Summation of extraocular motor unit tensions in the lateral rectus muscle of the cat

Contractile measures on 67 single muscle units in the cat lateral rectus muscle were made in response to motoneuron stimulation. Simultaneous activation of four to five additional units, using muscle nerve stimulation, allowed an examination of unit force summation. Linear force addition was found in 73% of the units, while 25% added only about half of their twitch force to the twitch force of the nerve-activated units. "Nonadditive" units had significantly weaker twitch tensions than the units which added linearly. Lengthening or shortening the whole muscle, from maximal isometric settings, reduced whole muscle twitch tension as well as muscle unit tension. Injury to the lateral rectus muscle did not significantly alter whole muscle tension. These findings suggest that the known serial and branching arrangement of these muscle fibers, as well as the complex interfiber matrix, may help explain the force reduction in some muscle units and the whole muscle's resistance to insult.     

Structure-function correlations in the human medial rectus extraocular muscle pulleys

PURPOSE: Fibroelastic pulleys function like the trochlea to fix the position and pulling direction of the recti extraocular muscles within the orbit. This study characterized the fine structure of the human medial rectus muscle pulley. METHODS: Human medial rectus muscle pulley tissue was dissected at autopsy, immersed in aldehyde fixative solution, and processed for and examined with light and electron microscopy. RESULTS: Pulley structure were located within posterior Tenon's fascia, closely surrounding the medial rectus muscle. Pulleys were comprised of a dense collagen matrix with alternating bands of collagen fibers precisely arranged at right angles to one another. This three-dimensional organization most likely confers high tensile strength to the pulley. Elastin fibrils were interspersed in the collagen matrix. Fibroblasts and mast cells were scattered throughout the relatively acellular and avascular collagen latticework. Connective tissue and smooth muscle bundles suspended the pulley from the periorbita. Smooth muscle was distributed in small, discrete bundles attached deeply into the dense pulley tissue. CONCLUSIONS: Fine structural observations confirm the existence and substantial structure of a pulley system in association with the medial rectus extraocular muscle. The presence of pulleys must be considered in models of the oculomotor plant. The cytoarchitecture and placement of pulleys suggest that they are internally rigid structures and are consistent with the idea that they determine functional origins for the extraocular muscles. However, the nature of the connective tissue-smooth muscle struts suspending the pulley system to the orbit supports the notion that the pulley position, and thus the vector force of the eye muscles, may be adjustable.     

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/ degrees and a mean frequency change of 4.7 Hz/ degrees 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 degrees.     

Signals of eye position and velocity in the first-order afferents from pigeon extraocular muscles

Responses of first-order afferents from the extraocular muscles of the pigeon were studied by extracellular recording in the ophthalmic part of the trigeminal ganglion of decerebrate, paralysed pigeons. The afferents responded to both the amplitude and velocity of ramp displacements of the intact eye with amplitude sensitivities ranging from 0.9 to 8 impulses/s/deg of eye displacement beyond the response threshold. Once a new stable position had been reached, the afferent signal depended only upon the absolute position of the eye within the orbit. The responses adapted in seconds rather than minutes so these units would not provide a continuous signal of the position of an immobile eye; they are best described as signalling position and velocity in relation to eye movements.     

The effect of temperature on contractile activation of intact and chemically skinned 'catch' muscle fibre bundles of Mytilus edulis

The effect of temperature (5-35 degrees C) on maximum force production was examined in intact and chemically skinned muscle fibre bundles (10-25 fibres) from the anterior byssus retractor muscle of Mytilus edulis. In intact fibre bundles, 10 microM acetylcholine induced a tonic contraction which had a magnitude of 65.4 +/- 4.0 N cm-2 (n = 30) at 23 degrees C. Activation by caffeine (20 mM) produced a force response which was 157.1 +/- 7.9% (n = 16) of the acetylcholine response at 23 degrees C and acetylcholine and caffeine together produced force which was not significantly different from the response to caffeine alone. At 5 degrees C the acetylcholine and caffeine responses were decreased by 9.6 +/- 3.4% (n = 6) and 14.6 +/- 2.8% (n = 8) compared with the respective responses at 23 degrees C. However, there was no significant reduction of the response induced by the combined action of acetylcholine and caffeine when the temperature was decreased from 23 degrees C to 5 degrees C. The 20-80% of peak force activation time increased by about one order of magnitude for all acetylcholine, caffeine and combined acetylcholine-caffeine-induced responses when the temperature was decreased from 23-5 degrees C. Repeated exposure of the intact preparation to caffeine caused a marked decrease in the caffeine-induced response (complete abolition of force after the third exposure to caffeine), but the response to caffeine could be fully restored following one acetylcholine-induced activation. The maximum Ca(2+)-activated force after skinning the preparation with saponin was not significantly different from the caffeine or combined acetylcholine-caffeine-induced responses before skinning. In the saponin skinned fibre preparation a drop in temperature from 23 degrees C to 15 degrees C or 5 degrees C decreased the maximum Ca(2+)-activated force by 13.2 +/- 1.4% (n = 8) and 41.4 +/- 3.1% (n = 5) respectively. The activation time between 20-80% of the peak Ca(2+)-activated force increased at 15 degrees C and 5 degrees C by a factor of 1.5 +/- 0.1 (n = 5) and 6.8 +/- 1.1 (n = 5) respectively when compared to corresponding values at 23 degrees C. The relaxation half-time decreased by a factor of 1.7 +/- 0.2 (n = 5) and 3.0 +/- 0.2 (n = 5) at 15 degrees C and 5 degrees C respectively compared with that at 23 degrees C.(ABSTRACT TRUNCATED AT 400 WORDS)     

