Role of Occlusal Vertical Dimension in Spindle Function
T. Yabushita1,2,*,
J.L. Zeredo2,
K. Toda2, and
K. Soma1
1 Orthodontic Science, Department of Orofacial Development and Function, Division of Oral Health Sciences, Graduate School, Tokyo Medical and Dental University, 1-5-45 Yushima, Bunkyo-ku, Tokyo 113-8549; and
2 Division of Integrative Sensory Physiology, Department of Developmental and Reconstructive Medicine, Graduate School of Biomedical Sciences, Nagasaki University, Nagasaki 852-8588, Japan;
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Figure 1. Schematic drawing of the experimental setting. (A) Barbiturate-anesthetized rats had their heads fixed to a stereotaxic frame in the left lateral decubitus. The masseteric nerve was exposed, hooked to an electrode, and preserved in a pool of liquid paraffin. A thread was attached to the mandible, and the masseter muscle was passively stretched by pulling the jaw open. (B) Sinusoidal stretch stimulation was applied with a maximum opening distance of 7.0 mm and a cycle duration of 4.0 sec (jaw-opening and -closing time of 2.0 seconds, followed by an interval of 2.0 sec). For data analysis, the stimulation (jaw-opening/-closing period) was divided into 3 phases (phases 1 to 3).
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Figure 2. Typical examples of responses from single masseter muscle spindle units to cyclic sinusoidal stretching. Vertical lines indicate the start of a stretch stimulation (continuous) and the first spike of a spindle unit response (dashed). Muscle spindle units were classified as primary or secondary according to their response patterns. Primary endings (A) discharged during stretch and were silent during release, while secondary endings (B) responded with tonic discharges in proportion to the change of muscle length during the entire stimulation period. Lower tracings show typical primary and secondary unit responses after 15 days of iOVD. Cycle histograms show averaged unit responses over 5 consecutive cycles (bins of 100 ms).
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Figure 3. Peak instantaneous frequencies during each phase of stimulation. (A) Primary endings (n = 43). (B) Secondary endings (n = 27). Significantly lower instantaneous frequencies were observed after 5 and 10 days of iOVD in secondary and primary endings, respectively, except for phase 3 in primary endings. Data points represent data averaged from 5 consecutive cycles in each unit. Error bars indicate SD. *Statistical significance vs. control; ANOVA followed by Scheffé’s post hoc test (5% significance level).
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Figure 4. Response thresholds in control and iOVD groups. (A) Primary endings (n = 43). (B) Secondary endings (n = 27). Response thresholds were analyzed by the amount of jaw opening required for the first spike to fire. Significantly higher threshold values were obtained after 5 days of iOVD in both primary and secondary endings. Error bars indicate SD. *Statistical significance vs. control; ANOVA followed by Scheffé’s post hoc test (5% significance level).
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