Effects of Ba ions on frog's isolated muscle spindle.

Dilute solutions of Barium Chloride have been tested on frog's isolated muscle spindles with the aim of clarifying the processes leading to the afferent discharge. In the presence of Ba++ the spindle properties are modified as follows: 1. The discharge of slack spindles increases and becomes extremely regular. 2. The phasic response to stretching is enhanced, the tonic responses being on the contrary virtually suppressed. Under Ba-treatment therefore the spindle changes from a phasic-tonic receptor to a merely phasic receptor. 3. The "silent period" following every propagated spike is prolonged while the safety margin of the "abortive" into propagated spike conversion becomes maximal under Ba-treatment. Additionally, this analysis showed that, when a "true" action potential arises in the spindle, it propagates not only to the stem fibre, but also to all the sensory endings in the spindle intracapsular network. Comparison between the effects of Ba++ and those produced by TEA and ouabain indicates that the action of Ba++ on muscle spindles results from conjunction of two factors : a) reduction of K permeability, and b) enhanced efficiency of an electrogenic Na/K-pump in spindle nerve membranes.     

Modulation of laryngeal and respiratory pump muscle activities with upper airway pressure and flow.

The hypothesis that upper airway (UA) pressure and flow modulate respiratory muscle activity in a respiratory phase-specific fashion was assessed in anesthetized, tracheotomized, spontaneously breathing piglets. We generated negative pressure and inspiratory flow in phase with tracheal inspiration or positive pressure and expiratory flow in phase with tracheal expiration in the isolated UA. Stimulation of UA negative pressure receptors with body temperature air resulted in a 10--15% enhancement of phasic moving-time-averaged posterior cricoarytenoid electromyographic (EMG) activity above tonic levels obtained without pressure and flow in the UA (baseline). Stimulation of UA positive pressure receptors increased phasic moving-time-averaged thyroarytenoid EMG activity above tonic levels by 45% from baseline. The same enhancement of posterior cricoarytenoid or thyroarytenoid EMG activity was observed with the addition of flow receptor stimulation with room temperature air. Tidal volume and diaphragmatic and abdominal muscle activity were unaffected by UA flow and/or pressure, whereas respiratory timing was minimally affected. We conclude that laryngeal afferents, mainly from pressure receptors, are important in modulating the respiratory activity of laryngeal muscles.     

Model-based prediction of fusimotor activity and its effect on muscle spindle activity during voluntary wrist movements

Muscle spindles provide critical information about movement position and velocity. They have been shown to act as stretch receptors in passive muscle, however, during active movements their behavior is less clear. In particular, spindle responses have been shown to be out-of-phase or phase advanced with respect to their expected muscle length-sensitivity. Whether this apparent discrepancy of spindle responses between passive and active movements is due to fusimotor (γ-drive) remains unresolved, since the activity of fusimotor neurons during voluntary non-locomotor movements are largely unknown. We developed a computational model to predict fusimotor activity and to investigate whether fusimotor activity could explain the empirically observed phase advance of spindle responses. The model links a biomechanical wrist model to length- and γ-drive-dependent transfer functions of type Ia and type II muscle spindle activity. Our simulations of two wrist-movement tasks suggest that (i) experimentally observed type Ia and type II activity profiles can to a large part be explained by appropriate, i.e. strongly modulated and task-dependent, γ-drive. That (ii) the empirically observed phase advance of type Ia or of type II profiles during active movement can be similarly explained by appropriate γ-drive. In summary, the simulation predicts that a highly task-modulated activation of the γ-system is instrumental in producing a large part of the empirically observed muscle spindle activity for voluntary wrist movements.     

