Motoneuron Firing Rate Research Articles

Evidence for chemicals sensitizing discharge from skeletal muscle afferents supporting cardiorespiratory reflexes during simulated exercise

the chemical metabolites in skeletal muscle that sensitize the activity of small-diameter afferents and support increases in cardiorespiratory function during exercise have been the subject of much investigation with a variety of methods. The chemicals that change within muscle with exercise have

Output of Human Motoneuron Pools to Corticospinal Inputs During Voluntary Contractions

This study investigated transmission of corticospinal output through motoneurons over a wide range of voluntary contraction strengths in humans. During voluntary contraction of biceps brachii, motor evoked potentials (MEPs) to transcranial magnetic stimulation of the motor cortex grow up to about 50% maximal force and then decrease. To determine whether the decrease reflects events at a cortical or spinal level, responses to stimulation of the cortex and corticospinal tract (cervicomedullary motor evoked potentials, CMEPs) as well as maximal M-waves (M(max)) were recorded during strong contractions at 50 to 100% maximum. In biceps and brachioradialis, MEPs and CMEPs (normalized to M(max)) evoked by strong stimuli decreased during strong elbow flexions. Responses were largest during contractions at 75% maximum and both potentials decreased by about 25% M(max) during maximal efforts (P < 0.001). Reductions were smaller with weaker stimuli, but again similar for MEPs and CMEPs. Thus the reduction in MEPs during strong voluntary contractions can be accounted for by reduced responsiveness of the motoneuron pool to stimulation. During strong contractions of the first dorsal interosseous, a muscle that increases voluntary force largely by frequency modulation, MEPs declined more than in either elbow flexor muscle (35% M(max), P < 0.001). This suggests that motoneuron firing rates are important determinants of evoked output from the motoneuron pool. However, motor cortical output does not appear to be limited at high contraction strengths.

Reduction in maximal firing rate of motoneurons after 1-week immobilization of finger muscle in human subjects

We investigated the effects of immobilization on the maximal motoneuronal firing rate recorded from the first dorsal interosseous (FDI) during voluntary isometric contraction. In five human subjects, the middle finger, index finger, and thumb were immobilized for 1 week in a fiber-glass cast, which kept FDI in a shortened position. During a maximal voluntary contraction, single muscle-fiber action potentials were recorded using a tungsten microelectrode, and mean firing rate was calculated for each action-potential train. Three recording sessions were held: before immobilization (pre), after immobilization (post), and after a 1-week recovery period (recovery). The mean firing rate of FDI motoneurons during maximal voluntary contraction was decreased immediately after the 1-week immobilization (pre: 39.0 ± 3.2 Hz, number of detected spike trains ( n) = 353; post: 33.1 ± 1.5 Hz, n = 285; p < 0.05), and there was a return to control after the recovery period (40.2 ± 3.4 Hz, n = 236). This suggests that the maximal motoneuronal firing rate achieved during maximal voluntary contraction is reduced after short-term immobilization. The functional implications and the contribution of this phenomenon to the immobilization-induced reduction in maximal voluntary force are discussed.

Computer modeling of a spinal reflex circuit

We used a set of properties of the interactions among the spinal neurons in order to develop a computer model for a spinal reflex circuit. The model equations take into account the synaptic characteristics of the somatodendritic membrane of neurons in a morphofunctional unity of the spinal reflex activity. This model is based on the idea that the responses of spinal alpha-motoneurons to a sensorial stimulation can be modulated by the serial activation of a motor command chain. We developed a Fortran program for simulating a physiological situation. The results are discussed in terms of available experimental data for the motoneuron firing rate.

Long Time-Constant Behavior of the Oculomotor Plant in Barbiturate-Anesthetized Primate

The mechanics of the extraocular muscles and orbital tissue ("oculomotor plant") can be approximated by a small number of viscoelastic (Voigt) elements in series. Recent analysis of the eye's return from displacement in lightly anesthetized rhesus monkeys has suggested a four-element plant model with time constants (TCs) of approximately 0.01, 0.1, 1, and 10 s. To demonstrate directly the presence of long (1,10 s) TC elements and to assess their contribution quantitatively, horizontal eye displacement was induced in Cynomolgus monkeys under deep barbiturate anesthesia that prevented interference from spontaneous eye movements. The displacement was maintained for either a prolonged (30 s) or brief (0.2 s) period before release. Return to resting position took 20-30 s after prolonged displacement but only 1-2 s after brief displacement, consistent with the presence of long TC elements that would only be substantially stretched in the former condition. Quantitative fitting of the release curves after prolonged displacement indicated that the two long TC elements contribute a substantial proportion (approximately 30%) of the total plant compliance. A model based on the estimated compliance values is shown to account quantitatively both for our release data and for Goldstein and Robinson's data on hysteresis of ocular motoneuron firing rates measured after centripetal saccades following prolonged eccentric fixation. Long time-constant elements in the plant thus make a substantial contribution to some types of eye movement, and their inclusion in plant models can help interpret the firing patterns of single units in the oculomotor system.

