Because primate studies provide data for the current experimental models of the human oculomotor system, we investigated the relationship of lateral rectus muscle motoneuron firing to muscle unit contractile characteristics in the squirrel monkey. Also examined was the correlation of whole-muscle contractile force with the degree of evoked eye displacement. A force transducer was used to record lateral rectus whole-muscle or muscle unit contraction in response to abducens whole-nerve stimulation or stimulation of single abducens motoneurons or axons. Horizontal eye displacement was recorded using a magnetic search coil.(1) Motor units could be categorized based on contraction speed (fusion frequency) and fatigue. (2) The kt value (change in motoneuronal firing necessary to increase motor unit force by 1.0 mg) of the units correlated with maximum tetanic tension. (3) There was some tendency for maximum tetanic tension of this unit population to separate into three groups. (4) At a constant frequency of 100 Hz, 95% of the motor units demonstrated significantly different force levels dependent on immediately previous stimulation history (hysteresis). (5) A mean force change of 0.32 gm/° and a mean frequency change of 4.7 Hz/° of eye displacement were observed in response to whole-nerve stimulation.These quantitative data provide the first contractile measures of primate extraocular motor units. Models of eye movement dynamics may need to consider the nonlinear transformations observed between stimulation rate and muscle tension as well as the probability that as few as two to three motor units can deviate the eye 1°.

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Children with hearing deficits frequently have delayed motor development. The purpose of this study was to evaluate saccular function in children with hearing impairments using the Vestibular Evoked Myogenic Potential (VEMP). The impact of the saccular hypofunction on the timely maturation of normal balance strategies was examined using the Movement Assessment Battery for Children (Movement ABC). Thirty-three children with bilateral severe/profound hearing impairment between 4 and 7 years of age were recruited from a three-state area. Approximately half of the sample had one or bilateral cochlear implants, one used bilateral hearing aids, and the rest used no amplification. Parents reported whether the hearing impairment was diagnosed within the first year or after 2 years of age. No VEMP was evoked in two thirds of the hearing impaired (HI) children in response to the bone-conducted stimulus. Children who were reportedly hearing impaired since birth had significantly poorer scores when tested with the Movement ABC.

Botulinum toxin type A (BTX) is often used as an alternative to surgery for the treatment of strabismus and many other motor or cosmetic problems. Although numerous studies established BTX as a powerful transmission-blocking agent at the neuromuscular junction, no evaluation of extraocular muscle (EOM) contractile properties after administration of BTX exists. Some anatomical studies on EOM fiber types suggested a long-term preferential effect of BTX on orbital layer, singly innervated muscle fibers. In this study, we examined the short-term effects of BTX on the contractile properties of normal lateral rectus muscle to determine the functional effect of BTX on muscle-force output over time. Measurements of muscle tension and the corresponding EMG evoked by stimulation of nerve VI were made hourly for up to 18 h following BTX administration. An intramuscular BTX injection of 2 U caused a dramatic decrease in maximum twitch and tetanic tension of the muscle in response to different frequencies of stimulation. This suppression developed gradually over time, with a concomitant reduction of EMG amplitude. No significant changes in muscle-speed-related characteristics (e.g., twitch contraction time, fusion frequency) were found. The results suggest a functional effect of BTX on all muscle fiber types, although, with the dose used, we did not observe complete muscle paralysis within the time of recording. The time course of muscle tension suppression by BTX also was frequency dependent, with the lower stimulation frequencies being more affected, suggesting that implementation of higher frequencies could still produce adequate eye movements.

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