Types of spontaneous discharges of muscle receptors

Characteristics of discharges of 462 muslce spindle receptors located in the deefferented triceps surae muscle with a divided tendon were analyzed in anesthetized cats. Of the total number, 205 units had spontaneous activity which, depending on its stability, could be divided into three types. A long continuous spike train was generated by 87% of units. Distributions of interspike intervals were unimodal and symmetrical. Spontaneous activity of this type was more regular in secondary than in primary endings. Action potentials in 8% of units were grouped in volleys and interval distributions were bimodal. Spontaneous activity of the remaining 5% of units was characterized by sporadically appearing spikes with long intervals between them. The first two types of spontaneous activity cannot be placed in the category of stochastic processes. Threshold discharges during static stretching of the muscle sufficient to cause the appearance of a long spike train were studied in 231 of 257 initially "silent" units. By the same criteria these discharges were divided into corresponding types. Comparison of the characteristics of spontaneous activity and evoked responses suggests that they share the same mechanism of origin. It is concluded that the spontaneous discharge can be regarded as a static response of the receptors due not to an externally applied stretching load, but to the action of internal factors, depending on interaction between extrafusal and intrafusal muscle fibers.I. P. Pavlov Institute of Physiology, Academy of Sciences of the USSR, Leningrad. Translated from Neirofiziologiya, Vol. 13, No. 3, pp. 315–321, May–June, 1981.     

Effect of hypertonic solutions on electrical activity of the isolated frog muscle spindle

Changes in electrical activity of the isolated frog muscle spindle were investigated in hypertonic solutions obtained by adding 400 mM sucrose, glucose, or glycerol to Ringer's solution. The spontaneous firing rate in hypertonic sucrose and glucose solutions increased at first (for 3–5 min) and then fell rapidly to zero; the receptor potential and evoked spike activity diminished under these conditions and disappeared. In the hypertonic solution with glycerol a similar effect was observed but, unlike in the first two media, in this case spike activity returned after its initial increase to the normal level; a second rise in the firing rate was then observed up to a steady value which was higher than normal. After rinsing out the hypertonic sucrose and glucose solutions with ordinary Ringer's solution the spontaneous and evoked activity gradually returned to normal with a small overshoot. During the rinsing out of the hypertonic glycerol solution a sharp and considerable rise in spontaneous activity was first observed, while the changes in frequency of the evoked activity were negligible. The spike activity then returned to normal. The observed changes in electrical activity of the muscle spindle in hypertonic media are attributed to deformation of the sensory terminals and intrafusal muscle fibers (in the glycerol medium), leading to depolarization of the receptor membrane.P. K. Anokhin Institute of Normal Physiology, Academy of Medical Sciences of the USSR, Moscow. Translated from Neirofiziologiya, Vol. 8, No. 3, pp. 291–299, May–June, 1976.     

The effects of cold-induced muscle spindle secondary activity on monosynaptic and stretch reflexes in the decerebrate cat.

The effects of muscle spindle secondary ending activity on the stretch reflex were studied in unanesthetized decerebrate cats. Activation of secondary endings was accomplished by reducing the muscle temperature. This has been shown to cause a sustained asynchronous discharge from secondary endings. Cooling of the medial gastrocnemius or lateral gastrocnemius-soleus muscles caused an increase in the phasic and tonic components of their stretch reflexes. Cooling of the relaxed medial gastrocnemius muscle caused similar increases in the components of the stretch reflex of the synergistic lateral gastrocnemius-soleus muscle and an increase in its monosynaptic reflex. It was concluded that the facilitatory autogenetic and synergistic effects of muscle cooling on the stretch and monosynaptic reflexes were brought about by activity in group II afferents from muscle spindle secondary endings and could not be ascribed to any other type of muscle receptor. These results support the concept of an excitatory role for the secondary endings of the muscle spindle in the stretch reflex of the decerebrate cat.     

Inhibitory and excitatory mechanisms of neurotensin action in canine intestinal circular muscle in vitro.

