The termination of callosal fibres in the auditory cortex of the rat. A combined Golgi-electron microscope and degeneration study

Abstract

When the corpus callosum of the rat is sectioned, the callosal fibres in the cerebral cortex undergo degeneration. In the auditory cortex (area 41) the degenerating axon terminals form asymmetric synapses, and the vast majority of them synapse with dendritic spines. Some other synapse with the shafts of both spiny and smooth dendrites, and a few with the perikarya of non-pyramidal cells. The degenerating axon terminals are contained principally within layer II/III, in which they aggregate in patches. Using a technique in which neurons within the cortex are Golgi-impregnated, then gold-toned and examined in the electron microscope, it has been shown that the dendritic spines of pyramidal neurons with cell bodies in different layers receive the degenerating callosal afferents. The spines arise from the main apical dendritic shafts and their branches, from the dendrites of the apical tufts, and in some cases from the basal dendrites of the pyramidal neurons. The shafts of some pyramidal cell apical dendrites also form asymmetric synapses with callosal afferents. Since we have encountered no spiny non-pyramidal neurons in Golgi preparations of rat auditory cortex, and because other types of non-pyramidal cells have few dendritic spines, it is concluded that practically all of the dendritic spines synapsing with callosal afferents originate from pyramidal neurons.

Influence of Core Auditory Cortical Areas on Acoustically Evoked Activity in Contralateral Primary Auditory Cortex

“…Although numerous studies have examined transcallosal communication properties [visual cortex: (Glickstein and Berlucchi, 2008;Innocenti, 1980); somatosensory cortex: (Clarey et al, 1996;Rema and Ebner, 2003;Pluto et al, 2005)], to date, the effects of auditory cortex deactivation on acoustically evoked responses of contralateral cortical regions has not been reported. The present study is the first investigation to demonstrate changes in A1 response characteristics induced by reversible cooling deactivation on contralateral auditory fields.…”

“…In the cat, the sources and terminations of callosally projecting neurons have been related to binaural and frequency maps (Imig and Brugge, 1978;Imig et al, 1986). Similarities of origins and terminations of auditory callosal projections in other species, including rat (Cipolloni and Peters, 1983;Games and Winer, 1988;Rüttgers et al, 1990), hamster (Ravizza et al, 1976, and marmoset (Aitkin et al, 1988) demonstrate consistency in connectivity profiles across animal models.…”

“…Founded upon models of common information processing across sensory systems (Olshausen and Field, 2004), reports of neuronal response strength decreases in primary cortical fields [visual (Payne et al, 1991); somatosensory (Clarey et al, 1996)] during deactivation of contralateral homologous regions, and known interhemispheric connections in the auditory system (Lee and Winer, 2008a), we hypothesized decreases in response strength of contralateral auditory cortex neurons during sup-pression of callosal afferent activity. The hypothesis was tested by examining the impact of individual and combined A1 and AAF reversible deactivation on contralateral A1 neuronal response activity in the cat.…”

Cortical and thalamic contributions to response dynamics across layers of the primary somatosensory cortex during tactile discrimination

“…Bilateral anatomical connections between S1 barrel cortex are most prominent in layers II/III, layer V, and in the septa of layer IV (Welker et al 1988;White and DeAmicis 1977). These afferents are thought to be excitatory (Cipolloni and Peters 1983;Giuffrida and Rustioni 1989). We and others have previously shown in anesthetized rats that S1 neurons in layers V/VI exhibit bilateral whisker responses that are dependent on the integrity of the contralateral S1 cortex (Li et al 2005;Rema and Ebner 2003;Shuler et al 2001).…”

Bilateral Integration of Whisker Information in the Primary Somatosensory Cortex of Rats

“…We further provide evidence that unilateral inactivation of one SI removes, in the intact SI, the suppressive influence previous ipsilateral stimulation exerts on contralaterally evoked activity. Because callosal connections are thought to be excitatory in the rat cortex (Cipolloni and Peters, 1983;Giuffrida and Rustioni, 1989), it is not likely that callosally transmitted ipsilateral input directly contributes to the subsequent inhibition in the opposite hemisphere. Rather, we propose that such inhibitory influence arises locally as a consequence of callosal input, although other sources of inhibition dependent on callosal activity cannot be ruled out.…”