RESUMEN
This review revolves primarily around segmental feedback systems established by muscle spindle and Golgi tendon organ afferents, as well as spinal recurrent inhibition via Renshaw cells. These networks are considered as to their potential contributions to the following functions: (i) generation of anti-gravity thrust during quiet upright stance and the stance phase of locomotion; (ii) timing of locomotor phases; (iii) linearization and correction for muscle nonlinearities; (iv) compensation for muscle lever-arm variations; (v) stabilization of inherently unstable systems; (vi) compensation for muscle fatigue; (vii) synergy formation; (viii) selection of appropriate responses to perturbations; (ix) correction for intersegmental interaction forces; (x) sensory-motor transformations; (xi) plasticity and motor learning. The scope will at times extend beyond the narrow confines of spinal circuits in order to integrate them into wider contexts and concepts.
Asunto(s)
Retroalimentación/fisiología , Músculo Esquelético , Red Nerviosa , Propiocepción/fisiología , Médula Espinal , Animales , Humanos , Mecanorreceptores/metabolismo , Mecanorreceptores/ultraestructura , Actividad Motora/fisiología , Fatiga Muscular , Husos Musculares/metabolismo , Husos Musculares/ultraestructura , Músculo Esquelético/citología , Músculo Esquelético/fisiología , Plasticidad Neuronal , Neuronas Aferentes/fisiología , Postura , Reflejo/fisiología , Médula Espinal/anatomía & histología , Médula Espinal/fisiologíaRESUMEN
The information transmission properties of ensembles of MSs and the effect of the gamma system on these properties were studied. Three converging lines of research were taken: (1) the development of information theoretic estimation tools, and the formulation of an "operational" interpretation for the information rate; (2) animal experiments in which the mutual information rate was estimated and the effect of the gamma system was quantified; (3) simulation of a muscle spindle model with gamma activation in order to corroborate the results of the animal experiments. The main hypothesis was that the gamma system will enhance information theoretic measures that quantify the quality of the sensory neural channel comprised from an ensemble of primary muscle spindle afferents. A random stimulus was applied to a muscle in the hind limb of a cat, while spike trains from several primary MS afferents were recorded simultaneously. The stimulus was administered twice, with an operative and a disconnected gamma system. The mutual information rate between the stimulus and spike trains, as well as other information theoretic measures, was estimated. The information rate of ensembles of MSs increased with increasing ensemble size. However, with an operative gamma system the "ensemble effect" was much higher. In addition, the ensemble effect was influenced by the stimulus spectrum. A muscle spindle population model with gamma activation was simulated with stimuli that were identical to that of the animal experiments. The simulation results supported the experimental results and corroborated the main hypothesis. The results indicate that the gamma system has an important role in enhancing information transmission from ensembles of MSs to the spinal cord.
Asunto(s)
Potenciales de Acción/fisiología , Neuronas Motoras gamma/fisiología , Husos Musculares/fisiología , Músculo Esquelético/inervación , Neuronas Aferentes/fisiología , Reflejo de Estiramiento/fisiología , Algoritmos , Animales , Gatos , Estimulación Eléctrica , Modelos Neurológicos , Músculo Esquelético/fisiología , Conducción Nerviosa/fisiología , Médula Espinal/citología , Médula Espinal/fisiología , Raíces Nerviosas Espinales/fisiología , Transmisión Sináptica/fisiologíaRESUMEN
The somatomotor and sympathetic nervous systems are intimately linked. One example is the influence of peripheral sympathetic fibers on the discharge characteristics of muscle spindles. Since muscle spindles play important roles in various motor behaviors, including rhythmic movements, the working hypothesis of this research was that changes in sympathetic outflow to muscle spindles can change rhythmic movement patterns. We tested this hypothesis in the masticatory system of rabbits. Rhythmic jaw movements and EMG activity induced by long-lasting electrical cortical stimulation were powerfully modulated by electrical stimulation of the peripheral stump of the cervical sympathetic nerve (CSN). This modulation manifested itself as a consistent and marked reduction in the excursion of the mandibular movements (often preceded by a transient modest enhancement), which could be attributed mainly to corresponding changes in masseter muscle activity. These changes outlasted the duration of CSN stimulation. In some of the cortically evoked rhythmic jaw movements (CRJMs) changes in masticatory frequency were also observed. When the jaw-closing muscles were subjected to repetitive ramp-and-hold force pulses, the CRMJs changed characteristics. Masseter EMG activity was strongly enhanced and digastric EMG slightly decreased. This change was considerably depressed during CSN stimulation. These effects of CSN stimulation are similar in sign and time course to the depression exerted by sympathetic activity on the jaw-closing muscle spindle discharge. It is suggested that the change in proprioceptive information induced by an increase in sympathetic outflow (a) has important implications even under normal conditions for the control of motor function in states of high sympathetic activity, and (b) is one of the mechanisms responsible for motor impairment under certain pathological conditions such as chronic musculoskeletal head-neck disorders, associated with stress conditions.
