Decline In Discharge Rate Research Articles

An attempt to explain the disappearance and decline in discharge rate of selected karst springs of the Opole region

An attempt to explain the disappearance and decline in discharge rate of selected karst springs of the Opole region

Discharge properties of abductor hallucis before, during, and after an isometric fatigue task

Abductor hallucis is the largest muscle in the arch of the human foot and comprises few motor units relative to its physiological cross-sectional area. It has been described as a postural muscle, aiding in the stabilization of the longitudinal arch during stance and gait. The purpose of this study was to describe the discharge properties of abductor hallucis motor units during ramp and hold isometric contractions, as well as its discharge characteristics during fatigue. Intramuscular electromyographic recordings from abductor hallucis were made in 5 subjects; from those recordings, 42 single motor units were decomposed. Data were recorded during isometric ramp contractions at 60% maximum voluntary contraction (MVC), performed before and after a submaximal isometric contraction to failure (mean force 41.3 ± 15.3% MVC, mean duration 233 ± 116 s). Motor unit recruitment thresholds ranged from 10.3 to 54.2% MVC. No significant difference was observed between recruitment and derecruitment thresholds or their respective discharge rates for both the initial and postfatigue ramp contractions (all P > 0.25). Recruitment threshold was positively correlated with recruitment discharge rate (r = 0.47, P < 0.03). All motor units attained similar peak discharge rates (14.0 ± 0.25 pulses/s) and were not correlated with recruitment threshold. Thirteen motor units could be followed during the isometric fatigue task, with a decline in discharge rate and increase in discharge rate variability occurring in the final 25% of the task (both P < 0.05). We have shown that abductor hallucis motor units discharge relatively slowly and are considerably resistant to fatigue. These characteristics may be effective for generating and sustaining the substantial level of force that is required to stabilize the longitudinal arch during weight bearing.

MOTOR UNIT DISCHARGE DIFFERS WITH INTENSITY AND TYPE OF ISOMETRIC CONTRACTION PERFORMED WITH THE ELBOW FLEXOR MUSCLES

Discharge characteristics of low-threshold motor units differ in isometric contractions when subjects maintain a target force (force task) compared with maintaining a target position (position task) (Jakobi et al, 2002). It is unknown if this difference exists at higher force levels. PURPOSE To compare changes in motor unit activity between a force task and a position task performed at high (≥ 30% MVC) and low (≤ 20% MVC) forces. METHODS Motor unit (n = 15) activity was recorded with branched wire electrodes that were inserted subcutaneously over the biceps brachii muscle in the non-dominant arm. Contractions were sustained for 60–300 s at a target force that was 5% of MVC greater than the recruitment threshold of the motor unit. The order of the tasks for each motor unit was randomized, and subjects rested for 15 min between tasks. RESULTS At low force levels, the initial mean discharge rate was similar for the force (12 ± 3 Hz) and position (12 ± 2 Hz) tasks. At high force levels, however, the initial mean discharge rate was greater during the force task (17 ± 1 Hz) compared with the position task (13 ± 2 Hz). The decline in discharge rate from the beginning to the end of the contraction was greater for the position task (low force 22% change, high force 33% change) compared with the force task (low force 12% change, high force 22% change), and was greater in both tasks for high-force contractions. Discharge rate variability increased during the contraction for both tasks and force levels; however, the increase was greater for high-force contractions (p ≤ 0.05). CONCLUSION Adjustments in the discharge rate of motor units in biceps brachii were more pronounced during high-force contractions compared with low-force contractions for both tasks. Supported by NIH grant AG09000 awarded to RME

Motor unit discharge rate following twitch potentiation in human triceps brachii muscle

It has been proposed that during brief voluntary contractions, twitch potentiation may sustain force output despite a decline in motor unit discharge rate. This study examined the evoked twitch force and motor unit discharge rates during submaximal voluntary contractions of the triceps brachii muscle before and after a 5 s conditioning contraction (CC) at 75% of maximal voluntary force. After the CC, twitch force potentiated (∼1.3–2-fold), and the discharge rate in 33 of 35 motor units declined significantly by 1–6 Hz. The increase in twitch force was significantly correlated with the decline in discharge rate ( r=−0.74). These findings suggest that the extent of the decrease in motor unit discharge rate following a CC is associated with the magnitude of twitch potentiation.

