Quantal Potential Fields around Individual Active Zones of Amphibian Motor-Nerve Terminals

Abstract

The release of a quantum from a nerve terminal is accompanied by the flow of extracellular current, which creates a field around the site of transmitter action. We provide a solution for the extent of this field for the case of a quantum released from a site on an amphibian motor-nerve terminal branch onto the receptor patch of a muscle fiber and compare this with measurements of the field using three extracellular electrodes. Numerical solution of the equations for the quantal potential field in cylindrical coordinates show that the density of the field at the peak of the quantal current gives rise to a peak extracellular potential, which declines approximately as the inverse of the distance from the source at distances greater than about 4 microm from the source along the length of the fiber. The peak extracellular potential declines to 20% of its initial value in a distance of about 6 microm, both along the length of the fiber and in the circumferential direction around the fiber. Simultaneous recordings of quantal potential fields, made with three electrodes placed in a line at right angles to an FM1-43 visualized branch, gave determinations of the field strengths in accord with the numerical solutions. In addition, the three electrodes were placed so as to straddle the visualized release sites of a branch. The positions of these sites were correctly predicted on the basis of the theory and independently ascertained by FM1-43 staining of the sites. It is concluded that quantal potential fields at the neuromuscular junction that can be measured with available recording techniques are restricted to regions within about 10 microm of the release site.

Electric fields of synaptic currents could influence diffusion of charged neurotransmitter molecules

“…Indeed, it has long been known that the postsynaptic current should exert a significant lateral voltage gradient in the cleft (see Eccles and Jaeger, 1958, and citations therein). This phenomenon has been addressed theoretically for the autonomic junction (Bennett et al, 1993) and the amphibian neuromuscular junction (Bennett et al, 2000) and, more recently, detected in experiments with loose-patch recordings near synaptic boutons in the sympathetic ganglia (Kearns et al, 2002). Detailed biophysical simulations predict that the lateral voltage gradient generated by the synaptic current in the narrow cleft could be substantial (Savtchenko et al, 2000).…”

Quantal Current Fields around Individual Boutons in Sympathetic Ganglia

“…The introduction of techniques for recording the electrical signs of transmission from chosen release sites within visualized nerve terminals has been successfully carried out for single active zones of the somatic neuromuscular junction (Bennett et al, 2000) as well as for varicosities of autonomic neuromuscular junctions (Lavidis & Bennett, 1992). In each of these cases, care has to be taken concerning the interpretation of the junctional currents observed.…”

“…Thus, the above analytic solution needs to be supplemented by a numerical one. A three-dimensional spherical polar mesh with grid spacings r, r and r sin is used and temporal updating on this mesh is performed using the &&leap-frog'' algorithm that we have employed in previous calculations of a similar nature (Bennett et al, 1993(Bennett et al, , 1999(Bennett et al, , 2000Bennett & Gibson, 1995;Henery et al, 1997). The grid spacings used were 0.24 m at the source and this spacing was maintained in the vicinity of the source in order to facilitate the incorporation of the loose-patch electrode [Fig.…”

“…This is clearly unphysical, being a consequence of the use of a point source rather than an extended source. This type of problem was investigated in Bennett et al (2000) with the conclusion that for distances of at least one mesh point away from the source, the numerical solution is meaningful. This means that values of <(r, t) at the source need to be interpreted with caution, as they depend on mesh size.…”

Seasonal regulation of quantal neurotransmitter release from amphibian neuromuscular junctions

“…The extracellular field associated with the discharge of a miniature EPC can only be detected within a radius of 5 -10 μm from the point of origin (Castillo & Katz 1956, Wernig 1975. Utilizing triple simultaneous extracellular recordings, it was demonstrated that the peak extracellular potential declines to 20% of its initial value in a distance of 6 μm, both along the length of the fibre and in the circumferential direction around the fibre (Bennett et al 2000). Focal extracellular loose patch electrodes have been used to record electrical currents immediately generated beneath the recording electrode (Forti et al 1997, Sheng et al 2012.…”