A Simulation Study to Examine the Effect of Common Motoneuron Inputs on Correlated Patterns of Motor Unit Discharge

The influence of common oscillatory inputs to the motoneuron pool on correlated patterns of motor unit discharge was examined using model simulations. Motor unit synchronization, in-phase fluctuations in mean firing rates known as 'common drive', and the coefficient of variation of the muscle force were examined as the frequency and amplitude of common oscillatory inputs to the motoneuron pool were varied. The amount of synchronization, the peak correlation between mean firing rates and the coefficient of variation of the force varied with both the frequency and amplitude of the common input signal. Values for 'common drive' and the force coefficient of variation were highest for oscillatory inputs at frequencies less than 5 Hz, while synchronization reached a maximum when the frequency of the common input was close to the average motor unit firing rate. The frequency of the common input signal for which the highest levels of synchronization were observed increased as motoneuron firing rates increased in response to higher target force levels. The simulation results suggest that common low-frequency oscillations in motor unit firing rates and short-term synchronization result from oscillatory activity in different bands of the frequency spectrum of shared motoneuron inputs. The results also indicate that the amount of synchronization between motor unit discharges depends not only on the amplitude of the shared input signal, but also on its frequency in relation to the present firing rates of the individual motor units.

Androgen-Induced Vocal Transformation in Adult Female African Clawed Frogs

Sex-specific behaviors of some vertebrates are reversible by androgen administered in adulthood. Such behavioral transformations in adulthood provide opportunities to identify how neural systems reconfigure to produce sex-specific behavior. In this study, we focused on the vocalizations of the African clawed frog, Xenopus laevis. Male and female adult Xenopus produce sexually distinct vocalizations; males produce series of rapid clicks, whereas females produce slow trains of clicks. The differences in click rate can be reduced to differences in the firing rate of laryngeal motoneurons in vivo. This behavioral dimorphism is accompanied by various sex-specific characteristics throughout the vocal pathways, including functionally distinct laryngeal muscles and motoneurons in the sexes. In this study, we first determined whether and how testosterone (T) modifies the vocalizations of adult females and then examined changes underlying the behavioral modification at the laryngeal muscle and motoneuron levels. Our results show that, in response to T, the vocalizations of females were transformed within 13 wk. Vocal transformation was preceded by complete masculinization of muscle contractile properties and motoneuron soma size by the fourth week of T treatment, which suggests that the vocal pathways' peripheral components masculinize earlier than the behavior. Therefore the rate of transformation of vocal behavior must reflect a functional transformation of neurons in the central vocal pathways, which leads to the generation of male-like motor rhythms.

Contribution of persistent sodium currents to spike-frequency adaptation in rat hypoglossal motoneurons.

In response to constant current inputs, the firing rates of motoneurons typically show a continuous decline over time. The biophysical mechanisms underlying this process, called spike-frequency adaptation, are not well understood. Spike-frequency adaptation normally exhibits a rapid initial phase, followed by a slow, later phase that continues throughout the duration of firing. One possible mechanism mediating the later phase might be a reduction in the persistent sodium current (I(NaP)) that has been shown to diminish the capacity of cortical pyramidal neurons and spinal motoneurons to sustain repetitive firing. In this study, we used the anticonvulsant phenytoin to reduce the I(NaP) of juvenile rat hypoglossal motoneurons recorded in brain stem slices, and we examined the consequences of a reduction in I(NaP) on the magnitude and time course of spike-frequency adaptation. Adding phenytoin to the bathing solution (> or =50 microM) generally produced a marked reduction in the persistent inward currents (PICs) recorded at the soma in response to slow, voltage-clamp triangular ramp commands (-70 to 0 mV and back). However, the same concentrations of phenytoin appeared to have no significant effect on spike-frequency adaptation even though the phenytoin often augmented the reduction in action potential amplitude that occurs during repetitive firing. The surprising finding that the reduction of a source of sustained inward current had no appreciable effect on the pattern of spike generation suggests that several types of membrane channels must act cooperatively to insure that these motoneurons can generate the sustained repetitive firing required for long-lasting motor behaviors.

