Effects of electrode penetrations into the abducens nucleus of the monkey: eye movement recordings and histopathological evaluation of the nuclei and lateral rectus muscles.

Two adult rhesus monkeys that had undergone 2 years of electrode penetrations into their abducens and vestibular nuclei, for chronic eye movement studies, were examined histologically. An analysis of their VIth nucleus neurons and lateral rectus muscles revealed the following. Twenty-two percent of the large neurons (approximately 30 microm in diameter), on average, were missing and extensive neuropil disruption and gliosis was evident in the experimental side abducens nuclei as compared with the control side in each animal. While the lateral rectus muscles showed small, but inconsistent, changes in total fiber number, the muscle fiber diameters were altered, leading to a more homogenous muscle and making the typical orbital and global subdivisions of the muscle less distinct. Eye movement records from before and after the electrophysiological studies were comparable. We discuss how the complex architecture of the extraocular muscles as well as the possibility of polyneuronal innervation of single muscle fibers could explain our results.

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 microm, mean = 37.2 +/- 6.2 microm), 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 microm, mean = 31.7 +/- 3.8 microm), 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.

Functional anatomy of the hypoglossal innervated muscles of the rat tongue: a model for elongation and protrusion of the mammalian tongue.

This anatomical investigation in the rat was designed to illustrate the detailed organization of the tongue's muscles and their innervation in order to elucidate the actions of the muscles of the higher mammalian tongue and thereby clarify the protrusor subdivision of the hypoglossal-tongue complex. The hypoglossal innervated, extrinsic styloglossus, hyoglossus, and genioglossus and the intrinsic transversus, verticalis and longitudinalis linguae muscles were observed by microdissection and analysis of serial transverse-sections of the tongue. Sihler's staining technique was applied to whole rat tongues to demonstrate the hypoglossal nerve branching patterns. Dissections of the tongue demonstrate the angles at which the extrinsic muscles act on the base of the tongue. The Sihler stained hypoglossal nerves demonstrate branches to the styloglossus and hyoglossus emanating from its lateral division while branches to the genioglossus muscle exit from its medial division. The largest portions of both XIIth nerve divisions can be seen to enter the body of the tongue to innervate the intrinsic muscles. Transverse sections of the tongue demonstrate the organization of the intrinsic muscle fibers of the tongue. Longitudinal muscle fibers run along the entire circumference of the tongue. Alternating sheets of transverse lingual and vertical lingual muscles can be observed to insert into the circumference of the tongue. Most importantly in clarifying tongue protrusion, we demonstrate the transversus muscle fibers enveloping the most superior and inferior portions of the longitudinalis muscles. Longitudinal muscle fascicles are completely encircled and thus are likely to be compressed by transverse muscle fascicles resulting in elongation of the tongue. We discuss our findings in relation to biomechanical studies, that describe the tongue as a muscular hydrostat and thereby define the "elongation-protrusion apparatus" of the mammalian tongue. In so doing, we clarify the functional organization of the hypoglossal-tongue complex.

Contractile properties of the tongue's genioglossus muscle and motor units in the rat.

The contractile characteristics of individual mammalian tongue muscles have rarely been investigated, in contrast to spinal cord-innervated and extraocular muscles. Therefore, whole muscle and motor unit contractile forces, plus muscle fiber types, were studied in the genioglossus, the major protrusor muscle, of the rat tongue. The muscle, exclusively composed of fast-contracting units, could be activated from rostroventral hypoglossal nucleus sites only. The following figures represent the means of the contractile measures. Whole muscle twitch tension was 7.02 g, contraction time was 14.22 ms, fusion frequency was 104 Hz, maximum tetanic tension was 37.22 g, and fatigue index was 0.72. Single motor unit twitch tension was 45. 9 mg, contraction time was 11.7 ms, fusion frequency was 94.8 Hz, maximum tetanic tension was 241.95 mg, and fatigue index was 0.68. The genioglossus muscle appeared qualitatively similar to the rat styloglossus muscle, one of the two major retractor muscles of the tongue. The delineation of motor unit contractile characteristics in tongue muscle is important in our understanding of the control of tongue movement.

