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1. The enzyme horseradish peroxidase was injected into identified lumbosacral alpha-motoneurones and Group Ia afferent fibres in cats anaesthetized with chloralose and paralysed with gallamine triethiodide. Subsequent histological examination allowed the determination of (a) the extent of the motoneuronal dendritic trees, (b) the number and location of Ia synapses upon the motoneurones. 2. alpha-motoneurones had seven to eighteen primary dendrites and each produced daughter branches up to the fourth to the sixth order. At dendritic bifurcations Rall's 3/2 Power Law was obeyed. There was little or no dendritic tapering up to about 800 micrometers from the soma. Beyond this distance, however, there was considerable tapering. 3. Horseradish peroxidase injections revealed that motoneuronal dendrites are much longer than previously thought. Individual dendrites could be traced for up to 1600 micrometers from the soma and dendritic trees were usually 2-3 mm from tip to tip. Nearly all the motoneurones had dendrites that entered the white matter of the cord. Dendrites could also reach as far dorsally as laminae V and VI. 4. Ia synapses upon motoneuronal somata were examined in cords counterstained with cresyl violet or methylene green. About 10% of Ia boutons in lamina IX were on somata and each Ia collateral terminated on 3.66 motoneuronal somata or the most proximal (30 micrometer) dendrites, with on average about two contacts per motoneurone. 5. Ten Ia afferent fibre-motoneurone pairs were injected with horseradish peroxidase. The following conclusions were drawn: (i) only one collateral of any given Ia axon makes contact with a motoneurone even though other collaterals from the same axon might pass through the dendritic tree, (ii) usually all contacts made between a Ia fibre and a motoneurone are at about the same geometrical distance from the soma, even when on different dendrites, (iii) between two and five contacts are made upon the dendritic tree (average 3.4) at distances of between 20 and 820 micrometers from the soma. 6. The results are discussed in relation to previous anatomical and electrophysiological work.
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