[Intrathalamic and mesencephalic afferent sources of the cat thalamic ventrobasal complex identified by using horseradish peroxidase].
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Biomedical subjects
Publications and source records attributed to V Iu Ermolaeva.
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The morphology and topography of the neurons in the ventro-basal complex (VB) projecting to the somatic sensory cortex (SI) were studied by the method of the retrograde axonic transport of horseradish peroxidase (HRP). Different types of HRP-labelled cells were found in this structure. The concentration of labelled cells was the greatest in the central part of the VB. The most peculiar feature in the distribution of HRP-positive neurons was their organization in two large populations in the dorsal and ventral portions of the VB. A comparison with the somatotopic map of the VB revealed that the central region of this complex is the area of the representation of the forelimb and the head. Our morphological results confirm the neurophysiological conception about the organization of the thalamo-cortical input on the "point-to-point" projection principles.
In 18 cats by means of two methods--anterograde degeneration and retrograde transport of exogenic horseradish peroxidase--cortico-cortical connections of the auditory fields to the cortical sensomotor area have been studied. These connections have been stated to terminate in layers V-III of certain parts of the sensomotor area corresponding to the projections of the foreleg and the head. Initial neurons of the connections studied are pyramidal cells in layers III and II. They are situated in rostral and caudal parts of the fields AI and AII, but within these levels they occur in different areas of the auditory fields.
The morphology and topography of neurons whose axons form the thalamic nonspecific input to the primary somatosensory cortex were studied in the cat thalamus by the method of the retrograde transport of the horseradish peroxidase. The labelled cells were found in the dorsolateral part of the nucleus ventralis anterior, in the nucleus centralis lateralis, in the lateral part of the nucleus dorsalis medialis, in the dorsal part of the nucleus centrum medianum. A very small number of slightly stained cells were found in the nucleus paracentralis. The variety of the shape, size and intensity of labelling of the HRP-positive cells was observed in each thalamic structure. The results of the experiments indicate that not only the ventrobasal complex but also the nonspecific thalamic nuclei are projected to the primary somatosensory cortex. These data demonstrate the existence of multiple afferent thalamic inputs to the primary somatosensory cortex.
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The functional properties of the auditory projections to the somatosensory zones S2 and S were studied by recording evoked potentials in anesthetized and vigil unrestrained cats. The thresholds of evoked potentials recorded from SI and SII were higher by 15--35 db than those from AI. No tonotopical localization was found in SI and SII. Signals about pure tones of different frequencies were conducted to SI and SII via area AI. The signals about clicks were ascending to SI and SII not only through this pathway, but also through other ones. It suggested from the time constants analysis of the first positive wave of the evoked potentials that the interneuronal organization of the cortical auditory projections to AI is not so comples as that to SI and SII. The differences in amplitudes of evoked potential recorded in SI indicate that the head projection areas receive higher portion of the auditory projections. This is confirmed by the morphological evidence.
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