An individual motor nerve fiber and its several muscle fibers constitute a motor unit. The electrical activity of motor units can be recorded by electromyography (EMG). An electromyogram is a useful research tool in the investigation of normal and abnormal innervation of eye muscles. A portable EMG device connected to an insulated needle is often used during injection of botulinum toxin into eye muscles to help the surgeon localize the appropriate muscle within the orbit, especially when the muscle has been operated on previously.
Recruitment during fixation or following movement
Recruitment is the orderly increase in the number of activated motor units, thus increasing the strength of muscle contraction. For example, as the eye moves farther into abduction, more and more lateral rectus motor units are activated and brought into play by the brain to help pull the eye temporally. In addition, as the eye fixates farther into abduction, the firing frequency of each motor unit increases until it reaches a peak (several hundred per second, for some motor units).
Excerpted from BCSC 2020-2021 series: Section 10 - Glaucoma. For more information and to purchase the entire series, please visit https://www.aao.org/bcsc.