The effect of neurotensin on canine ileal circular muscle devoid of myenteric plexus was investigated using single and double sucrose gap techniques. Similar results were obtained with microelectrode techniques. Neurotensin caused a temperature-sensitive and dose-dependent biphasic response, an initial hyperpolarization associated with inhibition of contractile activity, followed by an excitatory phase, usually consisting of spike discharge and tonic and phasic contractions, for which depolarization was not required. Neither response was affected by tetrodotoxin, phentolamine, propranolol, or atropine. The hyperpolarization was associated with decreased membrane resistance, blocked by 10(-7) M apamin, and converted to tonic depolarization by apamin (10(-6) M). Tachyphylaxis to neurotensin occurred when the stimulation interval was less than 20 min. After Ca2+ depletion, depolarization was observed instead of the hyperpolarization; this depolarization was not affected by nitrendipine and was gradually abolished with repetitive stimulation at 20-min intervals. When Ca2+ was present, nifedipine did not alter the hyperpolarizing phase of the response but inhibited spiking and blocked all contractions. The excitatory phase of the response was enhanced by Bay K-8644. Neuromedin N elicited a response identical with that of neurotensin. The responses of the two peptides were completely cross tachyphylactic. Inhibitory junction potentials were not affected by neurotensin tachyphylaxis. It is concluded that neurotensin and neuromedin N activate apamin-sensitive, calcium-dependent potassium channels in circular muscle, causing membrane hyperpolarization and inhibition of muscle contraction. Release of intracellular calcium is involved in the activation of these potassium channels.(ABSTRACT TRUNCATED AT 250 WORDS)     

Myosin heavy chain composition of muscle spindles in human biceps brachii.

Data on the myosin heavy chain (MyHC) composition of human muscle spindles are scarce in spite of the well-known correlation between MyHC composition and functional properties of skeletal muscle fibers. The MyHC composition of intrafusal fibers from 36 spindles of human biceps brachii muscle was studied in detail by immunocytochemistry with a large battery of antibodies. The MyHC content of isolated muscle spindles was assessed with SDS-PAGE and immunoblots. Four major MyHC isoforms (MyHCI, IIa, embryonic, and intrafusal) were detected with SDS-PAGE. Immunocytochemistry revealed very complex staining patterns for each intrafusal fiber type. The bag(1) fibers contained slow tonic MyHC along their entire fiber length and MyHCI, alpha-cardiac, embryonic, and fetal isoforms along a variable part of their length. The bag(2) fibers contained MyHC slow tonic, I, alpha-cardiac, embryonic, and fetal isoforms with regional variations. Chain fibers contained MyHCIIa, embryonic, and fetal isoforms throughout the fiber, and MyHCIIx at least in the juxtaequatorial region. Virtually each muscle spindle had a different allotment of numbers of bag(1), bag(2) and chain fibers. Taken together, the complexity in intrafusal fiber content and MyHC composition observed indicate that each muscle spindle in the human biceps has a unique identity.     

Separation and estimation of muscle spindle and tension receptor populations by vibration of the biceps muscle in the frog

Frog spinal cord reflex behaviors have been used to test the idea of spinal primitives. We have suggested a significant role for proprioception in regulation of primitives. However the in vivo behavior of spindle and golgi tendon receptors in frogs in response to vibration are not well described and the proportions of these proprioceptors are not established. In this study, we examine the selectivity of muscle vibration in the spinal frog. The aim of the study was (1) to examine how hindlimb muscle spindles and GTO receptors are activated by muscle vibration and (2) to estimate the relative numbers of GTO receptors and spindle afferents in a selected muscle, for comparison with the mammal. Single muscle afferents from the biceps muscle were identified in the dorsal roots. These were tested in response to biceps vibration, intramuscular stimulation and biceps nerve stimulation. Biceps units were categorized into two types: First, spindle afferents which had a high conduction velocity (approximately 20-30 m/s), responded reliably (were entrained 1:1) to muscle vibration, and exhibited distinct pauses to shortening muscle contractions. Second, golgi tendon organ afferents, which had a lower conduction velocity (approximately 10-20 m/s), responded less reliably to muscle vibration at physiologic muscle lengths, but responded more reliably at extended lengths or with background muscle contraction, and exhibited distinct bursts to shortening muscle contractions. Vibration responses of these units were tested with and without muscle curarization. Ensemble (suction electrode) recordings from the dorsal roots were used to provide rough estimates of the proportions of the two muscle afferent types.     

