Motoneuron Firing Rate Research Articles

Relationship between the discharge rate of a firing motoneuron and the effectiveness of excitatory la afferent volleys in man

The H-reflex was evoked after producing regular unit firing in the flexor carpi ulnaris set up by moderate voluntary isometric muscular contraction. The firing index was used to quantify the effectiveness of the monosynaptic afferent signal traveling to the firing motoneuron. An analysis was made of the 3.3–16.0 spikes/sec firing range characteristic of naturally occurring muscular contraction. Effectiveness of afferent signals for motor units in the "fast" muscles under study were found to depend on motoneuronal background firing rate; the former declined as the latter rose, as previously discovered during research into "slow" soleus muscle units [2]. Afferent signals were most effective for motoneurons belonging to the "fast" muscles over the entire range of firing rates. It was found from analyzing afferent signal efficacy in relation to its point of occurrence within the interspike interval that variations in motoneuronal excitability within this interval are the reason for this relationship.

Evidence for a fatigue-induced reflex inhibition of motoneuron firing rates.

1. In previous studies on the adductor pollicis and biceps brachii muscles we suggested that motoneuron firing rates are inhibited by a reflex from the muscle during fatigue, since: the firing rates decline during a sustained maximal voluntary contraction (MVC); recovery of MVC firing rates is prevented if the fatigued state of the muscle is preserved for 3 min by local occlusion of its blood supply; and full recovery occurs during this time once the blood supply to the peripheral muscle is restored. These findings were confirmed in the present study for quadriceps contractions. 2. These results do not necessarily imply an inhibitory reflex. The lower firing rates recorded from the muscle fibers during an MVC following 3 min of postfatigue ischemia may have been caused by either reduced subject effort (decreased muscle activation by the CNS) or impaired peripheral impulse transmission under these conditions. The present experiments, carried out on the quadriceps and adductor pollicis muscles, were designed to test this alternative explanation. 3. For both muscles, MVC contractions were sustained for 40 s with a blood pressure cuff inflated to 200 mmHg. This was followed by 3 min ischemic rest and a second 20-s MVC before cuff release. Three minutes after the blood supply to the muscle was restored a third 20-s MVC was made. Single shocks were delivered to the muscle throughout to record twitches from the relaxed muscle (Tr) before and after each MVC, and any twitches super-imposed on the voluntary contractions (Ts). The degree to which the muscle could be activated by voluntary effort was assessed from the ratio [1 - Ts/Tr]. For adductor pollicis, changes in the amplitude of the evoked M-waves were also measured. 4. Spike frequencies were only recorded during quadriceps experiments. These declined by 30% during the initial 40-s MVC. No recovery was seen in the second MVC following 3 min ischemic rest, but full recovery occurred within 3 min of cuff release. 5. Failure to retain full muscle activation was frequently seen in all three MVCs. However, for many well-motivated subjects twitch occlusion showed no reduction in the degree to which either the adductor pollicis or quadriceps muscles could be activated voluntarily during the MVC executed after 3 min of ischemic rest compared with that performed 3 min after the blood supply had been restored.(ABSTRACT TRUNCATED AT 400 WORDS)

The role of Renshaw cells in locomotion: antagonism of their excitation from motor axon collaterals with intravenous mecamylamine.

The contribution of Renshaw cell (RC) activity to the production of fictive locomotion in the mesencephalic preparation was examined using the nicotinic antagonist mecamylamine (MEC). After the i.v. administration of 3 doses of MEC (1.0 mg/kg) the following observations were made: 1) ventral root (VR) evoked discharge of RCs was decreased by up to 87.7%, 2) recurrent inhibitory postsynaptic potentials recorded in alpha motoneurons were greatly reduced or abolished, and 3) the rhythmic firing of RCs during the fictive step cycle was abolished in 83% of the cells examined. Locomotor drive potentials (LDPs) in motoneurons persisted during the fictive step cycle after MEC administration. Bursts of motoneuron firing during each fictive step cycle were characterized by increased frequency and number of spikes after MEC, although the burst duration was unaltered for similar step cycle lengths. A greater number and frequency of spikes per burst was also observed in Ia inhibitory interneurons (IaINs), which remained rhythmically active after MEC administration. It is concluded that Renshaw cells are not an integral part of the spinal central pattern generator for locomotion, nor do they control the timing of the motoneuron or IaIN bursts of firing during fictive locomotion. The data are consistent with a role for RCs in limiting the firing rates of motoneurons and IaINs during each burst.

