Steady Firing Research Articles

Factors influencing motoneuron rhythmic firing: results from a voltage-clamp study.

1. The rhythmic firing properties of cat lumbar motoneurons were determined by intracellular injection of constant-current pulses. The activation thresholds of various membrane current components were subsequently determined in the same neurons using the technique of somatic voltage clamp. Voltage steps were employed that traversed the same voltage range as the membrane potential between rhythmic spikes (the "pacemaker potential"). 2. At fast firing rates (e.g., secondary-range firing), the pacemaker potential remains entirely within the range of voltages over which a previously described (42), persistent, inward, calcium current (Ii) is activated during voltage clamp. Thus Ii is tonically activated and counters the repolarizing, outward, potassium currents during fast firing. At slower firing rates (e.g., primary-range firing), the pacemaker potential only partially enters the voltage range where Ii is activated, and this voltage range may not be entered at all the slowest firing rates. Cells in which Ii deteriorated could not be made to fire at fast rates although they could still fire at slow rates. 3. The use of two independent intracellular microelectrodes allowed accurate measurement of the somatic voltage at which spike initiation occurred ("firing level"). In all cells, firing level increased significantly as steady firing rate increased. During a given injected-current pulse, firing level also exhibited a more moderate variation with time. 4. The variation in firing level was caused by the accommodative properties of the axon initial segment. Except at the fastest firing rates, firing level occurs at less depolarized voltages than the somatic sodium conductance threshold. In addition, somatic sodium current shows minimal inactivation over the voltage range traversed by the pacemaker potentials during slower firing rates. An inactivation of about 50% is attained during the maximum firing rate. 5. We discuss the ways by which Ii activation and thr progressive rise in firing level influence motoneuron rhythmic firing. We propose that the basic role of Ii is to aid in maintaining a linear f-I curve, especially at faster firing rates. We hypothesize that the relative balance between persistent inward and outward ionic currents plays a major role in determining the f-I curve slope among different neurons and between primary- and secondary-range firing of cat lumbar motoneurons.

Activity of omnipause neurons in alert cats during saccadic eye movements and visual stimuli.

1. In the cats trained to follow a target spot with their eyes, activity was recorded from omnipause neurons (OPNs). OPNs discharge at a relatively high steady tonic rate (50-130 spikes/s) during visual fixation and smooth-pursuit eye movements but exhibit a complete cessation of discharge that begins before saccades in any direction. They are located in a compact region of the dorsal pontine tegmentum near the midline, just rostral to the abducens nucleus. 2. The average duration of the horizontal or vertical component of a saccade increases monotonically with pause duration, but a given pause duration is associated with a large range of individual saccade parameters and the timing of the pause, such as the latency from the pause onset to saccade onset or the interval from the maximum saccade velocity to the end of the pause, is no better. However, OPNs can be divided into two distinct groups on the basis of the timing of the pause relative to the parameters of the saccade. One group ceases discharging 32.4 +/- 4.6 ms, on average, before the saccade, while the second pauses 18.2 +/- 3.4 ms before the saccade. 3. Microstimulation at the site of OPNs affects the occurrence and trajectory of saccades but not smooth pursuit or fixation. Sustained electrical stimulation (20 micro A) lasting several seconds prevents the occurrence of saccades while brief trains (10-60 ms), timed to occur early in the saccade, interrupt it in midflight for the duration of the train. The latency to the interruption is about 26 ms. These data support the view that OPNs tonically inhibit the saccadic machinery between saccades and must be turned off to allow a saccade to occur. 4. Almost every (65 of 69) feline OPN exhibited a brief transient increase in activity for visual stimuli moving in any direction with a wide range of velocities. A moving 1 degree spot was generally more effective than a moving full-field, striped background. All units also showed a transient increase in firing when the spot was turned either on or off. Receptive fields plotted with the spot were greater than 250 deg2 and always included the area centralis. Two-thirds of the cells tested also responded to auditory stimuli. 5. Interaction between the excitatory visual input and the saccade-related pause was tested by comparing OPN activity and the saccadic trajectory during eye movements in the dark versus the light and by presenting brief flashes of light during a saccade. During saccades in the dark, the steady firing of OPNs was less than during saccades in the light. Only by stabilizing a flashed spot of light to occur on the area centralis at the beginning of the saccade was it possible to activate an OPN artificially to interrupt the saccade in midflight. Therefore, rather than being instrumental in specifically controlling the saccade trajectory, the visual input, along with the auditory and other sensory inputs, probably serves, under normal visual conditions, to help establish the tonic rate of OPNs. 6...