The response of chelonian muscle spindles to mechanical stimulation

Single unit recordings have been made from the muscle spindles of the extensor digitorum brevis I muscle of the chelonian emys orbicularis. The responses to ramp-type mechanical stretches (up to 5 mm s^?1 velocity and up to 1.6 mm extension) were compared to those of spindles from other groups. It is found that the spindles have lower rates of firing than those from the other groups with the exception of the snake spindles. Generally the spindles behaved like the secondary ending of mammalian spindles or the tonic type of snake spindle in terms of their response to the velocity of stretch. The results are consistent with the view that the tonic response arises from intrafusal muscle fibres in which the sensory region has a structure which is fairly uniform and similar to that of the polar regions and not interrupted by accumulations of nuclei.     

Oscillatory entrainment of striatal neurons in freely moving rats

Oscillations and synchrony in basal ganglia circuits may play a key role in the organization of voluntary actions and habits. We recorded single units and local field potentials from multiple striatal and cortical locations simultaneously, over a range of behavioral states. We observed opposite gradients of oscillatory entrainment, with dorsal/lateral striatal neurons entrained to high-voltage spindle oscillations ("spike wave discharges") and ventral/medial striatal neurons entrained to the hippocampal theta rhythm. While the majority of units were likely medium-spiny projection neurons, a second neuronal population showed characteristic features of fast-spiking GABAergic interneurons, including tonic activity, brief waveforms, and high-frequency bursts. These fired at an earlier spindle phase than the main neuronal population, and their density within striatum corresponded closely to the intensity of spindle oscillations. The orchestration of oscillatory activity by networks of striatal interneurons may be an important mechanism in the pathophysiology of neurological disorders such as Parkinson's disease.     

Gravity mechanisms in tonic motor system. Neurophysiological and muscle aspects

Nowadays it is widely believed that the animal motor system historically evolved under the powerful pressure of gravity forces. By the late 1970s, many manifestations of the microgravity effects on the motor system had already been known. At the same time, the basic sensorimotor relationships during exposure to zero-gravity remained unexplored. The article considers the main results of the studies of the scientific school of I.B. Kozlovskaya regarding the roles of the gravitational forces in the functioning of the tonic motor system in humans and other mammals. In these studies, it was demonstrated that the muscle tonic system is relatively independent and possesses its own structures and mechanisms at every level—from receptors to effectors. The support afferent input plays the main role in the regulation of the postural tonic system. The withdrawal of the support afferentation leads to the decline of the tonic motor units activity in extensor muscles and the alteration of the motor units recruitment patterns in the spinal cord. The decline of the tonic activity of the extensor motoneurons triggers on the development of the sensorimotor effects of microgravity including atony and muscle atrophy.     

Tracheobronchial muscle tonus and its reflex control

Airway smooth muscle tone is reinforced during the inspiratory phase of the breathing cycle and depends largely from neurogenic motor drive carried by the vagus nerve. This muscle tone seems to be produced mostly by a vago-vagal reflex loop initiated by the tonic discharge of tracheo-bronchial and/or alveolar receptors connected to thin sensory vagal fibres (non-myelinated or C-fibres). Inhibitory influences carried by large myelinated vagal fibres connected to tracheobronchial stretch receptors and also numerous afferents from the upper airways, systemic and pulmonary circulation, digestive tract and skeletal and respiratory muscles participate to the modulation of airway tone. The identification of neurotransmitters specific of the motor or sensory pathways helps to understand the peripheral modulation of airway motor drive and also the central integration of some peripheral informations.     

Cross-bridge mechanisms underlying the history-dependent properties of muscle spindles and stretch reflexes

The effects of prior movement on the force responses of skeletal muscle are compared with the effects of movement history on the changes in firing rate of muscle spindle receptors. Prior release results in the linearization of the mechanical properties of skeletal muscles, which can be provisionally explained by cross-bridge models of muscular contraction. The history-dependence of responses of muscle spindle receptors in unanesthetized decerebrate preparations appears to result from the kinetics of cycling and noncycling cross-bridges. The results of this comparison indicate that the integration of mechanical properties of muscle and spindle receptor promotes stiffness regulation.     