Cat hindlimb motoneurons during locomotion. II. Normal activity patterns.

Activity patterns were recorded from 51 motoneurons in the fifth lumbar ventral root of cats walking on a motorized treadmill at a range of speeds between 0.1 and 1.3 m/s. The muscle of destination of recorded motoneurons was identified by spike-triggered averaging of EMG recordings from each of the anterior thigh muscles. Forty-three motoneurons projected to one of the quadriceps (vastus medialis, vastus lateralis, vastus intermedius, or rectus femoris) or sartorius (anterior or medial) muscles of the anterior thigh. Anterior thigh motoneurons always discharged a single burst of action potentials per step cycle, even in multifunctional muscles (e.g., sartorius anterior) that exhibited more than one burst of EMG activity per step cycle. The instantaneous firing rates of most motoneurons were lowest upon recruitment and increased progressively during a burst, as long as the EMG was still increasing. Firing rates peaked midway through each burst and tended to decline toward the end of the burst. The initial, mean, and peak firing rates of single motoneurons typically increased for faster walking speeds. At any given walking speed, early recruited motoneurons typically reached higher firing rates than late recruited motoneurons. In contrast to decerebrated cats, initial doublets at the beginning of bursts were seen only rarely. In the 4/51 motoneurons that showed initial doublets, both the instantaneous frequency of the doublet and the probability of starting a burst with a doublet decreased for faster walking speeds. The modulations in firing rate of every motoneuron were found to be closely correlated to the smoothed electromyogram of its target muscle. For 32 identified motoneurons, the unit's instantaneous frequencygram was scaled linearly by computer to the rectified smoothed EMG recorded from each of the anterior thigh muscles. The covariance between unitary frequencygram and muscle EMG was computed for each muscle. Typically, the EMG profile of the target muscle accounted for 0.88-0.96 of the variance in unitary firing rate. The EMG profiles of the other anterior thigh muscles, when tested in the same way, usually accounted only for a significantly smaller fraction of the variance. Brief amplitude fluctuations observed in the EMG envelopes were usually also reflected in the individual motoneuron frequencygrams. To further demonstrate the relationship between unitary frequencygrams and EMG, EMG envelopes recorded during walking were used as templates to generate depolarizing currents that were applied intracellularly to lumbar motoneurons in an acute spinal preparation.(ABSTRACT TRUNCATED AT 400 WORDS)

Characterization of the Na +, K +-ATPase in a microsomal preparation from the abdominal ganglia of crayfish. Effect of neurodepressing hormone (NDH)

1. 1. The Na +, K +-ATPase activity of crayfish abdominal ganglia was characterized and the effect of neurodepressing hormone (NDH) on it was studied. 2. 2. At 20°C, 50% of the ATPase activity in the microsomal fraction was ouabain-sensitive. The enzyme activity decreased by pre-incubation at higher temperatures. 3. 3. The optimum monovalent cation concentrations were 40 mM NaCl and 12 mM KCl. The hydrolysis of ATP showed a strict dependence on Mg 2+. 4. 4. NDH increased the Na +, K +-ATPase activity and the enhancement was concentration- and time-dependent. 5. 5. Treatment of the membranes with sodium-dodecylsulphate (SDS) resulted in a 4-fold increase of ATPase specific activity. 75% of activity is ouabain sensitive. The SDS-treated enzyme was not responsive to NDH. 6. 6. NDH effect on the spontaneous firing rate of motoneurons in isolated abdominal ganglia shows a temperature-dependence similar to that of the effect on ATPase.

An improved neural-network model for the neural integrator of the oculomotor system: more realistic neuron behavior.

The discharge rates of premotor, brain-stem neurons that create eye movements modulate in relation to eye velocity yet firing rates of extraocular motoneurons contain both eye-position and eye-velocity signals. The eye-position signal is derived from the eye-velocity command by means of a neural network which functions as a temporal integrator. We have previously proposed a network of lateral-inhibitory neurons that is capable of performing the required integration. That analysis centered on the temporal aspects of the signal processing for a limited class of idealized inputs. All of its cells were identical and carried only the integrated signal. Recordings in the brain stem, however, show that neurons in the region of the neural integrator have a variety of background firing rates, all carry some eye-velocity signal as well as the eye-position signal, and carry the former with different strengths depending on the type of eye movement being made. It was necessary to see if the proposed model could be modified to make its neurons more realistic. By modifying the spatial distribution of afferents to the network, we demonstrate that the same basic model functions properly in spite of afferents with nonuniform background firing rates. To introduce the eye-velocity signal a double-layer network, consisting of inhibitory and excitatory cells, was necessary. By presenting the velocity input to only local regions of this network it was shown that all cells in the network still carried the integrated signal and that its cells could carry different eye-velocity signals for different types of eye movements. Thus, this model stimulates quantitatively and qualitatively, the behavior of neurons seen in the region of the neural integrator.

