Muscle End-plate

Peptides in the a-conotoxin family are inhibitors of nicotinic acetylcholine receptors. They were first isolated from C. geographus venom as components which cause paralysis in mice and fish when injected intraperitoneally (27). Early physiological experiments (28) indicated that a-conotoxins GI, GII, and GIA (see Table III) all act at the muscle end plate region. Mini end-plate potentials and end plate potentials evoked in response to nerve stimulation are inhibited in the presence of a-conotoxins in the nM to pM range. a-Conotoxin GI was subsequently shown to compete with rf-tubocurarine and a-bungarotoxin for the acetylcholine receptor (29). 

B. The receptor on skeletal muscle end plate is characterized as a nicotinic receptor. It responds to both nicotine and to acetylchohne. It does not respond to muscarine that is, it is not a muscarinic receptor. 

Nicotinic ACh receptor (AChR) at the muscle end plate. A. The AChR is a pentameric complex made up of five subunits surrounding a central conducting channel. Embryonic AChR, containing the 7-subunit as shown, is a low-conducting channel. Adult AChR has instead an e-subunit and is a high-conducting channel. 

The toxic effects of a large dose of nicotine are simple extensions of the effects described previously. The most dangerous are (1) central stimulant actions, which cause convulsions and may progress to coma and respiratory arrest (2) skeletal muscle end plate depolarization, which may lead to depolarization blockade and respiratory paralysis and (3) hypertension and cardiac arrhythmias. 

Pralidoxime Very high affinity for phosphorus atom but does not enter CNS Regenerates active AChE can relieve skeletal muscle end plate block Usual antidote for early-stage (48 h) cholinesterase inhibitor poisoning Intravenous every 4-6 h Toxicity Can cause muscle weakness in overdose... 

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. 

Dimethylphenylpiperazinium stimulates the autonomic ganglia, tetraethylammonium and hexamethonium block the autonomic ganglia, phenyltrimethylammonium stimulates skeletal motor muscle end plates, decamethonium produces neuromuscular blockade, and zZ-tubocurarine blocks both the autonomic ganglia and the motor fiber end plates. 

Nastuk, W.L. Membrane potential changes at a single muscle end plate produced by transitory application of acetylcholine with an electrically controlled microjet. 

Tattersall, J.E. (1990). Effects of organophosphores anticholinesterases on nicotinic ion channels at adult mouse muscle end-plates. Br. J. Pharmacol. 101 349-57. 

The po.ssible existence of a junction between muscle and nerve was suggested as early as 1856, when Claude Bernard ob.servcd that the site of action of curare was neither the nerve nor the muscle. Since that lime, it has been agreed that ACh mediates transmission at the neuromuscular junction by a sequence of events described above in this chapter. The neuromuscular junction consists of the axon impinging onto a specialised area of the mu.scle known as the muscle end plate. The axon is covered with a myelin sheath, containing the nodes of Ranvier, but is bare at the ending. The nerve terminal is separated from the end plate by a gap of 200 A. The subsynaptic membrane of the end plate contains the cholincigic receptor, the ion-conducting channels (which are opened under the influence of ACh), and AChE. 

These act as competitive antagonists at NM receptors at muscle end plate. Drugs include tubocurarine, atracurium, and pancuronium. 

Skeletal neuromuscular junction effectors act as competitive inhibitors of the NM muscle end-plate receptor or as depolarizing agonists. Tubocurarine, atracurium, and pancuronium are competitive antagonists, whereas succinylcholine is a depolarizing agent. Both the antagonists and agonists are... 

Used mainly in anesthesia protocols or in the ICU to afford muscle relaxation and/or immobility. Occasionally used to treat tetanus. These muscle relaxants interact with nicotinic receptors at the skeletal muscle end plate. Nicotinic receptors are comprised of five subunits, two of which (alpha) bind ACh, a requirement for opening of the Na+ channel. Most drugs in this class bind competitively to one of the alpha subunits to prevent depolarization (receptor antagonists) one drug (succinylcholine) binds noncompetitively and opens the Na+ channel, causing excessive depolarization and desensitization. 

