Neuromuscular junction calcium, role

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. 

Ribeiro JA, Sebastiao AM (1985) On the type of receptor involved in the inhibitory action of adenosine at the neuromuscular junction.Br J Pharmacol 84 911-18 Ribeiro JA, Sebastiao AM (1986) Adenosine receptors and calcium basis for proposing a third (A3) adenosine receptor. Prog Neurobiol 26 179-209 Ribeiro JA, Sebastiao AM (1987) On the role, inactivation and origin of endogenous adenosine at the frog neuromuscular junction. J Physiol (Lond) 384 571-85... 

Another important role that calcium ions play is as a second messenger . They can couple a chemical stimulus to a secretion, as in releasing neurotransmitters after receipt of a nervous impulse, whether at synapses or at a neuromuscular junction (Miledi and Slater, 1966), or in the release of epinephrine from the adrenal medulla at the instance of acetylcholine (Douglas,... 

Another important role that calcium ions play is as a second messenger . They can couple a chemical stimulus to a secretion, as in releasing neurotransmitters after receipt of a nervous impulse, whether at synapses or at a neuromuscular junction (Miledi and Slater, 1966), or in the release of adrenaline from the adrenal medulla at the instance of acetylcholine (Douglas, 1968). Likewise calcium is needed to couple the release of a neurotransmitter to a muscular contraction. Calcium seems also to initiate proliferation in many kinds of cells, such as muscle cells, lymphocytes, and fibroblasts. It also plays a role in phagocytosis. Those ionophores (Section 14. i) that transport calcium into cells can initiate many of these effects. Some think that, at the molecular level, calcium functions by giving ATPase access to its substrate (ATP), thus providing the energy for these various actions. 

---