Relation between electromyogram and force in fatigue.

The relationship between the surface electromyogram (SEMG) and force was examined during maximal voluntary contraction (MVC). Isometric MVC of elbow flexors were studied in 18 subjects who performed 27 trials, each consisting of six MVCs lasting 45 s at intervals of 30 s. There was a decrease in the median frequency (Fm) of the SEMG and of the compound action potentials (CAP) during MVC. The CAPs demonstrated that the fall in Fm was associated with a proportional increase in signal power, whereas CAP amplitude did not decrease, indicating intact neuromuscular transmission. The SEMG root-mean-square amplitude remained fairly constant, progressively deviating from force with time of contraction (r = 0.40). When SEMG amplitude was corrected for the Fm change, it tracked force more closely (r = 0.68), indicating a fall in motoneuron drive during MVC. The corrected SEMG was used to calculate the change in the generalized firing rate of motoneurons. The firing rate decreased 60% in the first and sixth contractions, tracked force closely, and corresponded to the firing rate fall seen in late adaptation of motoneurons (r = 0.90, P less than 0.001).

Changes in muscle contractile properties and neural control during human muscular fatigue.

The factors limiting force production and exercise endurance time have been briefly described, together with some of the changes occurring at various sites within the muscle and central nervous system. Evidence is presented that, in fatigue of sustained maximal voluntary contractions (MVC) executed by well-motivated subjects, the reduction in force generating capacity need not be due to a decline in central nervous system (CNS) motor drive or to failing neuromuscular transmission, but can be attributed solely to contractile failure of the muscles involved. However, despite this conclusion, both the integrated electromyogram (EMG) and the mean firing rate of individual motor units do decline progressively during sustained MVC. This, however, does not necessarily result in loss of force since the parallel slowing of muscle contractile speed reduces tetanic fusion frequency. It is suggested that the range of motoneuron firing rates elicited by voluntary effort is regulated and limited for each muscle to the minimum required for maximum force generation, thus preventing neuromuscular transmission failure and optimizing motor control. Such a CNS regulating mechanism would probably require some reflex feedback from the muscle.

Excitatory and inhibitory effects of repetitive stimulation of group I and II extensor afferents on homonymous motoneurons.

The repetitive firing of functionally isolated 143 extensor alpha motoneurons in response to homonymous group I and II tetanizations was studied in decerebrate cats. Of a total sample of 92 group I activated motoneuronal units, 76 motoneurons (82.6%) discharged phasically and 16 motoneurons (17.4%) discharged tonically at threshold or just suprathreshold tetanizations. Only 6 of the phasic motoneurons fired tonically when the stimulus strength was increased within the group I range. The firing rate of 8 phasic motoneurons increased at group I and II stimulations, but they retained their phasic discharge characteristics. The remaining 46 group I activated phasic motoneurons (60.1%) delivered tonic discharges when group II afferents were stimulated. Another group of 16 phasic motoneurons (21.1%) became tonic at group II tetanus, but their tonic discharge was usually interrupted by an inhibitory period, which coincided with an escape reaction of the animal. Out of 38 group II activated motoneuronal units, only 8 (21.1%) motoneurons discharged tonically at threshold or just suprathreshold tetanizations. The repetitive activity of these motoneurons was usually interrupted by an inhibitory period, which coincided with an escape reaction of the animal. This inhibition disappeared after procain application to the GS nerve, and was attributed to dominant inhibitory action of group II afferents mediating the sensation of pain. The remaining 30 (78,9%) motoneurons discharged phasically. Six of them remained phasic even at high threshold group II tetanus. The others gave tonic discharges when the stimulus strength was increased within the group II range. A great percentage of motoneurons discharged regularly with a fixed decoding ratio reflecting monosynaptic excitation. Some motoneurons discharged in multiple bands of impulse intervals due to a variable decoding ratio, which can be attributed to both monosynaptic and polysynaptic reflex discharge elicited by group I and/or II tetanizations. There were 6 motoneurons (4.2%), which discharged repetitively at stimulations of the GS, DP, and SUR nervers. These motoneurons were referred to "common motoneurons", which may participate in multiple motor actions such as fixation of a limb or as a part of the flexor reflex. SOL motoneurons usually discharged tonically at tetanizations within the group I range. The firing rate of these motoneurons usually decreased at tetanizations of the group II afferents of MG nerve. Conversely, tetanization of the group II afferents of SOL nerve usually exerted facilitatory effects on group II activated MG motoneurons.(ABSTRACT TRUNCATED AT 400 WORDS)