Electromyography of the opercularis muscle of Rana catesbeiana: An amphibian tonic muscle

The opercularis muscle of Rana catesbeiana originates on the suprascapular cartilage of the shoulder girdle and inserts on the otic opercular element. It is part of the levator scapulae musculature and lies dorsomedial to the levator scapulae superior and inferior muscles. Bipolar electrode recordings from all three muscles show electrical activity linked to cyclical firing of the posterior intermandibularis muscle, an important ventilatory muscle. The opercularis muscle shows low amplitude, erratic signals when animals are sumerged. Upon emergence of the snout region, the opercularis muscle shows rhythmic low amplitude activity at twice the rate of buccal pumping. Lung ventilation is synchronized with this rhythm and at ventilation the opercularis muscle shows higher amplitude activity. Upon submergence, opercularis activity again shows low level activity with no rhythmic pattern. Opercularis muscle activity has a major low frequency component (about 30 Hz) that probably corresponds to activity of tonic muscle fibers. Higher frequency signals (about 200–250 Hz) comparable to those of the levator scapulae muscles are also present and probably represent activity of phasic muscle fibers. Activity of the opercularis muscle is correlated with conditions in which aerial respiration is possible, and this pattern of activity supports an opercularis role in aerial hearing and/or detection of substrate vibrations. As far as we know, this is the first report of electromyographic analysis of a vertebrate tonic muscle.     

Effect of pH and calcium of the surrounding medium on electrical activity of the isolated muscle spindle

The effect of pH and the calcium ions of the surrounding medium on the receptor potential and unit activity of the isolated frog muscle spindle was investigated. With a decrease in pH of the medium the amplitude and duration of the hyperpolarization phase of the receptor potential increased and the amplitude of its depolarization phase decreased. The frequencies of the spontaneous and evoked activity were reversibly reduced under these conditions. A change in pH of the surrounding medium toward the alkaline side led to a decrease in the amplitude and duration of the hyperpolarization phase of the receptor potential. The firing rate initially increased and then decreased to 30–50% of normal. An increase in the calcium concentration in an alkaline medium led to an increase, but in an acid and normal medium to a decrease, in firing rate. The observed changes in electrical activity of the muscle spindle in solutions can be explained by changes in the velocity of active (electrogenic) and passive transport of sodium and calcium ions.     

Fusimotor reflexes influencing secondary muscle spindle afferents from flexor and extensor muscles in the hind limb of the cat.

The purpose of this study was to investigate secondary muscle spindle afferents from the triceps-plantaris (GS) and posterior biceps and semitendinosus (PBSt) muscles with respect to their fusimotor reflex control from different types of peripheral nerves and receptors. The activity of single secondary muscle spindle afferents was recorded from dissected and cut dorsal root filaments in alpha-chloralose anaesthetized cats. Both single spindle afferents and sets of simultaneously recorded units (2-3) were investigated. The modulation and mean rate of firing of the afferent response to sinusoidal stretching of the GS and PBSts muscle were determined. Control measurements were performed in the absence of any reflex stimulation, while test measurements were made during reflex stimulation. The reflex stimuli consisted of manually performed movements of the contralateral hind limb, muscle stretches, ligament tractions and electrical stimulations of cutaneous afferents. Altogether 21 secondary spindle afferents were investigated and 20 different reflex stimuli were employed. The general responsiveness (i.e. number of significant reflex effects/number of control-test series) was 52.4%, but a considerable variation between different stimuli was found, with the highest (89.9%) for contralateral whole limb extension and the lowest (25.0%) for stretch of the contralateral GS muscle. The size of the response to a given stimulus varied considerably between different afferents, and, in the same afferent, different reflex stimuli produced effects of varying size. Most responses were characterized by an increase in mean rate of discharge combined with a decrease in modulation, indicative of static fusimotor drive (Cussons et al., 1977). Since the secondary muscle spindle afferents are part of a positive feedback loop, projecting back to both static and dynamic fusimotor neurones (Appelberg Et al., 1892 a, 1983 b; Appelberg et al., 1986), it is suggested that the activity in the loop may work like an amplified which, during some circumstances, enhance the effect of other reflex inputs to the system (Johansson et al., 1991 b).     

