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In humans, an inhibitory via Ia afferent pathway from the medial gastrocnemius (MG) to the soleus (SOL) motoneuron pool has been suggested. Herein, we examined the relation between MG fascicle length changes and the SOL H-reflex modulation during passive knee movement. Twelve subjects performed static and passive (5 degrees s(-1)) knee movement tasks with the ankle immobilized using an isokinetic dynamometer in sitting posture. The maximal H- and M-waves were measured at four target angles (20 degrees, 40 degrees, 60 degrees, and 80 degrees flexion from full knee extension). The MG fascicles length and velocity were measured using a B-mode ultrasonic apparatus. Results demonstrated that the SOL Hmax/Mmax; i.e., ratio of the maximal H- to M-waves, was attenuated with increasing MG fascicle length in static tasks. The SOL Hmax/Mmax at 20 degrees was significantly attenuated compared with 60 degrees and 80 degrees with increasing MG fascicle length and lengthening velocity in passive knee extension. However, no significant differences in the SOL Hmax/Mmax were found across the target angles in the passive knee flexion task. In conclusion, as muscle spindles increase their discharge with lengthening fascicle velocity, but keep silent when fascicles shorten, our data suggest that lengthening the MG facilitates an inhibitory Ia pathway from MG to SOL, and modulates SOL motoneuron activity during movements.

Aim Following the electrical stimulation of mixed nerve during voluntary muscle contraction, temporarily suppressed EMG activation called mixt nerve silent period (MNSP). MNSP is interrupted by several bursts; first one is segmental reflex (SR) and others known as a long-loop reflexes(LLR). Vibration has been shown suppress the segmental H-reflex amplitude by increasing Ia afferent discharge. In this study, our aim was to provide insight into SR characteristics and the effects of vibration on other parameters. Method Eleven healthy volunteer (5 women, 33.36 ± 5.35 years) included the study. Vibration was applied for 2 min at 100 Hz with repositioning amplitude of 1 mm on right forearm. Median nerve stimulated at wrist during thumb abduction and MNSP recorded at baseline, during and after the vibration application. Latencies of SR, LLR and MNSP and amplitudes of SR and LLR compared for each session. Results Average SR and LLR amplitudes, MNSP durations before, during and after vibration were found as (1093.36 ± 606.16/877.22 ± 390.37/863.0 ± 604.51), (561.17 ± 353.8/1363.92 ± 1242.78/1100.0 ± 901.65) and (108.6 ± 12.41/113.55 ± 21.35/100.47 ± 18.03) respectively. MNSP durations, SR and LLR latencies were not significantly different between sessions. While SR amplitude tended to decrease, LLRA tended to increase. Conclusion Present study demonstrated that short-therm vibration is not effective on MNSP parameters. Our results may support that SR share similar characteristics with HR.

The amplitude of monosynaptic EPSPs produced by the action of single semitendinosus (ST) Ia afferent fibers in ST motoneurons display little or no increase in amplitude within hours of a spinal cord transection performed at the thirteenth thoracic segment. This finding is in marked contrast to results obtained at the medial gastrocnemius (MG) Ia fiber-motoneuron synapse, where EPSPs have been shown to be enlarged, even in preparations with normal ST EPSPs. The increase in EPSP amplitude reported previously at the MG Ia-motoneurons synapse is selective and therefore is probably not a direct consequence of injury to the isolated segment of the spinal cord.

In cats anesthetized with allobarbitone-urethane, Mn 2+ and Co 2+ (and occasionally La 3+ ) were released extracellularly from micropipettes while recordings were made of monosynaptic excitatory postsynaptic potentials evoked in lumbosacral motoneurones by Ia afferent stimulation. These excitatory postsynaptic potentials consistently showed a marked depression in their rate of rise (by an average of 44%) and an increase in half-amplitude duration (by an average of 50%). There was a less pronounced reduction in peak amplitude and increase in time-to-peak. After applications of Mn 2+ or Co 2+ , presynaptic potentials recorded in motoneurones showed no sign of any depression but the synaptic delay was clearly increased. It is concluded that the monosynaptic excitatory postsynaptic potential evoked by Ia afferents in cat motoneurones is probably mediated by chemical transmission.

