THE EXTENT OF THE DENDRITIC TREE AND THE NUMBER OF SYNAPSES IN THE FROG MOTONEURON

Frog motoneurons were intracellularly labelled with cobaltic lysine in the brachial and the lumbar segments of the spinal cord, and the material was processed for light microscopy in serial sections. With the aid of the neuron reconstruction system NEUTRACE, the dendritic tree of neurons was reconstructed and the length and surface area of dendrites measured. The surface of somata was determined with the prolate - oblate average ellipsoid calculation. Corrections were made for shrinkage and for optical distortion. The mean surface area of somata was 6710 mum2; lumbar motoneurons were slightly larger than brachial motoneurons. The mean length of the combined dendritic tree of brachial neurons was 29 408 mum and that of lumbar neurons 46 806 mum. The mean surface area was 127 335 mum2 in brachial neurons, and 168 063 mum2 in lumbar neurons. The soma-dendrite surface area ratio was 3 - 5% in most cases. Dendrites with a diameter of less-than-or-equal-to 1.0 mum constituted approximately 75% of the combined dendritic length in most of the neurons. Unlike in the cat, there was no correlation between the size of stem dendrites and the extent of daughter branches. From the synaptic density estimated in earlier electron microscope investigations of frog motoneuron dendrites (Antal et al., J. Neurocytol., 15, 303 - 310, 1986; 21, 34-49, 1992), and from the present data, the number of synapses on the dendritic tree was calculated. The calculations indicated 26 949 synapses on the smallest and 61 519 synapses on the largest neuron if the synaptic density was multiplied by the length of the dendritic tree. ff the synaptic density was multiplied by the surface area of the dendritic tree the calculation yielded 23 337 synapses for the smallest and 60 682 synapses for the largest neuron. More than 60% of the combined surface area of dendrites was > 600 mum from the soma. This suggests that about two-thirds of the synapses impinged upon distant dendrites > 600 mum from the soma. The efficacy of synapses at these large distances is investigated on model neurons in the accompanying paper (Wolfet al., Eur. J. Neurosci., 4, 1013 - 1021, 1992).