Structural and physiological properties of connections between individual reticulospinal axons and lumbar motoneurons of the frog

Although the direct, monosynaptic influence of brainstem projections onto m otoneurons is well-known, detailed morphological studies on the synaptic co ntact systems and a correlation with their functional properties are largel y lacking. In this work, 43 pairs, each formed by a reticulospinal fiber co ntacting a lumbar motoneuron, were identified and studied electrophysiologi cally. Four of these were successfully labeled intracellularly with horsera dish peroxidase HRP) or neurobiotin and reconstructed using a computer-assi sted camera lucida with high resolution. The mean amplitude of excitatory p ost-synaptic potentials (EPSPs) recorded in these four pairs varied from 10 0 to 730 muV, spanning most of the range obtained for all pairs (70-1,200 m uV; mean +/- SD: 400 +/- 250 muV). Between two and four collaterals of reti culospinal axons established 4-19 close appositions with st labeled motoneu ron. Mean distance from the origin of each collateral to any bouton on that collateral was 566-817 mum. A presynaptic action potential must pass 11 br anch points on average to reach it. Similarly, the boutons presumably conta cting motoneurons were on average 558-624 mum (9-11 branch points) from the origin of the collateral. The distributions of diameters of all boutons an d those making putative contacts with stained motoneurons were very similar . The dendritic surface of stained motoneurons was symmetrically distribute d along the rostrocaudal axis with more than half the surface being more th an 500 mum from the soma. However, the contacts from reticulospinal axons w ere concentrated ventromedially, 262-356 mum (range of average values for f our connections) from the motoneuron soma, in some instances on very proxim al dendritic segments. Thus, the location and size of putative contacts in relation to axonal collaterals was not distinguishable from location and si ze of other boutons, but they occupied specific positions on dendrites of l umbar motoneurons. The number of contacts formed by a reticulospinal axon o n a motoneuron in a particular location could be described as the product o f the available dendritic surface and the total number of presynaptic bouto ns in this region. Compartmental models of the reconstructed motoneurons we re created, and currents with the time course of an alpha function were inj ected at the sites of these putative contacts. Despite the restricted volum e occupied by contacts fi om a single fiber, a high variability of their co ntributions to somatic EPSPs owing to electrotonic attenuation was shown. T he coefficient of variation of quantal responses was estimated to be betwee n 60% and 120%, comparable to the variability of the path distance between contacts and soma (50-90%). (C) 2001 Wiley-Liss, Inc.