P. Hník

Work-induced potassium changes in skeletal muscle and effluent venous blood assessed by liquid ion-exchanger microelectrodes

SummaryUsing liquid ion-exchanger semimicroelectrodes with a side pore, we measured changes of extracellular potassium concentration (Ke+) in adult rabbit and cat gastrocnemius muscles and in venous effluent blood flowing from the cat gastrocnemius muscle during various bouts of activity induced by sciatic nerve stimulation.1.Isometric tetanic contractions (at 50 Hz) of various durations caused transient accumulation of Ke+ which was non-linearly related to the duration of muscle activity. The peak values of Ke+ in response to muscle stimulation were analogous in rabbits and cats, attaining values, e.g. after a 20-s isometric tetanus, between 8–9 mEq/lK+ in both species.2.Potassium concentration in venous effluent blood (Kven+) was transiently increased after isometric tetani. Since blood flow was measured at the same time, it was possible to calculate the amount of K+ lost by the muscle after tetani of various durations. A 32 g gastrocnemius muscle of the cat, for example, loses 9.36±1.52 μEqK+ after a 20-s isometric tetanus, which corresponds roughly to 0.5% of the total muscle potassium content. The loss of K+ in this muscle was 29.3 pEq K+/impulse/100 g fresh muscle tissue.3.There was no evident difference between the amount of K+ released during isometric tetani, or tetanic contractions performed under isotonic conditions. Single twitches evoked by indirect stimulation at 1 Hz for several minutes also induced a small rise in Kven+.4.If the loss of K+ from the muscle into the blood stream is transiently prevented by arterio-venous occlusion installed immediately before a 10-s isometric tetanus, most K+ is released subsequently when blood flow is renewed, if the occlusion lasts for 20–25 s. It is not until blood flow is occluded for 40–60 s that most K+ is apparently resorbed and only a minor portion is released and is to be found in the venous blood.5.The transient accumulation of muscle extracellular potassium may locally affect nerve endings, skeletal and smooth muscle cells.

The measurement of K e + concentration changes in human muscles during volitional contractions

Changes of extracellular potassium concentration ([K+]e) were measured in human muscles during volitional isometric contractions using liquid ion-exchanger electrodes. In principle, an intramuscular injection needle containing a microelectrode with a side-pore was inserted into the brachioradialis muscle. After insertion of the needle, the glass ion-selective microelectrode (ISM) could be moved out of the protective trocar shield into the muscle tissue. The average values of [K+]e in human muscles during maximal effort rose from 4.5 mmol/l K+ to 9.5 mmol/l K+. These values correspond closely to those previously found in muscles of experimental animals.

Changes in muscle spindle activity of the chronically de-efferented gastrocnemius of the rat

SummaryThe response to stretch of chronically de-efferented muscle spindles was studied in the rat gastrocnemius muscle 6 days (short-term) and 48 days (long-term) after ventral rhizotomy respectively. Chronic de-efferentation enhances both the dynamic and static component of the response. There was no difference between the short-term and long-term de-efferentation, i.e. the response to stretch was as high several days after de-efferentation as after 1–2 months, when the muscle had undergone considerable atrophy. The response of both spindle primaries and secondaries was enhanced, but more so in the latter group. This suggests that the probable site of these changes after ventral root section lies in the polar regions. Early adaptation of de-efferented group I endings remained unaltered throughout the whole experimental period. Group II and III afferents, on the other hand, exhibited less adaptation than those from control muscles. Fibrillation activity of intrafusal muscle fibres does not seem to be involved, since quinidine sulphate in doses suppressing fibrillations in extrafusal muscle fibres does not eliminate the enhanced response. Ultrastructural changes at the site of sensory nerve terminals on the receptor cells of the spindle caused by chronic de-efferentation are probably responsible.

Work-induced potassium changes in muscle venous effluent blood measured by ion-specific electrodes.

SummaryModified Walkers liquid ion-exchanger microelectrodes were employed for measuring changes of K+ concentration in venous effluent blood from the cat gastrocnemius muscle during and after isometric tetani of various duration induced by indirect stimulation. The time course of these changes was obtained and the overall loss of K+ from a working muscle could thus be estimated. By comparing present results in the venous blood and previous findings of K+ concentration changes in the muscle extracellular space, a concentration gradient was found between the muscle and venous effluent blood.

Reaching behavior in the rat : absence of forelimb peripheral input

In order to test whether peripheral input from a moving forelimb is essential for reaching in rats, the effects of dorsal rhizotomy C5-Th2 were examined. Rats were trained to reach for a food pellet in the horizontal tube or on a tray. Reaching attempts before and after bilateral forelimb deafferentation were monitored by continual recording using magnetic induction. Deafferented animals were able to initiate and generate the motor program of reaching, but modulation of its ongoing execution was lost. Peripheral input from moving forelimb was necessary for the effective performance of grasping; the duration of the manipulative part of reaching was significantly prolonged and the success of grasping was markedly decreased. Also, the aiming of forelimb was impaired, probably by disturbing of body forelimb postural coordination. No significant changes were found in the execution of forelimb protraction. It is concluded that somesthetic feedback is not required for execution of forelimb protraction, but it is necessary for grasping.

Postnatal development of conduction velocity and fibre size in the rat tibial nerve.