Characteristics of postsynaptic potentials produced in single human motoneurons by homonymous group 1 volleys.

Changes in the firing probability of voluntarily activated human tibialis anterior motor units occurred when the muscle nerve was stimulated below the threshold of motoneuron axons. A prominent period of increased firing probability was considered to result from the group 1 composite EPSP. The rise time of this composite EPSP, estimated from the duration of the period of increased firing probability, was between 2 and 5 ms. One estimate of the relative amplitude of the EPSP was derived from the cross-correlation and tested by inserting synthetic EPSPs into a computer simulation of a rhythmically discharging neuron. A second estimate of the amplitude was obtained by delivering stimuli with various delays within the interspike interval to determine the position (as a percentage of the mean interspike interval) at which the postsynaptic potential was capable of bringing the membrane potential to threshold. The two estimates were in reasonable agreement. The largest of these EPSPs could bring the motoneuron to threshold after 39% of the elapsed interspike interval. The falling phase of the postsynaptic potential was explored by delivering double stimuli within the interspike interval to produce temporal summation. The duration of the falling phase, so tested, was between 5 and 20 ms. An attempt was made to deduce the general form of the trajectory representing the effective distance of an EPSP from threshold during the interspike interval from the alterations in the cross-correlation profile that occurred when motoneuron firing rate was voluntarily altered.

The influence of temperature and deuterium oxide on the spontaneous activity of crayfish motoneurons

1. 1. The rate of spontaneous electrical activity of the superficial flexor motoneurons in isolated abdominal ganglia of crayfish increases with temperature between 14 and 19°C and decreases between 19 and 24°C. 2. 2. The decrement of firing rate induced by temperature beyond 19°C is abolished by ouabain and incubation in K-free solutions, which fail to affect the temperature-induced increments. 3. 3. An ATPase, ouabain-sensitive and activated by temperature is present in the abdominal ganglia. 4. 4. Deuterium oxide diminishes the firing rate of motoneurons at low temperatures and enhances it at high temperatures thus suggesting an action both on passive and ATPase dependent mechanisms in motoneurons.

Input-output properties of motor unit responses in muscles stretched by imposed displacements of the monkey wrist

Reflex responses are "servo-like" where the output is graded with the input or are "triggered" where the output is independent of input once an input threshold is exceeded. Imposed displacements of monkey or human upper limb joints result in prolonged EMG output in the muscles stretched by the displacements. The longer-latency portions of the prolonged output have been variously reported to be servo-like or to be triggered in nature. In monkeys and humans, angular wrist displacements imposed by step loads result in three peaks (M1, M2 and M3) in the gross EMG recorded from the stretched muscles. Each gross EMG peak largely results from the firing of a separately-responding subpopulation of single motor units (SMUs). We studied the responses of SMUs to loads that were presented to the monkeys in a random order as to magnitude, duration and onset time. Average response histograms were constructed for the SMU responses for individual step load magnitudes. Averages were also constructed for the simultaneously-recorded gross EMG responses for each step load magnitude. The input parameters used were the initial velocity of displacement or the magnitude of step load, while the output was taken as the probability of firing/millisecond/presentation above baseline for the SMUs or the area under the response peaks above baseline for gross EMG. The results establish: 1) That it is not possible to unambiguously determine the input-output properties of the responses to imposed displacements utilizing the analysis of gress EMG activity due to the response characteristics of the various subpopulations of motor units contributing to the gross response. 2) That the SMU activity during all of the peak intervals is monotonically graded with increases in magnitude of the step load or the initial velocity of displacement. Hence, the long-latency portions of the EMG responses are servo-like in nature and are not preprogrammed or triggered responses. 3) That the gain (output/input) of the gross EMG responses almost entirely reflects the variation in the number of motoneurons recruited by changes in magnitude of the step loads rather than variation in the firing rates of motoneurons during the reflex responses.

