Motor and Premotor Mechanisms of Licking
Section snippets
INTRODUCTION
FOR MANY animals the consummatory phase of ingestion is characterized by rhythmic oromotor activity. Functional morphologists have described these motor patterns as an intricate sequence of movements produced by the tongue, jaws and related structures. Variations in the motor sequence of consumption reflect both evolutionarily imposed adaptation of the musculo-skeletal system and the moment to moment adaptation to the changing nature of the ingesta 21, 46, 109. Thus, the consumption of solid
BEHAVIORAL ASPECTS OF LICKING
A description of the topography of licking provides useful behavioral endpoints for an analysis of central control mechanisms. In addition to the classical paradigm of water-bottle licking, licking-like behavior can be obtained by infusing fluids directly into a rat's mouth 34, 62, 114, as well as from electrical stimulation of the brain (ESB) 25, 30, 60, 71, 75, 76, 100, 104. Because all these approaches have been used to characterize central processes related to licking, it is necessary also
BRAINSTEM SUBSTRATES FOR LICKING AND MASTICATION
Both mastication and licking appear to be organized by brainstem CPGs. A brainstem substrate is evident from decerebrate preparations that lick and chew following peripheral stimulation 9, 35, 81. The concept of a CPG is invoked by the generation of rhythmic responses in motor nerves or nuclei in the absence of any sensory input 25, 104. These “fictive” motor responses induced by central stimulation in paralysed preparations proved that sensory feedback from contracting muscles was not
Brainstem sensory inputs
Inputs to a CPG for licking, organized and centered in the medullary RF, converge from a variety of sources, both local and distant. Within the brainstem, oral and visceral sensory nuclei including the NST 6, 7, 38, 101, 121, the parabrachial nuclei 42, 63 and the sensory trigeminal complex [101] all project extensively to PCRt. These pathways may mediate direct orosensory licking generated within the brainstem in decerebrates. Fig. 10, for example, shows the distribution of anterograde label
Acknowledgements
This review was based on a presentation given at a Symposium on Licking held in Pueblo, Colorado, August 1995. We wish to thank Susan Travers for her invaluable suggestions on this manuscript and Sandra Frankmann who helped conceive the idea of writing this review. Supported by DC 00417.
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