Human motoneuron responses to group 1 volleys blocked presynaptically by vibration (original) (raw)
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Responses of human soleus motor units to low-threshold stimulation of the tibial nerve
Experimental Brain Research, 2011
The peristimulus frequencygram (PSF) has recently been shown to illustrate postsynaptic potentials of motoneurones much more reliably than the peristimulus time histogram (PSTH). The aim of this investigation was to examine the profile of the postsynaptic potential (PSP) in soleus motoneurones in response to an H-reflex with and without accompanying M waves of different magnitude by using PSTH and PSF profiles of single motor units. Nine men and five women healthy subjects participated in this study. Electrical stimuli were delivered to the tibial nerve in the popliteal fossa. The reflex response of the soleus muscle was recorded using both surface electromyogram and single motor unit potentials. The PSTH analysis demonstrated that there were four different synaptic events following low-intensity stimulation of the tibial nerve: primary enhancement in firing probability (H-reflex or E1), primary reduction in firing probability (primary silent period or SP1), secondary reduction in firing probability (secondary silent period or SP2), and secondary enhancement in firing probability (E2). On the other hand, the PSF analysis indicated only two reflex responses, long-lasting enhancement in discharge rate including the H-reflex (LLE) and long-lasting decrease in discharge rate (LLD). The results of the two analyses methods are compared and contrasted. While the PSTH demonstrated that there was a silent period (SP1) immediately following the H-reflex, the PSF indicated an increase in discharge rate during the same period. The PSF also indicated that, during SP2 and E2, the discharge rate actually decreased (LLD). It was therefore suggested that LLD involved activation of several inhibitory pathways including the autogenic inhibition of units via the Golgi tendon organs. It was concluded that the PSF could indicate the details of the postsynaptic potentials and is very useful for bringing out previously unknown effects of electrical stimulation of muscle nerves.
Frontiers in human neuroscience, 2014
Motoneurons receive a barrage of inputs from descending and reflex pathways. Much of our understanding about how these inputs are transformed into motor output in humans has come from recordings of single motor units during voluntary contractions. This approach, however, is limited because the input is ill-defined. Herein, we quantify the discharge of soleus motor units in response to well-defined trains of afferent input delivered at physiologically-relevant frequencies. Constant frequency stimulation of the tibial nerve (10-100 Hz for 30 s), below threshold for eliciting M-waves or H-reflexes with a single pulse, recruited motor units in 7/9 subjects. All 25 motor units recruited during stimulation were also recruited during weak (<10% MVC) voluntary contractions. Higher frequencies recruited more units (n = 3/25 at 10 Hz; n = 25/25 at 100 Hz) at shorter latencies (19.4 ± 9.4 s at 10 Hz; 4.1 ± 4.0 s at 100 Hz) than lower frequencies. When a second unit was recruited, the discharge of the already active unit did not change, suggesting that recruitment was not due to increased synaptic drive. After recruitment, mean discharge rate during stimulation at 20 Hz (7.8 Hz) was lower than during 30 Hz (8.6 Hz) and 40 Hz (8.4 Hz) stimulation. Discharge was largely asynchronous from the stimulus pulses with "time-locked" discharge occurring at an H-reflex latency with only a 24% probability. Motor units continued to discharge after cessation of the stimulation in 89% of trials, although at a lower rate (5.8 Hz) than during the stimulation (7.9 Hz). This work supports the idea that the afferent volley evoked by repetitive stimulation recruits motor units through the integration of synaptic drive and intrinsic properties of motoneurons, resulting in "physiological" recruitment which adheres to Henneman's size principle and results in relatively low discharge rates and asynchronous firing.
Spinal reflex excitability and homosynaptic depression after a bout of whole-body vibration
Muscle & nerve, 2011
Although whole-body vibration (WBV) affects neuromuscular performance, it remains unclear whether the effects are due to spinal reflex potentiation or inhibition, or differ between muscle groups. This study aimed to identify the effect of WBV on measures of spinal reflex excitability (H-reflex) and homosynaptic depression (HD) in the soleus (SOL) and medial gastrocnemius (MG) muscles. H-reflex and HD measurements were made in the SOL and MG muscle of 20 participants before and after a bout of WBV. H-reflex and HD were measured every 15 seconds for 10 minutes post-WBV and averaged at 1-minute increments. H-reflex amplitude was depressed for the first minute post-vibration, whereas the effect of HD was reduced for the first 2 minutes post-vibration. WBV significantly decreases spinal reflex excitability and HD, but it does so transiently and independent of muscle group.