Whole-muscle and motor-unit contractile properties of the styloglossus muscle in rat.

Investigations of whole muscle and motor-unit contractile properties have provided valuable information for our understanding of the spinal cord and extraocular motor systems. However, no previous investigation has examined these properties in an isolated tongue muscle. The purpose of this study was to determine the contractile properties and muscle fiber types of the rat styloglossus muscle. The styloglossus is one of three extrinsic tongue muscles and serves to retract the tongue within the oral cavity. Adult male Sprague-Dawley rats (n = 19) were used in these experiments. The contractile characteristics of the whole styloglossus muscle (n = 9) were measured in response to stimulation of the hypoglossal nerve branch to the muscle. The average twitch tension produced was 3.30 g with a mean twitch contraction time of 13.81 ms. The mean maximum tetanic tension was 19.66 g and occurred at or near the fusion frequency, which averaged 109 Hz. The styloglossus muscle was resistant to fatigue [fatigue index (F. I.) = 0.76]. In separate experiments (n = 7), the contractile characteristics of 37 single motor units were measured in response to extracellular stimulation of hypoglossal motoneurons. The twitch tension generated by styloglossus motor units averaged 35.7 mg, and the mean twitch contraction time was 12.46 ms. The mean fusion frequency was 92 Hz. Maximum tetanic tension averaged 177.8 mg. Styloglossus single motor units were resistant to fatigue (F. I. = 0.74). The sites of stimulation that yielded a contractile response in the styloglossus muscle were consistent with the location of the styloglossus motoneuron pool reported in earlier anatomy studies. Muscle fiber typing was determined in three animals based on the myofibrillar ATPase reaction at pH 9.8, 4.6, and 4.3. The styloglossus muscle was composed of approximately 99% type IIA fibers with a few scattered type I fibers present in the study sample. On the basis of the combined findings of the physiology and histochemistry experiments, the styloglossus muscle appeared to be a homogeneous muscle composed almost exclusively of fast, fatigue-resistant motor units. These properties of the styloglossus muscle and its motor units were compared with findings in other rat skeletal muscles.

Organization of motoneurons in the dorsal hypoglossal nucleus that innervate the retrusor muscles of the tongue in the rat.

This anatomical investigation was prompted by the incomplete knowledge of the myotopic organization of the dorsal subdivison of the hypoglossal nucleus. Intrinsic muscle motoneurons were not segregated and labeled previously with regard to the lateral division of the hypoglossal nerve. Also, motoneuron number and cell size, in relation to the individual retrusor tongue musculature, were rarely addressed previously. Retrograde labeling ofretrusor muscle motoneurons in the dorsal subdivision of the rat hypoglossal nucleus was done. Cholera toxin conjugate horseradish peroxidase (CTHRP) was injected into the retrusor tongue muscles with only the lateral division of the hypoglossal nerve intact. The dorsal subdivision of the hypoglossal nucleus contained approximately 800 motoneurons ranging in cell body size from 19 to 41 microm. When either the styloglossus, hyoglossus, superior longitudinal, or inferior longitudinal muscle was isolated and injected with CTHRP, a separate motoneuron pool for each muscle was seen. The extrinsic muscle motoneurons, styloglossus and hyoglossus, were found rostrolateral and caudolateral respectively. In contrast, the intrinsic superior and inferior longitudinal muscle motoneurons were found more central and medial in the nucleus. Extrinsic muscle motoneurons were larger (approximately 30 microm) than intrinsic muscle motoneurons (approximately 26 microm; P < .0001). Intrinsic muscle motoneurons account for a great majority of the motoneurons in the dorsal aspect of the hypoglossal nucleus and their axons have been shown to be contained in the lateral (retrusor) division of the hypoglossal nerve. This study revealed the myotopic organization of the retrusor subdivision of the rat hypoglossal nucleus.

