Comparison of the depression of H-reflexes following previous activation in upper and lower limb muscles in human subjects

vation depression of the soleus H reflex measured using threshold tracking. J Neurophysiol 100: 3275-3284, 2008. First published October 15, 2008 doi:10.1152/jn.90435.2008. The interpretation of changes in the soleus H reflex is problematic in the face of reflex gain changes, a nonlinear input/output relationship for the motoneuron pool, and a nonhomogeneous response of different motoneurons to afferent inputs. By altering the stimulus intensity to maintain a constant reflex output, threshold tracking allows a relatively constant population of ␣-motoneurons to be studied. This approach was used to examine postactivation ("homosynaptic") depression of the H reflex (HD) in 23 neurologically healthy subjects. The H reflex was elicited by tibial nerve stimulation at 0.05, 0.1, 0.3, 1, and 2 Hz at rest and during voluntary plantar flexion at 2.5, 5, and 10% of maximum. A computerized threshold tracking procedure was used to set the current needed to generate a target H reflex 10% of M max . The current needed to produce the target reflex increased with stimulus rate but not significantly beyond 1 Hz. In three subjects, the current needed to produce H reflexes of 5, 10, 15, and 20% M max at 0.3, 1, and 2 Hz increased with rate and with the size of the test H reflex. HD was significantly reduced during voluntary contractions. Using threshold tracking, HD was maximal at lower frequencies than previously emphasized, probably because HD is greater the larger the test H reflex. This would reinforce the greater sensitivity of small motoneurons to reflex inputs.

Section: Hd Studied With Threshold Trackingcontrasting

confidence: 50%

Section: Hd Studied With Threshold Trackingmentioning

confidence: 85%

Section: R E S U L T S H Reflex Threshold Current At Restmentioning

confidence: 97%

See 1 more Smart Citation

Postactivation Depression of the Soleus H Reflex Measured Using Threshold Tracking

McNulty

Jankelowitz²,

Wiendels³

et al. 2008

“…It has been shown that the amplitude of H-reflex diminishes when stimuli are closely spaced in time. On the upper limb, the recovery between two stimulations is achieved in Ͻ1 s (Rossi-Durand et al 1999). In our experiment, the ISI could fluctuate according to the time necessary for subjects to initiate the task or follow the indicated path.…”

Section: Methodological Issuesmentioning

confidence: 99%

Physiological Basis of Limb-Impedance Modulation During Free and Constrained Movements

Damm

McIntyre²

2008

Damm L, McIntyre J. Physiological basis of limb-impedance modulation during free and constrained movements. J Neurophysiol 100: 2577-2588, 2008. First published August 20, 2008 doi:10.1152/jn.90471.2008. Arm stiffness is a critical factor underlying stable interactions with the environment. When the hand moves freely through space, a stiff limb would most effectively maintain the hand on the desired path in the face of external perturbations. Conversely, when constrained by a rigid surface, a compliant limb would allow the surface to guide the hand while minimizing variations in contact forces. We aimed to identify the physiological basis of stiffness adaptation for these two classes of movement. Stiffness can be regulated by two mechanisms: coactivation of antagonistic muscles and modulation of reflex gains. We hypothesized that subjects would select high stiffness (high coactivation and/or reflex gains) in free space and high compliance (low coactivation and reflex gains) for constrained movements. We measured EMG and the H-reflex during constrained and unconstrained movement of the wrist. As predicted, subjects coactivated antagonist muscles more when performing the unconstrained movement. Contrary to our hypothesis, however, Hreflex amplitude was higher for the constrained movement despite the a priori preference for lower reflex gains in this situation. In addition, the H-reflex depended on the task and the net force exerted by the limb on the environment, rather than showing a simple dependence on the level of muscle activation. Thus stiffness seems to increase in free space compared with constrained motion through the use of coactivation, whereas spinal loop gains are adjusted to better regulate the influence of afferences on the ongoing movement. These observations support the hypothesis of movement programming in terms of impedance.

“…The ISI was randomized from 4 to 70 ms in a uniform distribution. The interval between each double-stimulus sequence was set to 1 s, given that the H-reflex amplitude can fully recover within 200 ms (Rossi-Durand et al 1999). A total of 800 -1,000 double stimulations were applied.…”

Section: Experimental Data Collection and Analysismentioning

confidence: 99%

Estimating the time course of population excitatory postsynaptic potentials in motoneurons of spastic stroke survivors

Suresh

Rymer

2015

Hu X, Suresh NL, Rymer WZ. Estimating the time course of population excitatory postsynaptic potentials in motoneurons of spastic stroke survivors. J Neurophysiol 113: 1952-1957, 2015. First published December 24, 2014 doi:10.1152/jn.00946.2014.-Hyperexcitable motoneurons are likely to contribute to muscle hypertonia after a stroke injury; however, the origins of this hyperexcitability are not clear. One possibility is that the effective duration of the Ia excitatory postsynaptic potential (EPSP) is prolonged, increasing the potential for temporal summation of EPSPs, making action potential initiation easier. Accordingly, the purpose of this study was to quantify the time course of EPSPs in motoneurons of stroke survivors. The experimental protocol, which was based on parameters derived from simulation, involved sequential subthreshold electrical stimuli delivered to the median nerve of hemispheric stroke survivors. The resulting H-reflex responses were recorded in the flexor carpi radialis muscle. H-reflex response probability was then used to quantify the time course of the underlying EPSPs in the motoneuron pool. A population EPSP was estimated based on the probability of evoking an H reflex from the second electrical stimulus in the absence of a reflex response to the first stimulus. The accuracy of this time-course estimate was quantified using a computer simulation that explored a range of feasible EPSP parameters. Our experimental results showed that in all five hemispheric stroke survivors the rate of decay of the population EPSP was consistently slower in spastic compared with the contralateral motoneuron pools. We propose that one potential mechanism for hyperexcitability of motoneurons in spastic stroke survivors may be linked to this prolongation of the Ia EPSP time course. Our subthreshold double-stimulation approach also provides a noninvasive tool for quantifying the time course of EPSPs in both healthy and pathological conditions. EPSP time course; stroke; spasticity; reflex; double stimulation HYPEREXCITABILITY OF SPINAL motoneurons is potentially a major contributor to muscular hypertonia after a brain injury, such as a hemispheric stroke (Katz and Rymer 1989). This hyperexcitability can be induced by a change in either the passive or active electrical properties of these neurons, which can lead to sustained voltage changes for a given synaptic current input. These abnormal voltage changes can be captured as apparent alterations in the amplitude and time course of the excitatory postsynaptic potential (EPSP), namely a larger amplitude and/or longer decay than in normal EPSPs.The amplitude and time course of the EPSP can be measured directly in reduced animal models via intracellular recordings (Fetz and Gustafsson 1983). In contrast, such EPSP properties can only be inferred indirectly in humans, based on changes in the discharge probability of single or grouped motoneurons in response to controlled afferent stimulation (Ashby and Zilm 1982;Suresh et al. 2005;Turker and Cheng 1994). There have ...