Mary S. Shall

Extraocular Motor Unit and Whole-Muscle Responses in the Lateral Rectus Muscle of the Squirrel Monkey

Because primate studies provide data for the current experimental models of the human oculomotor system, we investigated the relationship of lateral rectus muscle motoneuron firing to muscle unit contractile characteristics in the squirrel monkey. Also examined was the correlation of whole-muscle contractile force with the degree of evoked eye displacement. A force transducer was used to record lateral rectus whole-muscle or muscle unit contraction in response to abducens whole-nerve stimulation or stimulation of single abducens motoneurons or axons. Horizontal eye displacement was recorded using a magnetic search coil. (1) Motor units could be categorized based on contraction speed (fusion frequency) and fatigue. (2) The kt value (change in motoneuronal firing necessary to increase motor unit force by 1.0 mg) of the units correlated with maximum tetanic tension. (3) There was some tendency for maximum tetanic tension of this unit population to separate into three groups. (4) At a constant frequency of 100 Hz, 95% of the motor units demonstrated significantly different force levels dependent on immediately previous stimulation history (hysteresis). (5) A mean force change of 0.32 gm/° and a mean frequency change of 4.7 Hz/° of eye displacement were observed in response to whole-nerve stimulation. These quantitative data provide the first contractile measures of primate extraocular motor units. Models of eye movement dynamics may need to consider the nonlinear transformations observed between stimulation rate and muscle tension as well as the probability that as few as two to three motor units can deviate the eye 1°.

Extraocular motor units: type classification and motoneuron stimulation frequency-muscle unit force relationships

Extracellular and intracellular techniques were used to study single motor units of the abducens nucleus and lateral rectus muscle in the cat. Using a combination of two motor unit properties, the fusion frequency and an index of fatigability, the population of twitch motor units could be separated into 4 subgroups: fast fatigable (FF), fast fatigue resistant (FR), slow fatigable (SF) and slow fatigue resistant (S). Nontwitch motor units, a fifth subgroup (NT), formed 10% of the total studied population. The twitch tension and the maximum tetanic tension of the FF motor unit type were significantly stronger than all other motor unit types. The use of frequency varying stimulation patterns did not further differentiate the motor unit types. The relation between a series of single motoneuron stimulation frequencies and the resultant single muscle unit forces generated a slope defined as a motor units kt value. Motor units with low kt values had higher twitch tensions, higher maximum tetanic tensions, higher fusion frequencies and lower fatigue indices than motor units with high kt values. Motoneuron recruitment was tested by electrical stimulation of the medial rectus subdivision of the contralateral oculomotor nucleus. No correlations were seen between recruitment order and the mechanical parameters of the single abducens motor units.

Chapter 20 Motor Units of Extraocular Muscles: Recent Findings

Publisher Summary The eye movement motor control system is characterized by exquisite precision and coordinated synchrony as the eyes acquire, pursue, and fixate visual targets. It is logical to think that the brain stem motoneurons and the muscle fibers that they innervate exhibit comparable precision and predictability. Consequently, the manner in which the final common path extraocular motor units are activated and their contractile forces assemble to produce these quick, stable, and repetitive movements is a significant research and clinical problem. This chapter discusses some of the progress made toward understanding the mammalian extraocular motor units. The principal goals of the studies over the past ten years have been to (1) compare and contrast extraocular motor unit types to spinal cord innervated units, (2) examine how single motor units are distributed within a particular muscle, (3) describe the motor unit forces when driven by stimulation paradigms that attempt to mimic motoneuron firing patterns observed during the eye movements in alert, behaving animals, and (4) determine how single motor unit forces summate as they contract in unison. Most of the findings over this period have been collected from deeply anesthetized, in vivo preparations involving the lateral rectus muscle in adult cats and monkeys.