Asunto(s)
Músculo Masetero/fisiología , Masticación/fisiología , Corteza Motora/fisiología , Contracción Muscular/fisiología , Husos Musculares/fisiología , Fibras Simpáticas Posganglionares/fisiología , Potenciales de Acción/fisiología , Animales , Estimulación Eléctrica , Electromiografía , Músculo Masetero/inervación , Inhibición Neural/fisiología , Dolor/etiología , Dolor/fisiopatología , Tractos Piramidales/fisiología , Conejos , Estrés Psicológico/complicaciones , Estrés Psicológico/fisiopatologíaRESUMEN
In the present study, c-fos expression in the spinal cord has been used as a marker of neuronal activation induced by capsaicin-sensitive sensory afferents from the dorsal neck muscles in cats (n = 6). The number of Fos-immunoreactive neurons, which were revealed using the avidin-biotin-peroxidase method, was significantly increased in the cervical and lumbar spinal cord. In contrast to the control group (n = 3), 2 h after intramuscular capsaicin injection, c-fos expression was more extensive ipsilaterally to the injected side in the C3-C6 segments, and bilaterally in the L4-L6 segments. Most labeled neurons in the cervical spinal cord were small and giant cells, predominantly located in the middle and lateral parts of lamina I and, additionally, at the neck of the dorsal horn (lamina V), i.e., within the zones of termination of high-threshold muscle afferents. The widespread distribution of labeled cells throughout the cervical cord within the intermediate zone (lamina VII) coincided with the sites of last-order premotor interneurons and cells of origin of long crossed and uncrossed descending propriospinal pathways to the lumbar spinal cord. These findings suggest possible mechanisms for spreading of nociceptive signals between cervical and lumbar regions.
Asunto(s)
Genes fos/genética , Músculos del Cuello/fisiología , Receptores de Droga/agonistas , Médula Espinal/metabolismo , Animales , Capsaicina/administración & dosificación , Capsaicina/farmacología , Gatos , Tamaño de la Célula , Vértebras Cervicales , Femenino , Histocitoquímica , Inyecciones , Vértebras Lumbares , Masculino , Músculos del Cuello/inervación , Músculos del Cuello/metabolismo , Vías Nerviosas/fisiología , Dolor/fisiopatología , Estimulación QuímicaRESUMEN
The present work was designed to check for the possibility of interactions between mechanical innocuous and chemically induced noxious muscle afferent inputs on discharge behavior of nociceptive superficial dorsal horn neurons (SDHNs) of the spinal cord in decerebrated cats. The innocuous and noxious stimuli were applied separately and in combination, so that the effects of the innocuous stimulus on nociceptive processing could be evaluated. The innocuous stimulus consisted of ramp-and-hold stretches of the gastrocnemius muscles, whereas the noxious stimulus consisted of i.a. injections of bradykinin (BK; 0.5-1 ml, 50 microg/ml) into the arterial circulation of same muscles. Only neurons up to approximately 1mm depth and those that responded to noxious pinch of the gastrocnemius muscles were selected for further analysis. The activity of 16 dorsal horn neurons was recorded extracellularly with high-impedance glass microelectrodes, out of which seven responded to stretch, while 12 neurons responded to bradykinin injections. The bradykinin injections induced three types of responses: excitatory, inhibitory and mixed. The majority of the neurons that showed excitatory and mixed responses to bradykinin were also influenced by stretches applied directly after the bradykinin injection. In these neurons, the stretch usually counteracted the bradykinin-induced response, i.e. shortening and reducing bradykinin-induced excitation and re-exciting the cells after bradykinin-induced inhibition. The mechanism of the stretch modulation is proposed to reside in a segmental spinal control of the nociceptive transmission.
Asunto(s)
Potenciales de Acción/fisiología , Contracción Muscular/fisiología , Husos Musculares/fisiología , Músculo Esquelético/fisiología , Células del Asta Posterior/fisiología , Potenciales de Acción/efectos de los fármacos , Animales , Bradiquinina/farmacología , Gatos , Contracción Muscular/efectos de los fármacos , Husos Musculares/efectos de los fármacos , Músculo Esquelético/efectos de los fármacos , Estimulación Física/métodos , Células del Asta Posterior/efectos de los fármacos , Estimulación QuímicaRESUMEN
The information transmission properties of single, de-efferented primary muscle-spindle afferents from the hind limb of the cat were investigated. The gastrocnemius medialis muscle was stretched randomly while recording spike trains from several muscle-spindle afferents in the dorsal root. Two classes of input stimuli were used: (i) Gaussian noise with band-limited flat spectrum, and (ii) Gaussian noise with a more "naturalistic" 1/f(n) spectrum. The "reconstruction" method was used to calculate a lower bound to the information rate (in bits per second) between the muscle spindles and the spinal cord. Results show that in response to the flat-spectrum input, primary muscle-spindle afferents transfer information mainly about high frequencies, carrying 2.12 bits/spike. In response to naturalistic-spectrum inputs, primary muscle-spindle afferents transfer information about both low and high frequencies, with "spiking efficiency" increasing to 2.67 bits/spike. A simple muscle-spindle simulation model was analyzed with the same method, emphasizing the important part played by the intrafusal fiber mechanical properties in information transmission.
