Classification of muscle spindle receptors

Effects of simulated hypo-gravity on lower limb kinematic and electromyographic variables during anti-gravitational treadmill walking

Introduction: This study investigated kinematic and EMG changes in gait across simulated gravitational unloading levels between 100% and 20% of normal body weight. This study sought to identify if each level of unloading elicited consistent changes-particular to that percentage of normal body weight-or if the changes seen with unloading could be influenced by the previous level(s) of unloading. Methods: 15 healthy adult participants (26.3 ± 2.5 years; 53% female) walked in an Alter-G anti-gravity treadmill unloading system (mean speed: 1.49 ± 0.37 mph) for 1 min each at 100%, 80%, 60%, 40% and 20% of normal body weight, before loading back to 100% in reverse order. Lower-body kinematic data were captured by inertial measurement units, and EMG data were collected from the rectus femoris, biceps femoris, medial gastrocnemius, and anterior tibialis. Data were compared across like levels of load using repeated measures ANOVA and statistical parametric mapping. Difference waveforms for adjacent levels were created to examine the rate of change between different unloading levels. Results: This study found hip, knee, and ankle kinematics as well as activity in the rectus femoris, and medial gastrocnemius were significantly different at the same level of unloading, having arrived from a higher, or lower level of unloading. There were no significant changes in the kinematic difference waveforms, however the waveform representing the change in EMG between 100% and 80% load was significantly different from all other levels. Discussion: This study found that body weight unloading from 100% to 20% elicited distinct responses in the medial gastrocnemius, as well as partly in the rectus femoris. Hip, knee, and ankle kinematics were also affected differentially by loading and unloading, especially at 40% of normal body weight. These findings suggest the previous level of gravitational load is an important factor to consider in determining kinematic and EMG responses to the current level during loading and unloading below standard g. Similarly, the rate of change in kinematics from 100% to 20% appears to be linear, while the rate of change in EMG was non-linear. This is of particular interest, as it suggests that kinematic and EMG measures decouple with unloading and may react to unloading uniquely.

Discriminability of multiple cutaneous and proprioceptive hand percepts evoked by intraneural stimulation with Utah slanted electrode arrays in human amputees

BACKGROUND

Electrical stimulation of residual afferent nerve fibers can evoke sensations from a missing limb after amputation, and bionic arms endowed with artificial sensory feedback have been shown to confer functional and psychological benefits. Here we explore the extent to which artificial sensations can be discriminated based on location, quality, and intensity.

METHODS

We implanted Utah Slanted Electrode Arrays (USEAs) in the arm nerves of three transradial amputees and delivered electrical stimulation via different electrodes and frequencies to produce sensations on the missing hand with various locations, qualities, and intensities. Participants performed blind discrimination trials to discriminate among these artificial sensations.

RESULTS

Participants successfully discriminated cutaneous and proprioceptive sensations ranging in location, quality and intensity. Performance was significantly greater than chance for all discrimination tasks, including discrimination among up to ten different cutaneous location-intensity combinations (15/30 successes, p < 0.0001) and seven different proprioceptive location-intensity combinations (21/40 successes, p < 0.0001). Variations in the site of stimulation within the nerve, via electrode selection, enabled discrimination among up to five locations and qualities (35/35 successes, p < 0.0001). Variations in the stimulation frequency enabled discrimination among four different intensities at the same location (13/20 successes, p < 0.0005). One participant also discriminated among individual stimulation of two different USEA electrodes, simultaneous stimulation on both electrodes, and interleaved stimulation on both electrodes (20/24 successes, p < 0.0001).

CONCLUSION

Electrode location, stimulation frequency, and stimulation pattern can be modulated to evoke functionally discriminable sensations with a range of locations, qualities, and intensities. This rich source of artificial sensory feedback may enhance functional performance and embodiment of bionic arms endowed with a sense of touch.

Functional organization and connectivity of the dorsal column nuclei complex reveals a sensorimotor integration and distribution hub

The dorsal column nuclei complex (DCN-complex) includes the dorsal column nuclei (DCN, referring to the gracile and cuneate nuclei collectively), external cuneate, X, and Z nuclei, and the median accessory nucleus. The DCN are organized by both somatotopy and modality, and have a diverse range of afferent inputs and projection targets. The functional organization and connectivity of the DCN implicate them in a variety of sensorimotor functions, beyond their commonly accepted role in processing and transmitting somatosensory information to the thalamus, yet this is largely underappreciated in the literature. To consolidate insights into their sensorimotor functions, this review examines the morphology, organization, and connectivity of the DCN and their associated nuclei. First, we briefly discuss the receptors, afferent fibers, and pathways involved in conveying tactile and proprioceptive information to the DCN. Next, we review the modality and somatotopic arrangements of the remaining constituents of the DCN-complex. Finally, we examine and discuss the functional implications of the myriad of DCN-complex projection targets throughout the diencephalon, midbrain, and hindbrain, in addition to their modulatory inputs from the cortex. The organization and connectivity of the DCN-complex suggest that these nuclei should be considered a complex integration and distribution hub for sensorimotor information.