In vivo-application of anti-proctolin-antiserum affects antennal flight posture in crickets.

Tethered crickets flying in a wind tunnel adopt a characteristic posture in which the antennae are pointed in parallel and anteriorly into the headwind. Although the firing rates of antennal motoneurons are largely reduced after the start of a flight sequence, the associated postural changes of the antennae are small. It is hypothesised that proctolin, which is present in antennal motoneurons, stabilises the prolonged antennal forward position. To test this hypothesis, proctolin was blocked by anti-proctolin antiserum injections into one antennal base in otherwise intact behaving crickets. The antiserum quickly led to prolonged backward deflections of the treated antennae in 65% of cases. It then took more than one hour for the deflected posture to revert to a normal flight posture. It appears that proctolin is necessary to produce muscle tension large enough to hold the antennae in a forward position and to compensate for the headwind drag. Proctolin, therefore, acts to generate force with reduced electrical activity of motoneurons and muscles.

Effects of evoked pain on the electromyogram and compound muscle action potential of the brachial biceps muscle

Muscle pain is often accompanied by a feeling of muscle fatigue and weakness. We examined the effect of experimental muscle pain on the electromyogram (EMG) during maximal voluntary contraction (MVC), and on the compound muscle action potential (CMAP) of the brachial biceps muscle. Twenty-one healthy subjects were injected intramuscularly with 0.1 ml/30 microg of the vanilloid receptor agonist (capsaicin) or 0.2 ml of 5% hypertonic saline. A Teflon-coated cannulated EMG needle was used to record the EMG interference pattern (IP) at MVC. The CMAP of the brachial biceps muscle was obtained by stimulation of the musculocutaneous nerve at the axilla using surface electrodes. Amplitude, mean frequency of the power spectrum, and turns/s of the interference pattern were reduced after pain induced by capsaicin or hypertonic saline. Latency, amplitude, and area-under-curve of the CMAP did not change after injection of either substance. Acute stimulation of muscle nociceptors thus produced a fatiguelike change in the interference pattern during MVC, possibly due to a decrease in motoneuron firing rate and increased muscle fatigability.

Control of flexor motoneuron activity during single leg walking of the stick insect on an electronically controlled treadwheel.

In the present study, motoneurons innervating the flexor tibiae muscle of the stick insect (Cuniculina impigra) middle leg were recorded intracellularly while the single leg performed walking-like movements on a treadwheel. Different levels of belt friction (equivalent to a change in load) were used to study the control of activity of flexor motoneurons. During slow leg movements no fast motoneurons were active, but a recruitment of these neurons could be observed during faster leg movements. The firing rate of slow and fast motoneurons increased with incremented belt friction. Also, the force applied to the treadwheel at different frictional levels was adapted closely to the friction of the treadwheel to be overcome. The motoneurons innervating the flexor tibiae were recruited progressively during the stance phase, with the slow motoneurons being active earlier than the fast (half-maximal spike frequency after 10-15% and 50-60% of the stance phase, respectively). The resting membrane potential was more hyperpolarized in fast motoneurons (64.6 +/- 6.5 mV) than in slow motoneurons (-52.9 +/- 5.4 mV). However, the threshold for the initiation of action potentials was not statistically significantly different in both types of flexor motoneurons. Therefore, action potentials were generated in fast motoneurons after a longer period of depolarization and thus later during the stance phase than in slow motoneurons. We show that motoneurons of the flexor tibiae receive substantial common excitatory inputs during the stance phase and that the difference in resting membrane potential between slow and fast motoneurons is likely to play a crucial role in their consecutive recruitment.

Amplitude and frequency dependence of spike timing: implications for dynamic regulation.

