Activity patterns of cochlear ganglion neurones in the starling

Spontaneous activity and responses to simple tonal stimuli were studied in cochlear ganglion neurones of the starling. Both regular and irregular spontaneous activity were recorded. Non-auditory cells have their origin in the macula lagenae. Mean spontaneous rate for auditory cells (all irregularly spiking) was 45 spikes s-1. In half the units having characteristic frequencies (CFs) less than 1.5 kHz, time-interval histograms (TIHs) of spontaneous activity showed regularly-spaced peaks or 'preferred' intervals. The spacing of the peak intervals was, on average, 15% greater than the CF-period interval of the respective units. In TIH of lower-frequency cells without preferred intervals, the modal interval was also on average about 15% longer than the CF-period interval. Apparently, the resting oscillation frequency of these cells lies below their CF. Tuning curves (TCs) of neurones to short tone bursts show no systematic asymmetry as in mammals. Below CF 1 kHz, the low-frequency flanks of the TCs are, on average, steeper than the high-frequency flanks. Above CF 1 kHz, the reverse is true. The cochlear ganglion and nerve are tonotopically organized. Low-frequency fibres arise apically in the papilla basilaris and are found near non-auditory (lagenar) fibres. Discharge rates to short tones were monotonically related to sound pressure level. Saturation rates often exceeded 300 spikes s-1. 'On-off' responses and primary suppression of spontaneous activity were observed. A direct comparison of spontaneous activity and tuning-curve symmetry revealed that, apart from quantitative differences, fundamental qualitative differences exist between starling and guinea-pig primary afferents.     

Modulation of smooth muscle activity by catecholamines.

The inhibitory and excitatory actions of catecholamines are compared in various types of smooth muscle. Inhibition is usually but not invariably associated with membrane hyperpolarization and a decrease in membrane resistance. It also has a metabolic component frequently involving an increase in tissue cAMP. In some cases, the metabolic component is related to a cation pump, but the nature of this pump is unclear. With the exception of intestinal muscle where inhibition results from the synergistic action of alpha and beta receptors, inhibition is caused by activation of beta receptors. Excitation is mediated by alpha activation and is usually accompanied by a decrease in membrane potential and membrane resistance. Only the uterus has a metabolic component. The specific ionic permeability changes accompanying excitation are different in various smooth muscles. Clarification of the mechanisms responsible for these differential actions in various types of smooth muscle is a challenge for future work.     

Modulation by prostaglandins of contractions in guinea-pig ileum

A high concentration of indomethacin (40μg/ml) substantially reduced contractions of guinea-pig isolated ileum in Krebs solution to nerve stimulation with electrical pulses or nicotine. Responses to acetylcholine and histamine were also inhibited, but to a smaller extent. Low concentrations of prostaglandin E₂ (2 or 4ng/ml) mainly restored all the excitatory responses. Using a modified bathing solution (lacking in phosphate and with some other changes) indomethacin 0.36μg/ml selectively inhibited nerve-mediated contractions. The results explain differences in various reports, and support the possibility that prostaglandins modulate the response to cholinergic nerve activity.     

Modulation of the soleus muscle H reflex caused by ballistic voluntary arm movements in humans

In healthy humans, we studied the influence of conditioning voluntary arm movements on the H reflex induced by transcutaneous stimulation of the tibial nerve and recorded from the soleus muscle. We examined the effects of flexion and extension of the forearm, as well as of finger clenching performed with the maximum rate. Conditioning arm movements were self-induced or realized upon presentation of a visual signal (light flash). We found that the pattern of changes in the H reflex is determined by the position of the subject’s body in the course of tests. The ipsilateral arm flexion in the elbow joint in the standing position resulted in depression of the H reflex lasting about 100 msec from the beginning of the movement, while the effect observed in the lying position (on the couch with the feet hanging free in the air) looked like a facilitation of the reflex lasting about 100 to 200 msec. The direction and dynamics of modifications of the H reflex under conditions of the use of different conditioning movements (forearm flexions/extensions and finger clenching of the ipsilateral arm, as well as contralateral forearm flexions in the elbow joint) were rather similar. We also showed that the observed facilitation of the H reflex began earlier than the voluntary arm movement (40 to 50 msec prior to the beginning). We hypothesize that these conditioning influences result from the action of central motor commands and represent the factor related to anticipatory postural rearrangements. Such rearrangements are directed toward the maintenance of equilibrium of the body in the course of a future movement. These commands depend significantly on the spatial position of the subject’s body. Neirofiziologiya/Neurophysiology, Vol. 40, No. 2, pp. 147–154, March–April, 2008.     

