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R Porter

Publications and source records attributed to R Porter.

At least 217 records · Page 12Linked to original sources
PubMed1971

Lack of involvement of fusimotor activation in movements of the foot produced by electrical stimulation of monkey cerebral cortex.

1. Contractions of the small muscles of the foot producing flexion and adduction of the hallux were elicited by brief trains of electrical stimulation of a motor point on the precentral gyrus of anaesthetized monkeys and these contractions were recorded myographically.2. The cortical stimulus intensities necessary to produce minimal muscle contractions were measured for different frequencies of stimulation at the cortical point, and the latency of the minimal muscle contraction was measured in each case.3. Section of all the relevant lumbar and sacral dorsal roots had no effect on the threshold stimulus currents necessary to produce minimal contractions or on the latencies of these responses. Hence, in the anaesthetized monkey, the power of the cortico-fusimotor activity stirred up by electrical stimulation of the cortex is inadequate to influence significantly the motor responses of the most accessible muscles.

Animals↗
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↗
PubMed1970

Early facilitation at corticomotoneuronal synapses.

1. Corticomotoneuronal EPSPs have been generated in lumbar motoneurones of the monkey by single and paired corticospinal volleys. The facilitation of the second of a pair of EPSPs with respect to the size of the first has been measured.2. The relationship between the degree of facilitation of the second response and the interval between the two volleys has been studied. Average facilitation of minimal EPSPs was found to be maximal about 2 msec after the arrival of the corticospinal volley and to decay roughly exponentially thereafter with a time constant of about 10 msec.3. The degree of facilitation varied from one minimal corticomotoneuronal EPSP to another but this facilitation was not statistically correlated with the time course of the individual EPSPs.4. Significant facilitation (0.4) was still present 10 msec after a corticospinal volley so that this phenomenon could play a part in the initiation of motoneuronal discharge by corticospinal activity at natural frequencies of the order of 100 impulses/sec.

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