Asunto(s)
Vías Aferentes/fisiología , Husos Musculares/fisiología , Músculo Esquelético/inervación , Neuronas Aferentes/fisiología , Potenciales de Acción/fisiología , Vías Aferentes/citología , Animales , Gatos , Estimulación Eléctrica , Mecanotransducción Celular/fisiología , Modelos Neurológicos , Neuronas Motoras gamma/fisiología , Contracción Muscular/fisiología , Husos Musculares/citología , Músculo Esquelético/fisiología , Neuronas Aferentes/citología , Distribución Normal , Raíces Nerviosas Espinales/citología , Raíces Nerviosas Espinales/fisiologíaRESUMEN
In the medial gastrocnemius muscle of the decerebrate cat, the spatial spread of fatigue between active and inactive muscle parts was studied. Conditioning fatiguing stimulation (CFS) was applied to a part of the muscle to test whether it had an effect on the contraction efficiency in an unstimulated part. To exclude somato-sympathetic reflexes during CFS, a full rhizotomy of the lumbo-sacral spinal cord was performed. The same ipsilateral ventral root, either L7 or S1, was divided into seven filaments, one of which was used for the test stimulation, and four or five for CFS. The CFS consisted of 12 s sessions of distributed stimulation of five (or four) filaments at a rate of 40 s(-1), the sessions were repeated, every 40 s, 15 or more times. The test consisted of 12 s of regular stimulation at a rate of 10 s(-1), preceded and followed by a single stimulus. The tests applied just after CFS showed a strong decline of both tension and electromyogram (EMG), amounting to only [mean (SD)] 0.45 (0.18) and 0.51 (0.19) (n = 15), respectively, of the corresponding values in the tests before CFS. It thus turned out that depressive fatigue-related effects could spread within the muscle. At the same time, control reactions recorded in the lateral gastrocnemius during stimulation of its cut nerve did not change. Subsequent repetitions of the tests usually revealed a tendency towards restoration. The EMG reactions recovered more quickly than tension. The depression of EMG after CFS was accompanied by a slowing of the constituent M-waves; their latencies decreased during restoration. Distinct changes in the systemic blood pressure were observed during CFS. These changes were usually correlated well with muscle tension changes. The factors possibly underlying the observed effects may include diffusion of metabolites from active to inactive muscle fibres, lowering of the efficiency of neuro-muscular transmission due to squeezing of efferent motor terminals and changes in outer metabolite content, as well as local hypoxia due to increases in intramuscular pressure.
Asunto(s)
Fatiga Muscular/fisiología , Músculo Esquelético/fisiología , Animales , Presión Sanguínea/fisiología , Gatos , Estimulación Eléctrica , Electromiografía , Femenino , Isquemia/fisiopatología , Contracción Isométrica/fisiología , Masculino , Músculo Esquelético/inervación , Sistema Nervioso Simpático/fisiologíaRESUMEN
Previous reports showed that sympathetic stimulation affects the activity of muscle spindle afferents (MSAs). The aim of the present work is to study the characteristics of sympathetic modulation of MSA response to stretch: (i) on the dynamic and static components of the stretch response, and (ii) on group Ia and II MSAs to evaluate potentially different effects. In anaesthetised rabbits, the peripheral stump of the cervical sympathetic nerve (CSN) was stimulated at 10 impulses s(-1) for 45-90 s. The responses of single MSAs to trapezoidal displacement of the mandible were recorded from the mesencephalic trigeminal nucleus. The following characteristic parameters were determined from averaged trapezoidal responses: initial frequency (IF), peak frequency at the end of the ramp (PF), and static index (SI). From these, other parameters were derived: dynamic index (DI = PF - SI), dynamic difference (DD = PF - IF) and static difference (SD = SI - IF). The effects of CSN stimulation were also evaluated during changes in the state of intrafusal muscle fibre contraction induced by succinylcholine and curare. In a population of 124 MSAs, 106 units (85.4 %) were affected by sympathetic stimulation. In general, while changes in resting discharge varied among different units (Ia vs. II) and experimental conditions (curarised vs. non-curarised), ranging from enhancement to strong depression of firing, the amplitude of the response to muscle stretches consistently decreased. This was confirmed and detailed in a quantitative analysis performed on 49 muscle spindle afferents. In both the non-curarised (23 units) and curarised (26 units) condition, stimulation of the CSN reduced the response amplitude in terms of DD and SD, but hardly affected DI. The effects were equally present in both Ia and II units; they were shown to be independent from gamma drive and intrafusal muscle tone and not secondary to muscle hypoxia. Sympathetic action on the resting discharge (IF) was less consistent. In the non-curarised condition, IF decreased in most Ia units, while in II units decreases and increases occurred equally often. In the curarised condition, IF in group II units mostly increased. The results have important functional implications on the control of motor function in a state of 'high' sympathetic activity, like excessive stress, as well as in certain pathological conditions such as sympathetically maintained pain.
