Cochlear implants: it's time to rethink.
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Cochlear Implants↗
Biomedical subjects
G E Loeb
Publications and source records attributed to G E Loeb.
Effect of neck posture on the activation of feline neck muscles during voluntary head turns.
1. To determine whether neck posture affects the usage of neck muscles during a specific motor task, we recorded the electromyographic (EMG) patterns of neck muscles in four cats, which made horizontal, head-turning movements to fixate eccentrically placed targets. In some trials, the cervical column was oriented vertically whereas in other trials, the cervical column was oriented more horizontally. 2. During horizontal head movements, five muscles (obliquus capitis inferior, splenius, levator scapulae, complexus, and biventer cervicis) displayed activation patterns that were consistent from cat to cat and did not change when the cats adopted a different neck posture. Most of these muscles are large, superficial muscles that attach to the skull and span many cervical joints. 3. Posturally dependent patterns of activation were observed in five other neck muscles (semispinalis cervicis, longissimus capitis, levator scapulae ventralis, scalenus anterior, and obliquus capitis superior). Most of these muscles lie deeper and more laterally within the neck musculature and generally span fewer cervical joints than the muscles that displayed invariant patterns of activation. 4. These results suggest that the set of invariantly activated muscles may compose part of a basic motor program that is triggered during head movements in the horizontal plane. This motor program appears to be modified by the selective activation of ancillary muscles, which are recruited in a manner related to the neck posture. The deep positioning of the ancillary muscles may permit them to regulate the mobility of the cervical column and to adjust the net muscular force applied across the neck to the skull. Organizing the motor output in this manner might simplify the task of computing the appropriate patterns of neck-muscle activation.
Animals↗
The computation of position sense from spindles in mono- and multiarticular muscles.
It is known that muscle spindles provide the majority of information about limb position, but little is known about how position sense is computed from their signals. We have developed a family of musculoskeletal models in order to determine some of the fundamental properties associated with transforming noisy spindle information into putative internal coordinate frames for position sense. A two-joint model was developed containing one biarticular and two monoarticular muscles with a total of 1000 sensors distributed among them. The sensors were assumed to function like spindle secondary afferents under fusimotor control designed to optimize their ability to encode static position in the presence of constant output noise. The optimal distribution of sensors was found to depend strongly on the coordinate frame in which position was measured (intersegmental angle, segment orientation, or end-point of the limb) and on the topology of the biarticular muscle with respect to the plane of motion. A similar analysis was performed for an anthropometric model of the human arm, using previously published counts of muscle spindles. In general, the actual distribution of spindles about the elbow and shoulder does not seem to favor any single coordinate frame for position sense. We also looked at the potential accuracy in detecting changes in joint angles based on the distribution of muscle spindles throughout the human body. The distribution of spindles about individual joints accounts well for psychophysical data showing a proximodistal descending gradient of angular resolution that partially reflects the relative importance of more proximal joints for determining the location of the end-point.
Extremities↗
Facial input to neck motoneurons: trigemino-cervical reflexes in the conscious and anaesthetised cat.
Cutaneous facial inputs influencing head movement were examined in the conscious and anaesthetised cat. EMG recordings were made in neck muscles of conscious, unrestrained cats in which an unexpected light cutaneous stimulus was applied to the glabrous skin of the planum nasale (PN). These observations established that head aversion movements were associated with synchronised activation of both deep and superficial dorsal neck muscles. In anaesthetised cats in which activity in the motoneurons of the large dorsal neck muscles was examined, mechanical stimulation of the PN or electrical stimulation of the infraorbital nerve (ION) produced a short latency, reflex activation. The reflex could be elicited by excitation of low threshold, rapidly conducting fibres in the ION. Intracellular recording from neck motoneurons showed that there is a short latency, probably disynaptic, excitatory pathway from low threshold nerves in the ION to neck motoneurons, but discharge of neck motoneurons occurred several milliseconds later, presumably as a result of activity in a longer multisynaptic pathway.
Anesthesia↗
The distal hindlimb musculature of the cat: interanimal variability of locomotor activity and cutaneous reflexes.