Compartmental organization of styloglossus and hyoglossus motoneurons in the hypoglossal nucleus of the rat.

Surgical techniques were used to isolate the extrinsic bellies of the styloglossus and hyoglossus muscles from the body of the tongue for cholera toxin HRP injection. An average of 53 styloglossus and 121 hyoglossus motoneurons in the dorsal subdivision of the hypoglossal nucleus were demonstrated using tetramethyl benzidine histochemistry. Styloglossus motoneurons were restricted to the rostral 25% of the nucleus while hyoglossus motoneurons occupied other regions of the dorsal nucleus.

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.

Orthodromic activation of juxta-abducens neurons following sixth nerve stimulation in the cat.

Intracellular electrophysiological and morphological (horseradish peroxidase) techniques were used to identify neurons just rostral and ventral to the cat abducens nucleus. These cells responded orthodromically to stimulation of the ipsilateral VIth nerve. Cells seen in previous investigations, which responded similarly, were thought to lie within the principal abducens nucleus. The neurons identified here may be involved in the transmission of some abducens afferent information to higher brain centers.

Motoneuron electrophysiological and muscle contractile properties of superior oblique motor units in cat.

Intracellular techniques were used to study single motor units of the trochlear nucleus and superior oblique muscle in the cat. Motoneuron electrophysiological properties were correlated with muscle-unit contractile characteristics assessed under isometric conditions. Two distinct motor-unit types were identified and designated as twitch and nontwitch. Nontwitch units made up 5% of the total population studied. They responded only to tetanic stimulation with graded force that increased as stimulus frequency was increased up to 300-400 Hz. These units made up a homogeneous population in that they were innervated by slowly conducting axons, produced weak tetanic tensions, and were extremely fatigue resistant. Twitch units made up the majority (95%) of units studied. These units responded to single pulse stimulation with typical twitch contractions. The contraction speed and tension ranges for these units were comparable with those obtained from other extraocular muscle single units. Superior oblique twitch units, mechanically comparable with multiply innervated conducting units, identified in the cat inferior oblique muscle (31) were not observed. The twitch-unit population was heterogeneous in terms of neuromuscular fatigue resistance. Unit fatigability was inversely related to maximal tetanic tension. Motoneuron conduction velocity was related to muscle-unit contractile properties in a way similar to that seen in extremity motor units. The slowest twitch units were weak, fatigue resistant, and innervated by slow conducting axons. The fastest units were, in general, innervated by faster conducting axons, produced greater tetanic tensions, and were more susceptible to fatigue. Correlations among input resistance, rheobase, and conduction velocity were also observed. At present, subdivisions of the twitch-unit population on the basis of any one or combination of unit properties does not seem appropriate.

The determination of dendrite morphology on lateral rectus motoneurons in cat.

Previously developed morphometric analysis of motoneurons (Ulfhake and Kellerth, '81, J. Comp. Neurol. 202: 571-583) was applied to lateral rectus motoneurons (LRMs). Total dendrite size was approximated from a single stem dendrite measurement. Fifteen dendrites from nine LRMs of the principal abducens nucleus intracellularly stained with HRP were morphometrically analyzed. The diameters and lengths along the extent of the dendrite were measured to calculate the surface area, volume, and combined length of the process. Linear correlation of stem dendrite diameter to these size parameters produced r values of .80, .84, and .61, respectively. Although the regression lines could be used to estimate dendrite size from the stem dendrite diameter, two morphologically distinct types were found among the 83 dendrites of the nine cells. Six dendrites differed from the other 77. Therefore, these six and a representative sample of the more common dendrite (nine) were included in the measurements. The rare dendrites consistently branched at about 40 micron from the soma into a rostrally and a caudally directed secondary dendrite. The secondary dendrites branched less and reduced more in diameter by tapering. Also, these dendrites exhibited a higher than expected total dendrite size to stem diameter ratio compared to "regular" dendrites. Statistical correlations of the stem diameter to surface area or volume within each dendrite type showed clear increases in r values from those of all 15. Significant differences were found between the size parameters of the two types. These qualitative and quantitative differences should be considered in accurate motoneuron size determinations in the abducens nucleus.

