Sue Bodine-Fowler

Mechanical and morphological properties of chronically inactive cat tibialis anterior motor units.

1. The lumbar spinal cord was functionally isolated in ten cats by cord transection at the junctions of segments T12‐T13 and L7‐S1 and cutting bilaterally all dorsal roots between the two transections. Two 24 h EMG recording sessions were used to verify that muscles in the lower limb were virtually electrically silent. The cats were maintained in excellent health for 6 months. 2. Six months after spinal cord isolation, an acute experiment was performed to isolate a single motor unit from the tibialis anterior of each hindlimb using ventral root splitting techniques. Each motor unit was characterized physiologically as either fast fatigable (FF, n = 11), fast fatigue resistant (FR, n = 4), fast intermediate (FI, n = 2), or slow (S, n = 1), and repetitively stimulated to deplete the motor unit of its glycogen. 3. Maximum tensions of the fast motor units were lower than mean maximum tensions of control, whereas the S motor unit remained within the range observed in controls. In general, the isometric contractile properties, as well as fatigability, were within the ranges for each of the motor unit types in control cats. The mean fibre cross‐sectional areas of the fibres within the FR and FF motor units were approximately 40 and 50% smaller than control, while the mean fibre size of the fibers within the S motor unit was similar to control. 4. Innervation ratios and specific tensions for all experimental motor units were within the ranges of those reported for tibialis anterior motor units in control cats. Thus, it appears that the decrease in maximum tension of the fast motor units was primarily related to a reduction in fibre size. 5. The spatial distribution of the fibres within fast motor units of a spinally isolated cat, as measured by interfibre distances of the motor unit fibres, was similar to that reported for control tibialis anterior motor units. 6. These data suggest that factors independent of activity play a prominent, if not dominant, role in maintaining the complement of motor unit types typical of adult cat muscles. In addition, normal innervation patterns appear to be maintained in the absence of activity.

Inaccurate projection of rat soleus motoneurons: a comparison of nerve repair techniques.

The objectives of this study were 1) to determine the degree to which soleus motoneurons find their appropriate target following crush and transection injuries to the sciatic nerve, and 2) to determine whether repair of a transected nerve with a silicone tube leads to greater specificity of reinnervation and recovery of muscle function than the standard epineurial suture repair method. Sixty adult female Sprague‐Dawley rats were randomly assigned to one of three sciatic nerve injury groups: crush injury, transection with epineurial suture repair, or transection with a silicone tube repair. The degree to which soleus motoneurons were able to find their appropriate target following a sciatic nerve injury was examined using a double labeling dye technique in which the original soleus motor pool was labeled with fast blue and reinnervating motoneurons were labeled with Dil. Soleus motoneurons were able to find their appropriate target following a crush injury. The accuracy of reinnervation following a transection injury and repair, however, was relatively poor. Only 14% of the original soleus motoneurons found the correct target following a transection injury. Repair of a lesioned nerve with a silicone tube and a 5‐mm gap as opposed to epineurial sutures did not increase the specificity of reinnervation or the degree of muscle recovery.

Skeletal muscle regeneration after injury: An overview

Skeletal muscle has a remarkable capacity for regeneration after injuries resulting in either partial or complete damage to the muscle fibers. Muscle damage occurs following a variety of injuries including direct injury caused by crushing, puncturing, cutting, or freezing; ischemia; direct application of local anesthetics; eccentric exercise, and a variety of neuromuscular diseases. Regardless of the injury, regeneration usually follows a characteristic sequence and is limited by three major factors that will be discussed in this overview of the processes involved in degeneration and regeneration of muscle. The major factors limiting the ability of skeletal muscle to regenerate after trauma or disease are a viable population of satellite cells, reinnervation, and revascularization.

Metabolic and morphologic properties of single muscle fibers in the rat after spaceflight, Cosmos 1887.