The maximum conduction velocity (CV) and fibre diameters (D) were determined in the tibial nerve of developing rats. In 1‐day‐old rats CV of the fastest motor and sensory fibres (assessed separately) was 1.4 m/sec on the average and increased to 35 m/sec by postnatal day 30. The maximum conduction rate in adult rats ranged from 60 to 84 m/sec. Diameters of at least 100 nerve fibres in each age group were measured in electronmicrographs. The calibre of myelinating fibres in 1‐day‐old rats was 0.5–1.5 μm. By day 90 after birth the range of myelinated fibre size extended to 1.5–12.5 μm. The factor relating conduction rate and total fibre diameter of the largest fibres (i.e. the value of CV/D) was found to vary with age, increasing from 1.1 to 6.2 between postnatal days 1 and 90. These results indicate that functional and morphological properties of peripheral nerve fibres in the rat undergo considerable changes during postnatal ontogeny until they reach adult values.

Response to stretch of proprioceptors in adult rat muscles de-efferented at brith

SummaryHind-limb muscles of new-born rats were de-efferented by removing the lumbosacral spinal cord. Spinal ganglia remained intact, together with their peripheral axon. The presence of sensory terminals in limb spindles, induces the full ultrastructural differentiation of muscle spindles, as has been shown previously. In the present paper we have shown by integrating the sensory discharges in the whole nerve from chronically de-efferented muscles that even several months after birth, muscle proprioceptors (probably mostly spindles) still maintain their basic mechanoreceptor properties. Although the limbs were completely immobilized throughout the whole experimental period, spindles from these chronically de-efferented muscles still responded as slowly adapting receptors. The dynamic component was also present in the integrated neurogram response during stretching. It thus appears that basic functional properties of rat muscle proprioceptors persist even when these receptors differentiate and survive without motor innervation and any adequate functional stimuli, i.e. under conditions of permanent disuse.

EMG changes in rat hind limb muscles following bilateral deafferentation

Bilateral section of dorsal roots was performed in 9 adult rats in order to ascertain whether the tendency to extension, the appearance of spontaneous electromyographic (EMG) activity in extensor muscles and other symptoms of postdenervation hypersensitivity after unilateral deafferentation are not due to the sensory inflow from the contralateral limb. EMG activity from the soleus (SOL) and tibialis anterior (TA) muscles was bilaterally recorded before and at various periods after deafferentation by a previously implanted electrode array. The postural activity in both SOL muscles disappeared after the operation. One to two days later, however, spontaneous tonic EMG activity, similar to that found in the SOL after unilateral deafferentation, appeared in these bilaterally deafferented muscles. The tonic spontaneous EMG activity in hind limb extensor muscles after deafferentation is apparently due to hypersensitivity of spinal neurones to supraspinal influences, since this activity completely disappears after myelotomy. The paradoxical inhibitory EMG response to stretch of the deafferented SOL frequently appeared 1–2 weeks after bilateral dorsal rhizotomy. The performance of movement was highly atactic after bilateral deafferentation. However, the basic locomotor EMG pattern persisted, although simultaneous activation of homonymous muscles was also occasionally recorded.

Function and structure of atypical muscle spindles after neonatal nerve crush in rats.

SummaryDuring the early postnatal period, the differentiation and maturation of muscle spindles in the rat is still dependent on their sensory innervation. When a nerve is crushed during this period, most spindles in the denervated muscles degenerate and after reinnervation only occasional spindles of atypical structure are to be found in these muscles. We determined the basic functional properties of these atypical spindles in adult rats and attempted to correlate them with their structural characteristics. The discharge rates of 13 afferent units from the soleus or lateral gastrocnemius muscles were evaluated in response to stretch. These units were capable of a slowly adapting response to 2–4 mm stretches. Their mean discharge frequencies at any point of the ramp-and-hold stretch were, however, on an average 50% lower than normal values. The conduction velocities of afferents from the atypical spindles were in the range of 10–40 m/s. Histological examinations revealed that 90% of the atypical muscle spindles found in the soleus or lateral gastrocnemius muscles had only 1 or 2 intrafusal fibres without any nuclear accumulations as compared to four intrafusal fibres in normal muscle spindles in the rat. The proportional decrease of the discharge rate in both the dynamic and static part of the response of these atypical spindles could be due to the decreased synaptic area between the sensory terminals and the intrafusal fibres and/or to altered structural properties of the intrafusal fibres.

Late Effects of Early Hind-Limb Denervation and Reinnervation in Rats: An EMG Study

Crushing of the sciatic nerve in new-born rats leads subsequently to a permanent decrease in the number and size of both sensory and motor nerve fibers. The number of motor axons and muscle fibers in the soleus is decreased by more than a half; the mean diameter of motor axons is reduced by 25 %, but that of muscle fibers is increased by 20 %. The number of encapsulated muscle receptors is dramatically reduced in the soleus (SOL), tibialis anterior (TA) and extensor digitorum longus (EDL) after neonatal nerve crush (Zelena and Hnik, 1963). This is in contrast to the peripheral nerve regeneration following nerve crush in adult animals. The recovery of muscle weight after neonatal nerve crush was also found to be rather poor as compared with the result of muscle reinnervation in adult animals.