Increase in Ib inhibition by antagonistic voluntary contraction in man

Reciprocal inhibition of the thigh muscles in humans: A study using transcutaneous spinal cord stimulation

Evaluating reciprocal inhibition of the thigh muscles is important to investigate the neural circuits of locomotor behaviors. However, measurements of reciprocal inhibition of thigh muscles using spinal reflex, such as H-reflex, have never been systematically established owing to methodological limitations. The present study aimed to clarify the existence of reciprocal inhibition in the thigh muscles using transcutaneous spinal cord stimulation (tSCS). Twenty able-bodied male individuals were enrolled. We evoked spinal reflex from the biceps femoris muscle (BF) by tSCS on the lumber posterior root. We examined whether the tSCS-evoked BF reflex was reciprocally inhibited by the following conditionings: (1) single-pulse electrical stimulation on the femoral nerve innervating the rectus femoris muscle (RF) at various inter-stimulus intervals in the resting condition; (2) voluntary contraction of the RF; and (3) vibration stimulus on the RF. The BF reflex was significantly inhibited when the conditioning electrical stimulation was delivered at 10 and 20 ms prior to tSCS, during voluntary contraction of the RF, and during vibration on the RF. These data suggested a piece of evidence of the existence of reciprocal inhibition from the RF to the BF muscle in humans and highlighted the utility of methods for evaluating reciprocal inhibition of the thigh muscles using tSCS.

The effects of opposite-directional static contraction of the muscles of the right upper extremity on the ipsilateral right soleus H-reflex

The objective of this study was to explore the neurophysiological remote after-effects of resistive static contraction (SC) of the muscles of the upper extremity, considering the resistant direction on the ipsilateral (right) soleus H-reflex. The participants included 12 normal subjects with a mean (SD) age of 23.8 (2.8) years. The subjects were asked to maintain their upper extremity against the traction force, at a level of resistance that was 50% of the maximal SC strength. A 20-s SC of the muscles of the upper extremity utilizing contraction of the upper extremity muscles using a diagonal flexion (shoulder flexion-adduction-external rotation) or extension (shoulder extension-abduction-internal rotation), a proprioceptive neuromuscular facilitation (PNF) pattern was induced. The traction force line of the diagonal flexion or extension direction ran parallel to the diagonal line from the left acromion process to the right ASIS. Three-way analysis of variance of the H/Mmax ratio with Scheffé's post-hoc tests revealed that the H/Mmacorrelatex ratio of SC via diagonal extension was significantly smaller than that via diagonal flexion and that the H/Mmax ratio during the 120-140 s phase after SC, as remote after-effect SC, was significantly smaller than that during SC. The induction of neurophysiological descending effects for inhibition requires consideration of the force direction.

Operant conditioning to increase ankle control or decrease reflex excitability improves reflex modulation and walking function in chronic spinal cord injury

Ankle clonus is common after spinal cord injury (SCI) and is attributed to loss of supraspinally mediated inhibition of soleus stretch reflexes and maladaptive reorganization of spinal reflex pathways. The maladaptive reorganization underlying ankle clonus is associated with other abnormalities, such as coactivation and reciprocal facilitation of tibialis anterior (TA) and soleus (SOL), which contribute to impaired walking ability in individuals with motor-incomplete SCI. Operant conditioning can increase muscle activation and decrease stretch reflexes in individuals with SCI. We compared two operant conditioning-based interventions in individuals with ankle clonus and impaired walking ability due to SCI. Training included either voluntary TA activation (TA↑) to enhance supraspinal drive or SOL H-reflex suppression (SOL↓) to modulate reflex pathways at the spinal cord level. We measured clonus duration, plantar flexor reflex threshold angle, timed toe tapping, dorsiflexion (DF) active range of motion, lower extremity motor scores (LEMS), walking foot clearance, speed and distance, SOL H-reflex amplitude modulation as an index of reciprocal inhibition, presynaptic inhibition, low-frequency depression, and SOL-to-TA clonus coactivation ratio. TA↑ decreased plantar flexor reflex threshold angle (-4.33°) and DF active range-of-motion angle (-4.32°) and increased LEMS of DF (+0.8 points), total LEMS of the training leg (+2.2 points), and nontraining leg (+0.8 points), and increased walking foot clearance (+ 4.8 mm) and distance (+12.09 m). SOL↓ decreased SOL-to-TA coactivation ratio (-0.21), increased nontraining leg LEMS (+1.8 points), walking speed (+0.02 m/s), and distance (+6.25 m). In sum, we found increased voluntary control associated with TA↑ outcomes and decreased reflex excitability associated with SOL↓ outcomes.

In humans Ib facilitation depends on locomotion while suppression of Ib inhibition requires loading

The role of force feedback during gait is still a matter of debate. From work on cats, it is known that input from Golgi tendon organs from triceps surae does produce Ib facilitation during locomotion instead of autogenic inhibition. In humans, Stephens and Yang (Stephens, M.J., Yang, J.F., 1996. Short latency, non-reciprocal group I inhibition is reduced during the stance phase of walking in humans. Brain Res. 743, 24-31) found that voluntary contraction results in a reduction of Ib inhibition. During gait, they even observed Ib facilitation in a subset of subjects. This raises the question whether the crucial elements involved in these changes are either loading of the leg or locomotion. To examine this question, Ib reflexes were investigated during sitting, lying supine, lying supine with 300 N pressure applied to the foot sole, standing, and a rhythmic loading and unloading task called "reduced" gait. Ib inhibition was obtained during sitting and lying supine. This inhibition was significantly reduced or disappeared during standing and when lying supine but loaded. During the stance phase of "reduced" gait, the inhibition disappeared in eight subjects, and even a facilitation was observed in six subjects. It is concluded that the decrease in Ib inhibition from gastrocnemius to soleus occurs during a load-bearing condition and does not require locomotion. In contrast, Ib facilitation requires locomotion at least in a rudimentary form.