Biophysical stimuli induced by passive movements compensate for lack of skeletal muscle during embryonic skeletogenesis

In genetically modified mice with abnormal skeletal muscle development, bones and joints are differentially affected by the lack of skeletal muscle. We hypothesise that unequal levels of biophysical stimuli in the developing humerus and femur can explain the differential effects on these rudiments when muscle is absent. We find that the expression patterns of four mechanosensitive genes important for endochondral ossification are differentially affected in muscleless limb mutants, with more extreme changes in the expression in the humerus than in the femur. Using finite element analysis, we show that the biophysical stimuli induced by muscle forces are similar in the humerus and femur, implying that the removal of muscle contractile forces should, in theory, affect the rudiments equally. However, simulations in which a displacement was applied to the end of the limb, such as could be caused in muscleless mice by movements of the mother or normal littermates, predicted higher biophysical stimuli in the femur than in the humerus. Stimuli induced by limb movement were much higher than those induced by the direct application of muscle forces, and we propose that movements of limbs caused by muscle contractions, rather than the direct application of muscle forces, provide the main mechanical stimuli for normal skeletal development. In muscleless mice, passive movement induces unequal biophysical stimuli in the humerus and femur, providing an explanation for the differential effects seen in these mice. The significance of these results is that forces originating external to the embryo may contribute to the initiation and progression of skeletal development when muscle development is abnormal.     

Modulation by prostaglandins of contractions in guinea-pig ileum.

A high concentration of indomethacin (40mu-g/ml) substantially reduced contractions of guinea-pig isolated ileum in Krebs solution to nerve stimulation with electrical pulses or nicotine. Responses to acetylcholine and histamine were also inhibited, but to a smaller extent. Low concentrations of prostaglandin E-2 (2 or 4ng/ml) mainly restored all the excitatory responses. Using a modified bathing solution (lacking in phosphate and with some other changes) indomethacin 0.36mu-g/ml selectively inhibited nerve-mediated contractions. The results explain differences in various reports, and support the possibility that prostaglandins modulate the response to cholinergic nerve activity.     

Modulation of electrical activity in gastrointestinal smooth muscle by peptides.

A rise in intracellular calcium is the predominant signal that leads to the activation of the contractile machinery in gastrointestinal smooth muscle. The primary sources of activating calcium are illustrated in Fig. 2. Voltage- and peptide-mediated release of intracellular calcium contribute to activation of some gastrointestinal smooth muscles. However, the primary source of activating calcium appears to be an influx of calcium across the plasma membrane. The degree of modulation of electrical activity by peptides varies depending upon the region of the gastrointestinal tract studied. Second messenger systems are undoubtly involved in the transduction pathway for receptor-mediated changes in ion channel activity in gastrointestinal smooth muscle. However, in comparison to other excitable cell types, little is known about the coupling mechanisms whereby peptide-receptor binding alters ion channel activity in gastrointestinal smooth muscle. This represents one of the challenging areas to be studied in the field of gastrointestinal smooth muscle. One disease in which a better appreciation of the regulation of ion channel activity could lead to therapeutic benefit is irritable bowel syndrome. A coupling of smooth muscle electrical activity to hypermotility in irritable bowel syndrome has been reported. CCK increases the level of spike activity which triggers hypermotility [40]. It would follow that inhibition of calcium influx should reduce spiking and, therefore, hypermotility. In fact, the calcium channel blockers nifedipine and nicardipine have been shown to decrease colonic motility in irritable bowel syndrome patients [62-64]. As our understanding of gastrointestinal smooth muscle ion channels expands, development of a gastrointestinal selective calcium channel blocker may be possible. This class of agents would be effective in the treatment of irritable bowel syndrome and potentially other peptide-related spastic smooth muscle disorders.     