Comments on reflex actions evoked by electrical stimulation of group II muscle afferents
Brain Research, 1977
In spinal cats electrical stimulation of group ll muscle afferents, irrespective of their muscle of origin, gives excitation in flexor and inhibition in extensor motoneurones, but sometimes the opposite actions dominate a,5,10. Recently, we postt, lated that the minimal linkage in excitatory group II pathways is disynaptic while the linkage in the corresponding inhibitory pathways is trisynaptic 8. However, Kirkwood and Sears 6, using spike triggered averaging (STA), found that impulses in secondary afferents produced homonymous monosynaptic EPSPs in ankle extensors and this finding has been confirmed 9. If adequate activity in secondaries evokes monosynaptic EPSPs then electrical stimulation of group I1 afferents would be expected to give the same effect. We have now re-analysed our material and indeed found previously overlooked evidence for homonymous monosynaptic group 1I EPSPs not only in extensor but also in flexor motoneurones which can be differentiated from the previously described interneuronally mediated EPSPs.
Studies on the receptor responsible for vibration induced inhibition of monosynaptic reflexes in man
Journal of Neurology, Neurosurgery & Psychiatry, 1975
SYNOPSIS A further attempt has been made to define the receptor responsible for the inhibition of monosynaptic reflexes by vibration in man. Vibration of the tendo Achillis will produce inhibition of the H reflex even when the muscles of the anterior compartment of the leg are denervated or blocked with local anaesthetic, implying that there are receptors in the posterior compartment capable of producing this effect. However, there is evidence that vibration spreads through the limb. The inhibition is greater when the anterior compartment is innervated indicating that there is a contribution from receptors in this compartment. Stretching the muscles of the posterior compartment alone, or the muscles of the anterior and posterior compartments reciprocally does not influence the inhibition of the monosynaptic reflex by vibration. These observations support the contention that the reduction of the monosynaptic reflex by vibration in man is due to presynaptic inhibition resulting from activation of primary spindle endings.
The afferent volleys responsible for spinal proprioceptive reflexes in man
The Journal of Physiology, 1983
1. To define the neural volleys responsible for the Achilles tendon jerk and the H reflex, muscle afferent activity was recorded using micro-electrodes inserted percutaneously into appropriate fascicles of the tibial nerve in the popliteal fossa. 2. The response of soleus muscle afferents to tendon percussion consisted of a dispersed volley, starting 3-5-7-0 ms after percussion, increasing to a peak over 6 5-1 1 0 ms, and lasting 25-30 ms, depending on the strength of percussion. Electrical stimuli to the sciatic nerve at a level adequate to evoke an H reflex but subthreshold for the M wave produced a more synchronized volley, the fastest fibres of which had conduction velocities of 62-67 m/s, and the slowest 36-45 m/s. 3. The wave of acceleration produced by percussion subthreshold for the ankle jerk spread along the skin at over 150 m/s. Midway between the bellies of the gastrocnemii it consisted of a damped oscillation with four to five separate phases and maximum amplitude approximately one-twentieth of that recorded on the Achilles tendon. 4. With ten primary spindle endings, tendon percussion subthreshold for the ankle jerk elicited two to five spike discharges per tap, the shortest interspike intervals being 4-7 ms. Tendon percussion elicited single discharges from two Golgi tendon organs, and altered the discharge pattern of a single secondary spindle ending. The degree of dispersion of the multi-unit muscle afferent volley can be explained by the pattern of discharge of primary spindle endings. 5. Percussion on the Achilles tendon evoked crisp afferent volleys in recordings from nerve fascicles innervating flexor hallucis longus, tibialis posterior, the intrinsic muscles of the foot and the skin of the foot. Electrical stimuli delivered to the tibial nerve in the popliteal fossa at a level sufficient for the H reflex of soleus produced either a volley in muscle afferents from the intrinsic muscles of the foot or a volley in cutaneous afferents from the foot. 6. For comparable stimuli in the two positions, the H reflex was inhibited but the Achilles tendon jerk enhanced when the ankle was dorsiflexed from 1050 to 900. 7. The duration of the rise times of the excitatory post-synaptic potentials (e.p.s.p.s) produced in soleus motoneurones by electrical stimulation, and by tendon percussion subthreshold for the H reflex and the ankle jerk respectively, was estimated from post-stimulus time histograms of the discharge of voluntarily D. BURKE, S. C. GANDEVIA AND B. McKEON activated single motor units in soleus. The mean e.p.s.p. rise times were 1-9 ms for electrical stimulation and 6-6 ms for tendon percussion. There was evidence that the duration ofthe electrically evoked e.p.s.p. was curtailed by an inhibitory post-synaptic potential (i.p.s.p.) of only slightly longer latency than the e.p.s.p. 8. The mechanically induced and electrically induced afferent volleys are not homogeneous volleys in group Ia afferents from triceps surae. The afferent volleys differ in so many respects that it is probably invalid to compare the H reflex and tendon jerk as a measure of fusimotor activity. It is suggested that neither reflex can be considered a purely monosynaptic reflex.