Load-regulating mechanisms in gait and posture: comparative aspects

How is load sensed by receptors, and how is this sensory information used to guide locomotion? Many insights in this domain have evolved from comparative studies since it has been realized that basic principles concerning load sensing and regulation can be found in a wide variety of animals, both vertebrate and invertebrate. Feedback about load is not only derived from specific load receptors but also from other types of receptors that previously were thought to have other functions. In the central nervous system of many species, a convergence is found between specific and nonspecific load receptors. Furthermore, feedback from load receptors onto central circuits involved in the generation of rhythmic locomotor output is commonly found. During the stance phase, afferent activity from various load detectors can activate the extensor part in such circuits, thereby providing reinforcing force feedback. At the same time, the flexion is suppressed. The functional role of this arrangement is that activity in antigravity muscles is promoted while the onset of the next flexion is delayed as long as the limb is loaded. This type of reinforcing force feedback is present during gait but absent in the immoble resting animal.

Changes in Ib heteronymous inhibition to soleus motoneurones during cutaneous and muscle nociceptive stimulation in humans

The effects of selective tonic cutaneous and muscle pain stimulation on heteronymous Ib pathways from the gastrocnemius medialis to the soleus motoneurones were studied in five healthy human subjects. Tonic pain stimulation, monitored by a psychophysical method, was obtained by local injection of 60 mg levo-ascorbic acid (L-AS) in a volume of 0.3 ml. Nociceptive cutaneous and muscle stimulation of the dorsal foot produced opposite, long-lasting changes in heteronymous Ib pathways to soleus motoneurones: Ib facilitation during cutaneous stimulation was reversed by disinhibition during muscle stimulation. The time-course of these Ib changes strictly paralleled that of subjective pain sensation. On the contrary, when the same nociceptive stimuli were applied to the leg (at pretibial level), the differences between cutaneous and muscle pain disappeared and similar short-lasting phases of facilitation and inhibition of Ib activity were observed. It is concluded that tonic discharge of cutaneous and muscle nociceptive afferents arising from the foot have specific and opposite effects on Ib inhibitory pathways to ankle extensor motoneurones. These interactions between muscle and cutaneous nociceptive and Ib pathways may be used to change muscle synergies, thus contributing to the establishment of appropriate adaptive locomotor strategies during pain.

Cutaneous nociceptive facilitation of Ib heteronymous pathways to lower limb motoneurones in humans

The effects of tonic pain stimulation on heteronymous Ib pathways from the gastrocnemius medialis (GM) to the soleus (Sol) and to the quadriceps (Q) muscles were investigated in four healthy human subjects. Tonic pain stimulation was performed by subcutaneous injection of 0.5 mg levo-ascorbic acid or vitamin C (L-LAS) in a volume of 0.5 ml on the dorsal surface of the ipsilateral foot. The mean curve of L-AS-induced pain sensation showed a steep rising phase reaching maximum intensity at 2-3 min, followed by a slow decay phase lasting about 15-20 min. Between about 5 and 20 min after injection, there was evidence of pure pain stimulation due to chemical activation of free nerve endings. During this interval, significant potentiation of Ib inhibition from GM to both Sol and Q motoneurones was observed. The time-course of these Ib heteronymous changes paralleled that of subjective pain sensation. These findings demonstrate that nociceptive discharge modifies the gain of Ib heteronymous effects in humans. Since the man function of these Ib pathways is to coordinate activity of muscles operating at different joints, it is suggested that nociceptive input may change muscle synergies by selecting specific subpopulations of Ib interneurones, thus contributing to establish appropriate adaptive motor strategies.

Cortical modulation of transmission in spinal reflex pathways of man

1. The motor actions in the lower limb of transcranial electrical stimulation of the motor cortex have been studied in sitting human subjects. 2. Cortical stimulation induced a short latency inhibition of H reflexes evoked in soleus motoneurones both at rest and during small voluntary contractions of soleus. 3. Spatial interaction between cortical inhibition of soleus motoneurons and inhibition evoked through identified spinal reflex machinery was investigated. 4. Interactions were found between cortically evoked inhibition and spinal Ia reciprocal inhibition, group I non-reciprocal inhibition and higher threshold components of longer latency reciprocal inhibition (D1 and D2 inhibitions). 5. Interactions were facilitatory when cortical and spinal inhibitory actions were weak and reversed to occlusion when both actions were strong. 6. It is concluded that the corticospinal pathway converges on the interneurones which subserve Ia reciprocal, group I non-reciprocal, D1 and D2 inhibition of soleus motoneurones. 7. No significant interaction was found under the present experimental conditions between cortical stimulation and group Ia-Ia presynaptic inhibition of soleus afferents. 8. The statistical significance of spatial interactions observed with H reflex conditioning was investigated using a control experiment.