Layer Specific Sub- And Suprathreshold Coding Of Voluntary Movements In The Motor Cortex Of Awake Mice

The primary motor cortex (M1) plays a prominent role in the initiation of voluntary movements and motor control. However, the intracortical mechanisms underlying motor command generation remain largely unknown. Here, we investigated the sublayer-specific membrane potential dynamics and output spike patterns of identified pyramidal cells in the forelimb region of M1, by performing whole-cell (WC) and cell-attached (CA) recordings from L2/3 and L5 pyramidal cells in awake, head-fixed mice during voluntary movement. We found that pyramidal neurons in L2/3 displayed large amplitude membrane potential (Vm) fluctuations during quiet wakefulness (Vm -60.6 ± 1.4mV; variance 25.8 ± 5.7µV2) and low spontaneous firing rates (0.3 ± 0.1 Hz; n = 11). During voluntary forelimb movements, the majority of L2/3 neurons (n = 10/11 cells) depolarized slightly (?Vm 3.6 ± 1.1mV; p=0.008) and displayed reduced membrane potential variance (11.2 ± 2.2µV2; p=0.003), without overall changes in the firing rate (0.2 ± 0.1Hz; p=0.41). In contrast, we found that L5 neurons displayed significantly higher firing rates during quiet wakefulness (5.5 ± 1.5Hz; n = 14 WC, 12 CA; p=0.002) and were approximately 7 mV more depolarised than L2/3 neurons (Vm-54.0 ± 0.4mV; n = 14 WC; p=0.0003). During voluntary movement, 6/14 neurons (WC) displayed an increase in firing rate (movement 12.4 ± 3.1Hz; p=0.01) due to a net depolarization of the membrane potential (?Vm 3.4 ± 0.7mV; p=0.005), with no change in membrane potential variance (?variance 1.4 ± 3.8µV2; p=0.73). In contrast, 5/14 neurons reduced their firing rate due to a decrease in membrane variance with no change in Vm (?variance-5.3 ± 2.1µV2, p=0.05; ?Vm0.5 ± 0.6mV, p=0.44). Our results suggest that L2/3 neurons operate in a fluctuation driven sparse coding regime while L5 neurons operate in a mean driven dense coding regime to generate output from the motor cortex.