The temporal relationship between intraocular pressure and extraocular muscle activation in cats

The temporal relationship between intraocular pressure and extraocular muscle activation was studied in cats in response to the administration of the depolarizing muscle relaxant, succinylcholine (i.e. bolus doses of 0.1 and 1.0 mg/kg). Simultaneous changes in intraocular pressure, extraocular muscle force, extraocular electromyograms (EMGs), limb muscle EMGs and hindlimb muscle afferent activity were recorded. Increases in intraocular pressure were associated with extraocular muscle activation and had two components: (1) an initial abrupt increase (lasting seconds) which correlated with fasciculations within the extraocular and hindlimb muscles; and (2) a latter more sustained component (minutes) presumably due to tonic muscle activation which correlated with increases in hindlimb muscle afferent activity (e.g. due to sustained activation of bag 1 intrafusal fibers by succinylcholine). In a separate group of animals, in which the extraocular muscles were detached from the right eye bilateral intraocular pressures were measured: depolarization by succinylcholine caused a significant increase in intraocular pressure only for the eye with intact muscles. Thus, increases in intraocular pressure following the administration of succinylcholine are directly related to the changes in extraocular muscle tension which is dependent on both tonic and phasic muscle fiber responses.