Structural and Physiological Properties of Connections Between Individual Reticulospinal Axons and Lumbar Motoneurons of the Frog (original) (raw)
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Connectivity changes of la afferents on axotomized motoneurons
Characteristics of EPSPs in cat spinal motoneurons axotomized by ventral root section were investigated by Kuno and Llin~s 10,11. The EPSPs elicited by electrical stimulation of single Ia afferents exhibited prolonged rise times and half widths. The authors suggested that this was due to the loss of axosomatic synapses ('somatic stripping'), since EPSPs (recorded in the soma) which are generated by synapses on the soma and proximal dendrites are characterized by shorter rise times and shorter half widths than EPSPs generated on more distal dendrites 13. There is anatomical evidence that synapses are stripped specifically from the soma and proximal dendrites of rat facial 3 and spinal a motoneurons following axotomy.
An analysis of the cable properties of spinal motoneurones using a brief intracellular current pulse
The Journal of Physiology, 1973
Abstract1. A brief intracellular current pulse, with duration less than 500 μsec, has been applied to lumbosacral motoneurones in anaesthetized cats. The resulting voltage transients have been analysed by the procedure suggested in Jack & Redman (1971b) to obtain the cable parameters for each motoneurone.2. Forty-three motoneurone responses were analysed. In all cases the voltage response indicated that the dendrites could be represented as uniform, finite length cables, with either a sealed distal end, or at least a high resistance distal termination. The electrical length of the equivalent uniform dendritic cable ranged from 1·0 to 2·1 space constants, with a mean value of 1·5.3. The initial decay of the membrane potential following the removal of the current pulse was more rapid than was predicted by the Rall model for the motoneurone, in approximately two thirds of the responses. Consequently a value of dendritic to soma conductance ratio could not be obtained for these motoneurones.4. The explanation given for the departure from the theoretical response to a brief current pulse is that the specific resistivity of the soma membrane is lower than the specific resistivity of the dendritic membrane. This explanation is complicated by the possibility of the electrode tip not lodging in the isopotential soma region. The contribution that each of these effects has on the early decay phase of the current pulse response has been assessed.5. It is concluded that the specific resistivity of the soma membrane could be as low as one third of the dendritic membrane resistivity. Tonic inhibitory activity restricted to the soma is suggested as an explanation.
Distribution of contacts from vestibulospinal axons on the dendrites of splenius motoneurons
The Journal of Comparative Neurology, 2005
Current descriptions of the organization of synapses on the dendritic trees of spinal motoneurons indicate that the inputs are arranged in several patterns: some are widely distributed; some are distributed to proximal dendrites; others are distributed based on the trajectory of the dendrites. However, the principles governing the organization of synapses on spinal motoneurons remain poorly defined. Our goal was to extend the descriptions of the distribution of synapses, identified by their source, on the dendritic trees of spinal motoneurons. We combined anterograde and intracellular staining techniques in cats to determine the distribution of contacts between excitatory axons from the rostral aspect of the descending vestibular nucleus and the dendrites of motoneurons supplying a dorsal neck muscle, splenius. In five of five motoneurons, the contacts were preferentially distributed on dendrites medial to the soma. This qualitative observation was confirmed by using Monte Carlo methods. The results from this analysis showed that the distribution of contacts can be explained not by the overall distribution of the dendritic membrane area but rather by a systematic innervation of the medial regions of the dendritic trees (P Ͻ 0.02). Despite this selectivity, there was no additional bias in the distribution of contacts to proximal vs. distal dendrites. By concentrating excitatory synapses in a restricted region of the dendritic tree, the actions of vestibulospinal connections on neck motoneurons may be increased as a result of a greater probability of activating persistent inward currents on the dendrites.
Neuroscience, 1985
addition to primary afferent tibres, the dorsal columns of the cat spinal cord contain ascending second-order axons which project to the dorsal column nuclei. The aim of the present study was to obtain mo~ho~o~cal evidence that certain primary afferent axons form monosynaptic contacts with cells of origin of this postsynaptic dorsal column pathway. In ten adult cats, neurones with axons ascending the dorsal columns were retrogradely labelled with horseradish peroxidase using a pellet implantation method in the thoracic dorsal columns. In the lumbosacral regions of the same animals, primary afferent fibres were labelled intra-axonally with ionophoretic application of horseradish peroxidase. Tissue containing labelled axons was prepared for light and combined light and eiectron microscopy. Ultrastructural examination demonstrated that slowly adapting (Type I), hair follicle, Pacinian corpuscle and group Ia muscle spindle afferents formed monosynaptic contacts with labelled cells and light microscopical analysis suggested that they also received monosynaptic input from rapidly adapting (Krause) afferents.
Experimental Brain Research, 1984
Intracellularly HRP-labelled cat hindlimb (x-motoneurones were reconstructed light microscopically from a series of 1 ~tm or 2 ~m thick consecutive sections. The volume and surface area of the soma as well as the size of the very proximal part of the dendritic and axonal processes were estimated morphometrically. Similar measurements were also made on adjacent unlabelled neurons in the same series of sections. A close relation was found between the soma volume and surface area on one hand the combined cross-sectional area of the proximal dendrites and axon on the other. The combined axonal and dendritic bases occupied on the average 16% of the soma surface. The accuracy in using the diameters and cross-sectional area of the cell body as indirect estimates of soma volume and surface area was analyzed. Combined measurements in both the transversal and sagittal planes were then found to yield more satisfactory estimates then when the measurements were confined only to the transversal plane. Several different formulas using the soma axes for indirect calculations of the soma volume and surface area were compared with respect to the accuracy of the results.
Velocity of supraspinal input and conduction velocity of axons of spinal motoneurons
Brain Research, 1977
Based on their observations of reflexes, Sherrington and his associates a inferred that red muscles were involved primarily in sustained postural contractions while pale muscles were used in fast voluntary movements. Further investigation 1,11,1a has shown that within mixed muscles, pale fibers contract quickly and are innervated by large motoneurons with fast axons, whereas red fibers contract more slowly and receive input from smaller, more slowly conducting motoneurons. One might expect motor units of different time characteristics to be activated in a selective manner according to the temporal requirements of the movement. If so, then large motoneurons should be accessible to faster signals from supraspinal structures than smaller motoneurons. The time taken in the conduction and transmission of impulses is, of course, of little consequence for steadily maintained contraction but may well be significant for the accurate timing of phasic movements. Time requirements of inhibition would be similar to those of excitation. A positive correlation could therefore be expected between the latency of synaptic potentials evoked by stimulation of descending pathways and the latency of antidromic invasion of motoneurons in response to stimulation of their axons. It should be emphasized that it is the total time required for a signal to travel from the b:ain to the spinal motoneuron which is important for the testing of this hypothesis and not just the conduction velocity of the descending fibers. An analogous correlation has been observed between velocities of input to and output from lateral geniculate neurons in the visual pathway 7.