Coupling between human muscle spindle endings and motor units assessed using spike-triggered averaging

Generalized Finger Motion Classification Model Based on Motor Unit Voting

Surface electromyogram-based finger motion classification has shown its potential for prosthetic control. However, most current finger motion classification models are subject-specific, requiring calibration when applied to new subjects. Generalized subject-nonspecific models are essential for real-world applications. In this study, the authors developed a subject-nonspecific model based on motor unit (MU) voting. A high-density surface electromyogram was first decomposed into individual MUs. The features extracted from each MU were then fed into a random forest classifier to obtain the finger label (primary prediction). The final prediction was selected by voting for all primary predictions provided by the decomposed MUs. Experiments conducted on 14 subjects demonstrated that our method significantly outperformed traditional methods in the context of subject-nonspecific finger motion classification models.

Quantitative input-output relationships between human soleus muscle spindle afferents and motoneurons

A method is described that, for the first time, allows instantaneous estimation of the Ia fiber input to human soleus motoneurons following electrical stimulation of the tibial nerve. The basis of the method is to determine the thresholds of the most and least excitable 1a fibers to electrical stimulation, and to treat the intervening thresholds as having a normal distribution about the mean; the validity of this approach is discussed. It was found that, for the same Ia fiber input, the percentage of soleus motoneurons contributing to the H (Hoffmann)-reflex differed considerably among subjects; when the results were pooled, however, there was an approximately linear relationship between Ia input and motoneuron output. Weak extension of the great toe diminished the soleus motoneuron reflex discharge in all but 2 of 16 subjects; the results for weak ankle plantarflexion were less consistent, but overall, there was a reduction in soleus motoneuron output also. The methodology should provide new insights into disorders of movement and tone, especially as it permits estimates of motoneuron depolarization to be made. NEW & NOTEWORTHY Assuming a normal distribution of Ia fiber thresholds to electrical stimulation and using the H-reflex, we determined for the first time an Ia input-α-motoneuron output relationship for the human soleus muscle. The relationship varies greatly among subjects but, overall, is approximately linear. Minimal contraction of a toe muscle alters the relationship dramatically, probably due to presynaptic inhibition of Ia fibers. Drawing on the literature, we can calculate changes in α-motoneuron membrane potential.

The representation of egocentric space in the posterior parietal cortex

The posterior parietal cortex (PPC) is the most likely site where egocentric spatial relationships are represented in the brain. PPC cells receive visual, auditory, somaesthetic, and vestibular sensory inputs; oculomotor, head, limb, and body motor signals; and strong motivational projections from the limbic system. Their discharge increases not only when an animal moves towards a sensory target, but also when it directs its attention to it. PPC lesions have the opposite effect: sensory inattention and neglect. The PPC does not seem to contain a "map" of the location of objects in space but a distributed neural network for transforming one set of sensory vectors into other sensory reference frames or into various motor coordinate systems. Which set of transformation rules is used probably depends on attention, which selectively enhances the synapses needed for making a particular sensory comparison or aiming a particular movement.

Does the nervous system depend on kinesthetic information to control natural limb movements?

This target article draws together two groups of experimental studies on the control of human movement through peripheral feedback and centrally generated signals of motor commands. First, during natural movement, feedback from muscle, joint, and cutaneous afferents changes; in human subjects these changes have reflex and kinesthetic consequences. Recent psychophysical and microneurographic evidence suggests that joint and even cutaneous afferents may have a proprioceptive role. Second, the role of centrally generated motor commands in the control of normal movements and movements following acute and chronic deafferentation is reviewed. There is increasing evidence that subjects can perceive their motor commands under various conditions, but that this is inadequate for normal movement; deficits in motor performance arise when the reliance on proprioceptive feedback is abolished either experimentally or because of pathology. During natural movement, the CNS appears to have access to functionally useful input from a range of peripheral receptors as well as from internally generated command signals. The unanswered questions that remain suggest a number of avenues for further research.

Implications of neural networks for how we think about brain function

Engineers use neural networks to control systems too complex for conventional engineering solutions. To examine the behavior of individual hidden units would defeat the purpose of this approach because it would be largely uninterpretable. Yet neurophysiologists spend their careers doing just that! Hidden units contain bits and scraps of signals that yield only arcane hints about network function and no information about how its individual units process signals. Most literature on single-unit recordings attests to this grim fact. On the other hand, knowing a system's function and describing it with elegant mathematics tell one very little about what to expect of interneuronal behavior. Examples of simple networks based on neurophysiology are taken from the oculomotor literature to suggest how single-unit interpretability might decrease with increasing task complexity. It is argued that trying to explain how any real neural network works on a cell-by-cell, reductionist basis is futile and we may have to be content with trying to understand the brain at higher levels of organization.

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.

