Does the motor nerve impulse evoke 'non-quantal' transmitter release?

Previous experiments have indicated that there is a continuous leakage of acetylcholine (ACh) from resting motor nerve terminals which can produce a small depolarization in anti-esterase treated endplates (Katz & Miledi 1977; Vyskocil & Illés 1978). This leakage might be expected to be intensified during the presynaptic action potential and also lead to a very small non-quantal endplate response. This hypothesis was examined, in frog and mammalian endplates, by stimulating the motor nerve in a calcium-deprived medium and recording the summated average response to several hundred stimuli. The result was completely negative; no trace of a non-quantal endplate potential was ever observed, with the limit of detection being always less than 10 microV, and sometimes as low as 2 microV. These experiments suggest that the leakage of ACh either does not originate predominantly from the synaptic region of the axon terminal, or that it occurs by a mechanism that is not directly influenced by the membrane potential.     

Attachment of acetylcholinesterase to structures of the motor endplate

Summary The kinetics of AChE solubilization from intact motor endplates of mouse diaphragm, by collagenase, papain and hyaluronidase, was studied in parallel with the ultrastructural localization of AChE in treated neuromuscular junctions. Hyaluronidase did not solubilize more AChE from isolated motor endplate regions than Ringer's solution itself. Residual AChE activity could be demonstrated histochemically in motor endplates even after the plateau of solubilization by collagenase or papain was reached. Less than 35% of junctional AChE is left after collagenase, and less than 20% after papain treatment, as estimated by the percentage of AChE activity left in the isolated endplate region of the diaphragm after protease treatment. Cytochemically, both proteases had a similar effect on postsynaptic AChE. Residual AChE activity was distributed randomly, adhering to the sarcolemma of junctional clefts. Presynaptic AChE localized in the gap between axon terminal and Schwann cell appears to be resistant to collagenase but not to papain treatment. The mode of AChE attachment or the composition of the intercellular material in this gap may differ from that of the primary and secondary clefts.     

Electrophysiological and morphological studies of a motor nerve in 'motor endplate disease' of the mouse

Motor nerves of mice affected with hereditary 'motor endplate disease' were examined by means of electrophysiological and morphological techniques. Conduction velocity was slower, the refractory period was prolonged and the temperature sensitivity higher in mutants as compared to controls of the same age. Most of the axons examined in the electron microscope showed signs of paranodal demyelination. The relationship between the morphological and the electrophysiological findings is discussed. We conclude that alterations in the motor axons can account for the failures in neuromuscular transmission described previously in the med mutation.     

Attachment of acetylcholinesterase to structures of the motor endplate.

The kinetics of AChE solubilization from intact motor endplates of mouse diaphragm, by collagenase, papain and hyaluronidase, was studied in parallel with the ultrastructural localization of AChE in treated neuromuscular junctions. Hyaluronidase did not solubilize more AChE from isolated motor endplate regions than Ringer's solution itself. Residual AChE activity could be demonstrated histochemically in motor endplates even after the plateau of solubilization by collagenase or papain was reached. Less than 35% of junctional AChE is left after collagenase, and less than 20% after papain treatment, as estimated by the percentage of AChE activity left in the isolated endplate region of the diaphragm after protease treatment. Cytochemically, both proteases had a similar effect on postsynaptic AChE. Residual AChE activity was distributed randomly, adhering to the sarcolemma of junctional clefts. Presynaptic AChE localized in the gap between axon terminal and Schwann cell appears to be resistant to collagenase but not to papain treatment. The mode of AChE attachment or the composition of the intercellular material in this gap may differ from that of the primary and secondary clefts.     

Acetylcholinesterase of the motor endplate and its response to muscle denervation

In innervated and denervated sternohyoid muscles of adult mice the AChE with a pH optimum at 7.2 was shown to occur in all three fiber types in two separate structural areals located: extrafibrillarly (synaptic cleft, postsynaptic folds, subsarcolemmal vesicles, T-tubules, interfibrillar space) and intrafibrillarly (perinuclear cisternae, SR including SR cisternae). There is not a stable connection between the two areas. The functional significance of the intrafibrillar AChE, in particular, is unknown. After muscle denervation, intrafibrillar AChE of the A and B fibers disappeares more quickly than that of C fibers. This phenomenon not only suggests a general, but possibly also a fiber-specific neurotrophic effect.Based on material presented at the Symposium Intercellular Communication Stuttgart, September 16–17, 1982     

On the nerve impulse equation: The dynamic responses of nerve impulse

With the assumption of dipole interaction with the membrane matrix, the dipole barrier under an applied field shows a minimum in its time transient. Kinetic equations governing the migration of ions are presented. Na⁺ activation, Na⁺ inactivation and K⁺ delay are all part of the same mechanism in this model with no other separate assumptions needed. Voltage Clamp equation and action potential equation are presented. Supported in part by Physics Research Center, National Science Council, The Republic of China.     

