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A prominent feature of thalamocortical circuitry in sensory systems is the extensive and highly organized feedback projection from the cortex to the thalamic neurons that provide stimulus-specific input to the cortex. In lightly sedated rats, we found that focal enhancement of motor cortex activity facilitated sensory-evoked responses of topographically aligned neurons in primary somatosensory cortex, including antidromically identified corticothalamic cells; similar effects were observed in ventral posterior medial thalamus (VPm). In behaving rats, thalamic responses were normally smaller during whisking but larger when signal transmission in brainstem trigeminal nuclei was bypassed or altered. During voluntary movement, sensory activity may be globally suppressed in the brainstem, whereas signaling by cortically facilitated VPm neurons is simultaneously enhanced relative to other VPm neurons receiving no such facilitation.

Physiological studies of the rodent somatosensory cortex have consistently described considerable heterogeneity in receptive field properties of neurons outside of layer IV, particularly those in layers V and VI. One such approach for distinguishing among different local circuits in these layers may be to identify the projection target of neurons whose axon collaterals contribute to the local network. In vivo, this can be accomplished using antidromic stimulation methods. Using this approach, the axonal conduction properties of cortical efferent neurons are described. Four projection sites were activated using electrical stimulation: (1) vibrissal motor cortex, (2) ventrobasal thalamus (VB), (3) posteromedial thalamic nucleus (POm), and (4) cerebral peduncle. Extracellular recordings were obtained from a total of 169 units in 21 animals. Results demonstrate a close correspondence between the laminar location of the antidromically identified neurons and their anatomically known layer of origin. Axonal properties were most distinct for corticofugal axons projecting through the crus cerebri. Corticothalamic axons projecting to either VB or POm were more similar to each other in terms of laminar location and conduction properties, but could be distinguished using focal electrical stimulation. It is concluded that, once stimulation parameters are adjusted for the small volume of the rat brain, the use of antidromic techniques may be an effective strategy to differentiate among projection neurons comprising different local circuits in supra- and infragranular circuits.

Rats actively sweep their whiskers back and forth to locate and palpate objects within their immediate environment. Microstimulation studies in anesthetized rats have demonstrated the presence of a large vibrissal motor representation in agranular cortex. However, the manner in which motor cortex neurons contribute to whisking behavior in the awake animal is unknown. This study represents an initial investigation of the relationship between the activity of task-related neurons in vibrissal motor cortex and the electromyographic (EMG) activity of the deep vibrissal pad muscles in the awake, freely whisking rat. Each animal was gently held in an experimenter's hands while the animal whisked the air. A spring-loaded, metal microelectrode mounted in a removable, miniature microdrive provided stabile recordings of motor cortex unit activity. Fine-wire electrodes implanted in the mystacial pad simultaneously recorded facial muscle activity. Results showed that the discharge of task-related neurons was correlated with changing levels of muscle output. Unit discharge was related in a tonic or phasic-tonic fashion to EMG activity. No units were found to discharge rhythmically in a 1:1 fashion with the periodicity of the whisking pattern. These findings support a role for vibrissal motor cortex in the initiation and modulation of the overall level of mystacial pad muscular output, but not in the generation of bursts of EMG activity responsible for individual whisking sweeps.