Why Firing Rate Distributions Are Important for Understanding Spinal Central Pattern Generators

Abstract

Networks in the spinal cord, which are responsible for the generation of rhythmic movements, commonly known as central pattern generators (CPGs), have remained elusive for decades. Although it is well-known that many spinal neurons are rhythmically active, little attention has been given to the distribution of firing rates across the population. Here, we argue that firing rate distributions can provide an important clue to the organization of the CPGs. The data that can be gleaned from the sparse literature indicate a firing rate distribution, which is skewed toward zero with a long tail, akin to a normal distribution on a log-scale, i.e., a “log-normal” distribution. Importantly, such a shape is difficult to unite with the widespread assumption of modules composed of recurrently connected excitatory neurons. Spinal modules with recurrent excitation has the propensity to quickly escalate their firing rate and reach the maximum, hence equalizing the spiking activity across the population. The population distribution of firing rates hence would consist of a narrow peak near the maximum. This is incompatible with experiments, that show wide distributions and a peak close to zero. A way to resolve this puzzle is to include recurrent inhibition internally in each CPG modules. Hence, we investigate the impact of recurrent inhibition in a model and find that the firing rate distributions are closer to the experimentally observed. We therefore propose that recurrent inhibition is a crucial element in motor circuits, and suggest that future models of motor circuits should include recurrent inhibition as a mandatory element.

Cell-type-specific origins of locomotor rhythmicity at different speeds in larval zebrafish

“…On the other hand, the speed-dependent dominance of reciprocal versus recurrent inhibition is flipped when considering pattern-forming V2a-B neurons. Until now recurrent inhibition was linked to limiting motor output (Hultborn et al, 1979), not to rhythmogenesis (see, however, Linden and Berg, 2021). The stronger recurrent inhibition we see in fast V2a-Bs during anguilliform swimming is consistent with this role.…”

“…8C). Until now recurrent inhibition was linked to limiting motor output (Hultborn et al, 1979), not to rhythmogenesis (see, however, Linden and Berg, 2021).…”

Population spiking and bursting in next generation neural masses with spike-frequency adaptation

“…Our analysis could be useful to develop new models of Central Pattern Generators, which are responsible for the generation of rhythmic movements, since these models are often based on two interacting oscillatory populations with adaptation, as reported for the spinal cord [64] and the respiratory system [65]. Finally, the model of the medullary circuitry reported in [66] can be taken as a cue for a further interesting application of coupled neural masses with SFA, like the modelization of rhythmic whisking in rodents and, in particular, the entrainment of whisking by breathing.…”

Viral strategies for targeting spinal neuronal subtypes in adult wild-type rodents

“…There is a large heterogeneity in spinal interneurons 13 , 14 , for which the functional role is unclear. Inhibitory interneurons in the spinal cord has been proposed to be providing a counter-balance of excitation 15 – 17 yet their role also remain unclear. The Dlx homeobox gene group is known to mark the neural crest, the telencephalic structures and placodes and primarily expressed in telencephalic structures 18 , hence it is unsure whether this enhancer will be specific in the spinal cord.…”