Parasympathetic Neurotransmitters & Effects on Target Organs

Parasympathetic preganglionic neurotransmitter

Acetylcholine (ACh).

Parasympathetic postganglionic neurotransmitter

Acetylcholine (ACh).

Parasympathetic neurotransmitter summary

Both preganglionic and postganglionic parasympathetic neurons release ACh at their synapses.

General effect of parasympathetic ACh

Generally excitatory.

Nicotinic receptors in parasympathetic pathways

Located on the membranes of all postganglionic parasympathetic neurons.

Muscarinic receptors in parasympathetic pathways

Located on the membranes of all parasympathetic target cells.

Parasympathetic preganglionic synapse

Preganglionic neuron releases ACh → ACh binds to nicotinic receptors on the postganglionic neuron.

Parasympathetic postganglionic synapse

Postganglionic neuron releases ACh → ACh binds to muscarinic receptors on the target cell.

Basic function of parasympathetic effects

Maintains homeostasis when the body is at rest.

General parasympathetic response

Heart rate and blood pressure decrease, digestion increases, and the eyes adjust for near vision.

Parasympathetic effect on heart rate

Decreases heart rate.

Parasympathetic effect on blood pressure

Lowers blood pressure as heart rate decreases.

Parasympathetic pathway to the heart

Preganglionic parasympathetic neurons travel through the vagus nerves and stimulate postganglionic neurons that reduce the rate of cardiac contraction.

Parasympathetic effect on pupils

Causes constriction of the pupils.

Parasympathetic nerve controlling pupil constriction

Oculomotor nerve (CN III).

Parasympathetic pupil constriction pathway

Oculomotor nerve fibers synapse in the ciliary ganglion and innervate the sphincter pupillae muscle.

Sphincter pupillae muscle

Contracts during parasympathetic activity to constrict the pupil and allow less light into the eye.

Parasympathetic effect on the lens

Produces accommodation of the lens for near vision.

Accommodation

Adjustment of the lens that allows the eye to focus on nearby objects.

Parasympathetic nerve involved in accommodation

Oculomotor nerve (CN III).

Ciliary muscle during near vision

Oculomotor nerve activity causes the ciliary muscle to contract, making the lens rounder for near vision.

Parasympathetic effect on bronchioles

Causes mild bronchoconstriction.

Bronchoconstriction

Narrowing of the bronchioles due to contraction of smooth muscle.

Parasympathetic nerve controlling bronchioles

Vagal neurons stimulate mild contraction of smooth muscle lining the airways.

Parasympathetic effect on digestive tract smooth muscle

Causes contraction of smooth muscle and increases movement through the digestive tract.

Parasympathetic effect on peristalsis

Vagal neurons trigger rhythmic smooth muscle contractions called peristalsis.

Peristalsis

Rhythmic contractions that propel digested food through the digestive tract.

Parasympathetic effect on digestive sphincters

Causes relaxation of digestive sphincters.

Parasympathetic effect on urinary sphincters

Causes relaxation of urinary sphincters.

Nerves involved in digestive and urinary sphincter relaxation

Vagal neurons and pelvic splanchnic nerves.

Result of digestive and urinary sphincter relaxation

Promotes urination and movement of digestive contents through the tract, ultimately promoting defecation.

Parasympathetic effect on penis and clitoris

Causes engorgement with blood.

Mechanism of parasympathetic engorgement

Pelvic splanchnic neurons cause smooth muscle in blood vessels of the penis or clitoris to relax, producing vasodilation.

Parasympathetic effect on most blood vessels

Most vasodilation during parasympathetic activity results from decreased sympathetic activity and removal of epinephrine from the bloodstream rather than direct parasympathetic innervation.

Parasympathetic effect on sweat glands

Has little to no effect because sweat glands are controlled by the sympathetic nervous system.

Facial nerve effect on lacrimal glands

Facial nerve (CN VII) stimulates tear production.

Facial nerve effect on mucus glands

Facial nerve (CN VII) stimulates mucus production from glands in the mucosa.

Parasympathetic effect on salivary glands

Facial and glossopharyngeal nerves stimulate saliva secretion.

Cranial nerves stimulating salivation

Facial nerve (CN VII) and glossopharyngeal nerve (CN IX).

Vagus nerve effect on digestive secretions

Vagal nerves stimulate secretion of enzymes and other products from cells of the digestive system.

Parasympathetic effect on metabolic rate

No direct effect on metabolic rate.

Parasympathetic effect on mental alertness

No direct effect on mental alertness.

Parasympathetic effect on skeletal muscle contraction force

No direct effect on the force generated by skeletal muscle contractions.

Parasympathetic effect on blood clotting

No direct effect on blood clotting.

Parasympathetic effect on adipocytes

No direct effect on adipocytes.

Parasympathetic effect on most endocrine secretions

No direct effect on most endocrine secretions.

Why many functions return to resting levels during parasympathetic activity

They return toward resting conditions because the sympathetic nervous system is no longer dominant.

Parasympathetic activity and energy storage

Resting periods allow the body to store glucose and other fuels for future sympathetic activity.

Overall parasympathetic cardiovascular effect

Decreases heart rate and blood pressure.

Overall parasympathetic eye effect

Constricts pupils and makes the lens rounder for near vision.

Overall parasympathetic respiratory effect

Causes mild bronchoconstriction.

Overall parasympathetic digestive effect

Increases peristalsis and digestive secretions while relaxing digestive sphincters.

Overall parasympathetic urinary effect

Promotes urination by relaxing urinary sphincters.

Overall parasympathetic glandular effect

Increases tears, mucus, saliva, and digestive secretions but has little to no effect on sweat glands.