The motor unit potential distribution over the skin surface and its use in estimating the motor unit location

Dextransucrase (DSRS) from Leuconostoc mesenteroides NRRL B-512F is a glucosyltransferase that catalyzes the synthesis of soluble dextran from sucrose or oligosaccharides when acceptor molecules, like maltose, are present. The L. mesenteroides NRRL B-512F dextransucrase-encoding gene (dsrS) was amplified by the polymerase chain reaction and cloned in an overexpression plasmid. The characteristics of DSRS were found to be similar to the characteristics of the extracellular dextransucrase produced by L. mesenteroides NRRL B-512F. The enzyme also exhibited a high homology with other glucosyltransferases. In order to identify critical amino acid residues, the DSRS sequence was aligned with glucosyltransferase sequences and four amino acid residues were selected for site-directed mutagenesis experiments: aspartic acid 511, aspartic acid 513, aspartic acid 551 and histidine 661. Asp-511, Asp-513 and Asp-551 were independently replaced with asparagine and His-661 with arginine. Mutation at Asp-511 and Asp-551 completely suppressed dextran and oligosaccharide synthesis activities, showing that at least two carboxyl groups (Asp-511 and Asp-551) are essential for the catalysis process. However, glucan-binding properties were retained, showing that DSRS has a two-domain structure like other glucosyltransferases. Mutations at Asp-513 and His-661 resulted in greatly reduced dextransucrase activity. According to amino acid sequence alignments of glucosyltransferases, alpha-amylases or cyclodextrin glucanotransferases, His-661 may have a hydrogen-bonding function.     

A case of malignant lymphoma with a metastasis to the lateral rectus muscle

We reported a 75-year-old woman with malignant lymphoma who had a metastasis to the right lateral rectus muscle. She was well until two months earlier, when a tumor in the left thigh began to enlarge. Ten days before admission, she noticed medial deviation of the right eyeball. Neurological examination showed the right esotropia with isolated paralysis of the right lateral gaze. She denied double vision. MR imaging demonstrated a swelling of the right lateral rectus muscle. Gallium scanning revealed abnormal accumulation in the right orbit and the left thigh. The tumor in the left thigh was histologically diagnosed as non-Hodgkin's lymphoma, diffuse large cell type. Discrete extraocular muscle metastasis is rare and unreported for malignant lymphoma. Reported cases of breast and thyroid cancers metastatic to the extraocular muscles did not develop diplopia similar to our case. The rapid growth of metastases to the extraocular muscles produces a large visual axes deviation, therefore no diplopia may be elicited.     

Cervical smooth muscle contractile activity after treatment with mifepristone and progesterone

The biodistribution and pharmacokinetics of intra-arterially administered N-isopropyl-p[123I] iodoamphetamine (123I-IMP) were prospectively evaluated in 38 patients with histologically proven pulmonary or hepatic tumors. Intra-arterially infused 123I-IMP was distributed initially in peripheral tissues in which the blood supply was maintained. Its concentration in malignant neoplasms was demonstrated to be higher than in normal tissues. In pulmonary cancer, the tumor uptake of the administered dose without a tissue attenuation correction (% uptake) of 123I-IMP at 1-2 min after injection was 14.7 +/- 5.7% (s.d.). The tumor to normal tissue ratio was 2.1 +/- 0.7 in hepatocellular carcinoma and 1.4 +/- 0.7 in metastatic tumors. The biodistribution of 123I-IMP was also compared to that of 99mTc-macroaggregated albumin (99mTc-MAA) in 9 cases of hepatic cancer. The distribution of 123I-IMP resembled that of 99mTc-MAA in 5 cases and was different in 4 cases. 123I-IMP was more concentrated in the tumor than 99mTc-MAA. Intra-arterial infusion scintigraphy with 123I-IMP seems to provide information on effective blood supply to neoplasms which are targeted in interventional radiology.     