Effect of a polarizing current on receptor potential of the isolated frog muscle spindle

The effect of a polarizing current on electrical activity of the isolated frog muscle spindle was studied. A depolarizing current increased the frequency and reduced the amplitude of afferent spike activity, both spontaneous and evoked by mechanical stimulation. A hyperpolarizing current produced the opposite effect. The amplitude of the receptor potential in response to a mechanical stimulus varied as a linear function of the intensity of the polarizing currentA. A. Ukhtomskii Physiological Institute, Leningrad State University. Translated from Neirofiziologiya, Vol. 5, No. 1, pp. 95–101, January–February, 1973.     

Experimental studies upon a bundle of tonic fibres in the locust extensor tibialis muscle

The bundle of tonic fibres situated at the proximal end of the locust metathoracic extensor tibialis muscle is innervated by the dorsal unpaired median neurone (DUMETi) as well as by the slow excitatory (SETi)) and common inhibitor (CI) neurones. It is not innervated by the fast excitatory neurone (FETi).These fibres contract spontaneously and rhythmically. The myogenic rhythm can be modified by neural stimulation.Spontaneous slow depolarizing potentials resembling the pacemaker potentials of insect cardiac muscle were demonstrated in these fibres.The actions of glutamate on the tonic muscle fibres are not compatible with its being a specific excitatory transmitter. Glutamate can stimulate weak contractions of the muscle, but this action is inhibited when chloride ions are removed from the saline.10^?6 M Octapamine hyperpolarizes the tonic fibre membrane. Octopamine, GABA and glutamate all inhibit the myogenic contractions and reduce the force of the neurally evoked contractions.The tonic muscle is very responsive to proctolin. At 5 × 10^?11 M proctolin enhances the force and increases the frequency of myogenic contractions. At 10^?9 M it depolarizes the muscle membrane potential, and at that and higher concentrations it causes the muscle to contract. At 2 × 10^?7 M proctolin induces contractures which resemble those evoked by sustained high-frequency neural stimulation. Iontophoretic experiments show that proctolin receptors occur at localized sites on the tonic fibre membrane.     

Spontaneous activity of passive muscle spindles of the cat triceps surae muscle

The hypothesis that one possible cause of the spontaneous discharge of muscle spindles in the totally relaxed and de-efferented triceps surae muscle, with its tendon divided, is the mechanical factor due to extrafusal-intrafusal interaction was tested in experiments on anesthetized cats. Responses of spontaneously active units were similar with respect to many indices to responses of silent receptors capable of generating a long discharge in response to an increase in static length of the muscle. Isotonic contraction of the relaxed muscle during direct or indirect stimulation was accompanied by a pause in spontaneous activity. The prolonged increase in discharge frequency sometimes arising as a result of pressure on the muscle in the region of the receptor also was abolished by weak isotonic contraction of the muscle. If a long posttetanic sensory discharge was induced in a spontaneously active receptor by intensive tetanization of the nerve to the muscle, and it was then abolished by short stretching of the muscle, the initial spontaneous discharge frequency was restored. The dynamic thresholds of spontaneously active receptors were lower than for silent receptors. In some spontaneously active receptors an initial slowing of the discharge was observed during linear or stepwise stretching of the muscle. It is suggested that the ability of sensory endings to generate a long spontaneous discharge is due to initial stretching of the spindle inside the relaxed muscle.I. P. Pavlov Institute of Physiology, Academy of Sciences of the USSR, Leningrad. Translated from Neirofiziologiya, Vol. 10, No. 3, pp. 287–294, May–June, 1978.