PubMed · 11022218
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M Shukla. 2000. Reprogramming the cortex.. https://doi.org/10.1007/bf02703924
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An analysis of central afferent projections to the dorsal cochlear nucleus (dCo), one of the three target nuclei of the auditory nerve, was made using retrograde axonal tracer, wheat germ agglutinin-horseradish peroxidase (WGA-HRP) in the rabbit. The findings showed that, in addition to its afferents from the brainstem auditory nuclei (they are not described herein), the dCo received sparse bilateral connections from the caudal three quarters of the vestibular nuclear complex (VNC). Following selective iontophoretic injections of WGA-HRP into the dCo, a small number of labelled neurones (from 2 to 28 per case) was found in the rostral and caudal portions of the medial vestibular nucleus and in the inferior vestibular nucleus. These neurones were observed mainly in the lateral regions of the nuclei. No labelling appeared in other nuclei which belonged to the VNC. The secondary vestibulocochlear connections have not been reported before in any species. With the anatomical method used, however, their functional role is difficult to explain. Further study is necessary to identify the type of neurotransmitter as well as the physiological properties of vestibular neurones projecting to the dCo, in terms of their responses to a change of the head position and to sound.
The efficacy of anterograde labeling of the central projections of primary afferent fibers were compared between biotinylated dextran amine (BDA), neurobiotin (NB) and Phaseolus vulgaris-leucoagglutinin (PHA-L) after injections into the L5 or T13 dorsal root ganglia (DRGs) of adult rats. Excellent labeling was obtained with BDA, which visualized fibers with fine terminal boutons in the L5 and T13 spinal cord segments, Clarke's nucleus and the gracile nucleus. Rarely observed crossed projections to the gracile nucleus and L5 ventral horn of the contralateral side could also be distinguished. Even in the most successful experiments, however, BDA labeled only about one-third of the axons originating from the injected dorsal root ganglion. BDA was also efficient as transganglionic tracer after application to the transected sciatic nerve. NB produced no significant labeling of the L5 primary afferents, and was only moderately effective on the T13 level. PHA injections resulted in sparse terminal labeling of the T13 and L5 afferents. Thus, BDA is an effective tracer for long-range labeling of primary afferent projections in the spinal cord and brain stem. Since not all stem fibers become labeled, however, the method does not allow quantification of all axon branches and terminals arising from the injected DRGs.
The spatial and temporal patterns of neocortex activation are determined not only by the dynamic character of the input but also by the intrinsic dynamics of the cortical circuitry. To study the role of afferent input frequency on cortical activation dynamics, the electrical activity of in vitro neocortex slices was imaged during white-matter electrical stimulation. High-speed optical imaging was implemented using voltage-sensitive dyes in guinea pig visual and somatosensory cortex slices concomitantly with intracellular recordings. Single white-matter electrical stimuli activated well-defined cortical sites with a radially oriented columnar configuration. This configuration was followed, over the next few milliseconds, by a lateral spread of excitation through cortical layers 5 and 6 and layers 2 and 3. Much of the optical response was eliminated in low extracellular calcium, indicating that it was primarily synaptically mediated. Repetitive stimuli at 10 Hz reproduced the spatiotemporal pattern observed for single stimuli. In contrast, repetitive stimulation in the gamma frequency range ( approximately 40 Hz) rapidly restrained the area of excitation to a small columnar site directly above the stimulating electrode. Intracellular recordings from cells lateral to the activated column revealed increased inhibitory synaptic activity and/or decreased excitatory responses during the train at 40 Hz, but not during a 10 Hz stimulation. Localized microinjections of GABA(A) antagonist produced a reorganization of the geometrical activity pattern that was dependent on the position of the microinjection site. These findings indicate that the frequency-dependent spatial organization of neocortex activation is determined by inhibitory sculpting attributable to local network dynamics.