Motor and Premotor Mechanisms of Licking

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Abstract

TRAVERS, J.B., DINARDO, L.A. AND KARIMNAMAZI, H. Motor and premotor mechanisms of licking. NEUROSCI BIOBEHAV REV 21(5) 631–647, 1997.—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. © 1997 Elsevier Science Ltd.

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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