Asunto(s)
Maxilares/fisiología , Músculo Masetero/inervación , Músculo Masetero/fisiología , Husos Musculares/fisiología , Sistema Nervioso Simpático/fisiología , Animales , Curare/farmacología , Estimulación Eléctrica , Músculo Masetero/irrigación sanguínea , Husos Musculares/efectos de los fármacos , Fármacos Neuromusculares Despolarizantes/farmacología , Fármacos Neuromusculares no Despolarizantes/farmacología , Neuronas Aferentes/fisiología , Conejos , Succinilcolina/farmacologíaRESUMEN
The distribution of Fos-immunoreactive (Fos-ir) and nicotinamide adenine dinucleotide phosphate-diaphorase (NADPH-d)-reactive neurons in the rat lumbar spinal cord was examined following muscle fatigue caused by intermittent high-rate (100 s(-1)) electrical stimulation of the triceps surae muscle or the ventral root L5 (VRL5) for 30 min. Following both types of stimulation, the fatigue-related c-fos gene expression was more extensive in the L2-L5 segments on the stimulated side, and the majority of Fos-ir neurons were concentrated in the dorsal horn. After direct muscle stimulation, the highest number of Fos-ir neurons were detected in two regions: layer 5, and superficial layers (1 and 2(o)), although many labeled cells were also found in layers 3, 4, 6, and 7. In response to VRL5 stimulation, the maximal density of Fos-ir neurons was detected in the middle and lateral parts of layers 1 and 2(o), the zone of termination of high-threshold muscle afferents(.) Statistically significant prevalence of Fos-ir cell number was also found in layers 5 and 7 on the stimulated side. A few Fos-ir neurons were detected in the ventral horn (layer 8 and area 10) on both sides. The lamellar distribution of NADPH-d-reactive neurons was similar over all experimental groups of animals. In the L3-L6 segments, such reactive cells were arranged in two distinct regions: dorsal horn (layers 2(i), 3, and 5) and area 10; in the L1 and L2 segments, an additional cluster of NADPH-d positive cells was found in the intermediolateral cell column (IML). Double-labeled cells were not detected. We suggest that c-fos expression in response to muscle fatigue reveals activity of functionally different types of spinal neurons which could operate together with NOS-containing cells in pre-motoneuronal networks to modulate the motoneuron output.
Asunto(s)
Fatiga Muscular/fisiología , NADPH Deshidrogenasa/análisis , Neuronas/enzimología , Proteínas Proto-Oncogénicas c-fos/biosíntesis , Médula Espinal/citología , Animales , Anticuerpos , Estimulación Eléctrica , Miembro Posterior , Vértebras Lumbares , Masculino , Músculo Esquelético/inervación , Músculo Esquelético/fisiología , Neuronas/química , Proteínas Proto-Oncogénicas c-fos/análisis , Proteínas Proto-Oncogénicas c-fos/inmunología , Ratas , Ratas Wistar , Raíces Nerviosas Espinales/fisiologíaRESUMEN
The information transmission properties of single, deefferented, primary muscle spindle afferents (MSAs) from the hind limb of the cat were investigated. Random stretches were delivered to the gastrocnemius medialis muscle, while recording spike trains from several MSAs near the dorsal root. Two classes of input stimuli were used: Gaussian noise with band-limited flat spectrum, and Gaussian noise with a more "naturalistic" 1/fn spectrum. The "reconstruction" method was used to calculate a lower bound to the information rate (in bit/ sec) delivered from MSAs to the spinal cord. Results show that in response to flat spectrum primary MSAs transfer information mainly about high frequencies, carrying 1.97 bits per spike. In response to naturalistic spectrum MSAs transfer information about both low and high frequencies, with "spiking efficiency" increasing to 2.99 bits per spike. A simple muscle spindle model was simulated, exemplifying the part of the intrafusal fiber mechanical properties in information transmission.