During stereotyped behaviors such as locomotion, patterns of muscle recruitment are usually quite consistent from animal to animal, even in the face of many surgical and pharmacological reductions. However, as studies of musculoskeletal structure, neuromuscular architecture, and sensorimotor circuitry become more detailed, it is important to ask whether there is some level of organization at which individual differences begin to dominate. This study concentrated on the small muscles of the foot and ankle, using standardized methods that consistently record stereotypical electromyographic activity from prime mover muscles and that permit well-calibrated stimulation of cutaneous nerves to elicit reflexes during treadmill locomotion. Some muscles (particularly the main ankle extensors, triceps surae, and plantaris) had stereotyped activity during both unperturbed locomotion and reflex responses. Others had stereotyped activity during locomotion but variable reflex patterns among animals (tibialis anterior, extensor digitorum longus, flexor hallucis longus, and peroneus brevis). Still others had variable locomotor activity but reflexes that were consistent (flexor digitorum longus) or variable for only peroneal nerve stimulation (peroneus longus), only plantar nerve stimulation (peroneus tertius), or the two (flexor digitorum brevis). Among muscles with interanimal variability, there seemed to be no particular correlation between locomotor and reflexive recruitment in a given animal. This functional heterogeneity is discussed in terms of the development of locomotor and reflex programs and in the context of structural heterogeneity of some of these muscles that is described in the companion paper.
Animals↗
The distal hindlimb musculature of the cat: multiaxis moment arms at the ankle joint.
The cat hindlimb muscles have been classified, traditionally, as flexors and extensors, based on their actions in the parasagittal plane and their patterns of recruitment during locomotion and reflex responses. This study provides a detailed examination of the relative magnitudes of the various moment arms of the cat ankle muscles and the interdependent effects of position in the various axes of motion. We used a method based on observing small sliding movements of tendon in response to small angular displacements of the joint. Surprisingly, we found that the ankle joint of the cat permits substantial motion in three axes (eversion/inversion and abduction/adduction as well as extension/flexion) and many muscles crossing the ankle joint have their largest moment arms about axes other than extension/flexion. These moment arms often depended on the joint position in the axis of the moment arm and, to a lesser degree, on the extension/flexion angle as well. For some muscles (notably peroneus longus) there was sufficient variability that the predominant action in neutral posture (axis with the largest moment arm) could change from animal to animal, which may be related to heterogeneities of locomotor and reflex recruitment reported in the companion paper.
Animals↗
Why cats pace on the treadmill.
There have been many studies suggesting that locomotion on a treadmill tends to be different than locomotion at similar velocities overground, but no satisfactory mechanical or neural mechanisms to account for the differences have been identified. The most prominent difference is the tendency to adopt a pacing gait for both walking and trotting speeds, in which the legs on one side of the body move in phase as lateral couplets rather than the typical diagonal couplet pattern seen overground. Using conventional video analysis, we quantified the gait patterns of intact, adult cats walking at various speeds overground and in a motorized treadmill. We noted that cats paced most frequently when they were at the front end of the treadmill enclosure, and that this gait was associated with an extended stride length that permitted the animals to maintain a higher duty factor of support (mean number of feet on the ground). We propose that the animal extends its stride specifically to improve the duty factor in anticipation of sudden stops of the treadmill belt and that it converts abruptly from diagonal to lateral gait because the extended stride results in collisions between ipsilateral hind and front feet.
Acceleration↗
Morphometry of human thigh muscles. Determination of fascicle architecture by magnetic resonance imaging.
A previous investigation suggested that striation patterns spanning individual muscles in longitudinally oriented MR images may represent the orientation of its fascicles. In this study, we confirmed that these striation patterns could be used to infer fascicle orientation and to compute other architectural features of muscles from MR images. The volumes of 14 muscles within a cadaveric thigh were shown to be estimated accurately from cross-sectional MR images by comparison with direct measures from muscle mass. The angles of striations were measured at several positions within vastus medialis and semimembranosus from sagittal and frontal-plane MR images. Mathematical techniques were developed to infer the 3-dimensional orientation of fascicles based on these striation angles. The angle of striations in a 3rd oblique plane was shown to agree with mathematical predictions based on these computed orientations. The pennation angle, defined as the angle between the fascicles and the line of action of the muscle, predicted from the MR images, was similar to directly measured values. Interestingly, the pennation angle of these fascicles varied along the length of the muscle; in vastus medialis, pennation angle ranged from 5 degrees to 50 degrees in a proximodistal direction. Procedures were developed and validated to compute fascicle length by projection of fascicle orientation across the 3D shape of the muscles. The use of MR images to estimate muscle morphometry could improve greatly the predictive capabilities of musculoskeletal modelling by reducing the number of unknown model parameters.