Modulation of arterial baroreflex control of muscle sympathetic nerve activity by muscle metaboreflex in humans

We aimed to investigate the interaction [with respect to the regulation of muscle sympathetic nerve activity (MSNA) and blood pressure] between the arterial baroreflex and muscle metaboreflex in humans. In 10 healthy subjects who performed a 1-min sustained handgrip exercise at 50% maximal voluntary contraction followed by forearm occlusion, arterial baroreflex control of MSNA (burst incidence and strength and total activity) was evaluated by analyzing the relationship between beat-by-beat spontaneous variations in diastolic arterial blood pressure (DAP) and MSNA both during supine rest (control) and during postexercise muscle ischemia (PEMI). During PEMI (vs. control), 1) the linear relationship between burst incidence and DAP was shifted rightward with no alteration in sensitivity, 2) the linear relationship between burst strength and DAP was shifted rightward and upward with no change in sensitivity, and 3) the linear relationship between total activity and DAP was shifted to a higher blood pressure and its sensitivity was increased. The modification of the control of total activity that occurs in PEMI could be a consequence of alterations in the baroreflex control of both MSNA burst incidence and burst strength. These results suggest that the arterial baroreflex and muscle metaboreflex interact to control both the occurrence and strength of MSNA bursts.     

Long-term repeatability of force, endurance time and muscle activity during isometric contractions.

We determined the repeatability and correlations between force, endurance and muscle activity during isometric contractions over three years. Twenty-six subjects, with and without complaints of the shoulder and neck, performed standardized maximal and submaximal shoulder-abduction contractions and wrist extension-contractions at yearly intervals from 1997 to 1999. Peak forces developed during maximal contraction and the endurance times of submaximal contractions during shoulder abduction and wrist extension were measured. Electromyography (EMG) of muscle activity was recorded bilaterally from the upper trapezius, middle deltoid, and forearm extensor muscles. Root mean square EMG amplitudes were calculated. We found statistically significant associations between peak forces developed during wrist extension and shoulder abduction, and between endurance times of submaximal wrist extension and shoulder abduction. No statistically significant changes in peak force and EMG(peak) were found over the measurement years. The responses were not statistically significantly influenced by gender, or neck and shoulder pain. However, we observed considerable intra-individual variation in the inter-year measurements particularly for the responses to submaximal contraction. Such large variations represent a challenge when attempting to use the responses to interpret the effects of therapies.     

Changes in the time course of potentiated and fatigued contractions of fast motor units in rat muscle

The contraction and relaxation times of the twitches and the last contractions within 32 unfused tetani of FF and 27 unfused tetani of FR motor units in the rat medial gastrocnemius muscle were studied during prolonged activity. The pattern of the MU stimulation included single pulses (to evoke twitches) and series of three trains of stimuli at 40, 50 and 60 Hz (to evoke unfused tetani), repeated 30 times. The analysis concerned changes of force and time parameters at the beginning of activity, during the potentiation and then during the fatigue. It was found that changes of force during the potentiation and the fatigue were mainly accompanied by changes in the course of relaxation. The significant prolongation of the half-relaxation time during the potentiation of either twitches or unfused tetani was revealed in both types of fast MU. The twitch contraction time did not change markedly, whereas significantly shortened in the last contractions of unfused tetani during the potentiation. These changes of time parameters correlated to the increase of the fusion degree. During the fatigue, the time parameters shortened, however, changes of the half-relaxation times were remarkably higher. The shortening of relaxation was responsible for the decrease of the fusion degree. Changes of the fusion index exceeding 0.75 during the potentiation or decreasing below this value during the fatigue, were accompanied by respective appearance or disappearance of the biphasic relaxation.     

Physiological characteristics of motor units in the brachioradialis muscle across fatiguing low-level isometric contractions.