Asunto(s)
Husos Musculares/fisiología , Reflejo de Estiramiento , Animales , Gatos , Estimulación EléctricaRESUMEN
Effects of low- and high-frequency fatigue were studied on muscle dynamics in isometric conditions of the cat gastrocnemius. Fatiguing sessions consisted of 25-28 repetitions of the standard tests that included an 18-s interval of continuous frequency-modulated stimulation preceded and followed by single stimuli evoking twitch contractions. The rate of the continuous part was changed in accordance with a symmetrical double-trapezoidal signal, including three successive phases of constant rate at 10, 40 and 10s(-1); between these phases, each lasting for 4s, the rate changed linearly within a 2-s interval. The following modes of muscle activation were applied: (i) stimulation of single filaments constituting approximately one-fifth to one-seventh of the total cross-section of the L(7) and S(1) ventral roots; (ii) the distributed stimulation of five similar filaments; and (iii) direct stimulation of muscle through bipolar wire electrodes. A relative drop in tension, the fatigue index, expressed as the ratio at the end of a fatigue session over its value at the beginning of the test, was used to quantify fatigue effects. The fatigue indices during low-rate stimulation were 0.56+/-0.03 (mean+/-S.D.) at the first phase and 0. 64+/-0.02 at the third phase, while during high-rate stimulation this parameter was only 0.32+/-0.02. The high-rate stimulation noticeably increased the mean tension during low-rate stimulation; the ratio between the reactions at the third and the first phases could be as much as two to three times greater than that at the beginning of the fatigue session. It was demonstrated that the potentiation was connected with after-effects of the rate-tension hysteresis. The hysteresis decreased with fatigue, the fatigue index for the rate-tension loop areas ranging from 0.39 to 0.52 (0.45+/-0. 05, mean+/-S.D.). The fatigue processes developed more quickly and intensively in the previously fatigued muscles: the obtained fatigue indices were 0.73+/-0.05 and 0.70+/-0.10 at the first and third phases, and 0.62+/-0.06 (mean+/-S.D.) at the second phase of stimulation, respectively. In the cases of distributed and direct stimulation applied to muscles in a fresh state, fatigue dynamics did not differ significantly from those observed during single-filament stimulation. In experiments with distributed stimulation applied to previously fatigued muscles, a powerful depression of the high-rate components was registered in several cases, which seemed to be connected with depressive effects at the level of nerve-muscle synaptic transmission. The effects of low- and high-frequency fatigue were studied in isometric conditions of muscle contraction. In addition to the well-known differentiation between low- and high-frequency fatigue effects, the complex pattern of efferent stimulation used allowed us to identify additional fatigue-related changes in the rate-tension hysteresis. This hysteresis seems to be one of the possible mechanisms directed to compensate for low-frequency fatigue in the muscle contraction.
Asunto(s)
Vías Eferentes/fisiología , Fatiga Muscular/fisiología , Músculo Esquelético/fisiología , Animales , Gatos , Estimulación Eléctrica , Femenino , Contracción Isométrica , Masculino , Músculo Esquelético/inervación , Raíces Nerviosas Espinales/fisiologíaRESUMEN
Changes in the compound muscle action potentials of cat gastrocnemius muscle were studied during low- and high-frequency fatigue. Fatiguing session consisted of 25-28 repetitions of the standard single fatigue tests (1.5min interval between the tests) that included the part of continuous frequency-modulated stimulation preceded and followed by single stimuli evoking twitch contractions in the muscle. The rate of the continuous part was changed in accordance with symmetrical double-trapezoidal signal, including three successive phases of constant rate at 10, 40 and 10s(-1); between these phases of 4s duration the rate changed linearly within a 2s interval. During fatigue relative changes in compound muscle action potential waves were usually smaller than changes in tension. Within the same fatigue procedure applied to a fresh muscle, the drop in tension was as much as 35% for high-rate stimulation and 59-71% for low-rate stimulation, whereas the decrease of the peak-to-peak compound muscle action potential waves amplitudes did not exceed 10-20%. Compound muscle action potential waves underwent the most pronounced depression during high-rate stimulation, the decrease proceeding during the following phase of low-rate stimulation. The tension changes during long-lasting activation were different for low- and high-frequency fatigue, with more pronounced depression during low-rate stimulation. As a rule, compound muscle action potential waves changes followed opposite patterns. Compound muscle action potential waves progressively split up, which was probably associated with a continuous slowing of the action potentials in the most fatigable motor units and the subsequent disappearance of the reactions at least in part of the motor units. Hysteresis effects in muscle contraction seem to be able, at least in part, to compensate for some of the depressive effects appearing during conduction of action potentials in muscle fibres. Changes in the compound muscle action potentials were studied during development of the muscle fatigue. These changes showed pronounced dependency on stimulation rate allowing differentiating effects of low- and high-frequency stimulation of the efferents supplying muscle under study. At the same time the fatigue-related changes in the action potentials were noticeably smaller than changes in tension, thus supporting existing concepts in the field arguing that fatigue effects are mainly connected with corresponding activity-dependent changes in muscle contraction machinery.
Asunto(s)
Potenciales de Acción/fisiología , Vías Eferentes/fisiología , Electromiografía , Fatiga Muscular/fisiología , Músculo Esquelético/fisiología , Animales , Gatos , Estimulación Eléctrica , Femenino , Masculino , Contracción Muscular/fisiología , Músculo Esquelético/inervaciónRESUMEN
Metabolites released during fatiguing muscle contractions excite group III IV muscle afferents which might inhibit skeleto-motoneuron firing, hypothetically via Renshaw cells. This was tested, in decerebrated, spinalized cats, by recording changes in Renshaw cell spontaneous discharges and responses to antidromic electrical stimulation of motor axons when small-diameter calf muscle afferents were excited by intra-arterially injected bradykinin, serotonin, lactic acid and KCI. Whenever such injections had an effect, it transiently raised or lowered the spontaneous firing rate and almost always decreased the antidromic response to motor axon stimulation. Injection of bradykinin and serotonin commonly decreased the blood pressure and concomitantly the spinal blood flow (as measured using laser Doppler flowmetry), which could have indirectly influenced Renshaw cell firing. But in general, blood pressure and flow changed after the Renshaw cell discharge did, which thus, appears to be modulated independently by group III-IV afferents. These results suggest that the Renshaw cell-mediated effects of neurochemically excited afferents would predominantly disinhibit rather than inhibit motoneurons.
