Skeletal muscle acetylcholine release

Botulism is a disease caused by ingestion of foods contaminated with Clostridium botulinum (food-borne botulism) or, very rarely, by wound infection (wound botulism) or colonization of the intestinal tract with Clostridium botulinum (infant botulism). The toxins block the release of acetylcholine. Botulism is characterized by generalized muscular weakness, which first affects eye and throat muscles and later extends to all skeletal muscles. Flaccid paralysis can lead to respiratory failure. 

Both the G- and V-agents have the same physiological action on humans. They are potent inhibitors of the enzyme acetylcholinesterase (AChE), which is required for the function of many nerves and muscles in nearly every multicellular animal. Normally, AChE prevents the accumulation of acetylcholine after its release in the nervous system. Acetylcholine plays a vital role in stimulating voluntary muscles and nerve endings of the autonomic nervous system and many structures within the CNS. Thus, nerve agents that are cholinesterase inhibitors permit acetylcholine to accumulate at those sites, mimicking the effects of a massive release of acetylcholine. The major effects will be on skeletal muscles, parasympathetic end organs, and the CNS. 

Stimulation of the motoneuron releases acetylcholine onto the muscle endplate and results in contraction of the muscle fiber. Contraction and associated electrical events can be produced by intra-arterial injection of ACh close to the muscle. Since skeletal muscle does not possess inherent myogenic tone, the tone of apparently resting muscle is maintained by spontaneous and intermittent release of ACh. The consequences of spontaneous release at the motor endplate of skeletal muscle are small depolarizations from the quantized release of ACh, termed miniature endplate potentials (MEPPs) [15] (seeCh. 10). Decay times for the MEPPs range between l and 2 ms, a duration similar to the mean channel open time seen with ACh stimulation of individual receptor molecules. Stimulation of the motoneuron results in the release of several hundred quanta of ACh. The summation of MEPPs gives rise to a postsynaptic excitatory potential (PSEP),... 

Measuring muscle-evoked responses to repetitive motor nerve electrical stimulation permits detection of presyn-aptic neuromuscular junction dysfunction. In botulism and the Lambert-Eaton syndrome, repetitive stimulation elicits a smaller than normal skeletal muscle response at the beginning of the stimulus train, due to impaired initial release of acetylcholine-containing vesicles from presyn-aptic terminals of motor neurons followed by a normal or accentuated incremental muscle response during repeated stimulation. This incremental response to repetitive stimulation in presynaptic neuromuscular disorders can be distinguished from the decremental response that characterizes autoimmune myasthenia gravis, which affects the postsynaptic component of neuromuscular junctions. 

The effect of Li+ upon the synthesis and release of acetylcholine in the brain is equivocal Li+ is reported to both inhibit and stimulate the synthesis of acetylcholine (reviewed by Wood et al. [162]). Li+ appears to have no effect on acetyl cholinesterase, the enzyme which catalyzes the hydrolysis of acetylcholine [163]. It has also been observed that the number of acetylcholine receptors in skeletal muscle is decreased by Li+ [164]. In the erythrocytes of patients on Li+, the concentration of choline is at least 10-fold higher than normal and the transport of choline is reduced [165] the effect of Li+ on choline transport in other cells is not known. A Li+-induced inhibition of either choline transport and/or the synthesis of acetylcholine could be responsible for the observed accumulation of choline in erythrocytes. This choline is probably derived from membrane phosphatidylcholine which is reportedly decreased in patients on Li+ [166],... 

Beside this there are some major differences with the neurotransmission in the autonomous nervous system The contractile activity of the skeletal muscle is almost completely dependent on the innervation. There is no basal tone and a loss of the innervation is identical to a total loss in function of the particular skeletal muscle. In contrast to the target organs of the parasympathetic nervous system the skeletal muscle cells only have acetylcholine receptors at the site of the so-called end-plate, the connection between neuron and muscle cell with the rest of the cell surface being insensitive to the transmitter. The release of acetylcholine results in a postjunctional depolarization which is either above the threshold to induce an action potential and a contraction or below the threshold with no contractile response at all. In contrast to the graduated reactions of the parasympathetic target organs, this is an all or nothing transmission. 

Skeletal muscles Methylxanthines facilitate neuromuscular transmission by increasing acetylcholine release. 

Autonomic nerves can regulate coronary arteriolar tone. Acetylcholine released from postganglionic parasympathetic nerves relaxes coronary arteriolar smooth muscle via the NO/cGMP pathway in humans as described above. Damage to the endothelium, as occurs with atherosclerosis, eliminates this action, and acetylcholine is able to contract arterial smooth muscle and produce vasoconstriction. Skeletal muscle receives sympathetic cholinergic vasodilator nerves, but the view that acetylcholine caused vasodilation in this vascular bed has not been verified experimentally. Moreover, NO, rather than acetylcholine, may be released from neurons. However, this vascular bed responds to exogenous choline esters because of the presence of M3 receptors on endothelial and smooth muscle cells. 

The cholinesterase inhibitors have important therapeutic and toxic effects at the skeletal muscle neuromuscular junction. Low (therapeutic) concentrations moderately prolong and intensify the actions of physiologically released acetylcholine. This increases the strength of contraction, especially in muscles weakened by curare-like neuromuscular blocking agents... 

This type of response may be caused by several mechanisms. For instance, the muscle relaxation induced by succinylcholine, discussed in more detail in chapter 7, is due to blockade of neuromuscular transmission. Alternatively, acetylcholine antagonists such as tubocurarine may compete for the receptor site at the skeletal muscle end plate, leading to paralysis of the skeletal muscle. Botulinum toxin binds to nerve terminals and prevents the release of acetylcholine the muscle behaves as if denervated, and there is paralysis. This will be discussed in more detail in chapter 7. 