Low-threshold afferent signalling of viscous loads during voluntary movements of the human digits

Humans can discriminate changes in load viscosity during voluntary contractions. The afferent signal origin is unknown. Microneurographic recordings from 83 single low-threshold afferents were made while participants performed triangular ramps either unloaded or with a viscous load. The neural discharges for each cycle were compared across load and velocity. Fifty-eight afferents did not respond. Afferents with sufficient activity were classified as ambiguous--discharges correlated to velocity and load (n=4), infinite viscosity--strong load and weak velocity signal (n=6), no viscosity--strong velocity and weak load signal (n=10) and those with neither (n=5). No single class of afferent provides a coherent signal of viscosity. These data suggest that the central nervous system compares the population response of different inputs to discriminate viscosity.

A study on synaptic coupling between single orofacial mechanoreceptors and human masseter muscle

The connection between individual orofacial mechanoreceptive afferents and the motoneurones that innervate jaw muscles is not well established. For example, although electrical and mechanical stimulation of orofacial afferents in bulk evokes responses in the jaw closers, whether similar responses can be evoked in the jaw muscles from the discharge of type identified single orofacial mechanoreceptive afferents is not known. Using tungsten microelectrodes, we have recorded from 28 afferents in the inferior alveolar nerve and 21 afferents in the lingual nerve of human volunteers. We have used discharges of single orofacial afferents as the triggers and the electromyogram (EMG) of the masseter as the source to generate spike-triggered averaged records to illustrate time-based EMG modulation by the nerve discharge. We have then used cross correlation analysis to quantify the coupling. Furthermore, we have also used coherence analysis to study frequency-based relationship between the nerve spike trains and the EMG. The discharge patterns of the skin and mucosa receptors around the lip and the gingiva generated significant modulation in EMGs with a success rate of 40% for both cross correlation and coherence analyses. The discharge patterns of the periodontal mechanoreceptors (PMRs) generated more coupling with a success rate of 70% for cross correlation and about 35% for coherence analyses. Finally, the discharges of the tongue receptors displayed significant coupling with the jaw muscle motoneurones with a success rate of about 40% for both analyses. Significant modulation of the jaw muscles by single orofacial receptors suggests that they play important roles in controlling the jaw muscle activity so that mastication and speech functions are executed successfully.

Evidence for strong synaptic coupling between single tactile afferents from the sole of the foot and motoneurons supplying leg muscles

It has been known for some time that populations of cutaneous and muscle afferents can provide short-latency facilitation of motoneuron pools. Recently, it has been shown that the input from individual low-threshold mechanoreceptors in the glabrous skin of the hand can modulate ongoing activity in muscles acting on the fingers via spinally mediated pathways. We have extended this work to examine whether such strong synaptic coupling exists between tactile afferents in the sole of the foot and motoneurons supplying muscles that act about the ankle. We recorded from 53 low-threshold mechanoreceptors in the glabrous skin of the foot via microelectrodes inserted percutaneously into the tibial nerve of awake human subjects. Reflex modulation of ongoing whole muscle electromyography (EMG) was observed for each of the four classes of low-threshold cutaneous mechanoreceptors (17 of 21 rapidly adapting type I; 2 of 4 rapidly adapting type II; 7 of 18 slowly adapting type I; and 4 of 10 slowly adapting type II). Reflex modulation of the firing probability in single motor units (5 of 11) was also observed. These results indicate that strong synaptic coupling between tactile afferents and spinal motoneurons is not a specialization of the hand and emphasizes the potential importance of cutaneous inputs from the sole of the foot in the control of gait and posture.

Transmission security for single kinesthetic afferent fibers of joint origin and their target cuneate neurons in the cat

Transmission between single identified, kinesthetic afferent fibers of joint origin and their central target neurons of the cuneate nucleus was examined in anesthetized cats by means of paired electrophysiological recording. Fifty-three wrist joint afferent-cuneate neuron pairs were isolated in which the single joint afferent fiber exerted suprathreshold excitatory actions on the target cuneate neuron. For each pair, the minimum kinesthetic input, a single spike, was sufficient to generate cuneate spike output, often amplified as a pair or burst of spikes, particularly at input rates up to 50-100 impulses per second. The high security was confirmed quantitatively by construction of stimulus-response relationships and calculation of transmission security measures in response to both static and dynamic vibrokinesthetic disturbances applied to the joint capsule. Graded stimulus-response relationships demonstrated that the output for this synaptic connection between single joint afferents and cuneate neurons could provide a sensitive indicator of the strength of joint capsule stimuli. The transmission security measures, calculated as the proportion of joint afferent spikes that generated cuneate spike output, were high (>85-90%) even at afferent fiber discharge rates up to 100-200 impulses per second. Furthermore, tight phase locking in the cuneate responses to vibratory stimulation of the joint capsule demonstrated that the synaptic linkage preserved, with a high level of fidelity, the temporal information about dynamic kinesthetic perturbations that affected the joint. The present study establishes that single kinesthetic afferents of joint origin display a capacity similar to that of tactile afferent fibers for exerting potent synaptic actions on central target neurons of the major ascending kinesthetic sensory pathway.