Ontogeny and sexual dimorphism of the sonic motor nucleus in the oyster toadfish.

Male oyster toadfish, Opsanus tau, produce a courtship boatwhistle call and have a larger sound-producing organ than females, who do not boatwhistle . We investigated the possibility of sexual dimorphism in ontogeny of the sonic motor nucleus (SMN) of the toadfish. Brain weight increases for life though the increase decelerates with increasing fish weight. Neuron number, ranging from 760 to 2,888 in the SMN, increases rapidly to about 3 years, more slowly to about 8 years, and then levels off. There are no sexual differences in regressions of brain weight and SMN neuron number against fish size or age. Neuronal soma size in the SMN increases from 8 to 35 micron in average diameter and 67 to 916 micron2 in area over a period of at least 7 years. Males have larger neurons than females (P less than .01). However, males can be separated by inspection into populations with large and small soma sizes. Neuron size is not different between females and males with small somas. Neurons in males with large somas are larger than neurons in females and neurons in males with small somas. Such male dimorphism is reminiscent of other behavioral and morphological dimorphisms , which have led to characterization of males into territorial and satellite forms in certain mating systems.

Zeroing in on write-offs.

Although collection activities had been successful at the Diagnostic Clinic of Houston, the medical group felt the need to maximize the efficiency of the collections department by focusing on the procedures involved in writing off accounts. Once the group had identified problem areas in this procedure, it engaged in an evaluation process that involved establishing goals, appraising the system, and analyzing the elements of the procedure. A recommendation was made to and accepted by the group's board of directors concerning alleviating problem areas. The result was a most favorable cash position and the ability to accurately measure collection effectiveness.

Retractor bulbi muscle responses to oculomotor nerve and nucleus stimulation in the cat.

This study demonstrates the presence of retractor bulbi motoneurons within the oculomotor nucleus which activate muscle units within all 4 slips of the cat retractor bulbi muscle. These muscle units are mechanically different and physiologically separate from retractor bulbi muscle units innervated by the abducens nerve. The refractor bulbi muscle, then, is innervated by two separate pools of motoneurons whose axons are carried in two different cranial nerves. These observations of mechanical properties of retractor bulbi muscle suggest that the oculomotor retractor bulbi motor units may be activated during patterned eye movements.

Relation between motoneuron position and lateral rectus motor unit contraction speed: an intracellular study in the cat abducens nucleus.

Thirty-one single muscle units of the cat lateral rectus muscle were activated through intracellular stimulation of their motoneurons in the principal abducens nucleus. Motoneurons encountered from dorsal to ventral in the nucleus tended to innervate muscle units with progressively slower contraction times. In addition, two cells innervated muscle units which did not exhibit twitch contractions and three cells innervated muscle units with unusually strong twitch contractions.

The effect of large unilateral cortical lesions on rubrospinal tract sprouting in newborn rats.

Many previous reports have demonstrated the development of aberrant neural connections in response to neonatal brain lesions. This investigation was undertaken to study possible alterations, particularly axonal sprouting, in rodent rubrospinal projections after neonatal destruction of the corticospinal tract through frontal cortical ablation. The neonatal ablations were made by aspiration in 1 to 2-day-old rats under hypothermic anesthesia. At three to six months after neonatal surgery, the rubrospinal tracts were ablated bilaterally in these small animals as well as in controls, by stereotaxically transecting the ventral tegmental decussation. Animals were killed two to six days after adult surgery, and rubrospinal projections were demonstrated using the Fink-Heimer degeneration stain. No differences in the pattern of rubrospinal projections were observed between animals with neonatal cortical lesions and controls. In all animals rubrospinal projections were located primarily in Rexed's lamina VI with a slight distribution into lamina V and the dorsal portion of lamina VII. Various hypotheses explaining the lack of rubrospinal sprouting after neonatal cortical lesions are presented, along with possible experiments to test these hypotheses.