State of monosynaptic reflex (H reflex) arc during voluntary muscle contraction in humans

Effect of an isometric voluntary contraction of the soleus muscle (5–6 sec in duration) on the H reflex was studied in persons aged 25–45 years. A sharp enhancement of the H reflex at a dynamic phase of fast contraction (in the force increase period) and its less pronounced increase at a static phase of contraction were shown. When a paired stimulation of the nerve was used (interstimulus interval, 500 msec), a voluntary contraction of the muscle fully abolished the first stimulus-related suppression of the second H reflex. When the muscle contracted against the background of vibration, the suppressed H reflex recovered up the the level observed in the resting muscle, but did not reach the level characteristic for the contracting muscle. The findings show that the two pathways controlling the H reflex (descending and afferent) function independently, and apparently there exist at least two corresponding groups of interneurons causing a presynaptic inhibition of the H reflex.Neirofiziologiya/Neurophysiology, Vol. 25, No. 5, pp. 365–371, September–October, 1993.     

Activity of propriospinal neurons during the scratch reflex in cats

Fictitious scratching, i.e., rhythmic activity of hind-limb motoneurons at the characteristic scratching frequency, was evoked by tactile stimulation of the ear in thalamic cats immobilized with flaxedil. Activity of propriospinal neurons in segments C₁, C₂, and T₄–T₇ was recorded extracellularly. The neurons were identified by their antidromic response to stimulation of their axons in segment L₁. Most neurons did not respond to stimulation of the ear. Some neurons, however, were activated during fictitious scratching. Neurons of the cervical segments responded not only to stimulation of the ear, but also to tactile stimulation of the forelimbs and also to passive movements of those limbs. Neurons of the thoracic segments were activated only by stimulation of the ipsilateral ear; these neurons were inhibited by stimulation of the contralateral ear. The role of the propriospinal neurons in the activation of the spinal mechanisms of scratching is discussed.Institute for Problems of Information Transmission, Academy of Sciences of the USSR, Moscow. M. V. Lomonosov Moscow State University. Translated from Neirofiziologiya, Vol. 9, No. 5, pp. 504–511, September–October, 1977.     

Synchronization of human motor unit firing during voluntary contraction of muscle and the tonic vibration reflex

We constructed cross-correlograms (CCGs) of action potentials of pairs of motor units (MUs) of human soleus, triceps brachii, and the first and second dorsal interosseous muscles. During voluntary muscle contraction, a pronounced peak in the zero bin was found in 21 out of 126 pairs investigated with the aid of the CCG; this indicates that the number of coincidental firings exceeded chance. The width of the peak did not exceed 5 msec (synchronization for a brief interval, i.e., short-term synchronization). When motoneurons of the soleus muscle were activated by vibration, correlations were found in 12 out 89 pairs of MUs investigated. On the CCGs of action potentials of MU pairs in two muscles (the first and second dorsal interosseous muscles), such correlations were found in four out of 10 pairs investigated. In all of these cases, the ratio of above-change coincidences relative to the total number of MU discharges was small, from 3.0 to 6.1%. Synchronization within a brief time interval can be considered a result of simultaneous creation of EPSPs in motoneourons reached by endings of a single pre-motor nerve fiber. In some pairs of MUs, long term synchronization (clustering) occurred, ie., synchronization lasting several tens of milliseconds. The long-term synchronization can be considered a manifestation of fatigue accompanying tremor.Institute for Problems of Information Transmission, Russian Academy of Sciences, Moscow. Translated from Neirofiziologiya, Vol. 23, No. 6, pp. 691–698, November–December, 1991.     

Activity of sensorimotor cortex neurons during a dopaminergic transmission block in cats performing conditional reflex

Activity of sensorimotor cortex neurons was recorded in chronic experements on cats trained to perform instrumental conditional reflex; records were made before, during, and after isolated iontophoretic applications of haloperidol or glutamate, or their combined application. Haloperidol was shown either to facilitate or to inhibit the background and evoked (related to acoustic stimulation and motor response) spike activity of cortical neurons. Aftereffects of haloperidol were observed too; they were still expressed 10–15 min after the cessation of the iontophoresis. Combined haloperidol and glutamate application was followed by a sharp suppression of the evoked responses potentiated earlier by glutamate. An adenylatecyclase system is supposed to mediate the facilitation evoked by glutamate application. Some modulators, including dopamine, probably activate adenylatecyclase and in this way ensure facilitation of the glutamate-induced responses.Neirofiziologiya/Neurophysiology, Vol. 26, No. 5, pp. 347–355, September–October, 1994.     

