Biomedical subjects

Randy M Bruno

Publications and source records attributed to Randy M Bruno.

5 recordsLinked to original sources

PubMed2006

Cortex is driven by weak but synchronously active thalamocortical synapses.

Sensory stimuli reach the brain via the thalamocortical projection, a group of axons thought to be among the most powerful in the neocortex. Surprisingly, these axons account for only approximately 15% of synapses onto cortical neurons. The thalamocortical pathway might thus achieve its effectiveness via high-efficacy thalamocortical synapses or via amplification within cortical layer 4. In rat somatosensory cortex, we measured in vivo the excitatory postsynaptic potential evoked by a single synaptic connection and found that thalamocortical synapses have low efficacy. Convergent inputs, however, are both numerous and synchronous, and intracortical amplification is not required. Our results suggest a mechanism of cortical activation by which thalamic input alone can drive cortex.

Action Potentials↗

PubMed2005

The role of thalamic inputs in surround receptive fields of barrel neurons.

Controversy exists regarding the relative roles of thalamic versus intracortical inputs in shaping the response properties of cortical neurons. In the whisker-barrel system, this controversy centers on the mechanisms determining the receptive fields of layer IV (barrel) neurons. Whereas principal whisker-evoked responses are determined by thalamic inputs, the mechanisms responsible for adjacent whisker (AW) responses are in dispute. Here, we took advantage of the fact that lesions of the spinal trigeminal nucleus interpolaris (SpVi) significantly reduce the receptive field size of neurons in the ventroposterior thalamus. We reasoned that if AW responses are established by these thalamic inputs, brainstem lesions would significantly reduce the receptive field sizes of barrel neurons. We obtained extracellular single unit recordings from barrel neurons in response to whisker deflections from control rats and from rats that sustained SpVi lesions. After SpVi lesions, the receptive field of both excitatory and inhibitory barrel neurons decreased significantly in size, whereas offset/onset response ratios increased. Response magnitude decreased only for inhibitory neurons. All of these findings are consistent with the hypothesis that AW responses are determined primarily by direct thalamic inputs and not by intracortical interactions.

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PubMed2003

Thalamocortical angular tuning domains within individual barrels of rat somatosensory cortex.

In the rodent somatosensory cortex, whisker-related barrels in layer IV are morphological counterparts of functional cortical columns that extend throughout the cortical depth. We used microelectrode recordings and spike-triggered averaging of field potentials evoked by single thalamic barreloid neurons to investigate functional thalamocortical microcircuits. The function of such circuits was probed by deflecting the principal whisker of a barrel in different angular directions. We found that individual barrels contain minicolumns of neurons preferring the same deflection angle. Angular tuning domains are established by convergent inputs from thalamocortical cells with corresponding angular preferences. Processing within such domains may depend on local connectivity among vertically aligned barrel neurons.

Action Potentials↗

PubMed2003

Response transformation and receptive-field synthesis in the lemniscal trigeminothalamic circuit.

To understand how the lemniscal trigeminothalamic circuit (PrV --> VPM) of the rodent whisker-to-barrel pathway transforms afferent signals, we applied ramp-and-hold deflections to individual whiskers of lightly narcotized rats while recording the extracellular responses of neurons in either the ventroposterior medial (VPM) thalamic nucleus or in brain stem nucleus principalis (PrV). In PrV, only those neurons antidromically determined to project to VPM were selected for recording. We found that VPM neurons exhibited smaller response magnitudes and greater spontaneous firing rates than those of their PrV inputs, but that both populations were similarly well tuned for stimulus direction. In addition, fewer VPM (74%) than PrV neurons (93%) responded with sustained, or tonic, discharges during the plateau phase of the stimulus. Neurons in both populations responded most robustly to deflections of a single, "principal whisker" (PW), and the majority of cells in both PrV (90%) and VPM (73%) also responded to deflections of at least one adjacent whisker (AW). AW responses in both nuclei occurred on average at longer latencies and were more temporally dispersed than PW responses. Lateral inhibition, as evidenced by AW-evoked activity suppression, was rare in PrV but prevalent in VPM. In both nuclei, however, suppression was weak, with AW responses being on average excitatory. Our results suggest that the receptive-field structures and response properties of individual VPM neurons can be explained in large part by input from one or a small number of PrV neurons, but that intrathalamic mechanisms act to further transform the afferent signal.

Action Potentials↗

PubMed2002

Feedforward mechanisms of excitatory and inhibitory cortical receptive fields.

Excitatory and inhibitory cortical layer IV neurons have distinctive response properties. Thalamocortical connectivity that may underlie differences was examined using cross-correlation analyses of pairs of thalamic and cortical neurons in the rat whisker/barrel system. Cortical layer IV cells discharging fast spikes, presumed inhibitory neurons, were distinguished from regular-spike units, presumed excitatory neurons, by the extracellular waveform shape. Regular-spike neurons fired less robustly and had smaller receptive fields (RFs) and greater directional tuning than fast-spike cells. Presumed excitatory neurons were less likely to receive thalamocortical connections, and their connections were, on average, weaker. RF properties of thalamic inputs to both cell types were equivalent, except that the most highly responsive thalamic cells contacted only fast-spike neurons. In contrast, the size and directional tuning of cortical RFs were related to the number of detectable thalamocortical inputs. Connected thalamocortical pairs were likely to have matching RF characteristics. The smaller, more directionally selective RFs of excitatory neurons may be a consequence of their weaker net thalamic drive, their more nonlinear firing characteristics and pervasive feedforward inhibition provided by strongly driven, broadly tuned inhibitory neurons.

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