Biology subjects

Shepherd, G. M. G.

Publications and source records attributed to Shepherd, G. M. G..

9 recordsLinked to original sources

bioRxiv2026

Specialized layer 5 cortical terminals target functional thalamic spines

Dendritic spines are ubiquitous morpho-functional elements of excitatory synaptic transmission and plasticity. Whether axo-spinous communication exists in the thalamus is presently unclear. Here we find that layer 5 (L5) corticothalamic terminals arising from frontal cortices selectively target thalamic spines with classical head and neck dimensions. The synapses of the L5 axo-spinous contacts were enriched in GluR1 subunit of AMPA receptor and their size correlated with their spine head volume. Synaptic stimulation of thalamic spines resulted in compartmentalized Ca2+ responses similar to that of hippocampal spines. Optogenetic activation of the frontal L5-thalamus pathway had strong impact on thalamic spiking even at high stimulation frequencies and could selectively recruit a subnetwork of thalamic cells in a behavioral state dependent manner. Optogenetic interference with the small L5-thalamic pathway perturbed motor learning. Our study reveals a hitherto unrecognized, effective top-down communication channel between the frontal cortex and thalamus that involves highly variable axo-spinous contacts.

neuroscience↗

bioRxiv2026

Multi-area activity in mouse motor cortex associated with one- and two-handed oromanual dexterity

Cortical dynamics during goal-directed dexterous hand movements are mainly understood from paradigms involving use of the contralateral hand. To study how movement-related activity changes when the ipsilateral or both hands are used, we exploited a natural form of rodent manual dexterity - food handling - that rodents can perform uni- or bimanually. We sampled kilohertz 3D kinematics as mice used either or both hands to manipulate food, while recording spiking activity in forelimb primary (fl-M1) and secondary (fl-M2) motor cortices, and in a lateral oral and manual (LOM) motor cortex area implicated in oromanual food handling. Unit- and population-level analyses showed that activity in fl-M1 and fl-M2 depended on both laterality (ipsi- vs contralateral) and "manuality" (uni- vs bimanual), with few differences between the two areas. By comparison, activity in LOM was largely laterality- and manuality-invariant. These results demonstrate how activity in multiple areas of mouse motor cortex varies as the same task is performed unimanually with either hand or bimanually with both. Our findings support a model in which fl-M1 and fl-M2 maintain separable information about both forelimbs for bimanual coordination, while LOM encodes ingestion-related forelimb parameters necessary for oromanual coordination.

neuroscience↗

bioRxiv2026

Circuit organization of the forelimb-related M2-to-M1 corticocortical pathway in the mouse

Communication from secondary (M2, premotor) to primary (M1) motor cortex is implicated in forelimb motor control. We investigated the underlying synaptic circuits in this corticocortical pathway in male and female mice using cell-type-specific optogenetic-electrophysiology methods, focusing on identifying the cell-type-specific synaptic connections in the excitatory and feedforward inhibitory circuits impinging on cervically projecting M1 corticospinal neurons. In forelimb M1 brain slices, recordings from layer 5B corticospinal neurons during brief photostimulation of M2 axons showed strong monosynaptic excitatory currents that, although accompanied by potent feedforward inhibitory currents, were capable of evoking action potentials (APs) in most neurons. In contrast, responses in layer 2/3 pyramidal neurons were generally much weaker. Parvalbumin-expressing neurons (PV), particularly in deeper layers, showed direct excitation from M2 axons without feedforward inhibition, and could fire APs robustly. Somatostatin (SST) neurons received generally weak inputs, whereas VIP and Ndnf neurons received stronger excitation and inhibition from M2 axons. Corticospinal neurons received little or no local inhibition from Ndnf and VIP interneurons, but relatively strong soma-targeting PV and dendrite-targeting SST inhibitory inputs, as functionally imaged by laser-scanning synaptic input mapping ("sCRACM"). The domains of PV and SST inputs were partly overlapping around the corticospinal somata, but broader for PV and more vertical for SST inputs. Collectively, the results provide a working model for the cell-type-specific synaptic circuits of this "top-down" corticocortical pathway, organized around direct M2 excitation and PV-mediated inhibition of M1 corticospinal neurons. SIGNIFICANCE STATEMENTCervically projecting corticospinal neurons in the primary motor cortex (M1) serve as the most direct conduits by which motor cortical activity reaches and influences spinal circuits controlling forelimb movements. Corticospinal activity is in turn influenced by inputs from multiple upstream areas. Here we studied the inputs from the secondary motor cortex (M2), a premotor-like area in the mouse, and characterized the patterns of synaptic connectivity formed by M2 axons onto multiple postsynaptic cell types in M1. The resulting "wiring diagram" suggests that these inter-areal circuits are configured to give premotor cortex privileged access to modulate M1 corticospinal output, through cell-type-specific connections and inhibitory mechanisms.

