ABSTRACT
The mechanics of the extraocular muscles and orbital tissue ("oculomotor plant") can be approximated by a small number of viscoelastic (Voigt) elements in series. Recent analysis of the eye's return from displacement in lightly anesthetized rhesus monkeys has suggested a four-element plant model with time constants (TCs) of approximately 0.01, 0.1, 1, and 10 s. To demonstrate directly the presence of long (1,10 s) TC elements and to assess their contribution quantitatively, horizontal eye displacement was induced in Cynomolgus monkeys under deep barbiturate anesthesia that prevented interference from spontaneous eye movements. The displacement was maintained for either a prolonged (30 s) or brief (0.2 s) period before release. Return to resting position took 20-30 s after prolonged displacement but only 1-2 s after brief displacement, consistent with the presence of long TC elements that would only be substantially stretched in the former condition. Quantitative fitting of the release curves after prolonged displacement indicated that the two long TC elements contribute a substantial proportion (approximately 30%) of the total plant compliance. A model based on the estimated compliance values is shown to account quantitatively both for our release data and for Goldstein and Robinson's data on hysteresis of ocular motoneuron firing rates measured after centripetal saccades following prolonged eccentric fixation. Long time-constant elements in the plant thus make a substantial contribution to some types of eye movement, and their inclusion in plant models can help interpret the firing patterns of single units in the oculomotor system.
Subject(s)
Eye Movements/physiology , Haplorhini/physiology , Macaca fascicularis/physiology , Models, Biological , Muscle Contraction/physiology , Oculomotor Muscles/physiology , Reaction Time/physiology , Animals , Barbiturates/administration & dosage , Computer Simulation , Eye Movements/drug effects , Motor Neurons/physiology , Muscle Contraction/drug effects , Oculomotor Muscles/drug effects , Reaction Time/drug effects , Time FactorsABSTRACT
Two adult rhesus monkeys that had undergone 2 years of electrode penetrations into their abducens and vestibular nuclei, for chronic eye movement studies, were examined histologically. An analysis of their VIth nucleus neurons and lateral rectus muscles revealed the following. Twenty-two percent of the large neurons (approximately 30 microm in diameter), on average, were missing and extensive neuropil disruption and gliosis was evident in the experimental side abducens nuclei as compared with the control side in each animal. While the lateral rectus muscles showed small, but inconsistent, changes in total fiber number, the muscle fiber diameters were altered, leading to a more homogenous muscle and making the typical orbital and global subdivisions of the muscle less distinct. Eye movement records from before and after the electrophysiological studies were comparable. We discuss how the complex architecture of the extraocular muscles as well as the possibility of polyneuronal innervation of single muscle fibers could explain our results.
Subject(s)
Abducens Nerve , Electrodes, Implanted , Eye Movements , Oculomotor Muscles/pathology , Pons/pathology , Animals , Electrophysiology/instrumentation , Electrophysiology/methods , Fixation, Ocular , Macaca mulatta , Magnetics , Pursuit, SmoothABSTRACT
This study aims to investigate the pattern of anticipatory postural adjustments associated with arm movements during balancing on unstable support surface as a mechanism of coordination between movement and posture in this specific motor task. The experiments were arranged in two series: (1) the subjects stood on a force platform, and (2) on unstable platform (see-saw). They performed self-paced arm elevations as fast as possible for each support condition. EMG activity of deltoideus anterior (DA), soleus (SOL), tibialis anterior (TA), biceps femoris (BF) and erector spinae (ES) muscles were recorded. The mean EMG of SOL and TA muscles during balancing and during the last 100 ms prior to DA EMG onset were measured. For both series the anticipatory postural adjustment of BF preceded the ES activation. During balancing on see-saw the following differences from the stable condition were found: (1) co-contraction of SOL and TA muscles; (2) significantly earlier recruitment of BF; (3) delayed related activity of ES; (4) decrease of SOL and increase of TA mean EMG activities during balancing and just prior to the onset of DA EMG activity. These data showed that posture control during balancing on unstable support surface consists mainly in adapting the motor program according to the postural requirements rather than in changes of the postural strategy.
