Masaomi Shindo

Changes in reciprocal Ia inhibition during voluntary contraction in man

SummaryReciprocal Ia inhibition from ankle flexors to extensors was studied during voluntary tonic isometric dorsiflexion and plantar flexion in five normal subjects. The Ia inhibition was examined as the short-latency suppression of the soleus H-reflexes by stimulation of the low-threshold afferents in the common peroneal nerve (Mizuno et al. 1971). At rest, weak Ia inhibition was demonstrated in four subjects out of five, the maximal amount being 14.1 ± 5.0% suppression of the control H-reflex. The absolute amount of inhibition, which was calculated by subtracting the mean size of the conditioned H-reflex from that of the control H-reflex and expressed as a percentage of the maximal M-response, increased during ankle dorsiflexion, and decreased or disappeared during plantar flexion in parallel with the amount of contraction. The neural mechanisms for facilitation of the Ia inhibitory pathway during dorsiflexion were considered to support the hypothesis of “α-γ-linkage in reciprocal inhibition”, i.e. combined facilitatory effects on the Ia inhibitory interneurone from the supraspinal centers directly and indirectly via the γ motoneurone — Ia afferent route. The mechanism for inhibition of the pathway during plantar flexion was considered to be inhibition of the Ia interneurone of the flexor side by Ia interneurone of antagonist extensors. A quantitative aspect of activity in the reciprocal Ia inhibitory pathway on the performance of voluntary movement is revealed in this study.

Progressive decrease in heteronymous monosynaptic la facilitation with human ageing

To evaluate functional change in the spinal reflex pathway with ageing, we studied heteronymous Ia facilitation from the quadriceps to soleus muscle in 30 normal volunteers (aged 24–68 years). The size of the test H-reflex of the soleus muscle was adjusted to 25% that of the maximal M-response. The conditioning stimulus was adjusted to 1.5-fold the motor threshold to stimulate all the Ia fibres in the femoral nerve. Facilitation was quantified as the slope of the very early part of facilitation, within 0.8 ms of onset. This procedure enabled us to evaluate the extent of monosynaptic Ia facilitation without contamination by other effects. The extent of facilitation decreased linearly with age. This decrease in facilitation could reflect a decrease in the number of Ia fibres and in their conduction velocities, and an increase in presynaptic inhibition on Ia terminals. The increase in presynaptic inhibition may be an adaptive phenomenon in the ageing of the neuromuscular system or, alternatively, a deteriorating process with decreasing flexible supraspinal modulation.

Inhibitory projections from pronator teres to biceps brachii motoneurones in human

Abstract Neural projections from the pronator teres (PT) muscle to biceps brachii (BB) motoneurones were studied in three healthy human subjects using a post-stimulus time histogram method. In 25 BB motor units, electrical stimulation to the PT nerve with intramuscular needle electrodes induced inhibition in nine units (36%), whereas facilitation was produced in 18 units (72%) by stimulation to the median nerve trunk with surface electrodes at the distal end of the intermuscular septum of the arm or in the cubital fossa. Six motor units (24%) received both inhibition (PT nerve stimulation) and facilitation (median nerve trunk stimulation). In the six, the latency of the inhibition was, on average, 1.2 ms longer than that of the facilitation. The stimulation site for the inhibition was, on average, 4.8 cm distal to that for the facilitation. The inhibition was evoked with an intensity well below the motor threshold. These findings suggest that BB motoneurones receive oligosynaptic inhibition of group I afferents from PT in human.

Inhibitory projection from brachioradialis to biceps brachii motoneurones in human

Neural projection from the brachioradialis to the biceps brachii motoneurones in human was studied using the method of post-stimulus time histogram. Electrical stimulation to the radial branch innervating the brachioradialis produced inhibition in 11 out of 21 biceps motor units. The central delays of the inhibition were 0.7–1.2 ms longer than those of the homonymous facilitation. The inhibition was evoked with the intensity below the motor threshold. Pure cutaneous stimulation provoked no effects on the motor-unit firing. These findings suggest that group I afferents from the brachioradials mediate an oligosynaptic inhibition of the biceps brachii motoneurones.

