Muscle tension during unrestrained human eye movements.

Abstract

SUMMARY1. Tensions in the horizontal rectus muscles have simultaneously and continuously been recorded during unrestricted eye movements in four strabismus patients, using force transducers small enough to be implanted in series between the tendons and their points of insertion on the globe.2. Levels of tension required to maintain fixation at each position of gaze vary from a minimum of 8-12 g approximately 150 outside the muscle's field of action to a maximum of around 40 g at extreme gaze within the muscle's field of action. When tension is plotted as a function of eye position, the static locus of fixation tension levels exhibits a parabolic relationship.3. Tensions recorded during smooth following movements parallel or slightly exceed those of fixation.4. At the onset of a saccade, tension in the agonist rises isometrically, then, as the eye moves, tension levels parallel those of fixation but with an isotonic increment of 15-25 g. At the end of the saccadic movement, tension falls essentially isometrically to the new fixation level.5. Tension in the antagonist reveals an unexpected peak at the onset of a saccade.6. For saccadic movements tension increments of 15-25 g above the fixation levels suffice to move the eye rapidly to a new position of gaze, regardless of the duration of the saccade and the location of the new fixation point.7. Maximum and minimum levels of tension during normal fixation, following and saccadic movements, plotted as a function of eye position, form an operational envelope which defines the limits of muscle forces during normal eye movements. The lowest level of this envelope is the parabolic static locus of fixation tensions.

A stretch reflex in extraocular muscles of species purportedly lacking muscle spindles

“…It has also been shown in decerebrated cats that different descending and reflex systems shift the static force-length characteristic of hind limb muscles by changing their SR thresholds (Matthews 1959;Feldman and Orlovsky 1972;Nichols and Steeves 1986;Capaday et al 1995;Archambault et al 2005). Strikingly similar, in terms of shape, force-length characteristics were recorded in EOM of intact humans (Collins et al 1975). Moreover, these researchers also showed that the force-length characteristic was shifted along the length coordinate when the subject changed the gaze direction.…”

“…Since EMG activity of EOM was not recorded by Collins and collaborators (1975), it remains unclear whether or not the repositioning of force-length characteristics resulted from shifts in the SR threshold. It seems unlikely that such repositioning of characteristics resulted from changing in the tonic EMG activity: in striated muscles (to which EOM belong), an increase in tonic stimulation results not only in repositioning but also in an increase of the slope (stiffness) of force-length characteristics (Rack and Westbury 1969), whereas, in the study by Collins et al (1975) the slope of such characteristics remained the same regardless of the shift. Therefore, one can suggest that the repositioning of EOM force-length curves (and thus shifts in gaze) can also be accomplished by central resetting of SR thresholds and that movements in both limb and ocular motor systems could be controlled in a similar manner.…”

Why do oVEMPs become larger when you look up? Explaining the effect of gaze elevation on the ocular vestibular evoked myogenic potential

“…For any eye position in the plane of an antagonist muscle pair (except neutral position), one muscle will have increased tonic activity and the other will have decreased activity (Björk and Kugelberg, 1953). Assuming relatively balanced reciprocal modulation of tonic activity at moderate gaze angles (Collins et al, 1975), when a transient eye movement is initiated from a non-neutral starting position, a larger muscle response in the tonically active muscle would automatically be offset by a smaller (and opposite polarity) response in its less-active pair. In this way an evoked eye movement should have approximately the same amplitude regardless of the starting point of the eye, as suggested by Welgampola et al (2009) …”

A influência do hormônio tireoideano nas proteínas estruturais da banda M no coração e no músculo esquelético de ratos.

“…Como resultado, parece que a proteína M e a EH-miomesina imprimem propriedades exclusivas à banda M. A banda M das fibras rápidas é caracterizada pela expressão de altos níveis de proteína M e miomesina. Tal composição molecular provavelmente melhora o alinhamento e firmeza dos filamentos espessos nas fibras rápidas, caracterizadas pelo rápido desenvolvimento da força (Collins et al, 1975;Gillis, 1996;Walmsley et al, 1978). É conhecido que tal stress (normalmente denominado contração excêntrica) pode levar ao dano das estruturas contráteis (Allen, 2001).…”

“…Uma característica peculiar de todos os músculos murinos que apresentam um regime funcional especial (por exemplo, diafragma, sóleo e EOM) são os baixos níveis de proteína M acompanhados de altos níveis de EH-miomesina. Todos esses músculos estão comprometidos à contratilidade contínua normalmente acompanhada de estiramento (Collins et al, 1975;Gillis, 1996;Walmsley et al, 1978). É conhecido que tal stress (normalmente denominado contração excêntrica) pode levar ao dano das estruturas contráteis (Allen, 2001).…”