Ganglion cells, parasympathetic

The enteric nervous system (see Chapter 6 Introduction to Autonomic Pharmacology) is composed of interconnected networks of ganglion cells and nerve fibers mainly located in the submucosa (submucosal plexus) and between the circular and longitudinal muscle layers (myenteric plexus). These networks give rise to nerve fibers that connect with the mucosa and deep muscle. Although extrinsic sympathetic and parasympathetic nerves project onto the submucosal and myenteric plexuses, the enteric nervous system can independently regulate gastrointestinal motility and secretion. Afferent fibers present in the mucosa and muscularis connect to cell bodies in the plexuses that mediate local reflexes. 

MR are present in, e.g. the central nervous system (CNS, for respiratory and cardiovascular activity, cognition and stress processing), peripheral nervous system (PNS, for smooth muscle contraction, control of heart rate, vasodilatation), as well as the sympathetic and parasympathetic ganglion cells [1], Five metabotropic cholinergic MR subtypes (M1-M5) were identified [1], but selectivity of TA is merely apparent [9] except for tiotropium and ipratropium [31]. 

Two distinct receptor groups have been identified for acetylcholine, the nicotinic and the muscarinic groups (Table 11.1). Furthermore, there are at least four subtypes of nicotinic and five subtypes of muscarinic receptors. Nicotinic receptors are ubiquitous and exist at the neuromuscular junctions of skeletal muscles and on ganglion cells in the autonomic nervous system. Nicotinic receptors located on cation-specific ion channels, when opened, evoke fast, transient depolarizations of the recipient cell. Muscarinic receptors are found in smooth muscle receiving parasympathetic innervation and elsewhere, and can be blocked by atropine. Muscarinic receptors are coupled indirectly to slow and fast ion channels via G proteins. 

It has been proposed that adrenergic fibers terminate at inhibitory B receptors on smooth muscle fibers, and at inhibitory a receptors on parasympathetic cholinergic (excitatory) ganglion cells of Auerbach s plexus. 

Nicotinic mechanism The mechanism of nicotinic action has been clearly defined. The ACh receptor is located on a channel protein that is selective for sodium and potassium. When the receptor is activated, the channel opens and depolarization of the cell (an excitatory postsynaptic potential EPSP) occurs as a direct result of the influx of sodium. These ACh receptors are present on ganglion cells (both sympathetic and parasympathetic) and the neuromuscular end plate. If large enough, the EPSP evokes a propagated action potential in the surrounding membrane. 

Cholinergic neurotransmission ChEs terminate cholinergic transmission in the central nervous system (CNS), in NMJs and in the autonomic system (the parasympathetic system, somatic motor nerves and pre-ganglionic sympathetic nerves). A few sensory cells and the NMJ in nematodes also include ChEs. 

Sympathetic, parasympathetic and sensory neurones, chromaffin, neuroblastoma and PC 12 cells (ganglionic nAChR, typically comprised of a3,a5,(32,(34 or al subunits)... 

Transmission through autonomic ganglia is more complex than neurotransmission at the neuromuscular and postganglionic neuroeffector junctions and is subject to numerous pharmacological and physiological influences. In some ganglionic synapses, especially at parasympathetic ganglia, there is a simple presynaptic to postsynaptic cell relationship in others, the presynaptic to postsynaptic cell relationship may involve neurons interposed between the presynaptic and postsynaptic elements (interneurons). 

Acetylcholine (ACh) The primary transmitter at ANS ganglia, at the somatic neuromuscular junction, and at parasympathetic postganglionic nerve endings. A primary excitatory transmitter to smooth muscle and secretory cells in the ENS. Probably also the major neuron-to-neuron ("ganglionic") transmitter in the ENS. 

The parasympathetic system is exclusively a cholinergic system. The neurotransmitter acetylcholine is produced at both pre- and post-ganglionic nerve cells. Acetylcholine is the neurotransmitter at the preganglionic neurons of the sympathetic system also, but the postganglionic sympathetic stimulation is by norepinephrine. 

The cell bodies of parasympathetic preganglionic neurons are located in the brainstem and the sacral spinal cord. Parasympathetic outflow is channeled from the brainstem (1) through the third cranial nerve (oculomotor n.) via the ciliary ganglion to the eye (2) through the seventh cranial nerve (facial n.) via the pterygopalatine and submaxillary... 

Two distinct factors have been isolated from muscle and spinal cord which clearly promote motoneuron survival and there is synergy between these factors (Dohrmann et al., 1987). Naturally occurring cell death is enhanced in sympathetic and parasympathetic ganglia after blockade of ganglionic neurotransmission with pem-pidine (Hendry, 1973 Maderdrut et al., 1988). On the other hand blockade of activity of the target with curare leads to an increase in survival of mo-... 

Maderdrut, J.L., Oppenheim, R.W. and Prevette, D. (1988) Enhancement of naturally occurring cell death in the sympathetic and parasympathetic ganglia of the chicken embryo following blockade of ganglionic transmission. Brain Res. 444 189-194. 

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