Modulation of ongoing EMG by different classes of low-threshold mechanoreceptors in the human hand

1. We have previously demonstrated that the input from single FA I and SA II cutaneous mechanoreceptors in the glabrous skin of the human hand is sufficiently strong to modulate ongoing EMG of muscles acting on the digits. Some unresolved issues have now been addressed. 2. Single cutaneous (n = 60), joint (n = 2) and muscle spindle (n = 34) afferents were recorded via tungsten microelectrodes inserted into the median and ulnar nerves at the wrist. Spike-triggered averaging was used to investigate synaptic coupling between these afferents and muscles acting on the digits. The activity of 37 % of FA I (7/19), 20 % of FA II (1/5) and 52 % of SA II afferents (11/21) evoked a reflex response. The discharge from muscle spindles, 15 SA I and two joint afferents did not modulate EMG activity. 3. Two types of reflex responses were encountered: a single excitatory response produced by irregularly firing afferents, or a cyclic modulation evoked by regularly discharging afferents. Rhythmic stimulation of one FA I afferent generated regularly occurring bursts which corresponded to the associated cyclic EMG response. 4. Selectively triggering from the first or last spike of each burst of one FA I afferent altered the averaged EMG profile, suggesting that afferent input modulates the associated EMG and not vice versa. 5. The discharge from single FA I, FA II and SA II afferents can modify ongoing voluntary EMG in muscles of the human hand, presumably via a spinally mediated oligosynaptic pathway. Conversely, we saw no evidence of such modulation by SA I, muscle spindle or joint afferents.

Evidence for strong synaptic coupling between single tactile afferents and motoneurones supplying the human hand

1. Electrical stimulation of digital nerves elicits short-latency excitatory and inhibitory spinal reflex responses in ongoing EMG in muscles acting on the fingers and thumb. Similar responses are elicited by stimulating a population of muscle spindles but not when a single muscle spindle is activated. The current study investigated whether short-latency EMG responses could be evoked from the discharge of a single cutaneous afferent. 2. Thirty-three tactile afferents were recorded via tungsten microelectrodes in the median nerve of awake humans. Spike-triggered averaging revealed EMG events time-locked to the afferent discharge. The afferents were activated by an external probe and the EMG was elicited by a weak voluntary contraction. 3. Eleven cutaneous afferents (33 %) showed a short-latency response in the ongoing EMG. Overt increases or decreases in EMG were observed for seven afferents (onset latency 20.0-41.1 ms). For four slowly adapting (SA) type II afferents, EMG showed a periodicity that was correlated to the afferent interspike interval (r = 0.99). 4. The EMG associated with two rapidly adapting (FA) type I afferents (29 %) showed a short-latency excitation while five showed neither excitation nor inhibition. Seven SA II afferents (39 %) showed excitation and 11 no response; and none of the six SA I afferents showed any response. 5. We conclude that, unlike muscle spindle afferents, the input from a single cutaneous afferent is strong enough to drive, via interneurones, motoneurones supplying muscles acting on the digits. The potent short-latency response we found supports the important role of cutaneous mechanoreceptors in fine motor control of the human hand.

Evaluation of reciprocal inhibition of the soleus H-reflex during tonic plantar flexion in man

Changes in reciprocal inhibition from ankle dorsiflexors to ankle plantar flexors were evaluated at increasing levels of tonic plantar flexion in 11 healthy subjects. Stimulation of the common peroneal nerve (CPN) evoked a short-latency depression of the rectified and averaged soleus electromyogram (average latency of depression: 40 ms) and a short-latency inhibition of the soleus H-reflex (conditioning-test interval: 2-3 ms). When the intensity of the CPN stimulation was below approximately 1.2 x motor threshold (x MT) the inhibition of both the soleus EMG (expressed as the amount of EMG during the inhibition as percentage of the background EMG) and the soleus H-reflex (expressed as the size of the conditioned reflex as percentage of the control H-reflex size) were seen to decrease with increasing levels of plantar flexion. At intensities of stimulation higher than approximately 1.2 x MT the inhibition of the EMG and the H-reflex was very strong and was not modulated with contraction. It is suggested that the decrease of reciprocal inhibition with increasing levels of plantar flexion is due to a decreased excitability of the Ia inhibitory interneurones which are responsible for the inhibition. It is emphasized that submaximal stimulation is necessary to demonstrate this modulation of inhibition and that the functional contribution of reciprocal inhibition to motor performance cannot be revealed from the amount of inhibition evoked by artificial electrical stimulation of a peripheral nerve.