The adaptation of a slow (soleus, Sol) and a fast (medial gastrocnemius, MG) skeletal muscle to spaceflight was studied in five young male rats. The flight period was 12.5 days and the rats were killed approximately 48 h after returning to 1 g. Five other rats that were housed in cages similar to those used by the flight rats were maintained at 1 g for the same period of time to serve as ground‐based controls. Fibers were classified as dark or light staining for myosin adenosine triphosphatase (ATPase). On the average, the fibers in the Sol of the flight rats atrophied twice as much as those in the MG. Further, the fibers located in the deep (close to the bone and having the highest percentage of light ATPase and high oxidative fibers in the muscle cross section) region of the MG atrophied more than the fibers located in the superficial (away from the bone and having the lowest percentage of light ATPase and high oxidative fibers in the muscle cross‐section) region of the muscle. Based on quantitative histochemical assays of single muscle fibers, succinate dehydrogenase (SDH) activity per unit volume was unchanged in fibers of the Sol and MG. However, in the Sol, but not the MG, the total amount of SDH activity in a 10‐μm‐thick section of a fiber decreased significantly in response to spaceflight. Based on population distributions, it appears that the α‐glycerophosphate dehydrogenase (GPD) activities were elevated in the dark ATPase fibers in the Sol, whereas the light fibers in the Sol and both fiber types in the MG did not appear to change. The ratio of GPD to SDH activities increased in the dark (but not light) fibers of the Sol and was unaffected in the MG. Immunohistochemical analyses indicate that approximately 40% of the fibers in the Sol of flight rats expressed a fast myosin heavy chain compared with 22% in control rats. Further, 31% of the fibers in the Sol of flight rats expressed both fast and slow myosin heavy chains compared with 8% in control rats. Immunohistochemical changes in the MG were minimal. These data suggest that the magnitude and direction of enzymatic activity and cell volume changes are dependent on the muscle, the region of the muscle, and the type of myosin expressed in the fibers. Further, the ability of fibers to maintain normal or even elevated activities per unit volume of some metabolic enzymes is remarkable considering the marked and rapid decrease in fiber volume.—Miu, B.; Martin, T. P.; Roy, R. R.; Oganov, V.; Ilyina‐Kakueva, E.; Marini, J. F.; Leger, J. J.; Bodine‐Fowler, S. C.; Edgerton, V. R. Metabolic and morphologic properties of single muscle fibers in the rat after spaceflight, Cosmos 1887. FASEB J. 4: 64‐72; 1990.

Treatment of acquired muscle spasticity using phenol peripheral nerve blocks

The use of phenol motor nerve blocks is advantageous in the early period of acquired spasticity (ie, that occurring following traumatic brain injury or incomplete spinal cord injury), when increased muscle tone is often the most severe. Because acquired spasticity is dynamic and usually improves slowly, a temporary treatment method used to ameliorate increased muscle tone is desirable. Phenol nerve infiltration provides a temporary motor nerve block that lasts for weeks or months. It allows passive limb mobilization in a comprehensive rehabilitation program that attempts to prevent fixed soft tissue contractures. Permanent or irreversible methods such as operative tendon lengthening, muscle release or recession, or neurectomy are usually best delayed until the spasticity has become static, when the need for surgical correction becomes more firmly indicated, and outcomes of operative intervention are more predictable. Although phenol nerve blocks were initially administered at the spinal cord level to control spasticity, the potential side effects have caused a loss of popularity of this method of administration. The safer and more common use of phenol infiltration at the peripheral nerve level is now more accepted for brain injury and spinal cord injury patients. This report reviews the indications, current concepts, and development of the different methods used to administer phenol nerve blocks. Comparisons to other methods to control spasticity are discussed.

Architectural and fiber type distribution properties of selected rhesus leg muscles: feasibility of multiple independent biopsies.

In experiments involving primates, e.g. before and after spaceflight, needle biopsies were thought to be a logical and feasible means of obtaining metabolic and morphological information from skeletal muscles. However, the feasibility of obtaining consistent, repeatable biopsies from individual muscles had to be demonstrated prior to the acceptance of this procedure. To study this approach, the architectural properties and the fiber type distributions at three levels and two regions along the proximo-distal axis of the soleus, medial gastrocnemius and tibialis anterior of adult rhesus monkeys were determined. In each muscle, biopsies were taken from specific regions where the fiber type distribution was determined. Within each region of each muscle, the fiber type populations were similar at the three levels studied. The percentage of fast or oxidative fibers in the biopsies and in the regions of the same muscle were highly correlated, i.e. r = 0.98 for both comparisons. In addition, based on normalized values (z scores), 25/26 and 22/26 biopsies were within the 95% confidence interval, i.e. the biopsies were a representative sample of the mean fiber type population of that region of the muscle. In all muscles, the mean fiber lengths were no more than one third the length of the muscle. Together, these data indicate the feasibility of obtaining independent, repeated biopsies having similar fiber types from each of the muscles studied.

Electromyographic (EMG) amplitude patterns in the proximal and distal compartments of the cat semitendinosus during various motor tasks

The electromyographic (EMG) signals recorded from the proximal (STp) and distal (STd) compartments of the cat semitendinosus muscle (ST) during treadmill running at various speeds, jumping and paw-shaking were quantified to assess the degree of independence of neural control of the two portions of the muscle. Five adult cats were implanted with intramuscular electrodes in the STp and STd. Raw EMG signals were sampled, rectified and a modified form of their running average was used to calculate the mean EMG every 20 ms. EMG amplitudes of each portion of the muscle were plotted and their relative density distributions were generated. The relative density distribution was used to represent a measure of the probability of any two amplitudes occurring simultaneously (i.e. joint probability density distribution). Based on the probability density distributions of the EMG signals from different movements, the patterns of recruitment from the STp and STd were similar. However, during jumping and paw shaking, two relatively vigorous tasks, some deviations in the pattern were apparent. These data, therefore, suggest that the two ends of the ST are subjected to similar, but not identical, control mechanisms during the motor tasks studied.

