Modulation of primary afferent discharge by dynamic and static gamma motor axons in cat muscle spindles in relation to the intrafusal fibre types activated

Monkey flexor and abductor pollicis brevis motoneuron pools: Proximal dendritic trees and small motoneurons

Transverse sections of the monkey cervical spinal cord from a previous study (Jenny and Inukai, 1983 [1]) were reanalyzed using Neurolucida to create a three-dimensional display of flexor pollicis brevis and abductor pollicis brevis (FAbPBr) motoneurons and dendrites that had been jointly labeled with horse radish peroxidase (HRP). These data were correlated with similar data from a reanalysis of an extensor digitorum communis (EDC) motoneuron pool (Jenny, Cheney, and Jenny, 2018 [2]). The FAbPBr motoneuron columns were located in the C8 (caudal) and T1 segments of the spinal cord and within the most dorsal and medial regions of the motor column pools that innervate hand muscles. Small motoneurons (cell body areas less than 500 µm² and presumed to be gamma motoneurons) comprised about four percent of the motoneurons and were located throughout the length of the motoneuron pool. HRP labeled dendrites extended radially (360°) from the motoneuron soma but greater numbers of dendrites were directed either dorsomedial to the base of the dorsal horn or medial to the ventromedial gray matter. The longer HRP labeled dendrites and their branch dendrites usually continued in the same radial direction as when originating from the cell body or proximal dendrite. As such we considered the radial direction of the longer HRP labeled dendrites to be a reasonable estimate of the radial direction of the more distal dendritic trees [2]. Both the EDC and FAbPBr motoneuron groups had a greater number of dendrites oriented in dorsal and medial directions from the motoneuron column. Our data continue to suggest that motoneuron dendritic trees have direction-oriented dendrites that extend toward functional terminal regions.

Proprioceptive Recognition with Artificial Neural Networks Based on Organizations of Spinocerebellar Tract and Cerebellum

Muscle kinematics and kinetics are nonlinearly encoded by proprioceptors, and the changes in muscle length and velocity are integrated into Ia afferent. Besides, proprioceptive signals from multiple muscles are probably mixed in afferent pathways, which all lead to difficulties in proprioceptive recognition for the cerebellum. In this study, the artificial neural networks, whose organizations are biologically based on the spinocerebellar tract and cerebellum, are utilized to decode the proprioceptive signals. Consistent with the controversy of the proprioceptive division in the dorsal spinocerebellar tract, the spinocerebellar tract networks incorporated two distinct inferences, (1) the centralized networks, which mixed Ia, II, and Ib and processed them together; (2) the decentralized networks, which processed Ia, II, and Ib afferents separately. The cerebellar networks were based on the Marr-Albus model to recognize the kinematic states. The networks were trained by a specific movement, and the trained networks were subsequently required to predict kinematic states of six movements. The results demonstrated that the centralized networks, which were more consistent with the physiological findings in recent years, had better recognition accuracy than the decentralized networks, and the networks were still effective even when proprioceptive afferents from multiple muscles were integrated.

Kinesio Tape and Shoulder-Joint Position Sense

CONTEXT

Joint position sense (JPS) is a key neuromuscular factor for developing and maintaining control of muscles around a joint. It is important when performing specialized tasks, especially at the shoulder. No researchers have studied how Kinesio Tape (KT) application affects JPS.

OBJECTIVE

To investigate the effects of KT application and no tape on shoulder JPS at increasing shoulder elevations in athletes.

DESIGN

Cross-sectional study.

SETTING

University laboratory.

PATIENTS OR OTHER PARTICIPANTS

A total of 27 healthy athletes who did not participate in overhead sports (age = 20.44 ± 1.05 years, height = 175.02 ± 11.67 cm, mass = 70.74 ± 9.65 kg) with no previous pathologic shoulder conditions volunteered for the study. All participants were from 1 university.

INTERVENTION(S)

Shoulder JPS was assessed at increasing elevations with and without KT application. Participants attempted to actively replicate 3 target positions with and without the KT and without visual guidance.

MAIN OUTCOME MEASURE(S)

We examined absolute and variable repositioning errors at increasing shoulder-elevation levels with and without KT application.

RESULTS

Data revealed an interaction between tape and position for absolute error (F2,52 = 4.07, P = .02); simple effects revealed an increase in error, with KT demonstrating a 2.65° increase in error at 90° of elevation compared with no tape (t26 = 2.65, P = .01). The effect size was medium (ω(2) = .135). Variable error showed no interaction of tape and position (F2,52 = .709, P = .50). Further analysis of simple effects was not needed. However, we still calculated the effect size and observed small effect sizes for tape (ω(2) = .002), position (ω(2) = .072), and tape by position (ω(2) = .027).

CONCLUSIONS

At 90° of elevation, shoulder JPS was impaired by the application of KT.