Asunto(s)
Vías Aferentes/efectos de los fármacos , Bradiquinina/farmacología , Neuronas Motoras/efectos de los fármacos , Músculos/metabolismo , Serotonina/farmacología , Animales , Presión Sanguínea/efectos de los fármacos , Gatos , Femenino , Inyecciones Intraarteriales , Ácido Láctico/farmacología , MasculinoRESUMEN
When humans voluntarily and maximally contract a muscle under isometric conditions, the average firing rate of motor units decreases from an initially high value over several tens of seconds. The mechanisms underlying the rate reduction are probably manifold. One mechanism could involve changes in the motoneuron afterhyperpolarization, another reflex effects of group III-IV muscle afferents that are excited during developing muscle fatigue. It appears possible that changes in motoneuron afterhyperpolarization are mediated by these afferent inputs. We therefore studied effects on motoneuron afterhyperpolarization of small-diameter muscle afferents excited by intra-arterially injected metabolites such as bradykinin and serotonin. In decerebrate and mostly spinalized cats, lumbosacral alpha-motoneurons were recorded intracellularly. Current pulses were injected to test for input resistance and elicit action potentials and afterhyperpolarizations. Afterhyperpolarizations were averaged from c. 10 successive stimulus repetitions. Measurements were taken of afterhyperpolarization amplitude, half-width and area; and exponential functions were fitted to the afterhyperpolarization decay phase to determine afterhyperpolarization decay time-constants. In selected cases, the entire afterhyperpolarization trajectory was fitted with a sum of two exponentials to assess more precisely changes in afterhyperpolarization trajectory. Small catheters were inserted into side-branches of the sural artery and the accompanying vein to apply substances like bradykinin, serotonin and KCl to the calf muscles. Concentrations were in the range of those used by other workers. Intra-arterial injection of bradykinin and serotonin usually decreased blood pressure, which may at times have affected mean motoneuron membrane potentials. Afterhyperpolarization amplitude usually changed with membrane potential in a way expected from ensuing changes in driving potential. Whenever excitation of group III-IV muscle afferents caused moderate to strong increases in motoneuron synaptic noise, afterhyperpolarization amplitudes were reduced, usually in parallel to decreases in input resistance. Afterhyperpolarization half-widths were mostly unaffected, but occasionally decreased. There was a significant trend for afterhyperpolarization decay time-constants to increase during increased synaptic noise, this increase being inversely correlated with the reduction in afterhyperpolarization amplitude. The reduction in input resistance was associated with a decrease in the membrane time-constant, which could therefore not account for the prolongation of the afterhyperpolarization decay time-constant. The afterhyperpolarization area decreased, indicating that the reduction of afterhyperpolarization amplitude outweighed the prolongation of afterhyperpolarization decay time-constant. During a prolonged fatiguing muscle contraction group III-IV afferents become increasingly excited, produce augmenting synaptic inputs in motoneurons, and will change afterhyperpolarization properties. On average, these changes per se tend to diminish the effect of afterhyperpolarization on motoneuron discharge.
Asunto(s)
Neuronas Motoras/fisiología , Músculo Esquelético/inervación , Neuronas Aferentes/fisiología , Animales , Presión Sanguínea/efectos de los fármacos , Bradiquinina/administración & dosificación , Bradiquinina/farmacología , Gatos , Estado de Descerebración/fisiopatología , Estimulación Eléctrica , Electrofisiología , Femenino , Inyecciones Intraarteriales , Masculino , Potenciales de la Membrana/fisiología , Neuronas Motoras/efectos de los fármacos , Músculo Esquelético/efectos de los fármacos , Músculo Esquelético/fisiología , Neuronas Aferentes/efectos de los fármacos , Cloruro de Potasio/administración & dosificación , Cloruro de Potasio/farmacología , Serotonina/administración & dosificación , Serotonina/farmacologíaRESUMEN
This article reviews presumed roles of recurrent inhibition in motor control, that have been proposed over the past five decades. The discussion is structured in an order of increasing complexity. It starts out with the simplest and earliest circuit, that is recurrent self-inhibition of skeleto-motoneurons, and related functions. It soon becomes clear that in order to understand recurrent inhibition, we must look beyond the simple self-inhibitory CNS circuit. First, recurrent inhibition must be seen in the context of other neural circuits. Second, some quantitative features appear to be correlated with features of the neuromusculo-skeletal periphery. Third, the aspect of lateral inhibition between different members of a motoneuron pool as well as between different motoneuron pools points to the essential multiple input-multiple output structure of recurrent inhibition that again can be understood only by correlating it with features of the neuromusculo-skeletal periphery. Another extension results from the discovery that recurrent inhibition affects not only skeleto-motoneurons, but also gamma-motoneurons, Ia inhibitory interneurons mediating reciprocal inhibition between antagonist motoneurons, other Renshaw cells and cells of origin of the ventral spinocerebellar tract (VSCT). Then the view broadens again, investigating the potential role that recurrent inhibition plays in two far-ranging theories of motor control, the inverse-dynamics approach and the equilibrium-point hypothesis. Finally, the present author tries to formulate, in broad strokes, a personal functional interpretation of recurrent inhibition. All the functional considerations, right or wrong, should yield ideas for new experiments, and this then is the last objective of this review.
