ARE JUNCTION POTENTIALS ESSENTIAL? DUAL MECHANISM OF SMOOTH MUSCLE CELL ACTIVATION BY TRANSMITTER RELEASED FROM AUTONOMIC NERVES

摘要: Stimulation of autonomic nerves supplying smooth muscle tissues often evokes either an e.j.p. or i.j.p. which involves a shift in the membrane potential such that entry calcium into cell through voltage-sensitive channels is likely to be increased decreased respectively. In freely discharges action potentials spontaneously response excitatory influences vivo, change will alter rate discharge and so tension developed by muscle. this way there modulation calcium, its release within cell, voltage-dependent mechanism entrained junction potential, as represents operation channels. muscles not discharging potentials, numbers open may with depolarization, even hyperpolarization, again brings play entry. However, dual neurotransmission seems operate at many nerve-smooth junctions voltage-independent mechanisms coexist mechanisms. It now generally accepted activation receptors on cells caused bathing solutions containing transmitters can bring processes are do depend discharge. Up present it has been no means certain when released from these (ACh, noradrenaline, substance P n.a.n.c. transmitters) could also act same - least because studies cellular level have dominated recording (by micro-electrode) revealed some form depolarization seemingly ubiquitous feature nerve cells. However, experiments show under conditions and/or abolished severely impaired drug application, substantial nerve-evoked contractions occur junctions. Conversely, severe impairment contraction cases excitatory-receptor antagonists without loss, any impairment, presumably depends different receptor type: transmitter evoking mainly responsible for contraction, if both evoked substance, then subtypes seem involved. In case inhibitory evidence less extensive. likely, however, involved. There ample evidence, example, β-adrenoreceptors, their agonist inhibits β-receptors activated nerve-released noradrenaline. transmitters, negligible occurs implies brought play. Activation (possibly more than one) well cause biochemical changes: rise cyclic GMP AMP levels occurring cases. How rises nucleotides related inhibition still obscure, but they simply consequence small might occur. It would many, perhaps all, possess electrically silent electrical changes unimportant, reinforce synergize