Fusimotor drive may adjust muscle spindle feedback to task requirements in humans

The aim of the present study was to investigate whether the fusimotor control of muscle spindle sensitivity may depend on the movement parameter the task is focused on, either the velocity or the final position reached. The unitary activities of 18 muscle spindle afferents were recorded by microneurography at the common peroneal nerve. We compared in two situations the responses of muscle spindle afferents to ankle movements imposed while the subject was instructed not to pay attention to or to pay attention to the movement, both in the absence of visual cues. In the two situations, three ramp-and-hold movements were imposed in random order. In one situation, the three movements differed by their velocity and in the other by the final position reached. The task consisted in ranking the three movements according to the parameter under consideration (for example, slow, fast, and medium). The results showed that paying attention to movement velocity gave rise to a significant increase in the dynamic and static responses of muscle afferents. In contrast, focusing attention on the final position reached made the muscle spindle feedback better discriminate the different positions and depressed its capacity to discriminate movement velocities. Changes are interpreted as reflecting dynamic and static gamma activation, respectively. The present results support the view that the fusimotor drive depends on the parameter the task is focused on, so that the muscle afferent feedback is adjusted to the task requirements.

Changes in human muscle spindle sensitivity during a proprioceptive attention task

The aim of the present study was to test whether fusimotor control of human muscle spindle sensitivity changed when attention was selectively directed to the recognition of an imposed two-dimensional movement in the form of a written symbol. The unitary activities of 32 muscle spindle afferents (26 Ia, 6 II) were recorded by microneurography at the level of the common peroneal nerve. The patterns of firing rate in response to passive movements of the ankle, forming different letters or numbers, were compared in two conditions: control and recognition. No visual cues were given in either condition, but subjects had to recognize and name the character in one condition compared with not paying attention in the control condition. The results showed that 58% of the tested Ia afferents presented modified responses to movements when these had to be recognized. Changes in Ia afferent responses included decreased depth of modulation, increased variability of discharge, and changes in spontaneous activity. Not all changes were evident in the same afferent. Furthermore, the percentage of correctly recognized movements amounted to 63% when changes were observed, but it was only 48% when the primary ending sensitivity was unaltered. The responses of group II afferents were only weakly changed or unchanged. It is suggested that the altered muscle spindle sensitivity is because of selective changes in fusimotor control, the consequence of which might be to feed the brain movement trajectory information that is more accurate.

Mathematical models of proprioceptors. I. Control and transduction in the muscle spindle

We constructed a physiologically realistic model of a lower-limb, mammalian muscle spindle composed of mathematical elements closely related to the anatomical components found in the biological spindle. The spindle model incorporates three nonlinear intrafusal fiber models (bag(1), bag(2), and chain) that contribute variously to action potential generation of primary and secondary afferents. A single set of model parameters was optimized on a number of data sets collected from feline soleus muscle, accounting accurately for afferent activity during a variety of ramp, triangular, and sinusoidal stretches. We also incorporated the different temporal properties of fusimotor activation as observed in the twitchlike chain fibers versus the toniclike bag fibers. The model captures the spindle's behavior both in the absence of fusimotor stimulation and during activation of static or dynamic fusimotor efferents. In the case of simultaneous static and dynamic fusimotor efferent stimulation, we demonstrated the importance of including the experimentally observed effect of partial occlusion. The model was validated against data that originated from the cat's medial gastrocnemius muscle and were different from the data used for the parameter determination purposes. The validation record included recently published experiments in which fusimotor efferent and spindle afferent activities were recorded simultaneously during decerebrate locomotion in the cat. This model will be useful in understanding the role of the muscle spindle and its fusimotor control during both natural and pathological motor behavior.

Shoulder joint position sense improves with elevation angle in a novel, unconstrained task

Proprioception, encompassing the submodalities of kinesthesia and joint position sense, is important in the maintenance of joint stability, especially in the shoulder. The purpose of this study was to examine the effects of plane and elevation angle on unconstrained shoulder joint position sense. Twenty-two subjects (12 male, 10 female) without a history of shoulder pathology were recruited from a university campus. Subjects attempted to replicate, with respect to plane and elevation angles, various target positions. Target positions consisted of five plane angles at 90 degrees of arm elevation and five arm elevation angles in the scapular plane. All target positions were tested twice to assess the reliability of the measurement. Intraclass correlation coefficients were generally low across target positions, possibly owing to the novelty and demanding nature of the task. No differences in repositioning errors were observed between plane angles (p = 0.255). Repositioning errors decreased linearly as the elevation angle increased from 30 degrees to 90 degrees (p = 0.007) and increased again from 90 degrees to 110 degrees of elevation (p = 0.029). Our results suggest that unconstrained joint position sense may be enhanced with increased muscular activation levels. Further, afferent feedback from musculotendinous mechanoreceptors may dominate over that from capsuloligamentous sources in unconstrained movements.

Glycogen depletion in intrafusal fibres in rats during short-duration high-intensity treadmill running

AIM

The recruitment patterns of the intrafusal and extrafusal fibres in the soleus (SOL) and extensor digitorum longus (EDL) muscles of rats were investigated during brief-intensity exercise by assaying their glycogen content histochemically.