Lateral rectus whole muscle and motor unit contractile measures with the extraocular muscles intact

Both extracellular and intracellular stimulation of single motoneurons were shown to be similarly effective and consistent in eliciting contractile responses in single lateral rectus muscle motor units. The whole muscle was activated by stimulating the sixth nerve in the brain stem. Both whole muscle and motor unit contractile characteristics, under isometric conditions, were found to remain consistent regardless of whether this extraocular muscle was detached or left attached to the globe. In addition, whole muscle twitch and maximum tetanic tension evoked by sixth nerve stimulation was significantly less than would be predicted by the linear summation of individual motor unit twitch and maximum tetanic tensions.

Motoneurons of the Lateral and Medial Rectus Extraocular Muscles in Squirrel Monkey and Cat

The goal of this study was to examine and compare the number and size of motoneurons in the cat and squirrel monkey abducens nucleus. We also examined medial rectus muscle motoneuron compartmentalization in the squirrel monkey oculomotor nucleus and compared those cells to abducens nucleus motoneurons. Retrograde labeling of the motoneurons, using cholera toxin conjugate of horseradish peroxidase (CTHRP) injected into cat and monkey lateral or medial rectus muscles, was observed after 24 h. The CTHRP was histochemically localized with tetramethylbenzidine. The slide-mounted sections were analyzed using a computerized imaging system. Cat abducens nucleus motoneurons showed a wide range of cell sizes (26.0–66.0 µm, mean = 37.2 ± 6.2 µm), four or more dendrites per cell and an average of 1,418 cells within a relatively loosely packed nucleus. Squirrel monkey abducens nucleus motoneurons were significantly smaller than those in the cat with a narrower range of cell sizes (20.0–44.0 µm, mean = 31.7 ± 3.8 µm), four or more dendrites per cell and an average of 2,473 cells densely packed within the nucleus. Squirrel monkey medial rectus muscle motoneurons were organized into MRa, MRb and MRc subgroups. MRa motoneurons comprise the primary innervation for the medial rectus muscle and were similar in size to abducens nucleus motoneurons while the MRc subgroup cells were significantly smaller in size. Similar relationships among medial rectus motoneurons have been seen in rhesus monkeys. The relationship of these anatomical findings to previous physiological results regarding the generation of extraocular muscle force in the squirrel monkey is discussed.

Extraocular Motoneuron Stimulation Frequency Effects on Motor Unit Tension in Cat

The contractile characteristics of 47 twitch and 3 nontwitch lateral rectus motor units in 11 cats were examined using two different stimulation paradigms derived from observed motoneuron firing frequencies in alert animals. The twitch units showed an average twitch tension (46.0 +/- 8.1 mg), contraction time (6.15 +/- 0.26 ms) and median fusion frequency (170 Hz) consistent with previous studies, kt value, defined as the slope of the relation between a series of constant frequency tetanic stimulation trains (lasting 200 ms) and tetanic tension, correlated with maximum tetanic tension (r = 0.984, p < 0.05). ktps value, defined as the slope of the relation between tetanic tension and a series of constant frequency stimulation trains (lasting 1975 ms) that immediately followed a 25-ms high-frequency burst (pulse/step paradigm), was similarly correlated (r = 0.853, p < 0.05) with maximum tetanic tension (x = 256.5 +/- 35.2 mg). ktps values were lower that kt values for each unit, but the units did not change their position in the numerical hierarchy. Eighty-four percent of the motor units produced different maximum tetanic tensions (x = 24 +/- 3%), when comparing pulse/step to constant frequency stimulation, but only 14% of those units produced a greater maximum tetanic tension during pulse/step stimulation. In contrast, 46% of the motor units contracted with more force during the step phase of the pulse/step paradigm than with constant frequency stimulation when the stimulation rate was below 120 Hz: 24% of the units contracted with less force. In addition, pulse/step stimulation frequency ranges above 120 Hz (step phase) were ineffective in increasing tension while higher frequencies continued to elicit tension increases during constant frequency stimulation. The impact of these tension variations on eye movement is discussed.

