Vestibular Contributions to Primate Neck Postural Muscle Activity during Natural Motion.

To maintain stable posture of the head and body during our everyday activities, the brain integrates information across multiple sensory systems. Here, we examined how the primate vestibular system, independently and in combination with visual sensory input, contributes to the sensorimotor control of head posture across the range of dynamic motion experienced during daily life. We recorded activity of single motor units in the splenius capitis and sternocleidomastoid muscles in rhesus monkeys during yaw rotations spanning the physiological range of self-motion (up to 20 Hz) in darkness.

Soleus responses to Achilles tendon stimuli are suppressed by heel and enhanced by metatarsal cutaneous stimuli during standing.

Key Points: We examined the influence of cutaneous feedback from the heel and metatarsal regions of the foot sole on the soleus stretch reflex pathway during standing. We found that heel electrical stimuli suppressed and metatarsal stimuli enhanced the soleus vibration response. Follow-up experiments indicated that the interaction between foot sole cutaneous feedback and the soleus vibration response was likely not mediated by presynaptic inhibition and was contingent upon a modulation at the ⍺-motoneuron pool level.

Lower-limb muscle responses evoked with noisy vibrotactile foot sole stimulation.

Aim: Cutaneous feedback from the foot sole contributes to the control of standing balance in two ways: it provides perceptual awareness of tactile perturbations at the interface with the ground (e.g., shifts in the pressure distribution, slips, etc.

Soleus single motor units show stronger coherence with Achilles tendon vibration across a broad bandwidth relative to medial gastrocnemius units while standing.

To probe the frequency characteristics of somatosensory responses in the triceps surae muscles, we previously applied suprathreshold noisy vibration to the Achilles tendon and correlated it with ongoing triceps surae muscle activity (recorded via surface EMG) during standing. Stronger responses to tendon stimuli were observed in soleus (Sol) relative to medial gastrocnemius (MGas) surface EMG; however, it is unknown whether differences in motor unit activity or limitations of surface EMG could have influenced this finding. Here, we inserted indwelling EMG into Sol and MGas to record the activity of single motor units while we applied noisy vibration (10-115 Hz) to the right Achilles tendon of standing participants.

Reliability of the diaphragmatic compound muscle action potential evoked by cervical magnetic stimulation and recorded via chest wall surface EMG.

Introduction: Stimulation of the phrenic nerve via cervical magnetic stimulation (CMS) elicits a compound muscle action potential (CMAP) that allows for assessment of diaphragm activation. The reliability of CMS to evoke the CMAP recorded by chest wall surface EMG has yet to be comprehensively examined.

Methods: CMS was performed on healthy young males (n=10) and females (n=10).

Frequency characteristics of human muscle and cortical responses evoked by noisy Achilles tendon vibration.

Noisy stimuli, along with linear systems analysis, have proven to be effective for mapping functional neural connections. We explored the use of noisy (10-115 Hz) Achilles tendon vibration to examine somatosensory reflexes in the triceps surae muscles in standing healthy young adults ( = 8). We also examined the association between noisy vibration and electrical activity recorded over the sensorimotor cortex using electroencephalography.

Cutaneous afferent feedback from the posterior ankle contributes to proprioception.

Cutaneous mechanoreceptors in skin surrounding joints can respond to the skin strain generated by movement, and thus provide proprioceptive cues. The objective of this experiment was to determine the contribution of skin feedback from the posterior ankle to position sense during passive movements. In 28 healthy adults (12 male), a topical anesthetized (n=14) or placebo cream (n=14) was applied to an area of skin on the posterior ankle that undergoes stretch and compression during ankle dorsi- and plantar-flexion.

Vibrotactile stimulation of fast-adapting cutaneous afferents from the foot modulates proprioception at the ankle joint.

It has previously been shown that cutaneous sensory input from across a broad region of skin can influence proprioception at joints of the hand. The present experiment tested whether cutaneous input from different skin regions across the foot can influence proprioception at the ankle joint. The ability to passively match ankle joint position (17° and 7° plantar flexion and 7° dorsiflexion) was measured while cutaneous vibration was applied to the sole (heel, distal metatarsals) or dorsum of the target foot.

Foot sole skin vibration perceptual thresholds are elevated in a standing posture compared to sitting.

Foot sole sensitivity is commonly assessed while individuals are seated or prone; however the primary role of foot sole cutaneous feedback is for the control of upright stance and gait. The aim of this study was to compare vibration perceptual thresholds across the foot sole between sitting and standing postures. Vibration perceptual thresholds were measured in sitting and standing postures in 18 healthy participants (8 male) using a custom vibration device.

Reliability of the Achilles tendon tap reflex evoked during stance using a pendulum hammer.

The tendon tap reflex (T-reflex) is often evoked in relaxed muscles to assess spinal reflex circuitry. Factors contributing to reflex excitability are modulated to accommodate specific postural demands. Thus, there is a need to be able to assess this reflex in a state where spinal reflex circuitry is engaged in maintaining posture.

Thresholds of cutaneous afferents related to perceptual threshold across the human foot sole.

Perceptual thresholds are known to vary across the foot sole, despite a reported even distribution in cutaneous afferents. Skin mechanical properties have been proposed to account for these differences; however, a direct relationship between foot sole afferent firing, perceptual threshold, and skin mechanical properties has not been previously investigated. Using the technique of microneurography, we recorded the monofilament firing thresholds of cutaneous afferents and associated perceptual thresholds across the foot sole.