The spike-time reliability of motoneurons in the Aplysia buccal motor ganglion was studied as a function of the frequency content and the relative amplitude of the fluctuations in the neuronal input, calculated as the coefficient of variation (CV). Measurements of spike-time reliability to sinusoidal and aperiodic inputs, as well as simulations of a noisy leaky integrate-and-fire neuron stimulated by spike trains drawn from a periodically modulated process, demonstrate that there are three qualitatively different CV-dependent mechanisms that determine reliability: noise-dominated (CV < 0.05 for Aplysia motoneurons) where spike timing is unreliable regardless of frequency content; resonance-dominated (CV approximately 0.05-0.25) where reliability is reduced by removal of input frequencies equal to motoneuron firing rate; and amplitude-dominated (CV >0.35) where reliability depends on input frequencies greater than motoneuron firing rate. In the resonance-dominated regime, changes in the activity of the presynaptic inhibitory interneuron B4/5 alter motoneuron spike-time reliability. The increases or decreases in reliability occur coincident with small changes in motoneuron spiking rate due to changes in interneuron activity. Injection of a hyperpolarizing current into the motoneuron reproduces the interneuron-induced changes in reliability. The rate-dependent changes in reliability can be understood from the phase-locking properties of regularly spiking motoneurons to periodic inputs. Our observations demonstrate that the ability of a neuron to support a spike-time code can be actively controlled by varying the properties of the neuron and its input.

No oculomotor plant, no final common path

The assumption that there is an oculomotor plant, a fixed relationship between motoneuron firing rate and eye position, is disproved by brainstem recording studies showing that this relationship depends on which supernuclear subsystem determines firing rate. But it remains possible that there is a final common path (FCP), a fixed relationship between firing rate and muscle force. But then, brainstem recording studies predict that lateral rectus (LR) forces (and probably medial rectus (MR) forces, as well) will be higher in converged than in unconverged gaze for a given eye position. We recently measured these forces and found that they are slightly lower in convergence, disproving the FCP hypothesis. Thus, even the relationship between motoneuron firing rate and muscle force is under supernuclear control. What peripheral oculomotor articulations could vary the relationship of firing rate to muscle force?: (1) Actively movable EOM pulleys could alter oculorotary muscle force for a given oculorotory innervation by altering muscle lengths. (2) ‘Outer’ motoneurons may function as gamma efferents in conjunction with palisade endings and non-twitch global EOM fibers. (3) Complex nonlinear interactions likely arise among both parallel and serially connected muscle fibers.

Correlation between CGRP immunoreactivity and firing activity in cat abducens motoneurons.

A relationship between motoneuron activity and calcitonin gene-related peptide (CGRP) expression was previously suggested based on indirect inferences. We show here a positive correlation between CGRP immunoreactivity and firing activity in an experimental model that used tetanus neurotoxin (TeNT) to alter basal firing levels. A low dose (0.5 ng/kg) of TeNT injected in the lateral rectus muscle raised the basal firing rate of ipsilateral abducens motoneurons, estimated as the firing rate at straight-ahead gaze (F(0)); the firing rate returned to control values after 2 weeks. In contrast, a high dose (5 ng/kg) of TeNT decreased basal firing, which recovered slowly over a 7-week period. Expression of CGRP immunoreactivity by abducens motoneurons, preferentially related to betaCGRP gene expression, was analyzed during these periods of altered firing activity. The number of CGRP-immunofluorescent abducens motoneurons increased to approximately 120% by 7 days after low-dose TeNT, to include all available motoneurons in the nucleus. In addition, the average CGRP immunofluorescence optical density inside motoneurons almost doubled after 4 days and returned toward control values in the following 2 weeks. In contrast, a high-dose injection of TeNT reduced the number of CGRP-immunofluorescent motoneurons to 5.4% of control 7 days post injection, and the number returned to 77.6% after 42 days. CGRP immunofluorescence intensity inside motoneurons was also reduced. Regression analysis of F(0) values with either the number of CGRP-immunolabeled motoneurons, their average immunofluorescence intensity, or both factors combined resulted in positive correlations with regression coefficients of 0.87 or higher. Therefore, CGRP expression and firing activity in abducens motoneurons are positively correlated.