The purpose of this study was to determine (i) if decomposition-based quantitative electromyography (DQEMG) could detect changes in motor unit potential (MUP) morphology and motor unit (MU) firing pattern statistics associated with muscle fatigue, (ii) if any detected changes are correlated with surface electromyographic (SEMG) signs of fatigue, and (iii) if significant fatigue-dependent changes are repeatable within individuals. Mean MU firing rates and the morphology of MUPs detected using needle and surface electrodes during constant-torque isometric contractions held until exhaustion were investigated in the brachioradialis (BR) muscle in 10 healthy volunteers (mean age=28.6 yr, SD+/-3.9). Time dependant changes were investigated using an analysis of variance with normalized time as a main effect. Partial correlation coefficients were computed using a repeated measures analysis of covariance to determine if changes in MU firing rates, needle-detected MUPs and surface-detected MUPs (SMUPs) were related to changes in SEMG signal amplitude and frequency parameters. Intraclass correlation coefficients (ICCs) were used to determine the within-subject repeatability of changes in MU firing rates, and MUP and SMUP parameters. Significant decreases in mean MU firing rates were found along with significant increases in various duration and area related parameters in both MUPs and SMUPs across the fatiguing contraction. The SEMG signal demonstrated the expected changes with fatigue: an increase in amplitude and a decrease in frequency content. SEMG amplitude was significantly positively correlated with SMUP peak-to-peak voltage (r=0.85, p<0.05), and SMUP area (r=0.86, p<0.05). Mean power frequency was significantly negatively correlated with SMUP negative peak duration (r=-0.74, p<0.05). The significant time-dependent changes were reliably observed (ICCs were 0.94 for MUP peak to peak amplitude, 0.97 for MUP area and 0.95 for MUP area to amplitude ratio, 0.95 for SMUP peak-to-peak voltage, 0.83 for SMUP area, 0.99 for SMUP negative peak amplitude and 0.88 for SMUP negative peak area). The decreases in mean MU firing rates measured along with the increases in amplitude, duration and area parameters of MUPs and SMUPs and their partial correlation with SEMG amplitude during submaximal fatiguing contractions of the BR, suggest that recruitment is a main cause of increased SEMG amplitude parameters with fatigue. We conclude that DQEMG can be effectively and reliably used to detect changes in physiological characteristics of MUs that accompany fatigue.     

Firing rates of motor units in human vastus lateralis muscle during fatiguing isometric contractions.

We investigated the firing rate of motor units in the vastus lateralis muscle in five healthy young men (mean = 21.4 yr, SD = 0.9) during a sequence of isometric constant-torque contractions repeated to exhaustion. The contractions were sustained at 20% of the maximal voluntary level, measured at the beginning of the test sequence. Electromyographic (EMG) signals were recorded via quadrifilar fine-wire electrodes and subsequently decomposed into their constituent motor unit action potentials to obtain the motor unit firing times. In addition, we measured the whole muscle mechanical properties during the fatigue task using electrical stimulation. The firing rate of motor units first decreased within the first 10-20% of the endurance time of the contractions and then increased. The firing rate increase was accompanied by recruitment of additional motor units as the force output remained constant. The elicited twitch and tetanic torque responses first increased and then decreased. The two processes modulated in a complementary fashion at the same time. Our data suggest that, when the vastus lateralis muscle is activated to maintain a constant torque output, its motoneuron pool receives a net excitatory drive that first decreases to compensate for the short-lived potentiation of the muscle force twitch and then increases to compensate for the diminution of the force twitch. The underlying inverse relationship between the firing rate and the recruitment threshold that has been reported for nonfatigued contractions is maintained. We, therefore, conclude that the central nervous system control of vastus lateralis motor units remains invariant during fatigue in submaximal isometric isotonic contractions.     

Modulation of smooth muscle activity by excitatory and inhibitory nerves in the guinea-pig stomach

• 1.1. In smooth muscle of the guinea-pig stomach, intramural nerve stimulation evoked cholinergic excitatory junction potential in the fundus and non-adrenergic non-cholinergic inhibitory junction potential in the antrum, yet cholinergic contractions in both regions.
• 2.2. This dissociation between electrical and mechanical responses was mainly due to different sensitivity of the membrane for depolarization to acetylcholine.

