The relative densities of monosynaptic pathways to the cell bodies and dendrites of the cat ventral horn

The amplitude, time course and charge of unitary excitatory post-synaptic potentials evoked in spinal motoneurone dendrites

1. Group Ia e.p.s.p.s were recorded from lumbosacral motoneurones in anaesthetized cats after almost complete section of the appropriate dorsal roots. The cable parameters of these same motoneurones were obtained from the voltage response to a brief intracellular current pulse, as described in Iansek & Redman (1973).2. A total of thirty-three e.p.s.p.s, recorded in thirty different motoneurones, were analysed. E.p.s.p.s which were recorded in motoneurones which were not studied using an intracellular current pulse, or in which the resting membrane potential fell below 50 mV, were not considered. Also, e.p.s.p.s whose time course indicated more than one synaptic site of origin were not analysed. The selected e.p.s.p.s were plotted on a semilogarithmic amplitude scale, and their 10-90% rise time, half-width and peak amplitudes were measured.3. Using the previously determined values of the cable parameters L, rho(infinity) and tau(m), the rise time and half-width of each e.p.s.p. were used to determine the synaptic location (X), and the synaptic current time course (alpha). Twenty-seven e.p.s.p.s had time courses which allowed a value of X and alpha to be determined within the constraints of the measured cable parameters. The remaining six e.p.s.p.s either required an extension of the dendritic cable to be localized, or their time course was not compatible with a brief synaptic current.4. The synaptic locations lie in the range 0 (soma) to 1.25 space constants. When expressed as a fraction of the length of the dendritic cable, all but four of the twenty-seven e.p.s.p.s were located on the proximal half of the dendrites.5. The time to peak of synaptic current for each e.p.s.p. ranged from 30 to 390 musec, although a majority (70%) lay in the range 50 to 200 musec. There was no significant correlation between time to peak of synaptic current and synaptic location.6. The peak amplitude of e.p.s.p.s at the soma showed no significant correlation with synaptic location.7. The peak amplitude, and the cable parameters for each e.p.s.p. were used to compute the time course and amplitude of each e.p.s.p. at its point of generation on various fractions of the total dendritic cable, using the results derived in Redman (1973). These calculations showed the greatly increased rate of decay of e.p.s.p.s at their point of generation. Assuming that the synaptic input was restricted to one tenth of the total dendritic tree, the range of peak amplitudes at the synaptic site was from less than 100 muV (soma) to 20 mV.8. The net inward positive charge crossing the synaptic junction was calculated from the voltage-time integral of the e.p.s.p., as was the net outward positive charge crossing the soma membrane. These calculations showed that dendritic synapses caused up to ten times more net charge to be displaced across the synaptic junction than did synapses on or near to the soma, for similar durations of synaptic current. Similarly, dendritic synapses were generally more effective than somatic synapses in displacing charge across the soma membrane. It was concluded that the average quantal content in the conductance change at dendritic synapses is significantly greater than for somatic synapses.9. Some implications of the results for general integrative mechanisms in dendrites are discussed.

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.