neuroscience↗

bioRxiv2024

Cortical dynamics in hand/forelimb S1 and M1 evoked by brief photostimulation of the mouses hand

Spiking activity along synaptic circuits linking primary somatosensory (S1) and motor (M1) areas is fundamental for sensorimotor integration in cortex. Circuits along the ascending somatosensory pathway through mouse hand/forelimb S1 and M1 were recently described in detail (Yamawaki et al., 2021). Here, we characterize the peripherally evoked spiking dynamics in these two cortical areas. Brief (5 ms) optogenetic photostimulation of the hand generated short ([~]25 ms) barrages of activity first in S1 (onset latency 15 ms) then M1 (10 ms later). The estimated propagation speed was 20-fold faster from hand to S1 than from S1 to M1. Amplitudes in M1 were strongly attenuated. Responses were typically triphasic, with suppression and rebound following the initial peak. Evoked activity in S1 was biased to middle layers, consistent with thalamocortical connectivity, while that in M1 was biased to upper layers, consistent with corticocortical connectivity. Parvalbumin (PV) inhibitory interneurons were involved in each phase, accounting for three quarters of the initial spikes generated in S1, and their selective photostimulation sufficed to evoke suppression and rebound in both S1 and M1. Partial silencing of S1 by PV activation during hand stimulation reduced the M1 sensory responses. Overall, these results characterize how evoked spiking activity propagates along the hand/forelimb transcortical loop, and illuminate how in vivo cortical dynamics relate to the underlying synaptic circuit organization in this system.

neuroscience↗

bioRxiv2022

Manipulation-specific activity in motor and somatosensory cortex as mice handle food

Food-handling offers unique yet largely unexplored opportunities to investigate how cortical activity relates to forelimb movements in a natural, ethologically essential, and kinematically rich form of manual dexterity. To determine these relationships, we recorded spiking activity in mouse forelimb M1 and S1 and tongue/jaw M1. Activity in all areas was strongly modulated in close association with discrete active manipulation events that occurred intermittently as mice fed. Each areas activity was also partly distinct in its overall timing and phasic/tonic temporal profile, attributable to area-specific composition of activity classes. Forelimb position could be accurately predicted from activity in all three regions. These results thus establish that cortical activity during food-handling is manipulation-specific, distributed, and broadly similar across multiple cortical areas, while also exhibiting area- and submovement-specific relationships with the fast kinematic hallmarks of this form of complex, free-object-handling manual dexterity.

neuroscience↗

bioRxiv2022

Multiscale model of primary motor cortex circuits reproduces in vivo cell type-specific dynamics associated with behavior

Understanding cortical function requires studying multiple scales: molecular, cellular, circuit and behavior. We developed a biophysically detailed multiscale model of mouse primary motor cortex (M1) with over 10,000 neurons and 30 million synapses. Neuron types, densities, spatial distributions, morphologies, biophysics, connectivity and dendritic synapse locations were tightly constrained by experimental data. The model includes long-range inputs from 7 thalamic and cortical regions, as well as noradrenergic inputs from locus coeruleus. Connectivity depended on cell class and cortical depth at sublaminar resolution. The model accurately predicted in vivo layer- and cell type-specific responses (firing rates and LFP) associated with behavioral states (quiet wakefulness and movement) and experimental manipulations (noradrenaline receptor blocking and thalamus inactivation). It also enabled evaluation of multiple mechanistic hypotheses underlying the observed activity. This quantitative theoretical framework can be used to integrate and interpret M1 experimental data and sheds light on the cell type-specific multiscale dynamics associated with a range of experimental conditions and behaviors.

neuroscience↗

bioRxiv2021

Circuit organization of the excitatory sensorimotor loop through hand/forelimb S1 and M1

