19.7: Parasympathetic Division of the ANS
The parasympathetic division of the autonomic nervous system (ANS) regulates rest and digestion functions in the body. It works in opposition to the sympathetic division, promoting relaxation, conservation of energy, and digestion. The parasympathetic division consists of preganglionic fibers originating from specific cranial nerves (III, VII, IX, X) and the sacral spinal nerves (S2-S4). These fibers synapse with postganglionic neurons in the terminal ganglia, innervating various organs and tissues.
The sacral part of the parasympathetic division plays a crucial role in regulating the pelvic organs' visceral functions and the large intestine's distal half It is characterized by preganglionic axons originating from the anterior roots of the second through fourth sacral spinal nerves. As these preganglionic axons travel through the sacral spinal nerves, they branch off to form pelvic splanchnic nerves. These nerves synapse with parasympathetic postganglionic neurons in the terminal ganglia present in the walls of the innervated viscera. From these terminal ganglia, parasympathetic postganglionic axons innervate the smooth muscle and glands in the walls of the colon, ureters, urinary bladder, and reproductive organs.
The parasympathetic division influences functions such as:
- Pupil Constriction: Parasympathetic stimulation causes pupil constriction (miosis), allowing less light to enter the eye and improving near vision.
- Salivation: Parasympathetic innervation stimulates salivary glands, promoting the production and release of saliva, which aids in digestion and facilitates swallowing.
- Digestion: The parasympathetic division enhances digestive processes by increasing intestinal motility and promoting the secretion of digestive enzymes and gastric juices, facilitating the breakdown of food.
- Slowing of the Heart Rate: Parasympathetic stimulation slows the heart rate by decreasing the rate of electrical impulses generated by the sinoatrial (SA) node, promoting relaxation.
- Stimulation of Glandular Secretions: Parasympathetic activity stimulates the secretion of various glands, including lacrimal glands (tear production), nasal glands (mucus production), and digestive glands (gastric, pancreatic, and intestinal secretions).
- Emptying of the Bladder: Parasympathetic fibers stimulate the detrusor muscle in the bladder, causing its contraction and promoting bladder emptying during urination.
