Motor nuclei and primary afferent projections of branchial cranial nerves were shown with the CoC1, staining technique. Motor nuclei constitute a discontinuous column a t the ventrolateral margin of the gray matter. The size of motoneurons decreases in the orocaudal direction within the column. The dendrites assemble in two main directions forming a dorsomedial and a ventrolateral dendritic array. The axons originate in the ventrolateral dendrites and run straight to the motor roots. Preganglionic vegetative neurons were found in a separate group at the level of the abducens nucleus, and intermingled with motoneurons a t the level of the glossopharyngeus and vagus nuclei. Their axons join the visceral sensory roots of the respective nerve.The tractus spinalis trigemini has a short and weak ascending limb, and a strong descending limb which decussates in the first and second segments of the spinal cord. A few uncrossed large fibers continue as far as the midthoracic level.In addition to trigeminal fibers, a small contingent of facial fibers and a significant number of vagus fibers contribute to the tractus spinalis. Small caliber fibers of the tractus and fine collaterals of large caliber fibers terminate in a continuous column which lies, in its largest part, in the neuropil medial to the tractus. The pattern of fiber terminations in this area is identical with that in the substantia gelatinosa. This area is called the nucleus spinalis trigemini. Coarse collaterals terminate in a discontinuous column of large and medium-sized neurons, extending caudally to the obex region. This area is the largest a t the level of the trigeminus nucleus and is called the nucleus principalis trigemini.The cells of origin of the mesencephalic tractus of the trigeminus lie in the second and fourth layers of the optic tectum. The descending fibers run ventral to the tractus spinalis in a separate bundle. They emit collaterals to the large fiber termination areas and to branchiomotor nuclei. Direct contacts are established with trigeminus motoneurons. In the obex region a significant number of collaterals invade the dorsomedial part of the gray matter.The fasciculus solitarius begins a t the level of the trigeminal nucleus and terminates in the commissura infima a t the medullospinal border. The nucleus solitarius begins a t the oral pole of the glossopharyngeus nucleus. The fasciculus receives fibers from the facialis (intermedius), glossopharyngeus and vagus nerves. The fibers spread over the total extent of the fasciculus: the facialis fibers exhibit a preference for a dorsomedial position and the glossopharyngeusvagus fibers for a ventrolateral position. There are several interchanging fibers between the somatic sensory and visceral sensory projecting areas.The results indicate a close similarity between amphibian and mammalian brain stem motor nuclei and sensory projections, though some parts are present in a primordial form in the frog.
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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 microm2; lumbar motoneurons were slightly larger than brachial motoneurons. The mean length of the combined dendritic tree of brachial neurons was 29 408 microm and that of lumbar neurons 46 806 microm. The mean surface area was 127 335 microm2 in brachial neurons, and 168 063 microm2 in lumbar neurons. The soma - dendrite surface area ratio was 3 - 5% in most cases. Dendrites with a diameter of = 1.0 microm 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. If 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 microm from the soma. This suggests that about two-thirds of the synapses impinged upon distant dendrites >600 microm from the soma. The efficacy of synapses at these large distances is investigated on model neurons in the accompanying paper (Wolf et al., Eur. J. Neurosci., 4 1013 - 1021, 1992).
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