Neuromuscular junction transmission

Tobacco smoke includes 4000 chemical species with varying potential which cause adverse effects. Nicotine is stimulating to the autonomic nervous system ganglia and neuromuscular junction. The most prominent effects relate to stimulation of the adrenal medulla, central nervous system (CNS), cardiovascular system (release of catecholamines), gastrointestinal tract (parasympathetic stimulation), salivary and bronchial glands, and the medullary vomiting center. There is subsequent blockade of autonomic ganglia and the neuromuscular junction transmission, inhibition of catecholamine release from the adrenal medulla, and CNS depression. 

Acetylcholine serves as a neurotransmitter. Removal of acetylcholine within the time limits of the synaptic transmission is accomplished by acetylcholinesterase (AChE). The time required for hydrolysis of acetylcholine at the neuromuscular junction is less than a millisecond (turnover time is 150 ps) such that one molecule of AChE can hydrolyze 6 105 acetylcholine molecules per minute. The Km of AChE for acetylcholine is approximately 50-100 pM. AChE is one of the most efficient enzymes known. It works at a rate close to catalytic perfection where substrate diffusion becomes rate limiting. AChE is expressed in cholinergic neurons and muscle cells where it is found attached to the outer surface of the cell membrane. 

In cerebellar Purkinje cells, a TTX-sensitive inward current is elicited, when the membrane was partially repolarized after strong depolarization. This resurgent current contributes to high-frequency repetitive firing of Purkinje neurons. The resurgent current results from open channel block by the cytoplasmic tail of the (34 subunit. The med Nav 1.6 mutant mice show defective synaptic transmission in the neuromuscular junction and degeneration of cerebellar Purkinje cells. 

In the periphery at the mammalian neuromuscular junction each muscle fibre is generally influenced by only one nerve terminal and the one NT acts on one type of receptor localised to a specific (end-plate) area of the muscle. The system is fitted for the induction of the rapid short postsynaptie event of skeletal muscle fibre contraction and while the study of this synapse has been of immense value in elucidating some basic concepts of neurochemical transmission it would be unwise to use it as a universal template of synaptic transmission since it is atypical in many respects. 

To achieve their different effects NTs are not only released from different neurons to act on different receptors but their biochemistry is different. While the mechanism of their release may be similar (Chapter 4) their turnover varies. Most NTs are synthesised from precursors in the axon terminals, stored in vesicles and released by arriving action potentials. Some are subsequently broken down extracellularly, e.g. acetylcholine by cholinesterase, but many, like the amino acids, are taken back into the nerve where they are incorporated into biochemical pathways that may modify their structure initially but ultimately ensure a maintained NT level. Such processes are ideally suited to the fast transmission effected by the amino acids and acetylcholine in some cases (nicotinic), and complements the anatomical features of their neurons and the recepter mechanisms they activate. Further, to ensure the maintenance of function in vital pathways, glutamate and GABA are stored in very high concentrations (10 pmol/mg) just as ACh is at the neuromuscular junction. 

The concept of chemical neurotransmission originated in the 1920s with the classic experiments of Otto Loewi (which were themselves inspired by a dream), who demonstrated that by transferring the ventricular fluid of a stimulated frog heart onto an unstimulated frog heart he could reproduce the effects of a (parasympathetic) nerve stimulus on the unstimulated heart (Loewi Navratil, 1926). Subsequently, it was found that acetylcholine was the neurotransmitter released from these parasympathetic nerve fibers. As well as playing a critical role in synaptic transmission in the autonomic nervous system and at vertebrate neuromuscular junctions (Dale, 1935), acetylcholine plays a central role in the control of wakefulness and REM sleep. Some have even gone as far as to call acetylcholine a neurotransmitter correlate of consciousness (Perry et al., 1999). 

Chemical transmission between nerve cells involves multiple steps 167 Neurotransmitter release is a highly specialized form of the secretory process that occurs in virtually all eukaryotic cells 168 A variety of methods have been developed to study exocytosis 169 The neuromuscular junction is a well defined structure that mediates the presynaptic release and postsynaptic effects of acetylcholine 170 Quantal analysis defines the mechanism of release as exocytosis 172 Ca2+ is necessary for transmission at the neuromuscular junction and other synapses and plays a special role in exocytosis 174 Presynaptic events during synaptic transmission are rapid, dynamic and interconnected 175... 

Ca2+ is necessary for transmission at the neuromuscular junction and other synapses and plays a special role in exocytosis. In most cases in the CNS and PNS, chemical transmission does not occur unless Ca2+ is present in the extracellular fluid. Katz and Miledi [16] elegantly demonstrated the critical role of Ca2+ in neurotransmitter release. The frog NMJ was perfused with salt solution containing Mg2+ but deficient in Ca2+. A twin-barrel micropipet, with each barrel filled with 1.0mmol/l of either CaCl2 or NaCl, was placed immediately adjacent to the terminal. The sodium barrel was used to depolarize the nerve terminal electrically and the calcium barrel to apply Ca2+ ionotophoretically. Depolarization without Ca2+ failed to elicit an EPP (Fig. 10-6A). If Ca2+ was applied just before the depolarization, EPPs were evoked (Fig. 10-6B). In contrast, EPPs could not be elicited if the Ca2+ pulse immediately followed the depolarization (Fig. 10-6C). EPPs occurred when a Ca2+ pulse as short as 1 ms preceded the start of the depolarizing pulse by as little as 50-100 (xs. The experiments demonstrated that Ca2+ must be present when a nerve terminal is depolarized in order for neurotransmitter to be released. 