Release. Certain drugs will increase synaptic activity by directly increasing the release of neurotransmitter from the presynaptic terminal. Amphetamines appear to exert their effects on the CNS primarily by increasing the presynaptic release of catecholamine neurotransmitters (e.g., norepinephrine). Conversely, other compounds may inhibit the synapse by directly decreasing the amount of transmitter released during each action potential. An example is botulinum toxin (Botox), which can be used as a skeletal muscle relaxant because of its ability to impair the release of acetylcholine from the skeletal neuromuscular junction (see Chapter 13). 

Injection of botulinum toxin is a rather innovative way to control localized muscle hyperexcitability. Botulinum toxin is a purified version of the toxin that causes botulism. Systemic doses of this toxin can be extremely dangerous or fatal because botulinum toxin inhibits the release of acetylcholine from presynaptic terminals at the skeletal neuromuscular junction. Loss of presynaptic acetylcholine release results in paralysis of the muscle fiber supplied by that terminal. Systemic dissemination of botulinum toxin can therefore cause widespread paralysis, including loss of respiratory muscle function. Injection into specific muscles, however, can sequester the toxin within these muscles, thus producing localized effects that are beneficial in certain forms of muscle hyperexcitability. 

Mechanism of action. The cellular actions of bot-ulinum toxin at the neuromuscular junction have recently been clarified.84 This toxin is attracted to glycoproteins located on the surface of the presynaptic terminal at the skeletal neuromuscular junction.33 Once attached to the membrane, the toxin enters the presynaptic terminal and inhibits proteins that are needed for acetylcholine release (Figure 13-4).84 Normally, certain proteins help fuse presynaptic vesicles with the inner surface of the presynaptic terminal, thereby allowing the vesicles to release acetylcholine via exocytosis. Botulinum toxin cleaves and destroys these fusion proteins, thus making it impossible for the neuron to release acetylcholine into the synaptic cleft.32,84 Local injection of botulinum toxin into specific muscles will therefore decrease muscle excitation by disrupting synaptic transmission at the neuromuscular junction. The affected muscle will invariably undergo some degree of paresis and subsequent... 

As indicated earlier, the relaxant effects of the toxin are likewise temporary, and these effects typically diminish within 2 to 3 months after injection.91 The effects apparently wear off because a new presynaptic terminal sprouts from the axon that contains the originally affected presynaptic terminal. This new terminal grows downward, reattaching to the skeletal muscle and creating a new motor end plate with a new source of acetylcholine. The effects of the previous injection are overcome when this new presynaptic terminal begins to function. Another injection will be needed to block the release from this new presynaptic terminal, thus allowing another 2 to 3 months of antispasticity effects. This fact raises the question of how... 

Acetylcholine was the first identified neurotransmitter. It is the neurotransmitter released by the motor neurons that innervate skeletal muscle, all preganglionic and many postganglionic autonomic neurons in the PNS, which innervate smooth muscle, cardiac muscle, and glands, and many neurons within the CNS. Extensive loss of cholinergic neurons in the CNS has been found in patients with Alzheimer s disease. The structure of acetylcholine is shown in Figure 11.6. 

The effects of curare develop rapidly after it enters the body. Victims develop rapid weakness of voluntary muscles followed by paralysis, respiratory failure, and death. The cause is a blockade of nicotinic cholinergic receptors at the neuromuscular junctions in skeletal muscle. Unlike botulinum toxin, release of acetylcholine by the cholinergic nerve terminals is not affected. When curare is present, however, the acetylcholine that is released cannot bind to the receptors because they are reversibly occupied by the curare. As a consequence, nerve-muscle communication fails and paralysis ensues. 

Q4 The ganglionic transmitter of both divisions of the autonomic nervous system is acetylcholine. The major postganglionic neurotransmitter of the sympathetic nervous system is norepinephrine (noradrenaline), but a small number of structures are innervated by sympathetic, cholinergic fibres. These fibres release acetylcholine and the structures innervated include the sweat glands and blood vessels supplying skeletal muscle. In the parasympathetic system the postganglionic neurotransmitter is acetylcholine. 

Skeletal muscle twitching is due to effects at the skeletal neuromuscular junction, which is innervated by the somatic nervous system, via motor nerves. The anticholinesterase prolongs and intensifies the actions of released acetylcholine at the junction, causing fasciculation (strong, jerky contractions) of skeletal muscle. Normally at the skeletal neuromuscular junction, the released acetylcholine is rapidly hydrolysed by cholinesterases to choline and acetate. This allows repolarization of the muscle membrane to occur following initial stimulation. In the presence of anticholinesterases the acetylcholine remains at the junction for a very prolonged period and produces repeated twitching of the muscle fibres via nicotinic receptors. 

Anticholinesterases such as malathion are used in commercial insecticide sprays. Unprotected operators may absorb malathion via the eyes, skin, respiratory tract and mucous membranes of the mouth. Effects include intestinal cramps and diarrhoea following stimulation of intestinal motility and secretion. Stimulation of lacrimal and salivary glands causes the eyes to water profusely (lacrimation) and saliva to drool. Bradycardia, bronchoconstriction, dyspnoea and increased sweating also occur. Skeletal muscle twitching (fasciculation) is due to the prolonged action of released acetylcholine at the skeletal neuromuscular junction. 

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