Amplification and Linear Summation of Synaptic Effects on Motoneuron Firing Rate

The aim of this study was to measure the effects of synaptic input on motoneuron firing rate in an unanesthetized cat preparation, where activation of voltage-sensitive dendritic conductances may influence synaptic integration and repetitive firing. In anesthetized cats, the change in firing rate produced by a steady synaptic input is approximately equal to the product of the effective synaptic current measured at the resting potential (I(N)) and the slope of the linear relation between somatically injected current and motoneuron discharge rate (f-I slope). However, previous studies in the unanesthetized decerebrate cat indicate that firing rate modulation may be strongly influenced by voltage-dependent dendritic conductances. To quantify the effects of these conductances on motoneuron firing behavior, we injected suprathreshold current steps into medial gastrocnemius motoneurons of decerebrate cats and measured the changes in firing rate produced by superimposed excitatory synaptic input. In the same cells, we measured I(N) and the f-I slope to determine the predicted change in firing rate (Delta F = I(N) * f-I slope). In contrast to previous results in anesthetized cats, synaptically induced changes in motoneuron firing rate were greater-than-predicted. This enhanced effect indicates that additional inward current was present during repetitive firing. This additional inward current amplified the effective synaptic currents produced by two different excitatory sources, group Ia muscle spindle afferents and caudal cutaneous sural nerve afferents. There was a trend toward more prevalent amplification of the Ia input (14/16 cells) than the sural input (11/16 cells). However, in those cells where both inputs were amplified (10/16 cells), amplification was similar in magnitude for each source. When these two synaptic inputs were simultaneously activated, their combined effect was generally very close to the linear sum of their amplified individual effects. Linear summation is also observed in medial gastrocnemius motoneurons of anesthetized cats, where amplification is not present. This similarity suggests that amplification does not disturb the processes of synaptic integration. Linear summation of amplified input was evident for the two segmental inputs studied here. If these phenomena also hold for other synaptic sources, then the presence of active dendritic conductances underlying amplification might enable motoneurons to integrate multiple synaptic inputs and drive motoneuron firing rates throughout the entire physiological range in a relatively simple fashion.

Effects of increased ventilatory drive on motor unit firing rates in human inspiratory muscles.

This study was designed to determine whether increased neural drive increases firing rates of inspiratory motoneurons uniformly in humans. The discharge of single motor units in the diaphragm, parasternal intercostal and scalene muscles was recorded with monopolar electrodes. Ventilation was increased threefold with an external dead space. The discharge of 516 motor units was sampled in four subjects. All but 4 units increased their discharge rate during inspiration with only 46 discharging tonically during expiration. With increased dead space, discharge frequencies of diaphragmatic motor units increased from 11.0 +/- 2.7 to 17.7 +/- 3.3 Hz (mean +/- SD; p < 0.001). However, firing rates increased for parasternal intercostals from 10.0 +/- 1.6 to only 11.9 +/- 1.9 Hz (p < 0.001), and for scalenes from 8.7 +/- 1.8 to only 9.5 +/- 1.2 Hz (p < 0.05). Proportionate increases in rib cage and abdominal expansion accompanied the increased ventilation with added dead space. These results suggest that previously reported predominant increase in firing rates of diaphragmatic motor units in patients with chronic airflow limitation reflects the normal response of respiratory motor output to increased neural drive. The motoneuron pools of the parasternal intercostals and scalenes may show more prominent recruitment than frequency modulation.

Motoneurons are altered in muscular dystrophy

The activity of motoneurons supplying the brachial biceps muscle was examined in eight control subjects and 26 patients affected by Duchenne muscular dystrophy. The patients were subdivided into two groups: one whose motor units (MU) fired with normal rates (N group) and the other whose MU firing rates were higher as compared to controls (I group). Firing rates of motoneurons of patients from group I increased more rapidly with increasing force level. The relationship between the standard deviation of interspike intervals and their mean value, σ( T m ), was shifted towards the shorter intervals and lower standard deviations in both groups of patients. The numerical values describing these changes correlated with the severity of disease. The MU recruitment was comparable for control subjects and for patients. Experimental results as well as computer simulations indicate that the break-point of the function σ( T m ) is correlated with motoneuronal properties, and in particular with the afterhyperpolarization (AHP) duration. In muscular dystrophy this break-point corresponds to the shorter interspike intervals. Therefore, we propose that the motoneurons in muscular dystrophy are altered either in response to the muscle degeneration or as a result of the disease itself.

Decorrelating actions of Renshaw interneurons on the firing of spinal motoneurons within a motor nucleus: a simulation study.

