270 SECTION III Central & Peripheral Neurophysiology
It contains sensory neurons innervating receptors in the
mucosa that respond to mechanical, thermal, osmotic, and
chemical stimuli. Motor neurons control motility, secretion,
and absorption by acting on smooth muscle and secretory
cells. Interneurons integrate information from sensory neu-
rons and feedback to the enteric motor neurons.
Parasympathetic and sympathetic nerves connect the cen-
tral nervous system to the enteric nervous system or directly
to the digestive tract. Although the enteric nervous system
can function autonomously, normal digestive function often
requires communication between the central nervous system
and the enteric nervous system.TABLE 17–2 Some drugs and toxins that affect autonomic activity.a
Site of Action Compounds That Augment Autonomic Activity Compounds That Depress Autonomic Activity
Autonomic ganglia Stimulate postganglionic neurons Block conduction
Nicotine Hexamethonium (C-6)
Low concentration of acetylcholine Mecamylamine (Inversine)
Inhibit acetylcholinesterase Pentolinium
DFP (diisopropyl fluorophosphate) Trimethaphan (Arfonad)
Physostigmine (Eserine) High concentration of acetylcholine
Neostigmine (Prostigmin)
Parathion
Postganglionic sympathetic terminals Release norepinephrine Block norepinephrine synthesis
Tyramine Metyrosine (Demser)
Ephedrine Interfere with norepinephrine storage
Amphetamine Reserpine
Guanethidineb (Ismelin)
Prevent norepinephrine release
Bretylium (Bretylol)
Guanethidineb (Ismelin)
Form false transmitters
Methyldopa (Aldomet)
Muscarinic receptors Atropine, scopolamine
α adrenergic receptors Stimulate α 1 receptors Block α receptors
Methoxamine (Vasoxyl) Phenoxybenzamine (Dibenzyline)
Phenylephrine (Neosynephrine) Phentolamine (Regitine)
Prazosin (Minipress) blocks α 1
Yohimbine blocks α 2
β adrenergic receptors Stimulate β receptors Block β receptors
Isoproterenol (Isuprel) Propranolol (Inderal) blocks β 1 and β 2
Atenolol (Tenormin) blocks β 1
Butoxamine blocks β 2aOnly the principal actions are listed.
bGuanethidine is believed to have two principal actions.