Aged↗
An intrinsic mechanism to stabilize posture--joint-angle-dependent moment arms of the feline ankle muscles.
The neuromuscular system can use sensory feedback to regulate motion, but these feedback loops involve relatively long delays (50-100 ms) and may produce undesirable oscillations. However, stabilizing changes in muscle force can also be provided intrinsically (i.e. without feedback) by 2 properties of the muscle itself, its force-length and force-velocity relationships. We have discovered another intrinsically stabilizing mechanism in the musculoskeletal architecture of the cat ankle joint. Many of its muscles have their predominant moment arms about the adduction/abduction axis, with smaller moment arms for inversion/eversion and about the principal axis of motion, dorsiflexion/extension. The magnitudes of the moment arms in ab/adduction and in/eversion depend strongly on joint angle, increasing for positions away from neutral that lengthen the muscles. Thus, co-activation of agonist-antagonist pairs, known to occur in these muscles, would provide immediate stabilizing changes in torque when the ankle is perturbed away from neutral position.
Anesthesia↗
Neuromuscular organization of feline anterior sartorius: II. Intramuscular length changes and complex length-tension relationships during stimulation of individual nerve branches.
The feline anterior sartorius is a long strap-like muscle composed of short muscle fibers. Nerve branches that enter this muscle contain the axons of motor units whose constituent muscle fibers are distributed asymmetrically within the muscle. In the present study, twitch and tetanic isometric contractions were evoked by stimulating individual nerve branches while muscle force was recorded and intramuscular length changes were monitored optically by the movement of reflective markers on the muscle. Contractions elicited by stimulating the parent nerve produced little change in the positions of the surface markers. Contractions elicited by stimulating the proximally or distally directed nerve branches caused the muscle to shorten at the end closest to the nerve branch and lengthen at the opposite end. Some muscles were supplied by a centrally directed nerve branch whose stimulation produced variable effects: in some cases a portion of the muscle shortened whereas the rest lengthened, but in other cases, the positions of the surface markers showed little change. The intramuscular length changes produced by stimulating single nerve branches were greater during isometric contractions at short whole-muscle lengths than at long whole-muscle lengths. The twitch and tetanic length-tension relationships obtained by stimulating the individual nerve branches were not congruent with the length-tension relationship produced when the parent nerve was stimulated. At short whole-muscle lengths, stimulation of a single nerve branch generated only a small fraction of the force that could be generated by the muscle when the parent nerve was stimulated. As whole-muscle length increased, an increased fraction of total muscle force could be generated by stimulating a single nerve branch. The results suggest that a complex relationship between passive and active elements contributes to the total muscle force and depends on the distribution of active and passive muscle units throughout the muscle.
Animals↗
Electromyographic studies of neck muscles in the intact cat. I. Patterns of recruitment underlying posture and movement during natural behaviors.
Natural head movements in alert, unrestrained cats were studied using video-filming, videofluoroscopy and electromyographic (EMG) recording methods. In each cat, up to sixteen neck muscles or neck-muscle compartments were implanted with recording electrodes. Patterns of muscle recruitment were examined during systematically-selected behavioral epochs in which the cat held a range of stationary postures, and when it performed volitional and exploratory behaviors such as flexion-extension or turning, grooming, eating, or headshaking. Patterns of muscular activity were interpreted with reference to simultaneous video images of head and neck movements. In separate, videofluoroscopic analysis, flexion-extension movements were examined to gain insight into the underlying movements of the skull and cervical vertebrae. These and other movements were found commonly to depend upon changes in joint angles between lower as well as upper cervical joints. Stationary postures in which the neck was held vertically were consistently associated with tonic EMG activity in only two long dorsal muscles, biventer cervicis and occipitoscapularis. Less consistent activity was also present in dorsal intervertebral muscles crossing lower cervical joints. When the neck was held horizontally, the long dorsal muscles increased their EMG activity and moderate activity was also recorded in deeper intervertebral and suboccipital muscles. When flexion-extension occurred around upper cervical joints, greatest activity was recorded in rectus capitis posterior and complexus, but when it involved the lower cervical joints, large changes in EMG activity could also be detected in biventer cervicis, occipitoscapularis, and the intervertebral muscles crossing lower cervical joints. During specialized, sagittal-plane movements such as grooming, well-defined patterns of synergy could be recognized that varied according to the degree of involvement of upper and lower cervical joint-sets. Movements in the horizontal plane were associated with EMG activity in a largely different subset of neck muscles including splenius, longissimus capitis and obliquus capitis inferior. The levels of EMG activity during flexion-extension or turning movements were much lower than those observed during other more vigorous behaviors, such as head shaking. Some neck muscles, such as clavotrapezius and sternomastoideus, could only be recruited during forceful or ballistic head movements. Results showed that the patterns of muscular activation were linked not only to the speed and trajectory of the movements of the skull, but also to the kinematics of the motion occurring across different parts of the cervical column.