     

Contour analysis of masticatory jaw movements and muscle activity in Macaca mulatta

Frontal plane mandibular movements during mastication and the associated electromyographic (EMG) activity for left and right superficial masseter, posterior temporalis, anterior temporalis, and anterior belly of the digastric (ABD) were studied for two adult male Macaca mulatta by the new technique of “contour” analysis. Contour analysis allowed graphic and quantitative portrayal of multiple chew cycle patterns of mandibular movement and EMG activity during active mastication. A series of computer programs (ATS, ATSED, ATSXYZ) facilitated the collection, editing and definition, and finally processing of these masticatory data into contour plots. These preliminary data indicated the essential symmetry of mandibular movement patterns, high chew cycle variability inferior to occlusion, multiple centers of intense EMG activity for balancing-side superficial masseter, and no difference between working-side anterior and posterior temporalis EMG patterns. Maximum EMG amplitude was found in the area of buccal phase power stroke (BPS). Maximum EMG amplitude for ABD was located medial and inferior to occlusion; all other muscle maximum amplitudes were buccal and inferior to occlusion. The location of maximum EMG amplitudes for superficial masseter and ABD were closer to occlusion (more superior) during mastication of carrot than were maximum amplitudes during biscuit mastication. The absence of any detectable shift of EMG maximum amplitude location between biscuit and carrot for posterior and anterior temporalis suggested, along with the continuous EMG activity of working-side posterior temporalis, a secondary role for the temporalis (compensation for superficial masseter activity) during active mastication.     

Relationship between work and electromyographic activity during repeated leg muscle contractions in orienteers

Contraction work (CW) was recorded for each of 200 repetitive isokinetic plantar flexions (1.05 rad.s-1) and knee extensions (1.57 rad.s-1) in 14 elite male orienteers. Simultaneous recordings of integrated electromyograms (iEMG) were obtained from the 3 parts of triceps surae and from 3 superficial portions of quadriceps femoris. CW in both muscle groups decreased significantly during the first 30 contractions (the fatigue phase), followed by a steady state level. The relative steady state level was higher for the plantar flexors (70 +/- 17%) than for the knee extensors (56 +/- 12%). For quadriceps a significant increase in iEMG occurred during the first 10 contractions followed by a decrease, whereas the iEMG of the plantar flexors showed a gradual decrease to the steady state level, which was similar for the two muscle groups (71-72%). The chosen expression of output/input balance (CW/iEMG) was constant throughout the plantarflexion test but decreased during the initial 20 knee extensions down to 82%. Thus, the fatigue phase of the knee extensions appeared to be divided into two; the first part had decreases in both CW and CW/iEMG and the second part with a decrease in CW alone. In contrast the plantar flexors only showed the characteristics of the second part.     

Electromechanical delay in isometric muscle contractions evoked by voluntary, reflex and electrical stimulation

Electromechanical delay (EMD) in isometric contractions of knee extensors evoked by voluntary, tendon reflex (TR) and electrical stimulation (ES) was investigated in 21 healthy young subjects. The subject performed voluntary knee extensions with maximum effort (maximal voluntary contraction, MVC), and at 30%, 60% and 80% MVC. Patellar tendon reflexes were evoked with the reflex hammer being dropped from 60°, 75° and 90° positions. In the percutaneous ES evoked contractions, single switches were triggered with pulses of duration 1.0 ms and of intensities 90, 120 and 150 V. Electromyograms of the vastus lateralis and rectus femoris muscles were recorded using surface electrodes. The isometric knee extension force was recorded using a load cell force transducer connected to the subject's lower leg. The major finding of this study was that EMD of the involuntary contractions [e.g. mean 22.1 (SEM 1.32) ms in TR 90°; mean 17.2 (SEM 0.62) ms in ES 150 V] was significantly shorter than that of the voluntary contractions [e.g. mean 38.7 (SEM 1.18) ms in MVC,P < 0.05]. The relationships between EMD, muscle contractile properties and muscle fibre conduction velocity were also investigated. Further study is needed to explain fully the EMD differences found between the voluntary and involuntary contractions.     

Limb movements generated by stimulating muscle,nerve and spinal cord

We have compared the movements generated by stimulation of muscle, nerve, spinal roots and spinal cord in anesthetized, decerebrate and spinalized cats. Each method produced a full range of movements of the cat's hind limb in the sagittal plane against a spring load, except for stimulation of the roots. Stimulation of the dorsal roots produced movements that were mainly up and forward, whereas stimulation of the ventral roots produced complementary movements (down and backward). Results from stimulation in the intermediate areas of the spinal cord were compared to predictions of the "movement primitives" hypothesis. We could not confirm that the directions were independent of stimulus amplitude or the state of descending inputs. Pros and cons of stimulating at some sites were provisionally considered for the reliable control of limb movements with functional electrical stimulation (FES) in clinical conditions.