Multiple mechanisms of spike-frequency adaptation in motoneurones

Spike-frequency adaptation is the continuous decline in discharge rate in response to a constant stimulus. We have described three distinct phases of adaptation in rat hypoglossal motoneurones: initial, early and late. The initial phase of adaptation is over in one or two intervals, and is primarily due to summation of the calcium-activated potassium conductance underlying the medium duration afterhyperpolarization (mAHP). The biophysical mechanisms underlying the later phases of adaptation are not well understood. Two of the previously-proposed mechanisms for adaptation are an increase in outward current flowing through calcium-activated potassium channels and increasing outward current produced by the electrogenic sodium-potassium pump. We found that neither of these mechanisms are necessary for the expression of the early and late phases of adaptation. The magnitude of the initial phase of adaptation was reduced when the calcium in the external solution was replaced with manganese, but the magnitudes of the early and late phases were consistently increased under these conditions. Partial blockade of the sodium-potassium pump with ouabain had no significant effect on any of the three phases of adaptation. Our current working hypothesis is that the magnitude of late adaptation depends upon the interplay between slow inactivation of sodium currents, that tends to decrease discharge rate, and the slow activation or facilitation of a calcium current that tends to increase discharge rate. Adaptation is often associated with a progressive decrease in the peak amplitude and rate of rise of action potentials, and a computer model that incorporated slow inactivation of sodium channels reproduced this phenomenon. However, the time course of adaptation does not always parallel changes in spike shape, indicating that the progressive activation of another inward current might oppose the decline in frequency caused by slow sodium inactivation.

Motor unit discharge rate is not associated with muscle relaxation time in sustained submaximal contractions in humans

The muscle wisdom hypothesis suggests that motor unit discharge rates decrease in parallel with the slowing of muscle contractile properties during fatigue. In this study, the discharge rates of single motor units and the muscle contractile properties were measured during a sustained submaximal contraction. The majority of motor units that were active from the beginning of the task demonstrated a decline in discharge rate in the absence of any slowing of muscle relaxation time. These findings suggest that the muscle wisdom hypothesis may not apply to sustained submaximal contractions.

Distinct mechanisms of modulation in a neuronal oscillator generate different social signals in the electric fish Hypopomus.

The medullary pacemaker nucleus of the gymnotiform electric fish, Hypopomus, is a relatively simple neuronal oscillator which contains pacemaker cells and relay cells. The pacemaker cells generate a regular discharge cycle and drive the relay cells which trigger pulse-like electric organ discharges (EODs). The diencephalic prepacemaker nucleus (PPN) projects to the pacemaker nucleus and modulates its activity to generate a variety of specific discharge patterns which serve as communicatory signals (Figs. 2 and 3). While inducing such signals by microiontophoresis of L-glutamate to the region of the PPN (Fig. 4) of curarized animals, we monitored the activity of neurons in the pacemaker nucleus intracellularly. We found that pacemaker cells and relay cells were affected differently in a manner specific to the type of EOD modulation (Figs. 5-10). The normal sequence of pacemaker cell and relay cell firing was maintained during gradual rises and falls in discharge rate. Both types of cells ceased to fire during interruptions following a decline in discharge rate. During sudden interruptions, however, relay cells were steadily depolarized, while pacemaker cells continued to fire regularly. Short and rapid barrages of EODs, called "chirps", were generated through direct and synchronous activation of the relay cells whose action potentials invaded pacemaker cells antidromically and interfered with their otherwise regular firing pattern.

EFFECT OF LIGHT ON ASCOSPORE DISCHARGE BY FIVE FUNGI WITH BITUNICATE ASCI

SummaryExperiments on ascospore discharge from naturally developed perithecia were conducted in a wind tunnel. Discharge by Venturia pirina, V. rumicis and Venturia sp. was stimulated by light, particularly by red, but also by green and blue. There was always a sharp decline in discharge rate after perithecia of these species and of V. inaequalis were plunged from light into darkness. In V. inaequalis a high rate of discharge persisted for several minutes after the change to darkness. It is postulated that in these bitunicate fungi a photoreaction is involved in the rupture of the exoascus tip. Coleroa circinans, another bitunicate species, discharged its ascospores equally readily in light and darkness.