Animals↗
Electromyographic studies of neck muscles in the intact cat. II. Reflexes evoked by muscle nerve stimulation.
Short-latency reflexes were studied in the neck muscles of four alert cats following electrical stimulation of nerves supplying biventer cervicis (BC), splenius (SP) or rectus capitis posterior (RCP). Reflexes were assessed by comparing levels of EMG activity of muscles before and after each stimulus, as the cats lapped milk, licked their paws or walked on a treadmill. When BC or SP nerves were stimulated at 1.5-4 times threshold (T) for their motor axons, no short-latency heteronymous reflexes could be identified in most neck muscles. However, stimulation of RCP nerves produced inhibitory effects as early as 3-4 ms in the ipsilateral BC, CM, and SP muscles and 6 ms in contralateral BC. At stimulus strengths above 4xT, a more complex pattern of inhibitory or excitatory effects was observed in CM, SP and the intervertebral muscle spinalis dorsi. The reflex effects were attenuated or abolished by partial or complete C1 dorsal rhizotomy (2 cats). Cervicocollic reflex data may need to be reevaluated to consider the possible effects of disinhibition rather than excitation in short-latency reflex pathways.
Afferent Pathways↗
Cartels, competition and activity-dependent synapse elimination.
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Animals↗
Effect of velocity and mechanical history on the forces of motor units in the cat medial gastrocnemius muscle.
1. Two fundamental aspects of the dynamic behavior of motor units of the cat medial gastrocnemius (MG) muscle were measured. Force-velocity (FV) relationships were measured with the use of constant velocity shortening and lengthening movements. Effects of mechanical history were assessed via comparisons of forces immediately after or during slow movements with standard isometric forces. Isometric force-length (FL) relations were also measured, and the effect of different stimulation rates on both FV and FL data was assessed. 2. Prior or concurrent movement greatly potentiated motor-unit force, but this movement potentiation was highly dependent on the amplitude of the unit's force. The smallest twitch forces of type S units (< 10 mN) were potentiated more than threefold, but no potentiation occurred for unit forces > 200 mN. It was tentatively concluded that movement potentiation may play little role in normal movements because it does not occur at forces > 1% of maximal isometric force of the MG. 3. During shortening, the normalized FV relations of type S units were relatively steeper than those of type FR or FF units. For lengthening, there was no evident relation between FV steepness and motor-unit type. 4. Stimulation rate affected both the FV and FL relationships of the motor units. The peak of the FL relationship (Lo) clearly shifted to shorter muscle lengths as stimulation rate was increased. The steepness of the FV relationship for shortening was decreased by increasing stimulation rate, but this effect was modest. 5. The shift in motor-unit Lo and the differences in motor-unit FV relationships were hypothesized to play significant roles during normal motor behavior. Realistic computer simulations of FL and FV functions for a population of motor units undergoing normal steady-state recruitment and rate modulation supported these hypotheses. As the level of simulated neural drive increased, the population Lo shifted to considerably shorter lengths, and the normalized FV function became much less steep. The significance of these results for models of muscle are discussed.
Animals↗
Distribution of motoneurons supplying cat sartorius and tensor fasciae latae, demonstrated by retrograde multiple-labelling methods.
