Figure 3: Potential points of descending control of coordinated locomotion at different speeds.

A. Threshold-based recruitment of rhythm generating neurons. Reticulospinal drive is distributed to all rhythm generating neurons. The low threshold rhythm generating neurons (RG_l) are activated by weak inputs (light gray) and activate inhibitory (V0_D) commissural neurons which function at low speeds for walking. Increased descending drive would recruit medium threshold rhythm generating neurons (RG_m) which may activate a distinct set of commissural interneurons, such as the disynaptic inhibitory pathway (via V0_v neurons) that is the main pathway for trot, an intermediate speed gait. High threshold rhythm generating (RG_h) neurons may be activated only when descending drive is strongest and control excitatory commissural neurons which synchronize the left and right rhythm generating centers in the fast bounding gait. Commissural pathways activated at distinct gaits have been previously described [6–8,22,67]. Arrows denote drive to or from a population, triangles are excitatory synapses, and circles are inhibitory synapses. B. Modular recruitment of rhythm generating neurons. Instead of being recruited based on threshold, separate modules of slow (RG_s), medium (RG_m), and fast (RG_f) rhythm generating neurons are activated by distinct reticulospinal populations. Each rhythm generating module would be related to a speed-dependent commissural pathway. Therefore, selective activation of particular modules would determine speed and locomotor gait. C. Nonspecific recruitment of rhythm generating neurons, selection of commissural coordinating neurons. Increasing reticulospinal drive increases the oscillatory frequency of the rhythm generating neurons but rhythmic drive to commissural populations is subthreshold. The speed-dependent commissural populations are selectively recruited by specific reticulospinal inputs, which allow oscillatory inputs from the rhythm generating neurons to bring them to threshold. In this case, lower speed circuits may also be actively inhibited at higher speeds (not shown).