Biomedical subjects

S J Redman

Publications and source records attributed to S J Redman.

43 records · Page 3Linked to original sources

PubMed1971

The propagation of transient potentials in some linear cable structures.

1. Analytical solutions have been given for the time course of voltage transients occurring in one-dimensional cable structures, with linear uniform membrane properties.2. These solutions show how the time course of the voltage transients generated at different distances are affected by variations in the time course of current injection (at one point on the cable) and by alterations in the length of the cable, with either sealed or open end terminations. These effects are illustrated with computed results.3. Simple measurements from the time course of a membrane potential displacement, occurring in a one-dimensional cable structure following a brief current injection across the membrane, are used to evaluate the parameters which describe the cable model. These parameters are the membrane time constant, the electrotonic length of the cable, the cable end termination, and in the case of a post-synaptic potential, the distance between the electrode and the active synapse.

Action Potentials↗

PubMed1971

An electrical description of the motoneurone, and its application to the analysis of synaptic potentials.

1. The Rall model of the motoneurone, which consists of a lumped resistance and capacitance, representing the soma, in parallel with a distributed resistance-capacitance network of finite length, representing the equivalent dendritic cable, has been used to investigate the effects of varying electrical and geometrical parameters on the time course of transients generated at the model soma.2. An analytical solution has been obtained for the voltage at the model soma, following a brief current injection at any point on the dendritic cable, in terms of the dendritic to soma conductance ratio, the electrotonic length of the cable, the membrane time constant, and the electrotonic distance between the point of current injection and the soma. This solution has been used to study the response at the soma to currents with a smooth time course, and to brief rectangular current pulses. Computations of these voltage transients are given to illustrate the effect of the above parameters on voltage time course.3. A method for determining the membrane time constant, the dendritic to soma conductance ratio, and the electrotonic length of the dendritic cable, is described. The method involves measurements from the decay time course of the transient at the soma following a brief current pulse being applied at the soma.4. A method is described whereby the time course of a synaptic potential, assumed to be generated by synaptic knobs located exclusively at the soma, may be used to determine the motoneurone parameters, and a parameter describing the time course of current injection.5. A method for estimating the distance between soma and origin of a non-somatic synaptic potential, once the parameters of the motoneurone are known, is described.

Computers↗

PubMed1971

The time course of minimal excitory post-synaptic potentials evoked in spinal motoneurones by group Ia afferent fibres.

1. Group Ia EPSPs were recorded from lumbosacral motoneurones in anaesthetized cats after almost complete section of the relevant dorsal roots. The EPSPs were usually of small amplitude (median value of 230 muV) and an averaging device was used to improve the definition of their time course.2. From a total of over 500 averaged EPSPs a smaller number (342) were subjected to analysis. The other EPSPs were rejected either because they showed signs of multiple origin in the rising phase of their time course (see Methods) or because the resting membrane potential of the cell was less than 50 mV. All the selected EPSPs had their rise time (from the 10 to the 90% level) and half-width measured, and a semilogarithmic plot of their decay time course was made.3. 252 of the EPSPs showed an exponential decline in their later time course and the slope of this line was used to give an estimate of the membrane time constant. The range of the time constant for different motoneurones was 2.3-12.9 msec, with a mean value of 5.8 msec.4. In ten cells an EPSP was recorded which was judged to be generated exclusively by synaptic knobs located on the soma. On this assumption measurements of the normalized rise time, half-width and break point time were used to estimate alpha, rho(infinity) and L by the method suggested in Jack & Redman (1971b). The estimated value of alpha ranged from 18 to 65. A positive correlation was found between alpha and tau(m), indicating that for these EPSPs the duration of current injection was independent of the membrane time constant. The peak time of the wave form of current injection was between 0.1 and 0.25 msec. The estimates of rho(infinity) were not thought to be very accurate. A lower limit of 4 was assumed and the highest measured value was 12, but in three cells the time course of the EPSP could not be fitted even with a very high value of rho(infinity). Some possible explanations for this discrepancy are mentioned in the Discussion. The electrotonic length of the dendrites (L) was usually greater than 1.0 lambda and ranged between 0.75 and 1.5 lambda. Evidence for an open-circuit termination of the dendrites was found in some cells.5. The normalized values of the rise time and half-width were used to make an electrotonic distance allocation to the 246 EPSPs which were judged to be non-somatic. The method of allocation was not precise because individual values of rho(infinity) and L were not available for these motoneurones. Instead, a maximum possible range was assumed: for rho(infinity), 4-25; for L, 0.75-1.5. The range of alpha was also assumed, from 12 to 100. With these values the motoneurone model (Jack & Redman, 1971b) was used to set limits within which the normalized rise time and half-width of all EPSPs, generated by current at a single point, should lie. Twenty of the 246 EPSPs lay outside these boundary lines and hence they did not receive a distance allocation. The remaining 226 were assigned values between 0.2 and 1.6 lambda (in 0.2 lambda steps); the majority of the allocations (183) were to the proximal electrotonic part of the dendrites (0.2, 0.4 or 0.6 lambda). The relationship of these distance allocations to the histological results of Conradi (1969) is discussed.6. It is concluded that there is no good evidence against the view that the main time course of minimal Ia EPSPs can be explained by their generation by a brief pulse of synaptic current and subsequent passive spread.

Animals↗

PubMed1969