Sartorius (SART) and tensor fasciae latae (TFL) in the cat hindlimb are functionally heterogeneous muscles with regions that differ in their skeletal actions and electromyographic recruitment during normal activity. The topographical organization of motoneurons supplying different regions of SART or TFL has been investigated by exposing cut nerve branches supplying different peripheral territories to a combination of retrograde tracers, including Fast Blue (FB), Fluorogold (FG), and horseradish peroxidase (HRP). Motoneurons supplying medial, central, and anterior regions of SART were intermixed extensively throughout a single columnar nucleus located in the ventrolateral part of segments L4 and L5. With this column, motoneurons supplying medial SART tended to lie more rostrally than those supplying anterior regions, but the gradient was modest and showed some cat-to-cat variation. Two major branches entered anterior SART at different proximodistal levels. When these two branches were exposed to different tracers, most motoneurons contained a single tracer; only a few double-labelled cells were apparent. The labelling suggests that anterior SART may contain two separate, in-series divisions of motor units. In TFL, motoneurons supplying nerve branches to posterior, central, and anterior parts of the muscle were intermingled indiscriminately in a single ventrolateral cell column in L6 and rostral L7. These results suggest that topographical organization in lumbar motor nuclei does not always reflect the highly ordered biomechanical and functional specialization evident in the peripheral organization of the muscles themselves.
Amidines↗
Functionally complex muscles of the cat hindlimb. I. Patterns of activation across sartorius.
The cat sartorius (SA) can be divided functionally into an anterior (SAa), knee extensor portion and a medial (SAm), knee flexor portion; it can be further subdivided anatomically by multiple nerve branches into parallel longitudinal columns that terminate in a distributed insertion at the knee with a continuous range of moment arms. Thus, SA may be controlled by a discrete number of motoneuron task groups reflecting a small number of central command signals or by a continuum of activation patterns associated with a continuum of moment arms. To resolve this question, the activation patterns across the width of the SA were recorded with an electrode array during three kinematically different movements--treadmill locomotion, scratching and paw shaking, in awake, unrestrained cats. Uniformity of activation along the longitudinal axis was also examined because individual muscle fibers do not extend the length of the SA. In addition, the cutaneous reflex responses were recorded throughout all regions of the SA during locomotion. Two fascial surface-patch arrays, each carrying 4-8 pairs of bipolar EMG electrodes, were sutured to the inner surface of the SA, one placed proximally and the other more distally. Each array sampled separate sites across the anterior to medial axis of SA. During locomotion, two basic EMG patterns were observed: the two burst-per-step-cycle pattern typical of SAa and the single burst pattern typical of SAm. There was an abrupt transition in the pattern of activation recorded in the two parts of SA during locomotion, and no continuum in the activation pattern was observed. Stimulation of both sural and saphenous cutaneous nerves during locomotion produced reflex responses that were uniformly distributed throughout SA, in contrast to the regional differences noted during unperturbed walking. Similarly, during scratching and paw shaking all parts of the SA were active simultaneously but with regional differences in EMG amplitude. The abrupt functional border between SAa and SAm coincided with the division of the SA into a knee flexor vs. a knee extensor. In all cases, the quantitative and qualitative differences in SAa and SAm EMGs were uniformly recorded throughout the entire extent of SAa or SAm; i.e., there was no segregation of activity within either SAa or SAm. Furthermore, the time course of EMG from each proximal recording site was nearly identical to the corresponding distal site, indicating no segregation of function along the longitudinal axis of SA. These results indicate that SAa and SAm constitute the smallest functional modules that can be recruited in SA.(ABSTRACT TRUNCATED AT 400 WORDS)
Animals↗
Functionally complex muscles of the cat hindlimb. II. Mechanical and architectural heterogenity within the biceps femoris.