Tensile properties of the neurorrhaphy site in the rat sciatic nerve

Forty-three epineurial rat sciatic neurorrhaphies were performed to gain insight into the duration necessary to protect a nerve repair. By killing animals at varying intervals after neurorrhaphy and harvesting the sciatic nerves, we investigated the timing of neurorrhaphy site tensile property recovery. There were 9 normal control nerves. No nerves ruptured after repair, even though the operated legs were not immobilized. Ultimate and maximal simulated in situ loads and elongations were measured, and stresses and strains were calculated from mechanical testing. Sixty-four percent of normal ultimate stress was gained during the first week after neurorrhaphy, with no significant increase for 10 to 12 weeks. Ultimate strains for control and repaired nerves for all time intervals ranged from 14.2% +/- 1.8% to 26.0% +/- 3.9%. Maximal simulated in situ stress and strain remained in the toe region of the stress versus strain curve, implying that no nerves ruptured because ultimate stress and strain were never approached. Caution must be exercised in extrapolating these data to the human clinical situation.

Evidence of incomplete neural control of motor unit properties in cat tibialis anterior after self‐reinnervation.

1. The mechanical, morphological and biochemical properties of single motor units from the anterior compartment of the tibialis anterior muscle in adult cats were studied six months after the nerve branches to that compartment were cut and resutured in close proximity to the muscle. 2. In these self‐reinnervated muscles, the maximum tetanic tensions were lower in slow than fast units, a relationship similar to that observed among motor units from control adult muscles. The maximum tetanic tensions produced by the fast units were larger than those produced by the same motor unit types in control muscles. Direct counts of muscle fibres belonging to a motor unit showed that factors controlling the number of muscle fibres innervated by a motoneurone type persist during the reinnervation process in that fast motoneurones reinnervated more muscle fibres than slow motoneurones. Thus, the number of muscle fibres reinnervated by a motoneurone principally accounted for the difference in the maximum tension outputs among motor unit types, a relationship similar to that observed in control tibialis anterior muscles. 3. Monoclonal antibodies for specific myosin heavy chains were used to differentiate fibre types. By this criterion, motor units from control muscles were found to contain a homogeneous fibre type composition. In contrast, a heterogeneous, yet markedly biased, fibre type composition was observed in each unit analysed from self‐reinnervated muscles. 4. Although not all of the muscle fibres of a motor unit developed the same type‐associated parameters after reinnervation, the relationships among myosin heavy chain profile, succinate dehydrogenase activity and the fibre size were similar in fibres of control and self‐reinnervated muscles. 5. The processes which dictate both motor unit size and the matching between motoneurone and muscle fibre type during the reinnervation process must be interdependent and result from a hierarchy of decisions which reflects their relative importance. The mechanisms responsible for these two processes may be a combination of: (1) selective innervation which may or may not incorporate a pruning process if multiple synaptic connections are initially formed and/or (2) conversion of enough fibres of a motor unit to form a predominant type.

Limited fiber type grouping in self-reinnervation cat tibialis anterior muscles

The percent and distribution patterns of three immunohistochemically identified fiber types within the anterior compartment of the cat tibialis anterior were determined 6 months after denervation and self‐reinnervation. After self‐reinnervation, mean frequencies of slow (9%) and fast (91%) fibers were similar to those in control (12% and 88%, respectively) muscles. However, a lower proportion of fast‐1 (26%) and a higher proportion of fast‐2 (65%) fibers were observed in self‐reinnervated than control (32% and 56%) muscles. Quantitation of adjacencies between fibers of similar myosin heavy chain (MHC) phenotype, a measure of type grouping, revealed that the frequencies of two slow or two fast‐1 fibers being adjacent in self‐reinnervated muscles were similar to control. In contrast, the frequency of fast‐2/fast‐2 fiber adjacencies found in self‐reinnervated muscles (45%) was significantly higher than in control muscles (37%). In both groups, the frequency of adjacencies between slow, fast‐1, or fast‐2 fibers was largely attributable to the number of each fiber type present. These data show that the incidence of grouping within each fiber type present was not altered after 6 months of self‐reinnervation. Minimal changes in the spatial distribution of fiber types following self‐reinnervation in adults suggests a limited degree of conversion of muscle fibers to a MHC phenotype matching the motoneuron characteristics.