The excitability of a firing human motoneurones was tested by Ia afferent excitatory volleys. An H reflex was evoked by stimulation of the tibial nerve while the subjects maintained the background rhythmic firing of single MU of the soleus and of the medialis gastrocnemius during weak voluntary contraction. The mean background firing rate of MUs ranged from 3.6 to 9.4 imp/sec. In order to estimate the ‘effectiveness’ of an afferent volley, PSTHs were constructed. The firing index was used as a measure of motoneurone excitability. It has been shown that the ‘effectiveness’ of an afferent volley decreases with an increase in background firing rate of a MU. Such a dependence was deduced from changes in current motoneurone excitability within an interspike interval. The results indicate that during natural muscle contraction at a low firing rate of a motoneurone, the discharge rate is one of the factors affecting the excitability of a firing motoneurone and, hence, its input-output relations. The method of testing changes in human motoneurone excitability within an interspike interval may prove useful in studies on interaction of volleys arriving at a motoneurone via different excitatory inputs.

The effects of lidocaine (1--20 microgram/ml) on afferent discharge patterns of primary endings of muscle spindles in cat tenuissimus muscle were investigated. Discharge from the endings, recorded in Ia afferent axons, was evoked by ramp stretch of the muscle, stimulation of single static or dynamic fusimotor axons or by a combination of stretch and fusimotor stimulation. Spontaneous discharge of the endings at the initial length of the muscles was reduced by 2--5 microgram/ml and abolished by 10--15 microgram/ml lidocaine. The static but not the dynamic discharge elicited by muscle stretch was blocked by concentrations of 10--15 microgram/ml. The same concentrations abolished static and dynamic fusimotor influences on primary ending discharge. However, in one experiment where the spindle was microscopically observed, fusimotor stimulation still resulted in contraction of the intrafusal muscle even when fusimotor stimulation failed to elicit changes in discharge response patterns of the primary endings. These findings indicate that lidocaine interferes with the encoding mechanism prior to block of impulse conduction in either the fusimotor or afferent axons.

Summary The conduction velocities of the α motor fibres and the IA sensitive fibres of the ulnar nerve have been studied in a group of full term newborn babies and during the neonatal period. At birth the average conduction velocity of the α motor fibres is 25,7 m/sec (σ = 2,4 m/sec) that of the IA afferent fibres is 35 m/sec (σ = 3,2 m/sec). The post natal evolution of the conduction velocity as a function of age has been established for both types of fibres. The time required for nervous conduction between the olecranon process and the wrist diminishes clearly in the course of the first two years because of a discordance between the augmentations of the lengtht and of the diameter of the nerve fibers. The evolution of this conduction time as a function of the age and of the length of the forearm, has been established for the period between birth and the age of six years. The Hoffmann reflex in the inferior limb is present in all infants a birth ; its latency is 14,2 msec ; the ratioR = Hmax/Mmax × 100 = 55,75 p. 100 is identical to that found in the adult. A Hoffmann reflex exists in 66 p. 100 of the population studied in the ulnar nerve. Its latency is 11,1 msec ; R = Hmax/Mmax is equal to 30 p. 100. This reflex disappears progressively during the development.

Although GABA and piperidine-4-sulphonate depolarize Ia afferent terminations in the lumbar spinal cord by activation of bicuculline-sensitive GABA receptors, no evidence was obtained for abicuculline-sensitive alteration by either GABAmimetic of the excitability of vestibulospinal terminations. This suggests that the terminal arborizations of vestibulospinal fibers, unlike their cell bodies. are devoid of GABA receptors having properties similar to those on either central neuron cell bodies, primary afferent cell bodies or their central terminations.