Physiology of neuromuscular transmission Dr. S. Parthasarathy MD, DA, DNB, Dip Diab.MD,DCA, Dip software based statistics, PhD (physiology) Go back slightly Bernard, in 1856, identified the gap between nerve and muscle, Dale et al. demonstrated that acetylcholine was the transmitter at the neuromuscular junction in Progressed slowly to current concepts. What is it ?? Nerve to muscle Nerve cell membrane presynaptic Muscle cell membrane postsynaptic Oval depression in the muscle cell membrane 20 30 mic.m. diameter Motor end plate Motor unit 5 2000 eye to thigh Motor unit ?? Fasciculation ?? IOP and scoline What happens ?? All the enzymes, proteins, the membrane of vesicles, needed for the nerve ending to synthesize, store and release the acetylcholine for chemical signal are made in the nerve cell body and are then transmitted to the nerve ending by axonal transport. Only the chemical substances such as the choline and acetate are obtained locally at the nerve ending to synthesise ACh. Junctional cleft 20 nm. Enzymes, proteins Membranes Choline, acetate 30 mic.m to 2000 mic.metre square by clefts Muscle side The peri-junctional zone has an enhanced ability to produce a wave of depolarisation to the muscle from that produced by the post-synaptic receptors. In simple terms Nerve electrical signal Mobilization and secretion of Ach through active zones Ach + receptors (post junctional) Na channels open in muscle Contraction Ach comes out of receptor Acetylcholinesterase NMJ = Prejunctional + Synapse + Post junctional + peri junctional The shoulders of the folds are densely populated with AChRs, about 5 million of them in each junction. AChRs are sparse in the depths between the folds. Instead, these deep areas contain sodium channels Presynaptic Choline is actively transported across the membrane into the cytoplasm by sodium dependent high affinity choline uptake system (SADAKU) stored in vesicles Released By exocytosis Quantal Theory Ach is released from vesicle in the form of quanta when an action potential comes VP2 close to active zone- readily available pool VP1 reserve pool tethered to actin, synapsin etc.. The amount of acetylcholine released by each nerve impulse is large, at least 200 quanta of about 5000 molecules each. More refined !! Nerve action potential Calcium P channels open entry of Ca Snare protein Vesicle move towards active zone Ach release Botulinum toxin inhibits snare protein Black widow spider venom calcium spurt in. Magnesium blunts ca P but calcium channel blockers blunt L !! open ca entry SNARE hypothesis N ethylmaleimide sensitive factor attachment receptor Synaptic cleft The synaptic cleft spans 50 nm from the nerve ending to muscle membrane Ach is destroyed by acetylcholinesterase Organophosphorous compound poisoning The post synaptic membrane Receptors AChRs are synthesized in muscle cells and are anchored to the end-plate membrane by a special 43.-kd protein known as rapsyn. five subunit proteins Two alpha units ach Channels open One alpha unit NDPs Antagonist Acetylcholinesterase Neostigmine - pour Ach everywhere competes and throws out NDPs Rapsyn Perijunctional The peri junctional zone is the area of muscle immediately beyond the junctional area, and it is critical to function of the neuromuscular junction. The perijunctional zone contains a mixture of receptors, including a smaller density of AChRs and a high density of sodium channels Perijunctional The admixture enhances the capacity of the perijunctional zone to respond to the depolarization (i.e., end-plate potential) produced by AChRs and to transduce it into the wave of depolarization that travels along the muscle to initiate muscle contraction. The density of sodium channels in the perijunctional area is richer than in more distal parts of the muscle membrane. The perijunctional zone is close enough to the nerve ending Miniature end plate potential MEPP 1/100 th Depolarizing muscle relaxant Scoline mimcs Ach -- Why ? Two molecules of Ach but not destroyed by cholinesterase Produces sustained depolarization But contraction followed by relaxation Why ?? Scoline Depolarization opens Na channel Voltage gated (VG) opens Na flows. Time gated(TG) closes It does not open until VG closes Depolarization relaxation TG VG Other drugs 4-aminopyridine acts on the pre junction area. By impeding the efflux of K+, it also prolongs the action potential of nerve ending. So antagonism of NMJ blockade Polymixin induced reversed !! Extra junctional Before nerve innervation, AChRs are present throughout the muscle membrane. After innervation, AChRs become more concentrated at the postsynaptic membrane and are virtually absent in the extrasynaptic area at birth. The innervation process progresses somewhat slowly during fetal life and matures during infancy and early childhood ( 2 years ) Structure The mature junctional receptor is a pentamer of two -subunits and one each of the -, -, and - subunits. The immature receptor consists of two -subunits and one each of the -, -, and -subunits EJ receptors Denervation, immobilization, burns, stroke, sepsis Re expression of gamma subunit Scoline is sensitive opens up channel for more time more K + ion efflux Nondepolarizers less effective Mature receptor life span is 2 weeks but EJ 24 hours Prejunctional receptors (32). Ach stimulates Na channels of Nerve cell Opens calcium channel More Ach to active zones Ready for next stimulus NMB cause blockade of prejunctional ACH receptors also Pre junctional- role PTC The augmentation of the twitch is thought to be caused by presynaptic mobilization of acetylcholine as a result of the positive feedback effect of the run of tetanus. Scoline does not block NDP s before scoline blocks fasciculation May play a role in FADE Other agents procaine, ketamine, inhaled anesthetics, or other drugs that dissolve in the membrane lipid may change the opening or closing characteristics of the channel. LA, inh. Agents, scoline, barbiturates, alcohol desensitizing block _ agonist binds channel does not open Alpha 7 receptors Choline activity good response channel opens Even two alpha are blocked, some more are ready Scoline induced hyperkalemia N Ach receptors Carotid body Ventilatory response to hypoxia Nm blockers also affects this receptor Residual curarization response blunted Keep it TOF 0.9 or above in patients where we need such response !! Summary Nerve depolarization terminal telodendria unmyelinated Calcium in, potassium out Snare proteins take vesicles to active zone to rupture Ach Ach quanta post junctional receptors and prejunctional Two alpha units channel open, cations flow Perijunctional area (acetyl cholinesterase ) Na channels activated, depolarization of muscle Close to actin myosin complex contraction NDP one alpha Suxa VG and TG persistent depolarization relaxation EJ receptors Thank you all