07.02.2 Effects of the Parasympathetic Nervous System

Many of the effects of the parasympathetic nervous system oppose those of the sympathetic nervous system. For instances, on the heart, acetylcholine is released from the parasympathetic nervous system to act at M₂ receptors on the sinoatrial node (the pacemaker in the atria) to slow the heart rate. Acetylcholine also acts on the M₂ receptors on the ventricles, but this only causes a small decrease in heart force. The predominant effect of the parasympathetic nervous system on the heart is to slow the heart beat.

On the lung, acetylcholine stimulates M₂ and M₃ receptors on trachea and bronchial smooth muscle to induce bronchoconstriction. On the lung, acetylcholine also stimulates M₂ and M₃ receptors on glands, to promote the release of secretions.

On the gut, acetylcholine stimulates M₂ and M₃ receptors to increase motility and tone. On the gut, acetylcholine also stimulates M₂ and M₃ receptors on sphincters to cause relaxation. Thirdly, on the gut, acetylcholine stimulates M₂ and M₃ receptors on glands to promote release of secretions.

On the urinary bladder, acetylcholine predominantly stimulates M₃-receptors on the detrusor muscle of the bladder (bulb part) to cause contraction (Figure 7.8). Acetylcholine predominantly stimulates M₃ on the trigone and sphincter muscle to cause relaxation, and release the urine (Figure 7.8). Thus, the overall effect of acetylcholine on the bladder leads to micturition (urination).

Figure 7.8 Sites of M3 recptors on bladder (Copyright QUT, Sheila Doggrell)

On the eye, acetylcholine Ach stimulates M₂ and M₃ receptors on the sphincter muscle of the iris to cause miosis (constriction of pupil). Acetylcholine also stimulates M₂ and M₃ receptors on the ciliary muscle to cause contraction for near vision. Finally, acetylcholine stimulates M₂ and M­ receptors on the lacrimal glands to increase the secretion of tears.

The effects of the parasympathetic nervous system can be mimicked with muscarinic agonists and prevented with the anti-muscarinic agents.