Anatomy and Physiology Resource Site

8.5 - ANS Physiology and Regulation

The sympathetic and parasympathetic divisions dominate under different circumstances. The organs and tissues of the body are said to be under the tone of either division.

Ex. During intense physical activity, the body is under sympathetic tone and consumes large quantities of energy. During sleep, the body is under parasympathetic tone and conserves energy.

Parasympathetic and Sympathetic effects on body organs

Organ or organ system	Sympathetic effects	Parasympathetic effects
Heart	Increased contraction rate and force	Decreased contraction rate and force (atria only)
Blood vessels	Constriction	Dilation
Lungs	Dilation	Constriction
Digestive System	Decreased rate of processing (decreased peristalsis and secretion, constriction of sphincters)	Increased rate of processing (increased peristalsis and secretion, relaxation of sphincters)
Urinary System (bladder)	Relaxation (filling)	Contraction (emptying)
Urinary System (sphincters)	Constriction (no urine flow)	Relaxation (urine flow)
Eye	Pupil dilation, far vision favored	Pupil constriction, near vision favored
Glycogen and fat stores	Broken down	None
Brain	Increased alertness	None
Genitals	Decreased blood flow	Increased blood flow

Chemical communication in PNS neurons

Different PNS neuron fibre types create and secrete different neurotransmitters.

Adrenergic fibres secrete norepinephrine. Most sympathetic postganglionic neurons are adrenergic. The most common types of receptors receiving input from adrenergic fibres are alpha and beta receptors.

Cholinergic fibres secrete acetylcholine. All preganglionic ANS neurons, all parasympathetic postganglionic neurons and the symapthetic postganglionic terminals at sweat glands and skeletal muscle blood vessels are cholinergic.

There are two major types of cholinergic receptors: muscarinic and nicotinic receptors. Nicotinic recptors receive excitatory inputs from both the parasympathetic and sympathetic systems. Muscarinic receptors are found on effector cell membranes and receive inputs from parasympathetic post-ganglionic fibres.

Pharmaceuticals (drugs) of different types can alter the responses of ANS receptors. Agonists mimic neurotransmitters and bind to neurotransmitter receptors. Antagonists block the response of a neurotransmitter by blocking its receptor.