Audio-motor reflex responses in orbicularis oris single motor units having different recruitment thresholds

There is some evidence that the magnitude of reflexes in single motor units is well correlated with their mechanical and electrophysiological properties. This design feature of somatic reflex systems may have significance for speech production if it is present in speech activated muscles. The objective of the present investigation was to determine whether the reflex responsiveness of motor units within the orbicularis oris inferior muscle (OOI) is dependent on recruitment threshold. Recruitment threshold was quantified as a percentage of maximum surface EMG at which a motor unit was recruited during a 6-s ramp contraction of OOI. The reflex responsiveness of each motor unit to auditory stimulation was tested with clicks presented binaurally at 75 dB SL during voluntary activation of the motor unit at a steady firing rate of approximately 20 ips. The magnitudes of reflex suppression and excitation were quantified by means of computer analysis of poststimulus time histograms obtained on each unit. Data obtained on 61 motor units to date indicate a positive correlation between recruitment threshold and the magnitude of both reflex suppression and excitation in two subjects, and a negative correlation in a third subject. [Work supported by NINCDS.]

Some effects of auditory stimulation on perioral motor unit discharge and their implications for speech production.

The objective of this investigation was to describe the electrophysiological properties of short-latency reflexive pathways existing between the auditory system and the motoneurons involved in the control of lower lip movements for speech. The general procedure involved binaural presentation of auditory clicks (60--75 dB SL) at constant rates while subjects maintained a steady firing rate in a perioral motor unit. Post-stimulus time histograms were used to assess the effects of stimulation on the probability of firing of individual motor units. Data collected on 33 motor units in four subjects revealed a significant short-latency change in the probability of firing of 16 of these units. The mean latency of this effect was 20.4 ms. There was some indication that the direction and latency of the initial response to stimulation was dependent on the location and function of individual motor units. The likely neural pathways mediating these effects and their potential role in speech production are discussed.

Facility of motor unit control during tasks defined directly in terms of unit behaviors

The phenomenon of recruitment order reversal among pairs of motor units during “single motor unit training” was studied in 11 normal subjects in abductor pollicis brevis, first dorsal interosseus, and extensor digitorum muscles of the hand. Feedback provided by visual and auditory cues allowed all subjects to control single-unit behavior directly, such as the production of single spikes, doublets, and steady firing at various frequencies. However, verified recruitment order reversal among motor unit pairs, recorded from the same fine-wire bipolar intramuscular electrode was obtained only during contractions of markedly different orientation in a multifunctional muscle. These results suggest that limited control of motor unit recruitment order is possible only in anatomically defined muscles that have multiple physiologic functions. Single motor unit frequency modulation was tested during an eight-target tracking task. The calculated information outflow rate achieved with motor unit frequency modulation performance was comparable to that obtained by modulation of the full-wave rectified surface electromyogram.

Role of primate flocculus during rapid behavioral modification of vestibuloocular reflex. II. Mossy fiber firing patterns during horizontal head rotation and eye movement.