Relationship of the Mechanical Properties of the Cat Inferior Oblique Muscle to the Anatomy of Its Motoneurons and Nerve Branches

Physiologically, the contractile characteristics and electromyography (EMG) of cat inferior oblique (IO) muscle fibers supplied by the medial and lateral IO muscle nerve branches were studied by direct nerve stimulation. Anatomically, the brain stem locations and sizes of IO motoneuron soma were evaluated after retrograde labeling by horseradish peroxidase (HRP) through whole IO muscle nerves and/or through each medial or lateral IO muscle nerve branch. Stimulation of the lateral nerve branch elicited significantly (p < 0.005) slower twitch contraction times (8.0 +/- 1.5 ms) and lower fusion frequencies (217 +/- 46 Hz) than when the medial branch of the IO nerve was stimulated (average twitch contraction time = 6.8 +/- 1.1 ms; average fusion frequency = 260 +/- 34 Hz). The EMG wave shape responses in the global and orbital layers could be differentiated when the individual nerve branches were stimulated, but the response differences were not consistent among animals. The average diameter of IO motoneuron soma with axons in the lateral branch of the nerve were significantly smaller (p < 0.001) than the average diameter of those IO motoneuron soma associated with the medial branch of the nerve (27.9 +/- 7.2 vs. 32.9 +/- 7.2 microns). Regardless of which nerve branch was labeled, the full range of motoneuron soma sizes was found, and these were distributed throughout the IO subdivision of the oculomotor nucleus. These findings showed that muscle contraction time and motoneuron soma diameter were correlated with the IO nerve branch subjected to stimulation or exposed to HRP. But no topographic organization of motoneurons was found within the IO division of the oculomotor nucleus.

Polyneuronal Innervation of Single Muscle Fibers in Cat Eye Muscle: Inferior Oblique

Single muscle fibers with multiple axonal endplates (multiply innervated fibers) are normally present in adult extraocular muscles (EOMs), while most other mammalian skeletal muscles contain fibers with a single myoneural junction. Recent findings by others led us to investigate for the presence of polyneuronal innervation (innervation of a single muscle fiber by >1 motoneuron) in the inferior oblique (IO) muscle of pentobarbital anesthetized cats. The IO muscle nerve branches, as they coursed through the orbit, were further divided for independent or simultaneous electrical stimulation with bipolar electrodes. Four of five established tests for polyneuronal innervation gave positive results. The sum of the twitch (1) and tetanic (2) tensions in response to individual nerve branch stimulation was greater than that for simultaneous (whole) nerve stimulation. The summed electromyographic (EMG) responses (3) gave a similar positive result. The result for crossed tetanic potentiation (4) was negative for polyneuronal innervation while the crossed fatigue (5) test was positive. These results are consistent with recent studies. That the EOMs exhibit polyneuronal innervation further explains the eye-movement systems functional integrity during some neuromuscular disorders as well as its ability to operate with precision after the loss of numerous motoneurons.

Effects of nandrolone on recovery after neurotization of chronically denervated muscle in a rat model.

OBJECT Suboptimal recovery following repair of major peripheral nerves has been partially attributed to denervation atrophy. Administration of anabolic steroids in conjunction with neurotization may improve functional recovery of chronically denervated muscle. The purpose of this study was to evaluate the effect of the administration of nandrolone on muscle recovery following prolonged denervation in a rat model. METHODS Eight groups of female Sprague-Dawley rats (15 rats per group, 120 in all) were divided into 3- or 6-month denervated hind limb and sham surgery groups and, then, nandrolone treatment groups and sham treatment groups. Evaluation of treatment effects included nerve conduction, force of contraction, comparative morphology, histology (of muscle fibers), protein electrophoresis (for muscle fiber grouping), and immunohistochemical evaluation. RESULTS Although a positive trend was noted, neither reinnervated nor normal muscle showed a statistically significant increase in peak muscle force following nandrolone treatment. Indirect measures, including muscle mass (weight and diameter), muscle cell size, muscle fiber type, and satellite cell counts, all failed to support significant anabolic effect. CONCLUSIONS There does not seem to be a functional benefit from nandrolone treatment following reinnervation of either mild or moderately atrophic muscle (related to prolonged denervation) in a rodent model.

Does partial muscle reinnervation preserve future re-innervation potential?

Late revision nerve surgery for incomplete motor recovery due to partial reinnervation would improve muscle function if all muscle fibers were protected from developing denervation atrophy.