The electromyogram is a record of the electric discharge of muscle, not of nerve. The physical substrate for this discharge is the anatomical motor unit consisting of the neuron cell body, its axon, and the group of muscle fibers innervated by that axon. When the electric impulse sweeps down the axon and strikes the motor endplates, all the muscle fibers discharge synchronously. The integrated voltage of this discharge constitutes the electric motor unit.⁴³,⁵⁴

The amplitude of a motor unit is dependent upon many factors, chief of which is the relation of the recording electrode to the discharging fibers. The further...

Polyphasic units consist of a series of oscillations which represent the non-synchronized discharge of a motor unit. As has been mentioned, they occur occasionally in normal peripheral skeletal muscle (3 percent) but are quite rare in normal extraocular muscle.^15,19They are thought to result from temporal or spatial dispersion of the nerve impulse and may indicate a lesion which disrupts the synchronous discharge of the muscle fibers of the unit. They frequently occur following denervation or during reinnervation of a muscle.

Some of the most diagnostic findings of electromyography are the fibrillation potentials of denervation.^55,62,141These are considered to be...

INNERVATIONAL CHARACTERISTICS (SEE FIGURES 4 TO 7). The response of the extraocular muscles to nerve stimulation in animals has demonstrated their great rapidity of contraction,⁵¹the short twitch duration being associated with a very high frequency required for tetanic fusion. High rates of motor unit firing would be expected and are found in the extraocular muscles of man.¹⁴A striking finding in extraocular muscle is the presence of constant tonic activity in the primary position during the waking state.^14,26This is in marked contrast to peripheral skeletal muscle, which is electrically silent at rest. The activity diminishes as the eye...

The tonic activity of the extraocular muscles, which is so characteristic and which falls off to zero or almost zero only in the extreme of gaze out of the field of the muscle, poses questions of considerable physiologic interest. Under what conditions do the ocular muscles achieve electric silence or complete rest? What maintains the tonic activity of the waking state?

SLEEP. It was observed that, during sleep, electric activity of extraocular muscle rapidly fell off to zero,¹⁵although it was interrupted by short bursts of single units or runs of units during lighter phases, possibly accompanying dream activity.³³ As...

The quantity of innervation to the extraocular muscles for a given position of gaze is constant and is the same no matter how the eye arrives at the given position. This holds true for all normal movements of the eye. In vergences, as will be seen later, this is true until the near point of convergence is approximated, at which time cocontraction may occur. It permits the statement of a new law of innervation; namely, “the electric activity of the extraocular muscles parallels the position of the eye.” This has been expressed by Breinin^34,39as follows: All forms of innervation,...

MID-LINE. During symmetrical mid-line convergence, the medial recti increase the firing rate, number, and amplitude of motor units. The lateral recti are correspondingly and reciprocally inhibited. When the near point of convergence is reached, there is an increased innervation to the lateral rectus of the diverging eye with a reciprocal inhibition of its medial rectus. As the eye diverges, the innervation parallels the changing position; that is, the lateral rectus increases and the medial rectus decreases in activity. The fixing eye may show little or no change of activity.^21,26

Several studies^106,107have shown that, at the breakpoint, following a short...

Although Sherrington believed that the extraocular muscles possess a proprioceptive mechanism giving rise to position sense,126 all the experimental studies that have been done in the last several decades have tended to disprove this notion.^{44,53,101,102,119}The information derived from vision and the innervational urge are considered the adequate basis of our awareness or lack of awareness of eye position. That the extraocular muscles should lack a mechanism for the recording of muscle tension, however, would place them so far apart from other skeletal muscle as to arouse wonder and doubt that they should be so distinguished.³⁰That they do have...

Much of the early investigation of the extraocular muscles was concerned with the problem of nystagmus.^{53,70,71,81,82,94,101,102,115,118,119}The question of extraocular muscle proprioception entered largely into the theoretical discussion of the causation of the fast and slow phases. It is generally accepted that the control of the rhythm of nystagmus is central and that the effector organ does not alter the pattern, so that proprioception can play no part in its genesis. This may not be altogether true of man where release of muscles from the globe modified the nystagmus timing and abolished the tonic outflow. It is unlikely that electrode...

The principal role of general electromyography has been to elucidate qualitative differences between normal and abnormal muscle. Electromyography is of prime importance in establishing the diagnosis of lower motor neuron disturbances. These disturbances are reflected by alterations of the electric wave form, amplitude, and frequency and the pattern of electric firing.

The following studies will demonstrate that extraocular and peripheral skeletal muscle respond similarly to disease.

NEUROGENIC PALSY (SEE FIGURE 26). Neurogenic paresis of moderate to severe degree is indicated by irregular or sparse recruitment, poorly sustained discharge, and loss of the interference pattern characteristically seen on effort.^17,29There may...

The extraocular muscles have long been singled out as differing structurally and physiologically from other striated skeletal muscles of mammals. The nature of the muscle fiber, the presence of large amounts of elastic tissue and unusual nerve endings in the extraocular muscles, plus the marked pharmacologic sensitivity to acetylcholine, choline, and nicotine have been cited in placing the extraocular muscles in an intermediate position between the skeletal muscle of birds and amphibia and that of mammals.^42,61Extraocular muscle is also said to demonstrate an affinity with smooth muscle.⁴²

Furthermore, the anatomical and histologic distinctions of extraocular muscle, its low innervation...

The necessity for quantitative techniques in assessing innervation in the extraocular muscles became apparent quite early, since kinesiology is such an important aspect of their physiology.^31,37,38The inspection of the electromyogram itself is a very inadequate method of determining the level of activity since frequency, amplitude, and the number of potentials all contribute to the electric sum of activity. It is extremely difficult to assess these various factors properly by visual methods. This is a general problem in neuromuscular electrophysiology and has resulted in the development of electronic integrators which accomplish the summing up of the total energy developed at...