The goal of this study was to analyze the architecture of the cat biceps femoris (BF), a multifunctional hamstring muscle, and to evaluate the relationships between muscle architecture, limb position, and muscle function during natural movement. The BF muscle consists of three neuromuscular compartments: anterior (BFa), middle (BFm) and posterior (BFp). Each compartment is innervated by a separate nerve branch. Nerve branch stimulation and 2-dimensional surface EMG recordings showed that individual compartment territories were discrete and non-overlapping with well-defined borders. Comparisons of the three compartments revealed consistent differences in architecture, relationship to the skeleton, and function. The BFa crossed only the hip joint and appears to function as a pure hip extensor. The BFm had equal lever arm lengths to the hip and knee joints, appears to function as a hip extensor, and may contribute to knee flexion or femoral rotation. The BFp had a greater lever arm to the knee, functions as a knee flexor, and may contribute to hip extension, femoral rotation or ankle extension. Measurements of individual fascicles from the three compartments revealed a surprising range of lengths, 3.3-12.0 cm. Microdissection of gold-stained tissue showed that fascicles from all compartments were comprised of interdigitated, short fibers (range: 0.6-5.0 cm; average 2.14 cm) arranged in-series in fascicles, running parallel to the origin-insertion axis of each muscle compartment. In regions of fiber interdigitation, the fiber endings were round and tapered (taper lengths: 1-11 mm) although flat, tapering endings like ribbons were occasionally found. As hip and knee joint angles were varied over physiological ranges corresponding to minimal to maximal muscle length, fascicles of the three compartments changed length disproportionately. Long BFa fascicles maximally lengthened 10-18%, consistent with in vivo length measures during treadmill locomotion. However, the long BFp fascicles lengthened 25-45%, and the relatively short fascicles near the BFm/BFp border maximally lengthened 45-53%. How do these unexpectedly large length changes affect sarcomere lengths? Using laser diffraction to measure sarcomeres, static fascicle and sarcomere lengths were compared in muscles that went into rigor mortis after fixing the hip and knee joint angles. Sarcomeres within the short BFm/BFp and long BFp fascicles consistently lengthened proportionately less than the whole fascicle. It remains to be determined how and where the fascicle length changes are dissipated in the connective tissue between the interdigitated muscle fibers and whether such a series-compliance operates during the large excursions over which this muscle normally works.
Animals↗
Functionally complex muscles of the cat hindlimb. IV. Intramuscular distribution of movement command signals and cutaneous reflexes in broad, bifunctional thigh muscles.
Similarities between the muscle synergies associated with the flexion reflex and locomotion in reduced preparations have suggested that spinal circuits subserving these two motor tasks might share common interneurons. To test this hypothesis in functionally complex muscles, we studied the interaction between low-threshold cutaneous afferents and the locomotor central pattern generator (CPG) during treadmill locomotion in awake, intact cats. Electrical stimuli were delivered via implanted nerve cuff electrodes at all phases of locomotion, and EMGs were recorded from fourteen intramuscular subregions in eight bifunctional thigh muscles (adductor femoris, biceps femoris, caudofemoralis, gracilis, semimembranosus, semitendinosus, tensor fasciae latae, and tenuissimus). In addition, the EMG patterns recorded during locomotion were compared with those recorded during two other centrally driven rhythmical behaviors, scratching and paw shaking, to determine whether the functional relationships among these intramuscular subregions were fixed or task dependent. Four of the five broad, bifunctional muscles studied (biceps femoris, gracilis, semimembranosus, and tensor fasciae latae) had functional subunits that could be differentially activated in one or more of the three movements studied; adductor femoris was consistently uniformly activated despite its distributed skeletal attachments. The pattern of recruitment of the intramuscular functional subunits was movement-specific. The locomotor CPG and cutaneous reflex pathways both similarly subdivided some bifunctional muscles, but not others, into intramuscular subregions. The results of the present study confirm that some combinations of muscle subregions and cutaneous nerves constitute simple reciprocal categories of flexors and extensors, as described originally by Sherrington (1910). "Typical" low threshold excitatory or inhibitory reflex responses were produced in muscles or muscle subregions that were recruited as "net" flexors of extensors, respectively. However, muscles with complex activation patterns during walking often had very individualized, complex reflex responses during locomotion that did not conform to the background locomotion synergies. All of the reflex responses observed were mediated by low threshold cutaneous afferents. These data indicate that there are multiple, low threshold, excitatory and inhibitory cutaneous reflex pathways that have highly specialized connections with flexor and extensor muscles and even their intramuscular subregions. It is also clear that the premotoneuronal circuits mediating these cutaneous reflex effects are not necessarily synonymous with those of the locomotor CPG. These two systems do interact powerfully, however, suggesting some convergence. The nature of the convergence between the CPG and the many independent subsets of spinal interneurons mediating cutaneous reflexes is specialized and muscle subregion-specific.
Afferent Pathways↗