1. Extracellular recordings were obtained from 113 mossu fibers (MFs) in the flocculus of alert monkeys trained to perform a visual tracking task during sinusoidal, horizontal head rotation. The analysis of MF discharge patterns was designed to allow quantitative comparison of the discharge properties of flocculus MFs with brain stem cell populations from which the MFs might originate and with flocculus Purkinje cells (P-cells). Based on their firing patterns, MFs were divided into two classes. Vestibular MFs discharged in relation to head velocity and, in some cases, also in relation to eye movement. Eye movement MFs discharged only in relation to one or more components of eye movement. 2. Vestibular MFs were subdivided into three classes. Vestibular-only MFs (n = 15) displayed a modulation in firing rate during head rotation but exhibited no relationship to spontaneous eye movements. Vestibular-plus-saccade MFs (n = 14) displayed a modulation in firing rate during head rotation that quantitatively resembled the modulation in vestibular-only MFs. In addition, a pause in firing rate interrupted the vestibular modulation during saccades in one or more directions. Vestibular-plus-position MFs (n = 4) exhibited steady firing rates that were linearly related to horizontal eye position in the absence of vestibular stimulation. Sinusoidal head rotation evoked a modulation ofiring rate above and below the firing rate set by the eye position. 3. during sinusoidal head rotation, vestibular MF firing rate led head velocity by an average of 24 degrees. The amplitude of MF firing-rate modulation increased as a function of the frequency of head rotation and, hence, maximum head velocity. Since these characteristics are similar to those displayed by P-cells during suppression of the VOR, vestibular MFs probably transmit the head velocity component of P-cell firing rate to the flocculus. Based on evidence from other mammals and a quantitative comparison of population discharge characteristics, it is likely that vestibular MFs originate from the vestibular nerve and from cells in the medial vestibular nucleus. 4. Based on their discharge patterns, eye movement MFs were also subdivided into three classes. Burst MFs (n = 14) emitted a high-frequency burst of spikes prior to and during saccades in one or more direction, but were silent during steady fixation. Burst-tonic MFs (n = 53) emitted a burst of spikes prior to saccades in a preferred ("on") direction, ceased firing during saccades in the opposite ("off") direction, and exhibited steady firing rates that increased as steady gaze shifted in the on direction. Tonic MFs (n = 13) displayed steady firing rates that increased as the position of steady gaze shifted in the on direction, and either paused or exhibited step changes in firing rate during saccades. 5. During steady fixation, 64% of tonic and burst-tonic MFs were recruited into maintained firing within +/- 10 degrees of the primary direction of gaze...

Repetitive firing of Renshaw spinal interneurons.

A model of the Renshaw spinal interneuron has been developed. The model consists of a nonhomogeneous cylinder divided into three compartments: dendrites, soma and axon initial segment (I.S). The soma and dendrites are represented as a cylindrical cable by the method of Rall (1962); anatomical data of Jankowska and Lindstrom (1971) from fluorescent dye injections were used to construct the cable. The soma and I.S. membranes are assumed to have Hodgkin-Huxley-like membrane activity. In comparison with our previous model of a tonic motorneuron (Traub, 1977), the Renshaw cell has a faster membrane time constant, faster Hodgkin-Huxley rate functions, ? h and β h shifted to the right on the voltage axis, and no slow potassium conductance. With appropriate input conductances, the Renshaw cell model exhibits the following features: it develops very high frequency bursts (over 1000 impulses per s) which trail off over a period of 10---20 ms; the second spike has small amplitude and successive spikes develop progressively larger amplitudes. Comparisons are drawn with the experimental observations of Eccles et al. (1961) and Willis and Willis (`966). With this model, it is feasible to compute the steady firing rate for a large number of steady synaptic excitatory and inhibitory conductances by direct integration of the differential equations.

Information processing in a cricket ganglion: The response of giant fibres to sound pulses

The response of giant fibres in the ventral nerve cord to stimulation of cercal afferents with pulses of sound was studied in the domestic cricket, Acheta domesticus. Pulses at 450 Hz gave the highest frequency response in several classes of units, and were therefore used as stimuli in subsequent experiments. In intact animals the response of the giant fibres to bilateral cercal stimulation showed a characteristic high frequency ‘on’ response followed by steady firing of some units for the duration of the sound pulse. The end of each pulse was followed by a short period of inhibition of the tonic units. Cercal amputation and other experiments showed that input from cercal afferents excites both large and small ipsilateral giants, and excites small and inhibits large contralateral giants. Descending input from higher neural centres in intact animals tends to reduce the responses to the stimuli. It is suggested that a function of the contralateral excitatory and inhibitory effects is to sharpen the ‘on’ response of the giant fibres to sound stimuli in intact animals.