It should be noted that at a neuromuscular junction or at a ganglionic synapse there is a delay in the transmission of the excitatory process of about 2 msec. As stated above, the im-... 

ACh is necessary for control of skeletal muscle in verterbrates, acting as the neurotransmitter at the neuromuscular junction. It is also involved in transmission in the autonomic nervous system (see below, under "Neuroanatomy"). Central ACh is produced in two general areas in the brain incuding the basal forebrain (medial septal nuclei, diagonal band... 

Neuromuscular Nicotinic receptors are responsible for transmission at the neuromuscular junction. While briefly causing stimulation, this phase is rapidly obscured by desensitization and neuromuscular blockade. Thus, nicotine has muscle-relaxant effects. 

All botulin neurotoxins act in a similar way. They only differ in the amino-acid sequence of some protein parts (Prabakaran et al., 2001). Botulism symptoms are provoked both by oral ingestion and parenteral injection. Botulin toxin is not inactivated by enzymes present in the gastrointestinal tracts. Foodborne BoNT penetrates the intestinal barrier, presumably due to transcytosis. It is then transported to neuromuscular junctions within the bloodstream and blocks the secretion of the neurotransmitter acetylcholine. This results in muscle limpness and palsy caused by selective hydrolysis of soluble A-ethylmalemide-sensitive factor activating (SNARE) proteins which participate in fusion of synaptic vesicles with presynaptic plasma membrane. SNARE proteins include vesicle-associated membrane protein (VAMP), synaptobrevin, syntaxin, and synaptosomal associated protein of 25 kDa (SNAP-25). Their degradation is responsible for neuromuscular palsy due to blocks in acetylcholine transmission from synaptic terminals. In humans, palsy caused by BoNT/A lasts four to six months. 

Perhaps the most prominent and well-studied class of synthetic poisons are so-called cholinesterase inhibitors. Cholinesterases are important enzymes that act on compounds involved in nerve impulse transmission - the neurotransmitters (see the later section on neurotoxicity for more details). A compound called acetylcholine is one such neurotransmitter, and its concentration at certain junctions in the nervous system, and between the nervous system and the muscles, is controlled by the enzyme acetylcholinesterase the enzyme causes its conversion, by hydrolysis, to inactive products. Any chemical that can interact with acetylcholinesterase and inhibit its enzymatic activity can cause the level of acetylcholine at these critical junctions to increase, and lead to excessive neurological stimulation at these cholinergic junctions. Typical early symptoms of cholinergic poisoning are bradycardia (slowing of heart rate), diarrhea, excessive urination, lacrimation, and salivation (all symptoms of an effect on the parasympathetic nervous system). When overstimulation occurs at the so-called neuromuscular junctions the results are tremors and, at sufficiently high doses, paralysis and death. 

Curare-like muscle relaxants act by blocking acetylcholine receptor sites, thus eliminating transmission of nerve impulses at the neuromuscular junction. There are two acetylcholine-like groupings in the molecules, and the drugs, therefore, probably span and block several receptor sites. The neurotransmitter acetylcholine is also a quaternary ammonium compound. The natural material present in curare is tubocurarine, a complex alkaloid that is a mono-quaternary salt. Under physiological conditions, the tertiary amine will be almost completely protonated (see Section 4.9), and the compound will similarly possess two positively charged centres. 

A quite large, diverse group of substances can affect skeletal muscle by acting both at the level of neuromuscular junctions as well as at various levels of the spinal cord and brain stem. A few of them influence transmission of nerve impulses at neuromuscular contacts and are capable of paralyzing skeletal muscle. They are used mainly as adjuvant snb-stances in anesthesia during minor surgical interventions. 

Neurotransmission can be blocked pharmacologically at the level of the neuromuscular junction either by an antagonist which competes with acetylcholine at the binding site without activating the receptor or by an agonist which induce an overstimulation of the receptor and thereby a blockade of the transmission. 

Some sedative-hypnotics, particularly members of the carbamate (eg, meprobamate) and benzodiazepine groups, exert inhibitory effects on polysynaptic reflexes and internuncial transmission and at high doses may also depress transmission at the skeletal neuromuscular junction. Somewhat selective actions of this type that lead to muscle relaxation can be readily demonstrated in animals and have led to claims of usefulness for relaxing contracted voluntary muscle in muscle spasm (see Clinical Pharmacology). Muscle relaxation is not a characteristic action of zolpidem, zaleplon, and eszopiclone. 

Pesticides derived from natural sources include nicotine, rotenone, and pyrethrum. Nicotine is obtained from the dried leaves of Nicotiana tabacum and N rustica. It is rapidly absorbed from mucosal surfaces the free alkaloid, but not the salt, is readily absorbed from the skin. Nicotine reacts with the acetylcholine receptor of the postsynaptic membrane (sympathetic and parasympathetic ganglia, neuromuscular junction), resulting in depolarization of the membrane. Toxic doses cause stimulation rapidly followed by blockade of transmission. These actions are described in Chapter 7. Treatment is directed toward maintenance of vital signs and suppression of convulsions. 

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