Showing papers on "Topographic map (neuroanatomy) published in 2015"

Imprecise Whisker Map in the Neonatal Rat Barrel Cortex

Abstract: The somatosensory barrel cortex in rodents contains a topographic map of the facial whiskers where each cortical barrel is tuned to a corresponding whisker. However, exactly when this correspondence is established during development and how precise the functional topography of the whisker protomap is at birth, before the anatomical formation of barrels, are questions that remain unresolved. Here, using extracellular and whole-cell recordings from the barrel cortex of 0- to 7-day-old (P0-7; P0 = day of birth) rat pups in vivo, we report a low level of tuning to the principal whisker at P0-1, with multiple adjacent whiskers evoking large multi- and single-unit responses and excitatory postsynaptic currents in cortical neurons. Additionally, we found broad and largely overlapping projection fields (PFs) for neighboring whiskers in the barrel cortex at P0-1. Starting from P2-3, a segregated whisker map emerged, characterized by preferential single whisker tuning and segregated whisker PFs. These results indicate that the functional whisker protomap in the somatosensory cortex is imprecise at birth, that for 2-3 days after birth, whiskers compete for the cortical target territories, and that formation of a segregated functional whisker map coincides with emergence of the anatomical barrel map.

T-type calcium channels cause bursts of spikes in motor but not sensory thalamic neurons during mimicry of natural patterns of synaptic input.

Abstract: Although neurons within intact nervous systems can be classified as ‘sensory’ or ‘motor,’ it is not known whether there is any general distinction between sensory and motor neurons at the cellular or molecular levels. Here we extend and test a theory according to which activation of certain subtypes of voltage-gated ion channel (VGC) generate patterns of spikes in neurons of motor systems, whereas VGC are proposed to counteract patterns in sensory neurons. We previously reported experimental evidence for the theory from visual thalamus, where we found that T-type calcium channels (TtCC) did not cause bursts of spikes but instead served the function of ‘predictive homeostasis’ to maximize the causal and informational link between retinogeniculate excitation and spike output. Here we have recorded neurons in brain slices from 8 sensory and motor regions of rat thalamus while mimicking key features of natural excitatory and inhibitory postsynaptic potentials. As predicted by theory, TtCC caused bursts of spikes in motor but not sensory thalamus. TtCC-mediated responses in motor thalamus were activated at more hyperpolarized potentials and caused larger depolarizations with more spikes than in visual and auditory thalamus. Somatosensory thalamus is known to be more closely connected to motor regions relative to auditory and visual thalamus, and likewise the strength of its TtCC responses was intermediate between these regions and motor thalamus. We also observed lower input resistance, as well as limited evidence of stronger hyperpolarization-induced (‘H-type’) depolarization, in nuclei closer to motor output. These findings support our theory of a specific difference between sensory and motor neurons at the cellular level.

The Rodent Vibrissal System as a Model to Study Motor Cortex Function

Abstract: The function of mammalian motor cortex was one of the first problems studied in neuroscience. But until today, the major principles of the workings of motor cortex have remained conjectural. It is clear that motor cortex holds a topographic map of body parts. However, does that necessarily imply that motor cortex itself undertakes the challenging task of converting movement plans (i.e. intended trajectories and effects of actions) into low level motor commands appropriate for driving the muscles? Many decades of research on motor function has shown that this is not entirely true by revealing the existence of dedicated networks, the so-called central pattern generators (CPGs) . Many, if not all of them, are located sub-cortically, and are likely to take over this task. Unfortunately the detailed circuitry and cellular elements of CPGs are only vaguely known. More recent work has elucidated continuous as well as discontinuous (discrete) mapping of motor cortex to movement. In the quest to understand motor cortex-CPG interactions, discontinuities are important because they allow us to dissect how neighboring motor cortex sites connect to different CPGs for different purposes—driving the very same muscles. The rodent whisker motor system is a decidedly modular system. Neighboring cortical areas drive very distinct whisker movements used by the animals in different contexts. We review the state of art in this system and argue that the modularity of the whisker system together with its great accessibility makes it a promising candidate for a model system for the investigation of motor cortex—CPG interactions on the cellular and network level—a highly valuable tool for the subsequent understanding of the more complex and continuously organized motor cortex of the arm/hand/finger system in primates.

Simultaneous microelectrode recording of layered structure of cortex and tonotopic structure of thalamus in the auditory pathway

Abstract: Simultaneous characterization of layer-specific activation in the cortex and topographically organized activation in the thalamus brings substantial benefits to advance the understanding of the sensory/motor functions. We designed a silicon-probe microelectrode array to simultaneously characterize the primary auditory cortex (A1) and thalamus (medial geniculate body; MGB), which are aligned in the dosro-lateral to ventro-medial axis perpendicular to the cortical surface of A1. The array had 3 shanks with 6-mm length. On each shank, 15 recording sites were made at the tip for MGB and 17 sites were made at the bottom for A1. A laser displacement meter was proved useful to make appropriate insertion of the array probe at a right angle with respect to the cortical surface. Our experiments in vivo demonstrated the capability of the designed array to investigate the thalamo-cortical interaction between tonotopically organized activities in the thalamus and layer-specific activities in the cortex.