Asunto(s)
Encéfalo/fisiología , Retroalimentación , Modelos Neurológicos , Actividad Motora , Neuronas Motoras/fisiología , Músculo Esquelético/inervación , Médula Espinal/fisiología , Animales , Humanos , Interneuronas/fisiología , PosturaRESUMEN
This review takes up and updates some major issues dealt with in the book "How brain-like is the spinal cord?" (Windhorst, U. (1988) How Brain-like is the Spinal Cord? Interacting Cell Assemblies in the Nervous System. Springer: Berlin). 1. The main subject dealt with in the book was on the code used by the central nervous system (CNS) to represent and convey information. Specifically, it was proposed that, apart from the generally accepted frequency code (rate of cell firing) and others, information is also encoded in the synchronized or otherwise correlated firing of assemblies of neurons. (ii) Since the temporary establishment of cell assemblies requires mechanisms of flexible neuronal interactions, the second important topic was neuronal plasticity and memory. In fact, plasticity is at the heart of the "population code" used for representation by neuronal assemblies. 2. The third topic was loosely related to a specific representation in the CNS, namely that by so-called "internal models" that represent aspects of the CNS' peripheral environment. 3. It was argued that the above mechanisms of representation and operation are used not only by higher, e.g. cortical, CNS structures, but also by the spinal cord. These topics are here dealt with in turn, always with regard to the special question as to what extent forebrain mechanisms appear at brainstem and spinal cord levels. There is accumulating evidence that in fact the spinal cord uses ensemble processing, synaptic plasticity and internal models to represent and handle its external environment.
Asunto(s)
Encéfalo/fisiología , Modelos Neurológicos , Médula Espinal/fisiología , Animales , Anuros/fisiología , Tronco Encefálico/fisiología , Gatos , Electrofisiología , Haplorrinos/fisiología , Humanos , Potenciación a Largo Plazo/fisiología , Plasticidad Neuronal , Periodicidad , Prosencéfalo/fisiología , Reflejo/fisiología , Respiración/fisiologíaRESUMEN
Experiments were performed to compare the ability of normal subjects and patients with spinal spasticity to suppress antagonist H reflexes during isometric ankle contractions. Soleus H reflex suppression was examined during tonic pretibial muscle contractions in which the torque levels were constant and during dynamic pretibial muscle contractions in which the torque followed a predetermined ramp. As well, subjects were instructed to alternately contract ankle plantarflexors and dorsiflexors at various frequencies to examine patterns of EMG activity during rhythmically alternating isometric contractions in antagonist muscles. Patients with incomplete spinal cord injury demonstrated reduced ability to suppress soleus H reflexes during pretibial muscle contraction. At slow speeds of alternating contraction, spinal cord injured patients retained the ability to perform alternating isometric pretibial/soleus muscle contractions. The patients demonstrated abnormal coactivation in soleus muscle during faster alternating isometric ankle muscle contractions. Furthermore, the patients who demonstrated the greatest impairment in natural reciprocal inhibition, also displayed the largest amount of coactivation. In general, the results would suggest that impairment of natural reciprocal inhibition is correlated with an increase in the amount of antagonist muscle coactivation seen during alternating isometric muscle contractions.