Age-related physiological and morphological changes of muscle spindles in rats

Age-related physiological and morphological changes of muscle spindles were examined in rats (male Fischer 344/DuCrj: young, 4-13 months; middle-aged, 20-22 months; old, 28-31 months). Single afferent discharges of the muscle spindles in gastrocnemius muscles were recorded from a finely split dorsal root during ramp-and-hold (amplitude, 2.0 mm; velocity, 2-20 mm s(-1)) or sinusoidal stretch (amplitude, 0.05-1.0 mm; frequency, 0.5-2 Hz). Respective conduction velocities (CVs) were then measured. After electrophysiological experimentation, the muscles were dissected. The silver-impregnated muscle spindles were teased and then analysed using a light microscope. The CV and dynamic response to ramp-and-hold stretch of many endings were widely overlapped in old rats because of the decreased CV and dynamic response of primary endings. Many units in old rats showed slowing of discharge during the release phase under ramp-and-hold stretch and continuous discharge under sinusoidal stretch, similarly to secondary endings in young and middle-aged rats. Morphological studies revealed that primary endings of aged rat muscle spindles were less spiral or non-spiral in appearance, but secondary endings appeared unchanged. These results suggest first that primary muscle spindles in old rats are indistinguishable from secondary endings when determined solely by previously used physiological criteria. Secondly, these physiological results reflect drastic age-related morphological changes in spindle primary endings.

Characterization of spindle afferents in rat soleus muscle using ramp-and-hold and sinusoidal stretches

The discharge properties of 51 afferents were studied in the rat soleus muscle spindles. Under deep anesthesia using a pentobarbital sodium solution (30 mg/kg), a laminectomy was performed and the right L(4) and L(5) dorsal and ventral roots were transected near their entry into the spinal cord. In situ, the minimal (L(min)) muscle length [3 +/- 0.08 (SE) cm] of the soleus was measured at full ankle extension. Unitary potentials from the L(5) dorsal root were recorded in response to ramp-and-hold stretches applied at 3 mm (S3) and 4 mm (S4) amplitudes and four stretch velocities (6, 10, 15, and 30 mm/s), sinusoidal stretches performed at four amplitudes (0.12, 0.25, 0.5, and 1 mm) and six stretch frequencies (0.5, 1, 2, 3, 6, and 10 Hz), and vibrations applied at 50-, 100-, and 150-Hz frequencies. These two kinds of stretches were performed at three different muscle lengths (L(min+10%), L(min+15%), and L(min+20%)), whereas vibrations were applied at L(min+20%) muscle length. Conduction velocity of the fibers was calculated but did not allow to discriminate different fiber types. However, the mean conduction velocity of the first fiber group (43.3 +/- 0.8 m/s) was significantly higher than that of the second fiber group (33.9 +/- 0.9 m/s). Three parameters allowed to differentiate the responses of primary and secondary endings: the dynamic index (DI), the discharge during the stretch release from the ramp-and-hold stretches, and the linear range and the vibration sensitivity from sinusoidal stretches. The slope histogram of the linear regression based on the DI and the stretch velocity was clearly bimodal. Therefore the responses were separated into two groups. During the stretch release at a velocity of 3 mm/s, the first response group (n = 26) exhibited a pause, whereas the second (n = 25) did not. The linear range of the second ending group (0.12-1 mm) was broader than that of the first (0.12-0.25 mm). The first ending group showed a higher sensitivity to high-vibration frequencies of small amplitude than the second. In comparison with the literature, we can assert that the first and the second ending groups corresponded to the primary and secondary endings, respectively. In conclusion, our study showed that in rat soleus muscle spindles, it was possible to immediately classify the discharge of Ia and II fibers by using some parameters measured under ramp-and-hold and sinusoidal stretches.

