Layer I of the Adult Human Acoustic Cortex

What is it about?

The cellular morphology and the three dimensional cytoarchitecture of layer I of the adult human acoustic cortex were studied by using sagittal, transverse and tangential Golgi preparations and electron microscopy. The prominent neuron of layer I of the acoustic cortex is the Retzius-Cajal cell which is a solitary neuron accompanied by satellite astrocytes located mostly below the pial surface in the upper third of the layer. This neuron demonstrates a marked polymorphism in the various regions of the acoustic cortex. The anterior part of the Hessl convolutions contains Retzius-Cajal cells which are horizontal. multipolar forming dense dendritic arborization by extending long horizontal dendrites to all directions within a tangential plane parallel to the surface of the convolutions. The axon of the Retzius-Cajal cell is a long myelinated nerve fibre which mostly run horizontally, parallel to the pial surface, descending finally to the deeper parts of the layer and projecting a large number of axonic collaterals which radiate in all directions forming dense axonic networks. The Retzius-Cajal cell in the posterior part of the Hessl convolutions is a large polyhedral or triangular cell which extends a large number of dendrites, forming two dendritic networks: one in the upper third of the layer and another in middle part of it. Both of the networks communicate with the axonic collaterals of the Retzius-Cajal cells, forming numerous axodendritic synapses as can be clearly seen in electron microscopy. Tertiary branches of the apical dendrites of the second and third layers of the acoustic cortex ascend to layer I and intermix with the dendritic network of the Retzius-Cajal cells, forming dendro-dentritic synapses en passant and axodendritic synapses with the terminals of the axonic collaterals of the Retzius-Cajal cells. Fine efferent fibres also ascend to layer 1, developing synapses with the dendritic spines of the tertiary dendritic branches of the Retzius-Cajal cell. Most of the axodendritic synapses in the upper third of layer I demonstrate a marked polymorphism of the synaptic vesicles. In spite of the rarity of neurons layer I of the acoustic cortex is a place of horizontal transmission of the efferent impulses along the dendritic networks and the numerous axonic collaterals of the Retzius-Cajal cells

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Why is it important?

The electron microscopy revealed that almost all the dendritic spines of the Cajal-Retzius cells develop round or disc-like synapses with only one presynaptic component, whereas in the deeper layers of the human acoustic cortex most of the postsynaptic components develop synaptic contacts with more than one presynaptic terminal. Complex-shaped perforated synapses were not seen in layer I of the acoustic cortex, although a large number of them have been found in layers I1 and V . It might be hypothesized that in the adult acoustic cortex the Cajal-Retzius cell develops a wide functional territory extending throughout the cortical layer I, developing synapses that are morphologically unmodified by the input of the continuous sensorial information, although by synaptic plasticity, the shape and size of the synaptic connections can be modified following senso-sensorial training.

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