ACh interacts with the nicotinic ACh receptor to initiate an end-plate potential (EPP) in muscle or an excitatory postsynaptic potential (EPSP) in peripheral ganglia (Chapter 6). The nicotinic receptor of vertebrate skeletal muscle is a pentamer composed of 4 distinct subunits a, /3, y, and S) in the stoichiometric ratio of 2 1 1 1, respectively. In mature, innervated muscle end plates, the y subunit is replaced by the closely related e subunit. The nicotinic receptor is prototypical of other pen-tameric ligand-gated ion channels, which include the receptors for the inhibitory amino acids (y-aminobutyric acid [GABA] and glycine) and S-HT serotonin receptors (Figure 9-1). 

Because physostigmine acts on the enzyme cholinesterase— which is present at all cholinergic synapses—this drug increases acetylcholine effects at the nicotinic junctions as well as muscarinic ones. Bethanechol, on the other hand, is a direct-acting agent that is selective for muscarinic receptors and has no effect on nicotinic junctions such as the skeletal muscle end plate. The answer is (B). 

C) Increased sodium influx into skeletal muscle end plate... 

Figure 27-2. Drug interactions with the ACh receptor on the skeietal muscle end plate. Top ACh, the normal agonist, opens the sodium channel. Bottom left Nondepolarizing blockers bind to the receptor to prevent opening of the channel. Bottom right Succinylcholine causes initial depolarization (fasciculation) and then persistent depolarization of the channel, which leads to muscle relaxation. (Reproduced, with permission, from Katzung BG [editor] Basic Clinical Pharmacology, 8th ed. McGraw-Hill, 2001.)...

Action potentials trigger calcium influx and the subsequent release of acetylcholine (ACh) from pres)m-aptic motor neurons. ACh diffuses across the membrane cleft and binds to nicotinic receptors on the muscle end plate. Nicotinic receptors have five subunits, two of which bind ACh. When both are occupied, the ion channel (the core of the ring formed by the 5 subunits) opens, allowing Na and Ca influx and... 

The nicotinic receptor of muscle tissue is a transmembrane glycoprotein consisting of tour types of subunits—a, 3, y(or z), and 5. Only the ai subtype of the a subunit is present in muscle. In a mature muscle end plate, the y subunit is replaced by an t subunit. This change in gene expression encoding the y and z subunits affects ligand selectivity along virith receptor turnover and/or tissue location. 

Drastic species differences in the subtypes of Ca channels expressed by different cell types have been found. For instance, the K -evoked Ca entry in brain cortex synaptosomes is controlled by N channels in the chick and by P channels in the rat [93], On the other hand, neurotransmitter release at the muscle end plate is controlled by N channels in fish [94-96] and amphibians [97] and by P channels in mammals [98]. 

At the neuromuscular junction, acetylcholine interacts with receptors on the muscle end-plate. The resultant depolarization triggers an action potential which is similar to a nervous impulse (described above). For about one millisecond, acetylcholine opens channels that allow sodium ions to flow inwards, there is a temporary loss of membrane potential, and the muscle begins to contract. The acetylcholine is then destroyed by acetylcholinesterase, which is located elsewhere in the end-plate. This process occupies only a few milliseconds and can be reproduced by administering acetylcholine close to the end-plate region. However, injection directly into the interior of the muscle cell produces neither depolarization nor contraction. 

The muscle end-plate is thus a chemically excitable membrane, and stands in contrast to the electrically excitable membrane which covers the bulk of every nerve fibre as described above. The conductance (permeability to ions) of a chemically excitable membrane is changed only by the specific chemical messenger (synaptic transmitter). These changes in ionic conductance produce membrane potential changes that are proportional to the concentration of the transmitter. 

The affinity of both toxins is a few nanomolar in each example, while the density of sodium channels varies considerably. For nerve fibres, there is a rough relationship between fibre diameter and channel density. It is not possible to make direct measurements of membrane conductance in small fibres, like those of the garfish olfactory nerve. Both in frog muscle and squid axon the channel conductance is a few fJicomho. This indicates that the turnover numbers at these channels are greater than one could reasonably expect from carrier-mediated transport, yet the turnover is less than at a single gramicidin pore or an ion channel in a muscle end-plate (see p. 7). These latter two, however, lack the ionic... 

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