Neural correlates of nystagmus in abducens nerve.

1. The firing rates of action potentials of abducens nerve single fibers were recorded in the cat's orbit during a variety of vestibular and optokinetic stimulations. 2. Comparison was made of the neural firing rates associated with agonist and antagonist responses during slow and fast components of vestibular and optokinetic nystagmus. It was found that the relationship between the motoneuron firing rates and the eye motion was independent of the reflex with which they were associated--vestibular or optokinetic, or the type of response--agonist or antagonist. No neurons were observed that responded only during the fast or only during the slow nystagmus phase. Motoneuron firing rates were proportional to both velocity and position of the eye in a ratio of 1 (spikes/s)/(deg/s) to 7.2 (spikes/s)/deg. The behavior of the motoneurons was compatible with the hypothesis that thier firing rates are sufficient to overcome both elastic and viscous forces by which the muscles and ligaments hold the eye in the orbit. 3. For low-frequency head rotations, eye displacement and neural responses showed a small phase angle difference. At higher frequencies, however, while the eyes maintained a fixed relationship to the head rotation, the neural responses showed an increasing phase lead. One component of this phase lead compensated for the phase lag introduced by the orbital mechanics. The other was modeled as a constant delay of approximately 70 ms, which may be accounted for by neuromuscular transmission and transduction.

Isometric force production by motor units of extensor digitorum communis muscle in man

Isometric force production by motor units of extensor digitorum communis muscle in man

The Relevance of the Correlation Between Tremor Frequency, Initial Firing Rate of Motoneurons and the Motor Unit's Contraction Time

A hypothesis about the origin of the physiological tremor on the basis of other hypotheses is presented. In order to find the muscle fiber candidates that produce the tremor. the muscle fibers and their motoneurons are classified into three groups: The silent fibers (1). the fibers that fire with their initial firing rate (2) and those firing with a high frequency (3). This classification is dynamical. A given muscle fiber can belong to anyone of the three groups. It depends on the threshold of the fiber and the actual excitation level of the motoneuron pool. to which group a fiber belongs. The group two fibers are responsible for, the genesis of the tremor. The reasons for this assumption are: 1. The twitches of the muscle fibers of group two are the least fused. 2. The motoneurons of this group have an augmented tendency to synchronisation. 3. The motor units of group two are the strongest (active) units. Therefore they are overrepresented in the total muscle force.

Firing rates of motoneurons with strong random synaptic excitation.

The expected time to firing of a nerve impulse when there is Poisson excitation is calculated exactly in Stein's model. This is done at various input frequencies and various ratios of threshold to epsp magnitude, extending some previous calculations. The appropriate conditions for the validity of the model are discussed. Details of a particular calculation are given which involves the solution of a differential-difference equation. The results are presented as variation of expected time to firing as a function of input frequency for a given threshold to epsp ratio. The experimental results of Redman et al. for Poisson monosynaptic excitation of cat spinal motoneurons lead to the estimation of the epsp size which was not measured. The magnitude of the epsps predicted is in good agreement with that expected under the given conditions of stimulation. The predicted variation of epsp magnitude with input frequency is in accordance with that obtained in other experiments. When the finite rise time of epsps is taken into account the predicted epsp sizes are in better agreement with their expected amplitudes.

Accommodative vergence in the alert monkey. Motor unit analysis

The behavior of both abducens nucleus and oculomotor nucleus motoneurons during accommodative vergence eye movements is examined in the alert monkey. This behavior is compared with that of the same neurons during versional eye movements. The behavior is found to be identical, which indicates that oculomotor unit discharge is determined by fixation angle without regard to the type of movement used to reach that angle. Detailed statistical analysis of unit firing rate also suggests that the separate inputs controlling vergence and version are probably combined at some supranuclear level before the motoneuron. The firing rates of motoneurons during the dynamic phase of a vergence movement is examined and compared to that during pursuit movements in an attempt to correlate neuron discharge with the mechanical dynamics of the orbit.

Membrane-potential trajectories between spikes underlying motoneuron firing rates.

Membrane-potential trajectories between spikes underlying motoneuron firing rates.

Excitability and inhibitability of motoneurons of different sizes.

Excitability and inhibitability of motoneurons of different sizes.