Dynamic characteristics of response in a vestibulomotor reflex

Automaticity of Postural Control while Dual-tasking Revealed in Young and Older Adults

Background: Postural control improvements in external focus and cognitive task conditions are thought to occur because directing attention away from postural control allows greater automaticity. We aimed to support this theory by using three dynamic measures of postural control that may reveal changes in the structure or composition of sway: the discrete wavelet transform, sample entropy, and rambling trembling analyses.Methods: We analyzed the center of pressure data from twenty-two healthy young adults (20.8 ± 2.82 years) and twenty healthy older adults (69.02 ± 3.47 years). Participants stood with feet together in five conditions: baseline standing, internal focus, external focus, easy cognitive task, and difficult cognitive task. Analyses of variance were used to examine the effect of condition and age on the three dynamic measures.Results: The wavelet transform revealed a shift toward greater contributions from higher frequency bands in cognitive task conditions, suggesting greater automaticity. Sample entropy was higher in cognitive task conditions, suggesting more complex sway and automatic control. The external focus and difficult cognitive tasks increased trembling in young adults, suggesting increased contributions from spinal reflex components.Conclusion: Results support the theory that stability improvements in cognitive task conditions were due to automaticity in young and older adults. They also suggest that tasks that are more difficult are better at promoting automaticity than tasks requiring less cognitive involvement.

Inhibitory connections of ipsilateral semicircular canal afferents onto Renshaw cells in the lumbar spinal cord of the cat

1. In intercollicularly decerebrate cats, the excitability of lumbar spinal Renshaw cells (tested by single shocks to ventral roots and deafferented muscle nerves) decreased for 600-1000 ms after conditioning electrical stimulation of ipsilateral semicircular canal nerves. 2. Conditioning stimulation of posterior canal afferents and combined stimulation of anterior and lateral canal afferents were equally effective in causing inhibition of Renshaw cells. No significant differences were observed for Renshaw cells excitable from hind-limb flexor or extensor nerves. 3. Inhibition appeared when one to five stimuli were applied to the canal afferents and arrived at the spinal segmental level 11-15 ms after the onset of conditioning stimulation. 4. Evidence is adduced to suggest that the inhibitory effects on Renshaw cells following stimulation of semicircular canal afferents were mediated directly, i.e. they were not caused by alterations of motoneurone activity. 5. Excitation of Renshaw cells due to stimulation of the canal afferents was rarely observed; it could not be excluded that it was secondary to motoneurone discharges. 6. It is suggested that vestibular inhibition of Renshaw cells ensure a high gain of hind-limb alpha-motoneurones during postural adjustments following a massive disturbance of body equilibrium.

The spectral characteristics of postural sway behaviour

To determine whether recognisable profiles of sway behaviour are characteristically found among normal subjects the energy content of a contiguous series of bandwidths were computed from power density spectra of the sway behaviour of 29 young men and 29 young women. No significant differences were observed over time either in the energy content or in the profile shape of the power spectral density (PSD) records when these were subjected to a multivariate profile analysis of variance. It was observed that the majority of energy in the power spectrum, viz. 90% (+/- 4%) in the antero-posterior and 95% (+/- 3%) in the lateral direction, was below 2.0 Hz. Three dominant frequencies of sway were identifiable in 93% of subjects. The modal frequencies in the antero-posterior direction were 0.30-0.45 Hz (primary), 0.60-0.75 Hz (secondary) and 1.05-1.20 Hz (tertiary), and in the lateral direction 0.30-0.45 Hz (primary), 0.45-0.60 Hz (secondary) and 0.75-0.90 Hz (tertiary). These three dominant frequencies were distributed over the full frequency range. An explanation of their origins was not sought but the suggestion is made that they could be driven by common dynamic physiological events. The patterns of energy distribution were simplified and quantified by the calculation of a difference-ratio index. The broad dispersion of the values of this index precludes the suggestion that there could be any clustering of profiles. It is concluded that the frequently postulated clustering of the sway patterns of young adults into recognisable and distinct profiles is without firm foundation. Instead it is suggested that sway behaviour is probably normally and randomly distributed about common respiratory, cardiac and perhaps also other dynamic physiological events.

Supraspinal and segmental input to lumbar epaxial motoneurons in the rat

Inputs to medial longissimus (ML) and lateral longissimus (LL) motoneurons were studied in urethane or urethane-chloralose anesthetized rats by recording from ML and LL nerves while stimulating ipsilateral lumbosacral dorsal roots, medial medullary reticular formation (RF), vestibular nuclei (VN), dorsal midbrain (MDBR), or ventromedial hypothalamus (VMH). Stimulation of appropriate dorsal roots produced short-latency responses (1.5-3.0 ms) in nerves to medial longissimus or lateral longissimus. The connections underlying these responses, which could be monosynaptic, are weak, since generally two or more stimuli were necessary for a response to occur. Short-latency LL nerve responses required more dorsal root stimuli than did ML nerve responses and stable LL responses sometimes could not be obtained, suggesting that segmental reflexes to a back muscle (LL) could be weaker than those to a proximal tail muscle (ML). Trains of conditioning stimuli delivered to the RF, VN, and MDBR facilitated segmental responses in ML nerves or LL nerves. Temporal profiles of facilitation of ML differed for the three regions. On one extreme, the facilitation produced by RF conditioning required few stimuli (median, 3 shocks) and peak facilitation occurred at short condition-test intervals (median, 1.5 ms). On the other extreme, facilitation produced by MDBR conditioning required long trains (median, 14 stimuli) and peak facilitation occurred at longer condition-test intervals (median, 10 ms). Stimulation within the VMH never facilitated ML or LL nerve activity. These results demonstrate excitatory connections from reticular formation, vestibular nuclei and the dorsal midbrain to medial longissimus and lateral longissimus. Such connections could be involved in behaviors mediated by midbrain, and in postural regulation through brain stem control of axial musculature. Motoneuron cell bodies for LL, ML and lumbar transversospinalis (TS) muscles were localized by ejecting dye at sites where unitary antidromic responses to muscle nerve stimulation were recorded extracellularly. ML cells were found ventrolaterally in the L6-S1 ventral horn. LL and TS cells were found medially in the ventral horn of the lumbar enlargement.

Dynamic relations between natural vestibular inputs and activity of forelimb extensor muscles in the decerebrate cat. I. Motor output during sinusoidal linear accelerations

Decerebrate cats were subjected to sinusoidal linear accelerations along the animal's horizontal and vertical axes, while recording the EMG activity of both triceps brachii muscles. This activity was found to be sinusoidally modulated in response to the accelerations and thus phase and gain relations between motor output and input acceleration could be obtained. They were found to be the same for accelerations along each of the three axes. In particular the gain dropped by 14-20 dB over a frequency range from 0.2 to 1.0 Hz and the phase of the motor output showed a lag of 40-60 degrees at 1.0 Hz. Thus, it was concluded that (1) the dynamic behavior of utricular and saccular receptors is the same, (2) the changes in motor activity observed during accelerations along the vertical axis are mostly due to the activation of saccular afferents, and (3) the motor output cannot simply result from vestibular afferent activities being relayed directly to the spinal motoneurons via the vestibulo-spinal tracts.