METHODS

Six adult male rats were assigned to each of four groups that ran up a 6 degrees incline on a motor-driven treadmill, at 40 m min(-1) for either 0.5, 1, 2, or 4 min. Six adult male rats in the control group did not run. Extrafusal and intrafusal fibres were classified by myosin ATPase staining. Optical densities for glycogen content were evaluated in serial periodic acid Schiff (PAS) stained-sections from the B and C regions of intrafusal fibres.

RESULTS

The glycogen content of type IIA fibres in the SOL and EDL muscles decreased significantly in the early phase of exercise whereas the glycogen content of type I fibres in these muscles decreased later than that of type IIA fibres. The glycogen content of bag2 fibres decreased after 1 min of exercise in the SOL muscle and after 2 min of exercise in the EDL muscle. On the other hand, the glycogen content of bag1 and chain fibres decreased significantly after 2 min in the SOL muscle but not in the EDL muscle.

CONCLUSION

The results suggest that during brief-intensity exercise, as the glycogen content of type IIA fibres is reduced earlier than that of type I fibres, bag2 fibres are most important early in this type of exercise.

Static and dynamic gamma-motor output to ankle flexor muscles during locomotion in the decerebrate cat

In locomotion, the flexor muscles of the leg are mainly concerned with the relatively constant task of raising the foot, whereas the extensors have the more variable task of support and propulsion at different speeds. This suggests that the way in which the fusimotor system works may differ between the two muscle groups. Observations previously made of the static and dynamic gamma-motor firing patterns in the ankle extensor medial gastrocnemius (MG) have therefore been repeated in the flexor tibialis anterior (TA). One or more single gamma-motor axons, dissected from a small filament of TA nerve, were recorded simultaneously with a number of single spindle afferents in dorsal rootlets. Cats were decerebrated and locomoted spontaneously on a treadmill. Identification of each gamma-motor axon depended on relating the changes in firing caused by midbrain stimulation to the changes in static and dynamic behaviour of the spindle afferents in response to repetitive ramp and hold stretches. Static gamma axons all showed a smooth modulation in frequency, increasing in phase with muscle shortening, superimposed on a minimum frequency of about 20-30 impulses s(-1). Dynamic gamma axons showed interrupted firing with the frequency rising abruptly from zero at the onset of shortening, and falling again to zero shortly after the onset of lengthening. The frequency during the active periods was relatively constant, even when movement amplitudes varied. The basic similarity in the static and dynamic gamma discharge patterns for the two muscles suggests that the strategy of gamma-motor control is common to both flexors and extensors. The static gamma pattern is thought to be a 'temporal template' of the expected movement, effectively expanding the dynamic response range of the spindles in active movements. The dynamic gamma pattern sensitizes the primary afferents to detect the onset of muscle lengthening and to detect departures from the intended movement trajectory.

The influence of bag2 and chain intrafusal muscle fibers on secondary spindle afferents in the cat

Static gamma-motor activity is strongly modulated by a particular phase relationship to the cyclic movements of locomotion, and this has a profound effect on the firing patterns of muscle spindle afferents. Whilst primary afferents are affected by both static and dynamic gamma-motor output,secondary afferents are affected significantly only by the static system acting via the intrafusal bag2 and chain fibres. It is therefore important to know how fluctuating patterns of static gamma-motor activity affect secondary afferents and to relate this to the actions of bagt and chain fibres. We have studied the action of single static gamma axons on secondary afferents in cat hindlimb muscles. Various physiological methods were explored to identify which of the intrafusal muscle fibres were being activated in each case, including the use of random stimulation and ramp frequency stimulation. The effects were also recorded of I Hz sinusoidally frequency-modulated gamma-axon stimuli and the amplitude and phase of the resulting afferent modulation related to the involvement of the bag2 and chain fibres. It was found that bag2 fibres are effective in biasing the secondary discharge, but their modulating action is relatively weak and involves a marked phase lag. Chain fibres acting alone cause strong modulation with very little phase lag. Mixed bag2 and chain-fibre action is most effective in modulating afferent discharge and causes intermediate values of phase lag. The results are discussed in relation to the control of natural movements and it is concluded that an important function of the static gamma motor system is to provide a signal to sum algebraically with the length-related signal. The results do not suggest that it could also usefully control stretch sensitivity.

Staining of human thyroarytenoid muscle with myosin antibodies reveals some unique extrafusal fibers, but no muscle spindles

This study describes the myosin composition of extrafusal and intrafusal muscle fibers found in the human thyroarytenoid (TA) and sternohyoid (control) muscles. We sought to determine the presence of muscle spindles in the TA muscle, and to identify unusual extrafusal fiber types, using the commonly accepted approach of tissue staining with myosin isoform specific antibodies. Extrafusal fibers are organized into motor units, which subsequently produce muscle movement, whereas intrafusal fibers compose muscle spindles, the primary stretch receptor that provides afferent (feed back) information to the nervous system for regulation of motor unit length and tonicity. Immunohistochemical identification of muscle spindles was confirmed in sternohyoid, but not in TA samples; however, some extrafusal fibers contained tonic myosin. These results indicate that human TA muscle functions similar to some mammalian extraocular muscle, performing unloaded (non-weight bearing) contractions without afferent information from native muscle spindles.