreases rate of conduction of electrical impulses.

Control of Pupil Diameter:

  • Relaxation of ciliary muscle causes:

    • Suspensory ligaments to contract.

    • Flattened lens.

It allows for focus at long distance.

  • Contraction of ciliary muscle causes:

    • Suspensory ligaments to relax.

    • Enlarged lens.

It allows for focus at near distances (accommodation).

  • Cycloplegia is the paralysis of ciliary muscle, leading to a loss of accommodation.

Clinical Modulation of Pupil Diameter:

  • Mydriatic Drugs

    • Muscarinic antagonists which cause mydriasis (loss of drive to constrictor pupillae) and cycloplegia.

  • Antiglaucoma and Miotic Drugs

    • Muscarinic agonists which cause:

      → Pressure in the eye to build up due to the production of aqueous humour, and a lack of drainage through the canal of Schlemm.

      → Pupil dilation - the iris occludes the canal.

      → Miosis, causing an increase in outflow, and a decrease in intraocular pressure.