(A) When viruses labelled with green and red fluorescent tags are injected into the left and right muscles that close the jaw, three different sets of premotor neurons can be seen when the fluorescence is imaged: those that project to the motoneurons that drive the muscle on the left (green); those that project to the motoneurons that drive the muscle on the right (red); and those that project to both sets of motoneurons in order to coordinate the movements of the two muscles (green and red). The virus can move across the synapse between the motoneurons and the premotor neurons when it is complemented by a glycoprotein (G) that is expressed in the motoneurons of the mice: this movement is indicated by the black arrows. (B and C) During chewing, the digastric muscles (which open the jaw) and the genioglossus muscle (which protracts the tongue) are active during the jaw opening (JO) phase, while the masseter muscles (which close the jaw) and the muscle that retracts the tongue are both active during the jaw closing (JC) phase. The green and red traces show that the jaw closing motoneurons (JCMN) and the protractor motoneurons (protractor MN) are active at different times. The results of Stanek et al. suggest that orofacial coordination is performed by multifunctional circuits rather than dedicated circuits. In dedicated circuits (B), motoneurons do not share their premotor neurons, and those that are active at the same time receive inputs from their central pattern generator (CPG), which inhibits activity of other circuits. In multifunctional circuits (C), sets of motoneurons driving different muscles can share their premotor neurons. The same CPG activates premotor neurons that are shared by the motoneurons that are active at the same time, and this same CPG also inhibits (as indicated by the black neurons that end in a black circle) the premotor neurons that project to motoneurons that are active at other times.
FIGURE CREDIT: PHILIPPE MORQUETTE