Behavior of Jaw Muscle Spindle Afferents During Cortically Induced Rhythmic Jaw Movements in the Anesthetized Rabbit

Abstract

The regulation by muscle spindles of jaw-closing muscle activity during mastication was evaluated in anesthetized rabbits. Simultaneous records were made of the discharges of muscle spindle units in the mesencephalic trigeminal nucleus, masseter and digastric muscle activity (electromyogram [EMG]), and jaw-movement parameters during cortically induced rhythmic jaw movements. One of three test strips of polyurethane foam, each of a different hardness, was inserted between the opposing molars during the jaw movements. The induced rhythmic jaw movements were crescent shaped and were divided into three phases: jaw-opening, jaw-closing, and power. The firing rate of muscle spindle units during each phase increased after strip application, with a tendency for the spindle discharge to be continuous throughout the entire chewing cycle. However, although the firing rate did not change during the jaw-opening and jaw-closing phases when the strip hardness was altered, the firing rate during the power phase increased in a hardness-dependent manner. In addition, the integrated EMG activity, the duration of the masseteric bursts, and the minimum gape increased with strip hardness. Spindle discharge during the power phase correlated with jaw-closing muscle activity, implying that the change in jaw-closing muscle activity associated with strip hardness was caused by increased spindle discharge produced through insertion of a test strip. The increased firing rate during the other two phases may be involved in a long-latency spindle feedback. This could contribute to matching the spatiotemporal pattern of the central pattern generator to that of the moving jaw.

Chewing variation in lepidosaurs and primates

“…We hypothesized that these behavioral changes improved chewing performance by minimizing variance in chew cycle duration around the chewing system's optimal frequency, minimizing the energetic cost of the work performed on the food, and allowing the system to operate for longer periods without fatigue (Ross et al, 2007a;Ross et al, 2007b). the magnitude of feed-forward control (Hidaka et al, 1999;Hidaka et al, 1997;Komuro et al, 2001a;Komuro et al, 2001b;Ottenhoff et al, 1992a;Ottenhoff et al, 1992b). What is feed-forward control and why would it be advantageous during the evolution of mammalian chewing?…”

“…We hypothesized that feed-forward control of mastication enables motor commands appropriate for the material properties of the food to be fed forward to the jaw muscles before the teeth make contact with the food (Hidaka et al, 1999;Hidaka et al, 1997;Ross et al, 2007b;Weijs and De Jong, 1977) dampening the effects of toothfood-tooth contact at the start of the slow close phase of the chewing cycle (Abbink et al, 1999;Ottenhoff et al, 1992a;Ottenhoff et al, 1996;Wang and Stohler, 1991). We hypothesized that this feedforward control facilitated rate-modulation rather than timemodulation of bite force (i.e.…”

Muscle Spindles Provide Servo-assistance to Jaw-closing Muscles for Chewing Hard Foods

“…The role of muscle spindle to increase jaw-closing muscle activity was also evident in anesthetized rabbit during rhythmic chewing movement induced by electrical stimulation in the cerebral cortex (Hidaka et al 1999). Muscle spindle discharge recorded in awake monkey (Matsunami & Kubota 1972) and freely eating rat (Yamamoto et al 1989) also supports this view.…”

“…During processing these foods, continuous changes of sensory information from sensory receptors modify the jaw movement and jawclosing muscle activities (Agrawal et al 1998;Hiiemae et al 1996). Muscle spindles of jaw closing muscles are reported to provide hardness related sensory information (Hidaka et al 1999). In most of the previous studies muscle spindle neuronal discharge was recorded when animal was chewing one kind of food or non-reducible test objects or lapping of liquid (Hidaka et al 1999;Masuda et al 1997;Taylor et al 1981).…”

“…Muscle spindles of jaw closing muscles are reported to provide hardness related sensory information (Hidaka et al 1999). In most of the previous studies muscle spindle neuronal discharge was recorded when animal was chewing one kind of food or non-reducible test objects or lapping of liquid (Hidaka et al 1999;Masuda et al 1997;Taylor et al 1981). However, natural foods are reducible and during natural chewing, the volume, hardness and size of the food particles vary between successive chewing cycles when the food is being triturated and hence food resistance varies from cycle to cycle (Agrawal et al 1998;Hiiemae et al 1996;Horio & Kawamura 1989).…”