This tutorial was designed to be used during a two week study of rat anatomy. It should not be used to replace hands-on experience in the laboratory. It is intended to enhance the laboratory experience and provide an alternate frame of reference for learning. You will get more benefit from this tutorial if you have already examined the material in the laboratory, however it can also be used to become familiar with the various systems.
Auditory cortex is the last in a series of structures dedicated to the interpretation of auditory input. Many subcortical auditory nuclei have specialized circuits or synapses that have no correlates in the other sensory systems. For instance, the medial nucleus of the trapezoid body (MNTB) contains large calyceal synapses specialized for rapid, precise synaptic transmission (Trussell 1997). Recent evidence (Smith and Populin 1999) suggests that the thalamic input layers of cat auditory cortex may differ from those of visual and somatosensory cortices. It is possible that rat auditory cortex also contains unique circuits and cells specialized for auditory information processing. Alternatively, auditory cortex may process stimuli using circuits similar to those found elsewhere in cortex. It is therefore important to keep these possible specializations in mind and study auditory cortex both in terms of its possible auditory functions and as a part of cerebral cortex.
Bipolar stimulating electrodes were used to activate axons in the white matter in coronal slices, with stimuli that were stepped from 10 to 150 V in 10-V increments, and with durations of 100 or 200 μs. In horizontal slices, the internal capsule and external capsule were stimulated separately, allowing isolation of thalamocortical from corticocortical inputs (Fig. 1). The space between the paired electrode tips was sufficient to span the width of the fiber tract to stimulate the maximum number of inputs possible. Occasionally, stimulation induced antidromic spikes from the recorded cell. If this was observed, the polarity of the electrode was switched or the stimulating electrode was moved.
Drawings of slice preparations. A: coronal hemisection illustrating thalamic landmarks and white matter stimulation site (stim). All recordings made in area Te1, primary auditory cortex. B: semihorizontal section illustrating thalamic and midbrain landmarks and internal capsule stimulation site (stim). The tissue is blocked higher rostrally and laterally to give this approximate plane of section. Scale bar is 1 mm. APT, anterior pretectal nucleus; CG, central gray; CPu, caudate putamen; Ent, entorhinal cortex; Hi, hippocampus; IC, inferior colliculus; MD, medial dorsal nucleus; Po, posterior thalamic nucleus; Oc2L, occipital cortex, area 2, lateral; Par1, parietal cortex, area 1; Par2, parietal cortex, area 2; SN, substantia nigra; Te1, primary auditory cortex; Te3, secondary auditory cortex; VP, ventral posterior thalamic nucleus; ec, external capsule; ic, internal capsule; ml, medial lemniscus.