Firing properties of muscle spindles supplying the intrinsic muscles of the foot in unloaded and free-standing humans

Human posture and locomotion are dependent on the sensory apparatus – involving muscle spindles, cutaneous afferents and the vestibular system – that provides proprioception. In my previous work with my Bachelor of Medical Research, I investigated the relationship between galvanic vestibular stimulation and the sensitivity of muscle spindles of the long muscles of the leg. While that study showed no correlation between these systems it was limited by the lack of subject postural threat. In order to record from muscle spindles directly during unsupported free-standing, a new methodology for microneurographic recording from the posterior tibial nerve at the ankle was developed. For the first time, we have been able to identify the firing properties of muscle spindle endings in the small (intrinsic) muscles of the foot, as well as mechanoreceptors in the skin of the sole, while the participant is standing unsupported. This thesis presents this methodology along with the recordings made. In Study 1, the firing properties of 26 muscle spindles supplying the intrinsic muscles of the foot are described in unloaded conditions. Their responsiveness to stretch and related joint movements is shown to be similar to those in the short muscles in the hand and the long leg muscles. Only 27% were spontaneously active, of which there was no consistent resting firing rate or discharge variability. In Study 2, activity from 12 muscle spindles supplying the intrinsic foot muscles in unsupported free-standing conditions is described. In this group 50% were spontaneously firing and 67% had activity correlated with changes of centre of pressure recorded by a force plate, primarily (88%) along the anteroposterior axis. In Study 3, the activity of 28 multiunit cutaneous afferent recordings, as well as of 15 single-unit cutaneous afferents, supplying the sole of the foot in unsupported free standing is described. Activity of cutaneous afferents was found to be dependent on receptor type and location of receptive field. The data presented in this report is proof of this novel methodology’s suitability for detailed study into the sensory sources in the foot contributing to maintaining the upright posture

Analysis and design methods for energy geostructures

Based on discussions at the international workshop on “Thermoactive geotechnical systems for near-surface geothermal energy”, hosted at École Polytechnique Fédérale de Lausanne (EPFL), Switzerland (http://www.olgun.cee.vt.edu/workshop/), this article attempts to provide a broad overview of the analysis methods used for evaluation of systems that use either boreholes or geo-structures for heat exchange. It identifies commonalities where knowledge transfer from the former to the latter can be made, and highlights where there are significant differences that may limit this cross-fertilisation. The article then focusses on recent developments and current understanding pertaining to the analysis of the thermo-mechanical interaction between a geostructure and the ground, and how this may be incorporated into the geotechnical design of energy geostructures

Assessing psychological flexibility: Validation of the open and engaged state questionnaire

Background: Despite increasing research on psychological flexibility (PF) and its importance to psychological health, only a few measures exist for assessing this construct, and they have shortcomings, particularly in construct validity.Methods: This study assessed the factor structure, construct validity, and predictive value of the Open and Engaged State Questionnaire (OESQ) on samples with panic disorder and/or agoraphobia (n=120), panic disorder with agoraphobia (n=46), and burnout (n=85).Results: The confirmatory factor analysis verified the expected one-dimensional structure and found good internal consistency in all three samples. Analysis of the construct validity revealed correlations to pathology, personality traits, and total functioning; it also revealed discrimination of PF from neuroticism when measured with the OESQ. Furthermore, we found that PF predicted outcomes of symptomatology.Conclusions: Considering the criticisms and suggestions for improvement in the literature on assessing PF, the OESQ proved to be a valuable operationalization of this construct. With better discriminant and incremental validity compared to other questionnaires as well as unique predictive value, the OESQ is important for future research on PF and acceptance and commitment therapy (ACT)

Functional properties of human muscle spindles

Muscle spindles are ubiquitous encapsulated mechanoreceptors found in most mammalian muscles. There are two types of endings, primary and secondary, and both are sensitive to changes in muscle length and velocity, with the primary endings having a greater dynamic sensitivity. Unlike other mechanoreceptors in the somatosensory system, muscle spindles are unique in possessing motor innervation, via y motoneurons (fusimotor neurons), that control their sensitivity to stretch. Much of what we know about human muscles spindles comes from studying the behavior of their afferents via intraneural microelectrodes (microneurography) inserted into accessible peripheral nerves. Here we review the functional properties of human muscle spindles, comparing and contrasting with what we know about the functions of muscle spindles studied in experimental animals. As in the cat, many human muscle spindles possess a background discharge that is related to the degree of muscle stretch, but mean firing rates are much lower (~10Hz). They can faithfully encode changes in muscle fascicle length in passive conditions, but higher-level extraction of information is required by the central nervous system to measure changes in muscle length during muscle contraction. Moreover, while there is some evidence supporting independent control of human muscle spindles via fusimotor neurons, any effects are modest compared to the clear independent control of fusimotor neurons observed in the cat

The vestibular system does not modulate fusimotor drive to muscle spindles in relaxed leg muscles of subjects in a near-vertical position

