Learning Objectives and quick synopsis:
- Compare and contrast the neurotransmitters and receptor types in the somatic motor division, parasympathetic autonomic efferent division, and sympathetic autonomic efferent division of the nervous system. Include the neurotransmitter / receptor pairs in the ANS two-neuron pathways.
Preganglionic nerve fibers exit the central nervous system (CNS) and release acetylcholine (ACh) on postganglionic neurons in autonomic ganglion. This second postganglionic neuron releases norepinephrine on targets if it is sympathetic, or ACh if it is parasympathetic. The ACh receptors on the ganglion are nicotinic type, whereas the distal receptors are muscarinic type on parasympathetic target cells and adrenergic on sympathetic target cells.
- Identify epinephrine / norepinephrine receptor types and their effects on various target organs
There are two categories of alpha adrenergic receptors and three types of beta adrenergic receptors. They are G-protein coupled receptors (GPCRs). You will learn the G proteins they each couple to and some principal cell types they are expressed in (see Tables).
- Compare nicotinic and muscarinic acetylcholine receptor (AChR) activation and identify acetylcholine receptor types and their effects on various target organs
Nicotinic nAChRs are fast, ligand-gated nonselective cation channels. Muscarinic mAChRs are GPCRs and not ion channels. You will learn the G proteins that M2 and M3 muscarinic receptors couple to and some principal cell types they are expressed in (see Tables).
- Identify the role of ATP and nitric oxide in smooth muscle relaxation and blood vessel-dilation.
ATP is released by exocytosis from synaptic vesicles as a cotransmitter from autonomic nerve fibers. It can initiate contraction of smooth muscles in arterioles and in the vas deferens. The diffusible gas nitric oxide (NO) is synthesized enzymatically in neurons of the genitalia and in endothelium of the cardiac coronary circulation. It relaxes smooth muscle, stimulates blood flow, and signals erection.
- Describe the baroceptor reflex in response to high or low blood pressure.
The baroceptor reflex is an autonomic reflex that excites the parasympathetic NS and inhibits the sympathetic NS when blood pressure rises. The reflex path is: Pressure receptor—>CNS—>ANS.
- Define orthostatic hypotension and discuss how the baroreflex counters it in healthy patients.
Orthostatic hypotension is a loss of blood pressure in the upper body that happens when you stand up. Pressure receptors mount a corrective response called the baroreflex.
The word efferent means “carrying messages away from the central nervous system (CNS).” The CNS communicates to the rest of the body through three major classes of efferent nerve fibers: (i) somatic, (ii) sympathetic preganglionic, and (iii) parasympathetic preganglionic. The first controls familiar voluntary muscle motor activity. The latter two belong to the autonomic nervous system (or simply, ANS), the subject of this chapter. The word autonomic implies involuntary, happening by itself. The agenda of these three efferent systems is different, crudely: (i) voluntary movement, (ii) fight & flight, and (iii) rest & These three classes of efferent nerve fibers have their cell bodies in the spinal cord or brain stem and send axons out as nerve bundles to the periphery. Such efferent nerve fibers all act by releasing the neurotransmitter acetylcholine (ACh) on their initial targets, so by definition they are called cholinergic (colored blue here). Here we focus on the neurotransmitters, receptors, and specialized cellular effector actions of ANS signaling. (ASIDE: Another branch of the peripheral nervous system related to the ANS is the Enteric Nervous System regulating the gut, which we will not deal with now. It lies in the peritoneum.)
(Unless otherwise noted, all figures are from: Kandel ER, Schwartz JH, Jessell TM 2012, Siegelbaum SA, Hudspeth AJ. ‘Principles of Neural Science, 5th ed. McGraw-Hill, New York.)