Acute nerve stretch and the compound motor action potential

In this paper, the acute changes in the compound motor action potential (CMAP) during mechanical stretch were studied in hamster sciatic nerve and compared to the changes that occur during compression.In response to stretch, the nerve physically broke when a mean force of 331 gm (3.3 N) was applied while the CMAP disappeared at an average stretch force of 73 gm (0.73 N). There were 5 primary measures of the CMAP used to describe the changes during the experiment: the normalized peak to peak amplitude, the normalized area under the curve (AUC), the normalized duration, the normalized velocity and the normalized velocity corrected for the additional path length the impulses travel when the nerve is stretched. Each of these measures was shown to contain information not available in the others.During stretch, the earliest change is a reduction in conduction velocity followed at higher stretch forces by declines in the amplitude of the CMAP. This is associated with the appearance of spontaneous EMG activity. With stretch forces < 40 gm (0.40 N), there is evidence of increased excitability since the corrected velocities increase above baseline values. In addition, there is a remarkable increase in the peak to peak amplitude of the CMAP after recovery from stretch < 40 gm, often to 20% above baseline.Multiple means of predicting when a change in the CMAP suggests a significant stretch are discussed and it is clear that a multifactorial approach using both velocity and amplitude parameters is important. In the case of pure compression, it is only the amplitude of the CMAP that is critical in predicting which changes in the CMAP are associated with significant compression.     

Comparison of motor endplate and central synapses. An ultrastructural study]

The fine structural characteristics of motor endplate (paradigm of a cholinergic synapse) and central synapses are briefly summarized giving major emphasis on recent observations in freeze fractured material. Special attention is paid to the concepts of "Active Zones" (presynaptic membrane complex) and "Specific Sites" (postsynaptic membrane complex). Comparison and functional interpretations are made specifically with respect to secretion and receptor mechanisms that are so characteristic of chemically transmitting junctions.     

Structure and ultrastructure of the frog motor endplate

Summary The frog motor endplate in its simplest form consists of an elongated, slender nerve ending embedded in a gutter-like depression of the sarcolemma. This nerve terminal contains the usual synaptic organelles. It is covered by a thin coating of Schwann cell cytoplasm which embraces the terminal with thin finger-like processes from both sides, thereby sub-dividing it into 300–1000 regularly spaced compartments. The individual synaptic compartments correspond to the strings of varicosities or grape-like configurations of motor nerve terminals in endplates of other species and in the cerebral neuropil of vertebrates.Each compartment contains one or more bar-like densities of the presynaptic membrane, active zones, which are associated with the attachment sites between synaptic vesicles and plasmalemma. Active zones have a regular transverse arrangement and occur at specific loci opposite the junctional folds. The attachment sites for synaptic vesicles are at the edges of the bars which are bilaterally delineated by a double row of 10 nm particles attached to the A-face. The structural appearance of vesicle attachment sites in freeze-etch replicas corresponds to that of micropinocytosis. The active zones are often fragmented and the frequency of their association with vesicle attachment sites is highly variable.The junctional folds are characterized by specific sites in which intramembranous particle aggregations occur at relatively high packing density (7500/m²). These sites are located opposite the active zones at the juxtaneural lips, a location where one would expect ACh-sensitive receptors on the postsynaptic membrane.This work was supported by the Deutsche Forschungsgemeinschaft (Sonderforschungsbereich 38, Projekt N), The Swiss National Foundation for Scientific Research (Grants Nr. 3 82372 and 3 77472) and the Dr. Eric Slack-Gyr Foundation Zürich.     

Calcium currents and calcium-activated potassium currents at the frog motor nerve endings

Intracellular recordings were made of evoked electrical response of the nerve endings during experiments on the frog cutaneous pectoral muscle. A delayed inward current was discovered when superfusing the neuromuscular preparation with a calcium-free solution containing tubocurarine in the response evoked at the nerve endings, using CaCl₂-filled electrodes. This was replaced by the opposite (outward) type of current when 4-aminopyridine was added to the external solution. The outward current was dependent on the calcium concentration at the electrode, decreased after local increase on potassium concentration at the electrode, and disappeared under the effects of cobalt. Local iontophoretic application of tetraethylammonium led to the disappearance of the outward current and the appearance of a powerful and protracted inward current. Similar readings of inward and outward currents were obtained when recording electrical signals using electrodes filled with SrCl₂, BaCl₂, and MgCl₂. It was deduced that the late inward current is carried through voltage-dependent calcium channels and outward delayed current through calcium-activated potassium channels at the nerve terminal. The part played by these currents in transmitter secretion from the motor nerve ending is discussed, together with the relationship between them.S. V. Kurashov Medical Institute, Ministry of Health of the RSFSR. V. I. Ul'yanov-Lenin State University, Kazan'. Translated from Neirofiziologiya, Vol. 19, No. 4, 1987, pp. 467–473, July–August, 1987.     