It has been shown that sinusoidal galvanic vestibular stimulation (sGVS) has no effect on the firing of spontaneously active muscle spindles in either relaxed or voluntarily contracting human leg muscles. However, all previous studies have been conducted on subjects in a seated position. Given that independent vestibular control of muscle spindle firing would be more valuable during postural threat, we tested the hypothesis that this modulation would become apparent for subjects in a near vertical position. Unitary recordings were made from 18 muscle spindle afferents via tungsten microelectrodes inserted percutaneously into the common peroneal nerve of awake human subjects laying supine on a motorized tilt table. All recorded spindle afferents were spontaneously active at rest and each increased its firing rate during a weak static contraction. Sinusoidal bipolar binaural galvanic vestibular stimulation (±2 mA, 100 cycles) was applied to the mastoid processes at 0.8 Hz. This continuous stimulation produced a sustained illusion of “rocking in a boat” or “swinging in a hammock” but no entrainment of EMG. The subject was then moved into a near-vertical position (75o) and the stimulation repeated. Despite robust vestibular illusions, none of the fusimotor-driven spindles exhibited phase-locked modulation of firing during sinusoidal GVS in either position. We conclude that this dynamic vestibular stimulus was insufficient to modulate the firing of fusimotor neurones in the near-vertical position. However, this does not mean that the vestibular system cannot modulate the sensitivity of muscle spindles via fusimotor neurones in free unsupported standing, when reliance on proprioceptive feedback is higher

Microneurography from the posterior tibial nerve : a novel method of recording activity from the foot in freely standing humans

The posterior tibial nerve, located behind the medial malleolus of the ankle, supplies the intrinsic muscles of the foot and most of the skin of the sole. We describe a novel approach for recording from this nerve via a percutaneously inserted tungsten microelectrode, and provide examples of recordings from presumed muscle spindle endings recorded in freely-standing human subjects. The fact that the angular excursions of the ankle joint are small as the foot is loaded during the transition from the seated position to standing means that one can obtain stable recordings of neural traffic in unloaded, loaded and freely standing conditions. We conclude that this novel approach will allow studies that will increase our understanding of the roles of muscle and cutaneous afferents in the foot in the control of upright posture

Firing properties of muscle spindles supplying the intrinsic foot muscles of humans in unloaded and free-standing conditions

We recently developed an approach to record from muscle spindles in the intrinsic muscles of the foot in free-standing humans by inserting a tungsten microelectrode into the posterior tibial nerve behind the medial malleolus of the ankle. Here, we characterize the behaviour of muscle spindles in the small muscles of the foot in (i) seated subjects with the leg horizontal and foot naturally plantarflexed and (ii) in standing subjects. In the first study recordings were made from 26 muscle spindle afferents located within flexor digiti minimi brevis (n=4), abductor digiti minimi (n=3), quadratus plantae (n=3), plantar interossei (n=4), flexor digitorum brevis (n=3), dorsal interossei (n=2) and lumbricals (n=2), with one each supplying abductor hallucis, adductor hallucis and flexor hallucis brevis. The identity of another two muscle afferents was unknown. The majority of the units were silent at rest; only 7 (27%) being spontaneously active. Because of the anatomic constraints of the foot, some spindles supplying muscles acting on the toes responded to movements of one or more digits. In the second study 12 muscle spindle afferents were examined during standing. The ongoing discharge of 8 spindle afferents covaried with changes in the centre of pressure during postural sway. We conclude that the majority of spindle endings in the small muscles of the foot are silent at rest, which may allow them to encode changes in conformation of the foot when it is loaded during standing. Moreover, these muscle spindle afferents can provide useful proprioceptive information during standing and postural sway

Random-amplitude sinusoidal linear acceleration causes greater vestibular modulation of skin sympathetic nerve activity than constant-amplitude acceleration

We tested the hypothesis that random variations in the magnitude of sinusoidal linear acceleration cause greater modulation of skin sympathetic nerve activity (SSNA), but not muscle sympathetic nerve activity (MSNA), than sinusoidal stimuli of the same frequency but constant amplitude. Subjects (n = 22) were seated in a sealed room mounted on a linear motor that could deliver peak sinusoidal accelerations of 30 mG in the antero-posterior direction. Subjects sat on a padded chair with their neck and head supported vertically, thereby minimizing somatosensory cues, facing the direction of motion in the anterior direction. Each block of sinusoidal motion was delivered at 0.2 Hz, either with a constant-amplitude (root mean square 14 mG) or randomly fluctuating amplitudes of the same mean amplitude. MSNA (n = 12) and SSNA (n = 10) were recorded via tungsten microelectrodes inserted into muscle or cutaneous fascicles of the common peroneal nerve. Cross-correlation analysis was used to measure the magnitude of vestibular modulation. The modulation index for SSNA was significantly higher during delivery of random vs constant-amplitude acceleration (31.4 ± 1.9 vs 24.5 ± 2.5%), but there was no significant difference in the modulation indices for MSNA (28.8 ± 2.9 vs 33.4 ± 4.1%). We conclude that the pattern of vestibular stimulation affects the magnitude of modulation of sympathetic outflow to skin but not to muscle. Presumably, this is related to the subperceptual development of nausea, which is known to be associated with greater vestibular modulation of SSNA but not MSNA