Potential changes recorded from the frog motor nerve terminal during its activation

255

180

SUMMARY1. We used external electrodes placed under precise visual control on motor endings of the mouse to record electrical activity promoted by nerve stimulation.2. Three types of wave form have been observed in relation to well-defined electrode emplacements: (i) at the transition between myelinated and non-myelinated parts of the axon, the wave form consists of two negative deflexions preceded by a small positivity (preterminal response), (ii) at the main part of the terminal branches, we obtained a two component positive wave form (terminal response) and (iii) electrode positions in a narrow area between the former and the latter yielded triphasic (positive-negative-positive) wave forms (intermediate responses).3. Since these responses could not be readily interpreted in terms of classical description of membrane currents associated with propagating action potentials, we used specific channel blocking agents to identify wave form components.4. Bath application of tetraethylammonium or aminopyridines, or, better, a combination of both, suppressed delayed positive deflexions of terminal and intermediate responses and the late negative component of preterminal responses. Local inophoretic drug application showed that K channels are present only at the terminal part of the endings. K+ outflux promotes a local circuit whose sink is located at the preterminal part where it generates the late negative deflexion of the preterminal response.5. Local application of tetrodotoxin suppressed the first negative component of preterminal responses but failed to affect electrical activity at the terminal part of the endings. This indicates that Na channels, and, therefore, action potential generation, are restricted to the preterminal part.6. Suppression of K conductance revealed a slow inward current at the terminal part of the endings which could be identified as a Ca current. Ba2+ and Sr2+ could substitute for Ca2+ as inward current carriers.7. Activation of spatially separated Na channels, on one side, and of K and Ca channels, on the other, generated ionic currents and separated local circuit currents which flow between preterminal and terminal parts (and vice versa). Thus, the signals recorded at-each point of motor endings correspond to the sum of ionic and passive currents entering (or leaving) the membrane at that point.8. The present results represent a further example of heterogeneity of axonal membrane.

Section: Discussionmentioning

confidence: 99%

Section: Methodsmentioning

confidence: 99%

Section: Introductionmentioning

confidence: 99%

See 1 more Smart Citation

Presynaptic currents in mouse motor endings

Brigant

Mallart

1982

255

180

“…Repetitive as well as single pulse activation of synapses in central and peripheral nervous systems almost invariably influence in some way the magnitude of subsequent test responses (Job & Lundberg, 1953;Liley & North, 1953;Curtis & Eccles, 1960;Hubbard, 1963;Braun & Schmidt, 1966;L0mo, 1971b;Richards, 1972). While such phenomena have been closely examined at the neuromuscular junction, there have been few systematic attempts to characterize them at synapses in mammalian brain (e.g.…”

Section: R Creager T Dunwiddie and G Lynch Tntroductionmentioning

confidence: 99%

Paired‐pulse and frequency facilitation in the CA1 region of the in vitro rat hippocampus

Creager

Dunwiddie

Lynch

1980

322

164

SUMMARY1. Several types of facilitation of evoked synaptic responses were investigated in the CAI region of the in vitro rat hippocampus. Homosynaptic paired-pulse facilitation and heterosynaptic frequency facilitation were characterized and found to be differentiable processes on the basis of several characteristics.2. Paired-pulse facilitation, which occurs when the same input is stimulated twice in rapid succession, is manifested as an increase in both the extracellularly recorded population spike and the field e.p.s.p., and is specific to the set of afferents excited by the first impulse. Responses to other excitatory afferents show no facilitation by a heterosynaptic conditioning pulse.3. At intervals less than 200 msec, the degree of facilitation produced by a preceding impulse appears to decline as a first order exponential function of time. Facilitation is increased by lowering calcium or raising magnesium concentrations in the bathing medium, with no apparent change in the time constant of the decay process.4. The phenomenon that has sometimes been termed frequency facilitation, and which occurs during the early phase of repetitive stimulation, appears to be an extension of paired-pulse facilitation. It is seen as an increase in amplitude of both the e.p.s.p. and population spike in response to stimulation of homosynaptic inputs, can be predicted with fair accuracy by assuming that the residual paired-pulse facilitation produced by each impulse adds linearly with that from previous impulses, and is affected by calcium and magnesium ions in the same manner as is paired-pulse facilitation. These two types of facilitation, which apparently share a common mechanism, are termed synaptic or primary facilitation.5. Another type of facilitation, which we suggest might more properly be called frequency facilitation, develops slowly during the course of repetitive stimulation. It is the result of an increase in cell firing in response to any excitatory input, either homo-or heterosynaptic, at time points at which the field e.p.s.p. is typically depressed.6. Increases in the potassium concentration of the perfusion medium produce effects similar to those observed with frequency facilitation; stimulation-evoked increases in the extracellular concentration of this ion are hypothesized to underlie this type of generalized facilitation.t Address for correspondence.

“…However, it has recently been shown that potassium may increase m.e.p.p. frequency by an action distinct from its depolarizing effect (Gage & Quastel, 1965a), and, moreover, that the quantal content of end-plate potentials (e.p.p.s) and presynaptic spike size can change independently (Hubbard & Schmidt, 1963;Martin & Pilar, 1964;Braun & Schmidt, 1965;Katz & Miledi, 1965b).…”

Section: Introductionmentioning

confidence: 99%

Competition between sodium and calcium ions in transmitter release at mammalian neuromuscular junction

Gage¹,

Quastel²

1966

106

[Ca]. It was thought that this could be due to differences in chelation of Ca which were more apparent at low Ca concentrations.9. The similarities between the effects of Na, Ca and K on m.e.p.p. frequency and the effects of these ions on Ca-influx in heart muscle led to the suggestion that transmitter release is proportional to the concentration of a negatively charged complex of a carrier X with one calcium ion (CaX) at the internal surface of the membrane and that changes in membrane potential affect transmitter release by changing the distribution or location of CaX in the membrane.