Proprioception from a spinocerebellar perspective

This review explores how proprioceptive sensory information is organized at spinal cord levels as it relates to a sense of body position and movement. The topic is considered in an historical context and develops a different framework that may be more in tune with current views of sensorimotor processing in other central nervous system structures. The dorsal spinocerebellar tract (DSCT) system is considered in detail as a model system that may be considered as an end point for the processing of proprioceptive sensory information in the spinal cord. An analysis of this system examines sensory processing at the lowest levels of synaptic connectivity with central neurons in the nervous system. The analysis leads to a framework for proprioception that involves a highly flexible network organization based in some way on whole limb kinematics. The functional organization underlying this framework originates with the biomechanical linkages in the limb that establish functional relationships among the limb segments. Afferent information from limb receptors is processed further through a distributed neural network in the spinal cord. The result is a global representation of hindlimb parameters rather than a muscle-by-muscle or joint-by-joint representation.

Light microscopic observations of the contacts made between two spindle afferent types and alpha-motoneurons in the cat trigeminal motor nucleus

Previous studies indicate that cat jaw-muscle spindle afferents can be divided into two types (type I and II) on the basis of their axonal trajectories. The present study examined the relationship between spindle afferent fibers and their target masseter alpha-motoneurons in the cat by using the intracellular horseradish peroxidase (HRP) injection technique, and provided several new findings on the synaptic organization generated between the two. Five type I afferent fiber-motoneuron pairs and nine type II afferent-motoneuron pairs were well stained with HRP. The following conclusions were drawn: 1) A motoneuron received contacts from only one collateral of any given spindle afferent. 2) The number of contacts made between an afferent and a motoneuron ranged from one to three. 3) The contacts made by a spindle afferent were on the same dendrite or dendrites branching from the same primary dendrite. 4) The vast majority of the contacts made by an afferent on a motoneuron were distributed in the dendritic tree within 600 microns from the soma, i.e., in the proximal three fourths of the dendritic tree. The differences observed between the two afferent types were as follows. First, type II afferent terminals made contacts on more distal dendrites of the motoneurons than did type I afferent terminals. Second, the contacts made between a type I afferent and a motoneuron were clustered together, but those made between a type II afferent and a motoneuron were widely dispersed. The present results provided the general rules of synaptic contacts between the spindle afferents and masseter alpha-motoneurons, and demonstrated that the spatial distribution of synaptic contacts on the dendritic tree was different between type I and type II afferents.

The classification of afferents from muscle spindles of the jaw-closing muscles of the cat

1. The effects of the muscle-depolarizing drug succinylcholine (SCh) on the stretch responses of jaw-closer muscle spindle afferents were studied in the anaesthetized cat. Using ramp and hold stretches repeated every 6 s the basic measurements made were: initial frequency (IF), peak frequency (PF) and static index (SI), the frequency 0.5 s after the end of the ramp of stretch. Derived from these were: dynamic difference (DD) = PF-IF, dynamic index (DI) = PF-SI and static difference (SD) = SI-IF. Increases in these measures caused by a single I.V. dose of SCh (200 micrograms kg-1) are symbolized by the prefix delta. 2. In a population of 234 units, delta DD and delta IF were each distributed bimodally, but were uncorrelated, thus defining four subgroups. 3. delta DD was argued to be an index of the effect of bag1 intrafusal fibre contraction and delta IF to be an index of the effect of bag2 fibre contraction. On this basis it is proposed that units can be divided into four groups according to the predominant influences of the bag1, bag2 and chain fibres as b1c (6.8%), b1b2c (22.2%), b2c (54.3%) or c (16.7%). 4. Testing with sine wave stretches at 1 Hz showed that changes in mean frequency and amplitude of response caused by SCh correlated with delta IF and delta DD respectively, but separated groups of units much less effectively than did ramp and hold testing. 5. Evidence is presented to indicate that the control value of DD in passive spindles does not relate to the potential strength of bag1 fibre effects in fully activated spindles. The bag1 fibre appears to contribute little to responses of spindle afferents in the passive state. DD is superior to DI as a measure of bag1 effects. 6. Conduction velocity was unimodally distributed in masseter spindle afferents and was not correlated with delta DD or delta IF and was therefore of no value in classifying them. 7. Neither the threshold of afferents to quick transient stretch nor the coefficient of variation of interspike intervals provided any significant additional help in classification. 8. The unexpectedly high proportion of units of b2c type is thought to include primaries lacking appreciable bag1 fibre contacts and secondaries with more or less substantial bag2 contracts.

Classification of muscle spindle afferents in the peroneus brevis muscle of the cat

Muscle-spindle afferents are commonly classified according to their conduction velocity. Under certain conditions such classifications may not be feasible and another form of identification is required. In this study 5 tests, comprising either quantitative or qualitative criteria, have been evaluated as a means of classifying spindle afferents. The choice of these tests was made on the basis of predicted physiological differences arising from the structural variations in the endings. Prior conditioning of the spindles was found to enhance the distinction between the two types of afferent. All the tests generated similar identifications with a maximum of 10% of afferents being classified differently by any two tests.