Asunto(s)
Espasticidad Muscular/fisiopatología , Inhibición Neural/fisiología , Traumatismos de la Médula Espinal/fisiopatología , Adulto , Tobillo/fisiología , Electromiografía , Humanos , Contracción Isométrica/fisiología , Masculino , Persona de Mediana Edad , Espasticidad Muscular/etiología , Músculo Esquelético/inervación , Músculo Esquelético/fisiología , Traumatismos de la Médula Espinal/complicacionesRESUMEN
Spinal recurrent inhibition influences the discharge patterns of motoneurons and spinal interneurons. The precise pattern of this influence depends on the static and dynamic characteristics of this feedback system. It is thus of importance to quantify its characteristics as well as possible. We here compare nonlinear features (hysteresis) in Renshaw cells and recurrent inhibition in response to cyclic stimulation of motor axons. In pentobarbitone-anaesthetized or decerebrate cats, intracellular recordings were obtained from 26 hindlimb muscle nerves skeleto-motoneurons and extracellular recordings from nine Renshaw cells. Various hindlimb muscle nerves (dorsal roots cut) or ventral roots (dorsal roots intact) were prepared for electrical stimulation to elicit recurrent inhibition in motoneurons or discharges in Renshaw cells. Stimulus patterns consisted of repetitive pulse trains whose rates varied cyclically between around 10 pulses/s and several tens of pulses/s, at modulation frequencies between 0.1 and 1.0 Hz, in one of two waveforms: triangular or sinusoidal. Recurrent inhibitory potentials in motoneurons and discharge patterns of Renshaw cells were averaged with respect to triggers (cycle-triggers) marking a fixed phase in the stimulation cycle. In another two experiments, motor axons to hindlimb muscles (soleus and medial gastrocnemius) were stimulated with sinusoidal and distorted temporal patterns to show their effects on force production. Most often the cycle-averaged motoneuron membrane potential changed in a temporally asymmetrical way, i.e. it fairly rapidly hyperpolarized early in the stimulus cycle (during increasing rate) and then depolarized more slowly throughout the rest.(ABSTRACT TRUNCATED AT 250 WORDS)
Asunto(s)
Interneuronas/fisiología , Neuronas Motoras/fisiología , Médula Espinal/citología , Médula Espinal/fisiología , Animales , Gatos , Estado de Descerebración/fisiopatología , Estimulación Eléctrica , Retroalimentación/fisiología , Femenino , Masculino , Potenciales de la Membrana/efectos de los fármacos , Músculos/inervación , Músculos/fisiologíaRESUMEN
This chapter reviews several mechanisms that the CNS may use to mitigate muscle fatigue, including intrinsic motoneuron properties and feedback systems. The emphasis is on the effects of sensory inputs on spinal cord interneurons including: Renshaw cells; Ib inhibitory interneurons; interneurons mediating presynaptic inhibition; Ia inhibitory interneurons; and interneuronal networks constituting central pattern generators for locomotion. This exercise brings out how little is known about the operation of these circuits in dealing with muscle fatigue.
Asunto(s)
Neuronas Motoras/fisiología , Fatiga Muscular/fisiología , Animales , Interneuronas/fisiología , Médula Espinal/fisiologíaRESUMEN
A considerable number of theoretical and experimental studies have been undertaken to establish quantitative relationships between the time course of postsynaptic potentials in a neuron and the change in firing probability thereby induced. Depending on background synaptic noise level, the time course of the postsynaptic potential per se as well as its time derivative are both of importance in varying proportion. We have recently begun to study recurrent inhibitory potentials in cat hindlimb motoneurons during rhythmically varying rates of stimulation of motor axons. The amplitude-rate relationship exhibits hysteresis in that amplitudes are usually larger during augmenting than decrementing rates in the cycle. We here report results on the other important variable, that is the slope of recurrent inhibitory potential development, which need not a priori be correlated with amplitude. We found that the slope has a relation to stimulus rate similar to amplitude, so that both parameters are correlated. In pentobarbitone anaesthetized or decerebrate cats, intracellular recordings were obtained from hindlimb skeleto-motoneurons. Various hindlimb muscle nerves were prepared for electrical stimulation to elicit recurrent inhibitory potentials, with dorsal roots cut. Test stimulus patterns consisted of repetitive pulse trains whose rates varied, at modulation frequencies between 0.1 and 1.0 Hz, in one of two waveforms: triangular or sinusoidal. Modulation depths were either "full", with rates varying between a minimum of less than 10 and a maximum of around 50 pulses per s. Or they were about "half" this depth, with mean rates shifted into a "low", "medium" or "high" rate region. Recurrent inhibitory potentials were averaged with respect to stimuli occurring during different phases of the stimulation cycle. Most often when, throughout the cycle, the amplitude changed in a consistent way, so did the slopes of the inhibitory potentials. That is, when the amplitudes rhythmically declined with increasing and recovered with decreasing stimulus rate, the rate of hyperpolarization followed the same pattern. With prominent hysteresis in amplitude, a corresponding hysteresis appeared in slopes. Hence, amplitude and slopes were correlated, occasionally showing a hysteresis among themselves. To a certain extent, these results can be explained by Renshaw cell behaviour, the contribution of the Renshaw cell-motoneuron synapse being unknown and difficult to assess experimentally. For the inhibitory effect of Renshaw cells on motoneurons (and reciprocal Ia inhibitory interneurons), both its magnitude and its time course probably play an important role in determining the efficacy of counteracting local excitatory inputs. The change in slope of inhibitory potentials, and likely its underlying conductance, during cyclic motoneuron activation can be presumed to significantly contribute to the temporal pattern of discharge of motoneurons, in particular in relation to the prevention of synchronization leading to enhanced tremor.