Sensitivity of the motor endplate to carbachol in the presence of sulfhydryl reagents

The effect that bath application of sulphydryl reagents (SR) exerts on frog sartorius motor endplate sensitivity to iontophoretically applied carbachol (CCh) has been studied. Sensitivity to CCh is expressed as the ratio of the CCh potential (mV) to the nanocoulombs delivered by the iontophoretic pulse and has been determined before and after addition of SR to the bath. Two groups of SR have been tested: oxidizing reagents, o-iodosobenzoate and reducing agents, dithiothreitol (DTT). CCh was applied iontophoretically by means of a microelectrophoretic programmer with constant current source. Exposure of the muscle to 1 mM DTT in a bath pH range of 7-8 for 2 to 85 min showed no significant differences in endplate sensitivity to CCh before and after addition of the reducing agent. o-Iodosobenzoate at a 1 mM bath concentration (pH 7) for 2 to 19 min strongly decreases endplate sensitivity to CCh. The statistical methods used were Wilcoxon rank tests and linear regression. Since previous studies have shown that oxidizing and reducing SR evoke depolarizations when applied iontophoretically at the endplate region, these results suggest that activation of the receptor is achieved only when SR are delivered iontophoretically, and that discrepancies observed can be attributed mainly to the different techniques of drug application.     

Muscle fibrillation caused by Cytochalasin-B applied to the motor nerve

Cytochalasin-B, a drug known to interfere with axoplasmic transport, evoked fibrillary potentials in the geniohyoid muscle when applied to its motor nerve. Despite this denervation-like effect, neuromuscular transmission remained normal. Some contractile characteristics of the muscle were studied. It was found that contraction time, isometric twitch tension, and half-relaxation time were not altered by the drug treatment. The present findings show that neurogenic molecular factors conveyed by axoplasmic transport to the nerve terminal are involved in the regualtion of some muscle membrane characteristics but do not modify the muscle contractile features.     

Four types of potassium currents in motor nerve terminals of snake

The experiments were perfomed on transvcrsus abdominis muscle of Elaphe dione by subendothelial recording. The results indicate that in snake motor nerve endings there exist four types of K* channels, i.e. voltage-dependent fast and slow K channels, Ca2 -activated K channel and ATP-sensitive K channel, (i) The typical wave form of snake terminal current was the double-peaked negativity in standard solution. The first peak was at-tributed to Na influx (INa) in nodes of Ranvier. The second one was blocked by 3, 4-aminopyridine (3, 4-DAP) or te-traethylammonium (TEA), which corresponded to fast K outward current (IKF) through the fast K* channels in terminal part, (ii) After IKF as well as the slow K current (IKS) were blocked by 3, 4-DAP, the TEA-sensitive Ca2 -dependent K current (IK(Ca)) passing through Ca2 -activated K channel was revealed, whose amplitude depended on [K ]and [Ca2 ] It was blocked by Ba2 , Cd2 or Co2 . (iii) IK.F and IK(Ca) were blocked by TEA, while IK.S was retained. It     

Action of exogenic acetylcholine on the potassium currents of frog motor nerve endings

Using nerve-muscle preparations of the cutaneous pectoris muscle of the frog, and recording extracellular evoked electrical responses from a nerve ending (NE), we have studied the mechanisms by which exogenic acetylcholine (ACh) influences the ion currents of a motor nerve ending. We have established that ACh in concentrations of 0.1–0.6 mmoles/liter causes an increase in the third phase of the NE response, and in concentrations of 0.7–2.0 mmoles/liter, suppresses it. We have found that tubocurarine and atropine do not block the effects of ACh. It has been shown that the ACh-induced increase in the amplitude of the third phase of the NE response is seen against a background of calcium channel blockers, and it can be eliminated by 4-aminopyridine (4-AP) and tetraethylammonium. The inhibitory effect of ACh on the third phase of the NE response is not present in calcium-free solutions, nor when the calcium current is blocked. A discussion is given of the mechanisms of exogenic and endogenic ACh action on the potential-dependent and calcium-activated potassium current of the NE.S. V. Kurashov Medical Institute, Ministry of Health of the Russian Federation, Kazan. Translated from Neirofiziologiya, Vol. 24, No. 6, pp. 678–683, November–December, 1992.     

Intramembrane events during the nerve impulse

In current literature the cell adhesion to solid surfaces has been treated in the context of basic physicochemical forces. However, in all these reports the concept of solid surface force has not been properly analyzed. The surface forces acting across an interface formed when two phases meet has been shown to consist of dispersion (attraction) forces and polar forces (arising from different interactions). Current theories have repeatedly neglected the role of polar forces in the cell adhesion. In order to clarify this concept, the particular case, i.e. adhesion of cells on polystyrene surfaces with varying degree of polar groups is described. In this case, the adhesion of cells was reported to increase with polarity of polystyrene, and this agrees with the present study that the solid polar force component increased in the same manner.