The influence of age on the assessment of motor unit activation in a human hand muscle

Two indices of motor unit recruitment, the ramp-force and repetitive-discharge thresholds, were compared in the first dorsal interosseus muscle of ten young and twelve elderly subjects. The purpose was to determine the effect of age on the relationship between the two recruitment thresholds and the spike-triggered average force of motor units. Each subject performed three tasks requiring isometric abduction of the left index finger: a maximum voluntary contraction (MVC), a ramp-and-hold contraction, and a repetitive-discharge task. The elderly subjects used coactivation of the antagonist muscle (second palmar interosseus) more frequently than the young subjects during the ramp-and-hold contraction. Many elderly subjects expressed difficulty with the controlled ramp-down phase of the ramp-and-hold contraction and preferred a coactivation strategy to a derecruitment strategy for this task. There were no differences due to age or gender in the ramp-force thresholds between the various groups. However, the normalized repetitive-discharge threshold was significantly less for the younger subjects and for the male subjects. Nonetheless, the two recruitment thresholds were able to predict the spike-triggered average force with similar success for both the young and the elderly subjects. These data suggest that the recruitment threshold of a motor unit in first dorsal interosseus was characterized equally well by either the ramp-force or repetitive-discharge measurement for both young and elderly subjects but that coactivation was used more frequently by the elderly subjects during the ramp-and-hold task.     

Combined paresis and restriction of the extraocular muscles after orbital fracture: a study of 16 patients

Sixteen patients with combined paresis and restriction of extraocular muscle(s) orbital fracture repair were studied before and after in order to determine the clinical features and management of such patients. All 16 patients showed limited ductions of the involved eye in the field of action of the entrapped, paretic muscle and of the antagonist muscle after orbital fracture. Single extraocular muscles (13 patients) and two extraocular muscles (three patients) were demonstrated adjacent to the fracture site on orbital computed tomography (CT). In three patients prior to orbital surgery, a deviation in primary position was present. After fracture repair with release of the entrapped muscle in all patients, evidence of paresis of the muscle was demonstrated by underaction in its field of action and overaction in the field of its antagonist. There was a resultant manifest tropia or phoria in the primary position. In seven patients, the paresis gradually improved with no tropia and little diplopia in the functional fields of gaze. Three patients had minimal deviations and required no further treatment. Six patients with significant deviations required prisms (three patients) or strabismus surgery (three patients). The latter three patients had two muscles involved. Results of this study demonstrate that the ophthalmologist must appropriately diagnose patients with paresis and restriction of an extraocular muscle and counsel them that "new" diplopia may occur after orbital fracture repair and that this diplopia may require additional therapy.     

Longitudinal study of the contractile and electrical properties of single human thenar motor units

Serial motor unit number estimates have shed important light on the extent and rates of motoneuron losses in aging and amyotrophic lateral sclerosis. However, the estimates alone provide few clues to the health and functional status of surviving motor units. A reliable means for assessing the functional status of the surviving motor units would therefore by a welcome addition to our present tools for studying motor units. Examining the physiological properties of samples of motor units drawn at intervals during the course of a motoneuronal disease suffers from the important limitation that the samples may not be representative of one another. The latter problem could be circumvented by serially studying the same motor units. This study describes a noninvasive technique capable of longitudinally tracking the contractile and electrical properties of specific single thenar motor units in healthy subjects, in some instances over several years. The technique proved to be reasonably reliable and provided information on a wide range of contractile and electrical properties of motor units. Such an approach could serve as a potentially powerful and sensitive means of studying the life histories of single motor units in aging, diseases of the motoneuron, and in the latter instances, the responses of the motoneurons to treatment.     

Dissociation of the electrical and contractile properties in single human motor units during fatigue

The surface EMG area often exhibits progressive enlargement during a submaximal fatiguing contraction, but the underlying reasons still remain uncertain. Fatigue-induced changes in the surface-detected motor unit action potentials (S-MUAPs) of 10 human thenar motor units (MUs) with widely differing physiological properties were examined. After 2 min of repetitive 40-Hz stimulation, the size of the S-MUAPs of all MUs increased, the magnitude of which was negatively correlated with their tetanic tension changes. These findings suggest that during muscle fatigue, in addition to reflecting recruitment of new MUs and increases in firing rates of the active MUs, the surface EMG may also be markedly influenced by changes in the S-MUAPs, especially in fast fatigable muscles.