A model of nonlinear motor cortical integration and its relation to movement speed profile control

It is recognized that natural point-to-point movements are characterized by bell-shaped speed profiles. However, the neural basis of this smooth, substantially symmetric time course is unknown. Here it is demonstrated via a simplified compartmental model of tufted layer V (TL5) pyramidal neurons, the principal output units of the motor cortex, that nonlinear integration may underlie the bell-shaped profile. Specifically, it is shown that TL5 neuronal output depends upon an approximately multiplicative relationship between inputs to its apical or basal regions (zones A and B, respectively) and those to its central zone (C). This is because the latter facilitate Ca²⁺dependent bursting that enhances responsiveness to other inputs. As a result, when part of TL5 output returns to zones A and C via thalamocortical and cerebrocerebellar feedback, TL5 neuronal firing rate initially increases before decreasing, rather than progressively decrease as would the output of a linear integrator. This yields a sigmoidal position vs. time response in the musculoskeletal plant and therefore a bell-shaped speed curve. Because of this mechanism, smooth movements may be triggered and modulated by step-like and tonic inputs to zone C as might be received from SMA or basal ganglia. The model thus gives possible insight into the basis of certain features of motor dysfunction in Parkinson's and cerebellar disease.