Sensory-guided limb control relies on communication across sensorimotor loops. For active touch with the hand, the longest loop is the transcortical continuation of ascending pathways, particularly the lemnisco-cortical and corticocortical pathways carrying tactile signals via the cuneate nucleus, ventral posterior lateral (VPL) thalamus, and primary somatosensory (S1) and motor (M1) cortices to reach corticospinal neurons and influence descending activity. We characterized excitatory connectivity along this pathway in the mouse. In the lemnisco-cortical leg, disynaptic cuneate[->]VPL[->]S1 connections excited mainly layer (L) 4 neurons. In the corticocortical leg, S1[->]M1 connections from L2/3 and L5A neurons mainly excited downstream L2/3 neurons, which excite corticospinal neurons. The findings provide a detailed new wiring diagram for the hand/forelimb-related transcortical circuit, delineating a basic but complex set of cell-type-specific feedforward excitatory connections that selectively and extensively engage diverse intratelencephalic projection neurons, thereby polysynaptically linking subcortical somatosensory input to cortical motor output to spinal cord.

neuroscience↗

bioRxiv2019

Manual dexterity of mice during food-handling involves the thumb and a set of fast basic movements

The small first digit (D1) of the mouses hand resembles a volar pad, but its thumb-like anatomy suggests ethological importance for manipulating small objects. To explore this possibility, we recorded high-speed close-up video of mice eating seeds and other food items. Analyses of ethograms and automated tracking with DeepLabCut revealed multiple distinct microstructural features of food-handling. First, we found that mice indeed made extensive use of D1 for dexterous manipulations. In particular, mice used D1 to hold food with either of two grip types: a pincer-type grasp, or a "thumb-hold" grip, pressing with D1 from the side. Thumb-holding was preferentially used for handling smaller items, with the smallest items held between the two D1s alone. Second, we observed that mice cycled rapidly between two postural modes while feeding, with the hands positioned either at the mouth (oromanual phase) or resting below (holding phase). Third, we identified two highly stereotyped D1-related movements during feeding, including an extraordinarily fast ([~]20 ms) "regrip" maneuver, and a fast ([~]100 ms) "sniff" maneuver. Lastly, in addition to these characteristic simpler movements and postures, we also observed highly complex movements, including rapid D1-assisted rotations of food items and dexterous simultaneous double-gripping of two food fragments. Manipulation behaviors were generally conserved for different food types, and for head-fixed mice. Wild squirrels displayed a similar repertoire of D1-related movements. Our results define, for the mouse, a set of kinematic building-blocks of manual dexterity, and reveal an outsized role for D1 in these actions.

animal behavior and cognition↗

bioRxiv2017

Corticocortical signaling drives activity in a downstream area rapidly and scalably

Quantitative analysis of corticocortical signaling is needed to understand and model information processing in cerebral networks. However, higher-order pathways, hodologically remote from sensory input, are not amenable to spatiotemporally precise activation by sensory stimuli. Here, we combined parametric channelrhodopsin-2 (ChR2) photostimulation with multi-unit electrophysiology to study corticocortical driving in a parietofrontal pathway from retrosplenial cortex (RSC) to posterior secondary motor cortex (M2) in mice in vivo. Ketamine anesthesia was used both to eliminate complex activity associated with the awake state and to enable stable recordings of responses over a wide range of stimulus parameters. Photostimulation of ChR2-expressing neurons in RSC, the upstream area, produced local activity that decayed quickly. This activity in turn drove downstream activity in M2 that arrived rapidly (5-10 ms latencies), and scaled in amplitude across a wide range of stimulus parameters as an approximately constant fraction (~0.2) of the upstream activity. A model-based analysis could explain the corticocortically driven activity with exponentially decaying kernels (~20 ms time constant) and small delay. Reverse (antidromic) driving was similarly robust. The results show that corticocortical signaling in this pathway drives downstream activity rapidly and scalably, in a mostly linear manner. These properties, identified in anesthetized mice and represented in a simple model, suggest a robust basis for supporting complex non-linear dynamic activity in corticocortical circuits in the awake state.\n\nSIGNIFICANCE STATEMENTThe signaling properties of corticocortical connections are not well understood, particularly for higher-order inter-areal pathways. Here, we developed a paradigm based on parametric optogenetic photostimulation, linear-array electrophysiology, and mathematical modeling to characterize signaling along corticortical connections linking retrosplenial cortex to posterior secondary motor cortex (M2) in anesthetized mice. The results indicate that corticocortically driven activity in the downstream area followed the optogenetically evoked upstream activity in a rapid and scalable manner, and could be described with a simple linear integrator model. These findings suggest that this pathway, when activated selectively in the unconscious state, supports intrinsically linear inter-areal communication.

neuroscience↗