Continuous Inhibition of Excessive Polyol Pathway Flux in Peripheral Nerves by Aldose Reductase Inhibitor Fidarestat Leads to Improvement of Diabetic Neuropathy

We investigated the effects of three aldose reductase (AR) inhibitors, fidarestat, epalrestat and zenarestat, on the slowing of sensory nerve conduction velocity (SNCV), motor nerve conduction velocity (MNCV), and minimal F-wave latency prolongation in streptozotocin (STZ)-induced diabetic rats. Two weeks after STZ injection, SNCV and MNCV in the diabetic rats were significantly slower than in normal rats. Fidarestat (0.25-2 mg/kg/day), epalrestat (48 to 96 mg/kg/day) or zenarestat (10-40 mg/kg/day) was administered orally for the following 2 weeks, and SNCV, MNCV and F-wave latency were measured 3 h after final administration. Significant prolongation of minimal F-wave latency, as well as slowing of SNCV and MNCV, was found in the untreated diabetic rats 4 weeks after STZ injection. At a dose of 0.5 mg/kg/day or more fidarestat showed significant effects on these nervous dysfunctions, effects that were more potent than those shown by the other inhibitors. Furthermore, following the 2-week administration of fidarestat (1 mg/kg/day), epalrestat (48 mg/kg/day) or zenarestat (20 mg/kg/day), which began 2 weeks after STZ injection, sorbitol content in the sciatic nerve, produced by AR, a rate-limiting enzyme in the polyol pathway, was determined at 3, 8, 12, and 24 h after final administration. At each point in time, sorbitol content in the untreated diabetic rats was much higher than that in the normal control rats. Fidarestat suppressed sorbitol accumulation remarkably and continuously until 24 h after administration. On the other hand, the inhibitory effect by zenarestat declined in a time-dependent manner, and epalrestat did not decrease sorbitol content. Therefore, these results suggest that continuous inhibition of increased polyol pathway flux can improve diabetic neuropathy more potently.

Lack of modulation of Ib inhibition during antagonist contraction in spasticity

Objective: To examine the modulation of non-reciprocal group I (Ib) inhibition during tonic contraction of antagonist muscles in patients with spasticity vs normal subjects. Methods: The authors studied 10 patients with spastic paraplegia due to cervical compression myelopathy and 16 age-matched normal subjects. Ib inhibition to soleus motoneurons was recorded as the change in size of the H-reflex of the soleus, evoked by conditioning stimulus to the nerve innervating the medial gastrocnemius muscle. The extent of inhibition was studied at rest and during tonic contraction of the pretibial muscles of variable strength. Results: In the resting state, the extent of inhibition in the patients did not differ from normal controls. During antagonist contraction, the extent of inhibition increased both in the normal subjects and patients. The increment was smaller in the patients, especially in those with severe spastic gait. The smaller increment in the inhibition was correlated with the time required to walk 10 m in the patients. Conclusion: The authors observed a lack of modulation of Ib inhibition during tonic antagonist contraction in patients with spasticity, especially those with gait disturbance. Disturbed central modulation of non-reciprocal (Ib) interneurons may be responsible for spasticity.

Serial changes of sensory nerve conduction velocity and minimal F-wave latency in streptozotocin-induced diabetic rats

We studied the serial changes of sensory nerve conduction velocity (SNCV) in the caudal nerve of streptozotocin (STZ)-induced diabetic rats using a new technical method. Minimal F-wave latency was also studied by stimulating the tibial nerve. The SNCV in the diabetic rats was slower than that in the normal rats 2 weeks after STZ injection, and minimal F-wave latency was prolonged compared to normal rats 4 weeks after STZ injection. Treatment of the diabetic rats with insulin for 14 days inhibited SNCV slowing and minimal F-wave latency prolongation. This new method to measure SNCV is useful for various studies, and improvement of diabetic neuropathy with insulin treatment is indicated by recovery from SNCV slowing and minimal F-wave latency prolongation.