Coupling between single muscle spindle afferent and EMG in human wrist extensor muscles: physiological evidence of skeletofusimotor (beta) innervation

OBJECTIVES

This study was conducted to find physiological evidence of skeletofusimotor innervation in man.

METHODS

Discharges of 38 single muscle spindle afferents from m. extensor carpi radialis were recorded from 9 subjects during steady isometric contractions of wrist extension. Correlation between these afferents and rectified surface EMG was investigated by estimating cumulant density function.

RESULTS

In the cumulant density estimate between spindle afferent and EMG, a positive EMG peak was obtained prior to afferent firing between -30 and -10 ms in 15 afferents (39%).

CONCLUSIONS

The present finding indicates coupling between spindle afferents and extrafusal activity and suggests the widespread skeletofusimotor innervation in human muscle spindle.

Multiple action potential waveforms of single units in man as signs of variability in conductivity of their myelinated fibres

Percutaneous microneurography was performed with concentric needle electrodes to record neural activity from myelinated fibres in human peripheral nerves. Template matching techniques were used together with interspike interval analysis and studies on functional class, receptive field characteristics, conduction velocities and other single fibre properties to classify single units. Sometimes the same fibres exhibited different action potentials at the same time. The potentials had some common features, but differed either in their waveform types or only in duration. There was a correlation between the occurrence of the different potential shapes and firing frequency of the studied unit. The outcome of the studies suggested that there was a common denominator which could explain the observations. Most likely, momentary fluctuations in excitability of the myelinated fibres occurring during the relative refractory period or the supernormal period were responsible for the variations in complexity of the studied units due to a partial block of fibre propagation probably caused by the recording electrode. Thus, action potentials deriving from the same axon may not always have the same shapes. Methods for unit classification, such as template matching, are discussed in the light of our findings.

Responses of human masseter motor units to stretch

1. The reflex responses to stretch were studied in single motor units and the surface electromyogram in human masseter. 2. Controlled stretches of the isometrically contracting jaw-closing muscles evoked short-latency (10-15 ms) and long-latency (35-70 ms) excitatory reflex responses in the masseter surface electromyogram. 3. The majority (65%) of tonically active masseter motor units were excited in both short- and long-latency phases of the reflex. The timing of the stimulus determined whether the unit discharged in the short- or long-latency phase. If a non-tonically active motor unit was recruited by the stimulus, it invariably discharged in the long-latency phase. 4. Although short-latency responses were strongly time-locked to the stimulus, there was very little shortening of interspike intervals (ISIs) in this phase of the reflex. The shortening of ISIs was more prominent and prolonged during the long-latency phase, which explains why this phase produces most of the reflex force changes following the stretch. 5. Within pairs of concurrently active motor units there was a tenfold range in the size of the short-latency response to the same stretch. 6. A substantial proportion (35%) of the twenty-two masseter motor units tested had no statistically significant short-latency reflex response. 7. In contrast to other human muscles, there was no functional connection between a population of Ia afferents and some masseter motoneurons. There are two possible explanations for this result. The short-latency, presumably monosynaptic, Ia afferent inputs may not be uniformly distributed to human masseter motoneurons. Alternatively, these inputs may be subject to tonic presynaptic inhibition that is not uniformly distributed throughout the masseter motoneuron pool.

Lateral dominance and motor unit firing behavior

Twelve subjects were classified as left-handed (LH) or right-handed (RH) using Annett's hand dominance classification. Motor unit recordings were obtained from the first dorsal interosseous (FDI) muscle of each hand using a quadrifilar needle electrode. Firing occurrences of individual motor units were then identified and the firing rates of all motor units recorded during the contraction were cross-correlated. The results demonstrated significantly greater firing rate cross-correlation scores in the dominant hand than in the non-dominant hand for both LH and RH subjects. This association between hand dominance and the common drive of motor unit firing rates lends credence to the idea that one or more CNS sites may influence conjoint motor unit firing behavior.

Microneurography for the recording and selective stimulation of afferents: an assessment

The purpose of this study was to examine whether a microneurography electrode could record from and then selectively stimulate the same afferent fiber in cat sural and tibial nerves. Fiber activity was recorded distally with microneurography needle electrodes and proximally with hook electrodes. Records from the hook electrodes allowed the waveform shape and latency following natural stimulation to be compared with that produced by electrical stimuli delivered through the needle electrode. The action potentials from approximately 50% of the recorded fibers failed to propagate beyond the needle electrode. When propagation did occur and the comparison could be made, only 25% of the afferents recorded could be selectively stimulated. This relatively low success rate suggests that a typical microneurography needle is not particularly good for selectively stimulating identified afferents. Thus in human experiments, attempts to microstimulate identified afferents may often include stimulation of unidentified alternative, or additional, afferent fibers at thresholds of sensory perception.