A simulation of spinal motoneurons and Renshaw cells was constructed to examine possible functions of recurrent inhibition. Recurrent inhibitory feedback via Renshaw cells is known to be weak. In our model, consistent with this, motoneuron firing was only reduced by a few pulses per second. Our initial hypothesis was that Renshaw cells would suppress synchronous firings of motoneurons caused by shared, dynamic inputs. Each motoneuron received an identical pattern of noise in its input. Synchrony coefficients were defined as the average motoneuron population firing relative to the activity of selected reference motoneurons; positive coefficients resulted if the motoneuron population was particularly active at the same time the reference motoneuron was active. With or without recurrent inhibition, the motoneuron pools tended to show little if any synchronization. Recurrent inhibition was expected to reduce the synchrony even further. Instead, it reduced the variance of the synchrony coefficients, without a comparable effect on the average. This suggests-surprisingly-that both positive and negative correlations between motoneurons are suppressed by recurrent inhibition. In short, recurrent inhibition may operate as a negative feedback mechanism to decorrelate motoneurons linked by common inputs. A consequence of this decorrelation is the suppression of spectral activity that apparently arises from correlated motoneuron firings due to common excitatory drive. Without recurrent inhibition, the power spectrum of the total motoneuron pool firings showed a peak at a frequency corresponding to the largest measured firing rates of motoneurons in the pool. Recurrent inhibition either reduced or abolished this peak, presumably by minimizing the likelihood of correlated firing among pool elements. Renshaw cells may act to diminish physiological tremor, by removing oscillatory components from aggregate motoneuron activity. Recurrent inhibition also improved coherence between the aggregate motoneuron output and the common drive, at frequencies above the frequency of the "synchronous" peak. Sensitivity analyses demonstrated that the spectral effect became stronger as the duration of inhibitory synaptic conductance was shortened with either the magnitude or the spatial extent of the inhibitory conductances increased to maintain constant net inhibition. Overall, Renshaw inhibition appears to be a powerful way to adjust the dynamic behavior of a neuron population with minimal impact on its static gain.

Phrenic motoneuron firing rates before, during, and after prolonged inspiratory resistive loading.

Phrenic motoneuron firing rates during brief inspiratory resistive loading (IRL) are high, and nearly all the motoneurons are recruited. Diaphragmatic fatigue has been difficult to demonstrate during IRL. Furthermore, evidence from studies in limb muscles has shown variable motoneuron responses to prolonged high-intensity loads. We studied phrenic motoneuron firing rates before, during, and after prolonged IRL in anesthetized rabbits. Of 117 phrenic axons, only 2 axons were not recruited; 41 axons were silent during unloaded breathing but were recruited at higher loads. Silent axons showed a more rapid increase in firing rate as the load increased. Phrenic motoneuron firing rates increased throughout the period of loading, whereas airway pressure swings did not. After prolonged IRL, higher motoneuron firing rates were needed during brief loads to produce the same airway pressure. No evidence of a decline in motoneuron firing rates was seen at any point. We conclude that the respiratory muscles can be shown to demonstrate physiological responses consistent with fatigue during prolonged IRL, and activation rates are high and remain so throughout this prolonged loading.

Simulated recruitment of medial rectus motoneurons by abducens internuclear neurons: synaptic specificity vs. intrinsic motoneuron properties.

Ocular motoneuron firing rate is linearly related to conjugate eye position with slope K above recruitment threshold theta. Within the population of ocular motoneurons K increases as theta increases. These differences in firing rate between motoneurons might be determined either by the intrinsic properties of the motoneurons, or by differences in synaptic input to them, or by a combination of the two. This question was investigated by simulating the input signal to medial rectus motoneurons (MR-MNs) from internuclear neurons of the abducens nucleus (INNs). INNs were represented as input nodes in a two-layer neural net, each with weighted connections to every output node representing an MR-MN. Individual simulated MR-MNs were assigned parameters corresponding to an intrinsic current threshold I(R) and an intrinsic frequency-current (f-I) slope gamma. Their firing rates were calculated from these parameters, together with the effective synaptic current produced by their synaptically weighted INN inputs, with the use of assumptions employed in computer simulations of spinal motoneuron pools. The experimentally observed firing rates of MR-MNs served as training data for the net. The following two training conditions were used: 1) synaptic weights were fixed and the intrinsic parameters of the MR-MNs were allowed to vary, corresponding to the situation in which each MR-MN receives a common synaptic drive and 2) intrinsic MR-MN properties were fixed and synaptic weights were allowed to vary. In each case, the varying quantities were trained with a form of gradient descent error reduction. The simulations revealed the following three problems with the common-drive model: 1) the recruitment of INNs produced nonlinear responses in MR-MNs with low thetas; 2) the range of I(R)s required to reproduce the observed range of theta were generally larger than those measured experimentally for cat ocular motoneurons; and 3) the intrinsic f-I slope gamma increased with I(R). Experimental data from cat indicate that gamma decreases with I(R). When synaptic weights were allowed to vary, all three problems with the common-drive model were overcome. This required MR-MNs receiving selective input from INNs with similar firing rate thresholds. These results suggest that the differences in firing rate properties among MR-MNs in relation to steady-state eye position cannot be derived from their intrinsic properties alone but result at least partly from differences in their synaptic inputs. An MR-MN's individual set of synaptic inputs constitutes, in effect, a premotor receptive field.