Decrease in presynaptic inhibition on heteronymous monosynaptic Ia terminals in patients with Parkinson's disease

Heteronymous Ia facilitation from the quadriceps to the soleus was studied to clarify central motor control through presynaptic inhibition (PSI) on Ia terminals of spinal motoneurons in Parkinsons disease.

Increase in reciprocal Ia inhibition during antagonist contraction in the human leg: a study of motor units and the H reflex.

1. The change in reciprocal Ia inhibition of soleus motoneurones produced by stimulation of the common peroneal nerve was investigated by the use of twenty‐three soleus motor units as well as the soleus H reflex in six normal subjects during tonic pretibial contraction. 2. In the motor unit experiments, motoneuronal excitability was measured as the ‘critical firing stimulus’ (CFS), which is the difference between the test stimulus intensity needed to reach the threshold for the lowest threshold Ia fibres and the intensity which evokes firing of a motor unit with the probability of 50%. The conditioning effect, assessed from the change in the CFS, was expressed as a percentage of the unconditioned CFS. 3. At a conditioning intensity of 0.95 times the motor threshold value, there was Ia inhibition in sixteen of the twenty‐three motor units (69.6%) at rest. Of these sixteen motor units, twelve showed increases in inhibition at intervals below 2.0 ms during pretibial contraction. In four of the remaining seven units, inhibition first appeared during contraction. There was no significant decrease in inhibition at any time during contraction. 4. Based on the conventional H reflex, reciprocal Ia inhibition increased during very weak (below 2% of the maximum) voluntary dorsiflexion and continued to increase at a slightly stronger (3‐8% of the maximum) contraction, then decreased continuously when contraction was strengthened further. Maximal inhibition occurred at a relatively strong contraction when a weak conditioning stimulus was used, and vice versa. 5. We conclude that the activity of reciprocal Ia inhibitory interneurones increases during tonic antagonist contraction. The previous controversy about this inhibition is the result of occlusion at the Ia interneuronal level.

Increase in Ib inhibition by antagonistic voluntary contraction in man.

1. Ib inhibition from gastrocnemius medialis (GM) muscle to soleus (Sol) muscle was studied at rest and at the onset of phasic voluntary contraction of antagonistic pretibial muscles in seventeen normal subjects. 2. In twelve out of seventeen subjects there was inhibition of Sol H reflex by GM conditioning stimulation at rest with a latency of 1.5‐3.0 ms and a threshold of 0.85‐1.00 times the motor threshold (MT). The amount of inhibition at 0.95‐1.05 x MT, which was calculated by subtracting the size of the conditioned reflex from that of the unconditioned one, ranged from 0.8 to 5.6% of the maximal M‐response or 2.9‐18.3% of control H reflex. This inhibition was ascribed to Ib inhibition, taking into account its latency and threshold. 3. On weak pretibial contraction the inhibition either increased in amount or newly appeared in all the subjects. When the strength of voluntary contraction was graded from 1 to 20% of the maximum, the increment in the amount of inhibition decreased or almost disappeared at strengths of more than several per cent. These facts imply that at least some of the Ib interneurones are facilitated to fire by descending commands alone without peripheral Ib impulses. Contralateral ankle dorsiflexion did not modify the inhibition. 4. Soleus muscle H reflex was not modulated at all by cutaneous stimulation instead of GM stimulation at rest, nor was it affected by cutaneous stimulation on ipsilateral antagonistic contraction. 5. It is concluded that activity in the Ib inhibitory pathway is facilitated at the onset of antagonistic voluntary contraction. This suggests that control of the Ib inhibitory pathway may be utilized in ordinary voluntary movement, and is presumably beneficial for smooth execution of movement.