Nonuniform volume changes during muscle contraction.

We measured dynamic changes in volume during contraction of live, intact frog skeletal muscle fibers through a high-speed, intensified, digital-imaging microscope. Optical cross-sections along the axis of resting cells were scanned and compared with sections during the plateau of isometric tetanic contractions. Contraction caused an increase in volume of the central third of a cell when axial force was maximum and constant and the central segment was stationary or lengthened slightly. But changes were unequal along a cell and not predicted by a cell's resting area or shape (circularity). Rapid local adjustments in the cytoskeletal evidently keep forces in equilibrium during contraction of living skeletal muscle. These results also show that optical signals may be distorted by nonuniform volume changes during contraction.     

Responses of bulbospinal neurons during expiration reflex in cats

The responses of 129 bulbospinal (BS) neurons were investigated an anesthetized cats during the expiration reflex evoked by mechanical stimulation of the vocal folds or electrostimulation of the superior laryngeal nerve. It was found that in addition to expiratory and inspiratory neurons, a large number of nonrespiratory BS cells localized bilaterially, primarily in the giant-cell tegmental fields, are activated. It is shown that expulsive expiration during this reflex is to a considerable extent provided by activation of non-respiratory BS neurons participating also in the transmission of descending influences from high-threshold afferents of other modalities.A. A. Bogomolets Institute of Physiology, Academy of Sciences of the Ukrainian SSR, Kiev. Translated from Neirofiziologiya, Vol. 23, No. 1, pp. 88–98, January–February, 1991.     

In vivo electrophysiological responses of pedunculopontine neurons to static muscle contraction

The pedunculopontine nucleus (PPN) has previously been implicated in central command regulation of the cardiorespiratory adjustments that accompany exercise. The current study was executed to begin to address the potential role of the PPN in the regulation of cardiorespiratory adjustments evoked by muscle contraction. Extracellular single-unit recording was employed to document the responses of PPN neurons during static muscle contraction. Sixty-four percent (20/31) of neurons sampled from the PPN responded to static muscle contraction with increases in firing rate. Furthermore, muscle contraction-responsive neurons in the PPN were unresponsive to brief periods of hypotension but were markedly activated during chemical disinhibition of the caudal hypothalamus. A separate sample of PPN neurons was found to be moderately activated during systemic hypoxia. Chemical disinhibition of the PPN was found to markedly increase respiratory drive. These findings suggest that the PPN may be involved in modulating respiratory adjustments that accompany muscle contraction and that PPN neurons may have the capacity to synthesize muscle reflex and central command influences.     

Control of respiration and bioenergetics during muscle contraction

¹H-NMR experiments have determined intracellular O₂ consumption (O₂) with oxymyoglobin (MbO₂) desaturation kinetics in human calf muscle during plantar flexion exercise at 0.75, 0.92, and 1.17 Hz with a constant load. At the onset of muscle contraction, myoglobin (Mb) desaturates rapidly. The desaturation rate constant of 30 s reflects the intracellular O₂. Although Mb desaturates quickly with a similar time constant at all workload levels, its final steady-state level differs. As work increases, the final steady-state cellular PO₂ decreases progressively. After Mb desaturation has reached a steady state, however, O₂ continues to rise. On the basis of current respiratory control models, the analysis in the present report reveals two distinct O₂ phases: an ADP-independent phase at the onset of contraction and an ADP-dependent phase after Mb has reached a steady state. In contrast to the accepted view, the initial intracellular O₂ shows that oxidative phosphorylation can support up to 36% of the energy cost, a significantly higher fraction than expected. Partitioning of the energy flux shows that a 31% nonoxidative component exists and responds to the dynamic energy utilization-restoration cycle (which lasts for only milliseconds) as postulated in the glycogen shunt theory. The present study offers perspectives on the regulation of respiration, bioenergetics, and Mb function during muscle contraction. myoglobin; nuclear magnetic resonance; glycogen; oxygen; exercise     

