Kinesthetic Mechanisms in the Trigeminal System

Capra, Norman

Abstract

The long-term objectives of this research proposal are to define anatomical and functional properties of sensory receptors in the orofacial region that contribute to the perception of mandibular position at rest and during oral behaviors including mastication, swallowing and speech. The experimental approach is directed toward morphological description of relevant sensory receptors, their primary afferent neurons, and their central representation in the brain stem. From a clinical perspective, the motivation for these studies is derived from observations that alterations in normal craniomandibular relationships and the presence of occlusal interferences produce changes in the sensory environment which in turn have profound effects on postural muscle activity. Such alterations may be involved in the etiology of temporomandibular and myofascial pain disorders. The proposed studies recognize recent progress in the understanding of how intramuscular receptors function. Experimental studies suggest that intramuscular receptors, particularly neuromuscular spindles, are important in both perceptual and segmental motor mechanisms. One of the aims of this research is to describe the fine structure of muscle spindles and other specialized sensory structures in masticatory muscles using light microscopic and ultrastructural methods. Morphometric features of these structures will be evaluated using computerized image analysis. The specific source of innervation to these structures will be determined using anterograde transport methods. The relative location of muscle receptors will be determined by analysis of extrafusal fiber architecture and histochemistry using routine techniques for muscle fiber-typing. Functional properties of central neurons that receive input from muscles of mastication will be studied with extracellular microelectrodes. Neurons that transmit muscle information to higher regions of the neuraxis, and possibly play a role in kinesthesia, will be identified by antidromic stimulation from the ventrobasal thalamus. Local circuitry of regions such as the supratrigeminal nucleus and dorsal part of the main sensory nucleus, will be defined by light and electronmicroscopic studies of their synaptic connections. Study of brain stem trigeminothalamic neurons activated by jaw movements, and description of their connections with local circuit neurons will further our understanding of the anatomical substrate for afferent modulation of motor behavior and kinesthesia.