326 BRAIN MECHANISMS AND LEARNING 



In the ganglionic nervous system of Invertebrates, the rule is that the 

 perikaryon iiei'cr takes part in the synapses, as it is located in the periphery 

 of the ganglion, so tightly enclosed in a glial capsule (Fig. i) that it even 

 shows deep membranous glial invaginations in the neurone. In addition, 

 neither dendrites nor axons reach the cellular body; all these extensions, 

 together with the glial ramifications towards the centre of the ganglion, 

 constitute a dense, delicate and heterogeneous plexus improperly named 

 'neuropilc'. In the nervous centre of Invertebrates there are only three 

 synaptic types to be seen: [a) axo-dendritic ; (/)) axo-axonic and (c) dendro- 

 dendritic synapses. In contrast, in the nervous system of vertebrates the 

 perikaryon takes part in the synapse, with the only exception of the 

 neurones of the sensitive ganglia along the neuraxis and the mesencephalic 

 nucleus of the trigeminal nerve which, by the morphology of its neurones, 

 is equivalent to a spinal ganglion. 



In the nervous system of Vertebrates, intimate contacts occur in the three 

 morphological regions of a neurone; perikaryon, dendrites and axon. 

 These contacts are iioiiiolo(^oiis or lu'tcrolo(^oiis according to whether the 

 membranes are in contact with the same or different morphological 

 elements. The homologous contacts are [a) dendro-dendritic, [b) axo- 

 axonic, (f) somato-somatic. The heterologous contacts are [a] axo-den- 

 dritic, (/)) axo-somatic, (c) dcndro-somatic, [d) dcndro-somato-dendritic, 

 those of neurones with sensory or' receptor cells and of neurones with 

 effectors, namely secretory cells (crinocytcs), contractile fibres (myo-fibres), 

 etc. The contact, either homologous or heterologous may be terminal, 

 de passable ('de trayecto') or mixed. 



In addition to the types of synapses described above we call para- 

 synapses the axo-axonic, pre-terminal contacts between axons having 

 different origins but converging upon a common post-synaptic element 

 (Figs. 14, 15, 16). Parasynapses are as frequent as synapses; they enable a 

 slender pre-terminal fibre to influence not only its own terminal synapse, 

 but also the synapse of all the pre-terminal fibres with which it has a 

 parasynaptic contact. Parasynapses may guarantee synaptic security. 



In the next part of our study we wish to emphasize the importance of 

 dendro-dendritic, somato-somatic, dendro-somatic, dendro-somato- 

 dendritic and axo-dendritic cross synapses. The frequency of dendro- 

 dendritic contacts in the cerebellar glomeruli, in the horizontal neurones of 

 the retina, in the sympathetic ganglia, in the medulla, in the pons and in 

 general in all the nervous centres explored is such that there is no place for 

 the interpretation that these contacts are aberrant, exceptional or even 

 physiologically meaningless. 