Structural dynamics of frog muscle during isometric contraction

Intensity fluctuation autocorrelation functions of laser light scattered by actively contracting muscle were measured at points in the scattered field. They were reproducible and showed characteristics which depended on the physiological state of the muscle and the parameters of the scattering geometry. The autocorrelation functions had large amplitudes and decay rates that varied significantly with the phase of the contraction-relaxation cycle. The dependence of the autocorrelation function on scattering geometry indicated many elements with diameters on the order of 0.5 mum (presumed to be myofibrillar sarcomeres or their A bands or I bands) undergo independent random changes in their axial positions and their internal distribution of optical polarizability during the plateau of an isometric tetanus. The experimental results are interpreted in terms of a model in which most of the scattering elements in isometrically contracting muscle have random fluctuating axial velocities of average magnitude 20 nm/ms that persist for a few milliseconds at least. In addition to these axial motions there are local fluctuations in polarizability. Similar intensity fluctuation autocorrelation functions were observed throughout the active state on two muscle preparations, whole sartorius muscle and small bundles of single fibers (three to eight) of semitendinosus muscle. These results imply that the tension developed during an isometric tetanus contains a fluctuating component as well as a constant component.     

Alkalinization of phosphorylase kinase-deficient muscle during tetanic contraction

The intracellular pH of resting and stimulated muscle was monitored by two independent methods: measurement of pH iniodacetate-treated homogenates of freezeclamped tissue and the absorbance at 550–443 nm of intracellular neutral red dye in vivo. During tetanic stimulation, muscle of phosphorylase kinase-deficient mice shows a transient alkalinization whereas muscle in normal mice becomes more acid under similar conditions. The alkalinization appears to be caused by abnormally rapid AMP deamination associated with adaptation to phosphorylase kinase deficiency.     

Development of muscle spindles deprived of fusimotor innervation

Summary Muscle spindles of limb muscles were deefferented in neonatal rats by sectioning ventral roots or by removal of the lumbosacral spinal cord.Ten to 56 days after the operation, muscle spindles were examined in the medial gastrocnemius, extensor digitorum longus and soleus muscles. The differentiation of muscle spindles was not affected by deefferentation. The number of spindles in the investigated muscles was not reduced. Intrafusal fibres increased in number from two at birth to four per spindle on the average, as in normal muscles. The characteristic ultrastructural distinctions of nuclear bag and nuclear chain fibres developed as under normal conditions. However, intrafusal fibres atrophied slowly after fusimotor denervation, their polar zones becoming reduced in diameter by about 25% in comparison with control fibre diameters. Spindle capsules, on the other hand, increased in size and attained diameters comparable with normal spindles, appearing even somewhat distended.As intrafusal fibres degenerate after complete denervation at birth (Zelená, 1957), but differentiate in the absence of fusimotor innervation, it can be concluded that sensory nerve terminals induce and support their development. It is assumed that the morphogenetic influence of sensory terminals is mediated by release and uptake of a trophic substance at the synaptic junction. The occurrence of light and dense core vesicles in the sensory terminals and of coated invaginations and vesicles at both the axonal and plasma membrane speak in favour of such a possibility.The authors wish to thank Mrs. M. Sobotková, Dr. Z. Liková, Mr. H. Kunz and Ing. M. Doubek for their skillful technical assistance.     

Protein phosphorylation during spontaneous contraction of smooth muscle

The relationship between spontaneous contraction and protein phosphorylation of rat uterine smooth muscle was studied. Myometrial strips from estrogen-dominated rats were incubated in [³²P]orthophosphate and then frozen at various levels of isometric tension. Proteins were separated by gel electrophoresis and the incorporation of ³²P was measured. Contraction was associated with the phosphorylation of one major protein (20,000 M_r). This phosphorylation preceded maximal tension development and dephosphorylation preceded complete spontaneous relaxation. Two-dimensional gel electrophoresis indicates that the 20,000-M_r protein is the myosin light chain which has been implicated in the regulation of smooth muscle contraction.