Motor and Premotor Mechanisms of Licking

The location, organization and anatomical connections of a central pattern generator (CPG) for licking are discussed. Anatomical and physiological studies suggest a brainstem location distributed within several subdivisions of the medullary reticular formation (RF). The involvement of widespread RF regions is evident from brainstem recording experiments in awake freely moving preparations and studies employing electrical stimulation of the frontal cortex to produce ororhythmic activity. The complex multifunctional properties of RF neurons producing licking are indicated by their activity during licking, swallowing and the rejection of an aversive gustatory stimulus. Anatomical studies place descending inputs to a brainstem CPG for licking to widely distributed areas of both the medial and lateral RF. In contrast, most projections originating from brainstem orosensory nuclei terminate primarily within the lateral RF. Because many pre-oromotor neurons appear concentrated largely in the intermediate zone of the RF (IRt), it is hypothesized that neurons from both lateral and medial sites converge within the IRt to control oromotor function.