Premium
Topographical relations between ipsilateral cortical afferents and callosal neurons in the second somatic sensory area of cats
Author(s) -
Barbaresi Paolo,
Minelli Andrea,
Manzoni Tullio
Publication year - 1994
Publication title -
journal of comparative neurology
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 1.855
H-Index - 209
eISSN - 1096-9861
pISSN - 0021-9967
DOI - 10.1002/cne.903430408
Subject(s) - axon , cats , anatomy , horseradish peroxidase , sensory system , axoplasmic transport , cortex (anatomy) , biology , cerebral cortex , corpus callosum , neuroscience , medicine , biochemistry , enzyme
Experiments were carried out on the second somatic sensory area (SII) of cats to study (1) the laminar distribution of axon terminals from the ipsilateral first somatic sensory cortex (SI); and (2) the topographical relations between their terminal field and the callosal neurons projecting to the contralateral homotopic cortex. To label simultaneously in SII both ipsilateral cortical afferents and callosal cells, cats were given iontophoretic injections of Phaseolus vulgaris ‐leucoagglutinin (PHA‐L) in the forepaw zone of ipsilateral SI, and pressure injections of horseradish peroxidase (HRP) in the same zone of contralateral SII. The possibility that ipsilateral cortical axon terminals synape callosal neurons was investigated with the electron microscope by combining lesion‐induced degeneration with retrograde HRP labelling. Fibers and terminations immunolabelled with PHA‐L from ipsilateral SI were distributed in SII in a typical patchy pattern and were mostly concentrated in supragranular layers. Labelled fibers formed a very dense plexus in layer III and ramified densely also in layers I and II. Labelled axon terminals were both en passant and single‐stalked boutons. Counts of 8,303 PHA‐L‐labelled terminals of either type showed that 82.40% were in supragranular layers. The highest concentration was in layer III (43.99%), followed by layers II (30.22%) and I (8.09%). The remaining terminals were distributed among layers IV (6.96%), V (4.93%), and VI (5.68%). The same region of SII containing anterogradely labelled axons and terminals also contained numerous neurons retrogradely labelled with HRP from contralateral SII. Callosal projection neurons were pyramidal, dwelt mainly in layer III, and were distributed tangentially in periodic patches. Patches of anterograde and retrograde labelling either interdigitated or overlapped both areally and laminarly. In the zones of overlap, numerous PHA‐L‐labelled axon terminals were seen in close apposition to HRP‐labelled pyramidal cell dendrites. Combined HRP‐electron microscopic degeneration experiments showed that in SII axon terminals from ipsilateral SI form asymmetric synapses with HRP‐labelled dendrites and dendriticc spines pertaining to callosal projection neurons. These results are discussed in relation to the layering and function of the SI to SII projection, and to the evidence that SII neurons projecting to the homotopic area of the contralateral hemisphere have direct access to the sensory information transmitted from ipsilateral SI. © 1994 Wiley‐Liss, Inc.