A model of a human neuromuscular system for small isometric tensions.

A model is constructed of the motor units in the human first dorsal interosseus (FDI) muscle. Each motorneuron is simulated using a pseudo-steady-state model that omits the membrane capacity and the events underlying the action potential. Properties of individual twitches in the corresponding muscle units are based on the data of Milner-Brown et al. for the FDI, while the transduction between steady firing rate and percentage of maximum tension in a muscle unit is based on the work of Rack and Westbury on the cat soleus muscle. Since we are concerned only with small isometric tensions, we ignore effects due to muscle spindles and to recurrent inhibition. The model allows one to to determine, by simulation, the tension-time functions produced by different "programs" of input to an entire pool of 120 motorneurons. Thus, for example, in order to produce tension rising linearly with time, it suffices to deliver to each neuron in the pool a non-linearly rising conductance; the conductance can be the same for all neurons in the pool, but can NOT be scaled in proportion to the surface area of the respective neurons. The input may be delivered to any part of the neuron's dendritic tree, as long as the electrotonic distribution of input is the same for all the neurons. For a linearly rising force produced in this way, most of the motorneurons yield similar slopes for their frequency-force curves, as observed by Milner-Brown et al. To produce tensions greater than about 1 kg, mechanisms not included in this model must come into play, i.e. perhaps introduction of phasic motorneurons. The most important data needed to improve this model are sets of isometric frequency-force curves for muscle units of different twitch tensions.

A description of adaptation in excitable membranes

1. (1) A general model of the adaptation, based on the presence of a slowly changing ionic current, is presented. 2. (2) It is shown that it is possible to distinguish between two main types of adaptation: an impulse dependent and a stimulus dependent. 3. (3) When stimulated by a constant current the impulse dependent type is able to fire continuously, and the relation between firing frequency and stimulus is almost linear. The time course of the firing frequency after a step of constant current consists of a rapidly falling part which abruptly breaks into an almost constant part. 4. (4) The stimulus dependent type falls into two cases: With a weak adaptation steady firing is possible for large stimuli, but not for small. With a strong adaptation steady firing is never possible. The relation between firing frequency and stimulus is, with the weak adaptation, nonlinear and the decrease in frequency with time is smooth—almost exponential. 5. (5) The firing elicited by a sudden, transient change in the magnitude of a constant stimulus is considered. For membranes exhibiting a steady firing there is an amplification of the stimulus change; so the apparent change, judged from the firing pattern, is larger than the actual change. Especially, for the impulse dependent type this amplification can be very large. 6. (6) Only for the stimulus dependent type a threshold for detecting stimulus changes can be found; and the threshold is almost proportional to the magnitude of the stimulus. 7. (7) As another dynamic property of the membrane the accommodation is described, and it is shown how the accommodative behaviour of the membrane might arise from the properties of its adaptation. 8. (8) The limitations of the model are discussed and other types of adaptation are considered.

Responses of fibers in medial longitudinal fasciculus (MLF) of alert monkeys during horizontal and vertical conjugate eye movements evoked by vestibular or visual stimuli.

Extracellular recordings were obtained from 37 histologically identified MLF fibers near the trochlear nucleus in alert monkeys trained to perform a visual tracking task and subjected to adequate horizontal and vertical vestibular stimulation. The behavioral paradigm permitted independent quantitative assessment of a fiber's response to eye or head movements. 2. According to their discharge pattern, almost all MLF fibers were placed in one of two classes: 1) Horizontal burst-tonic fibers (n = 20). During both vestibular and visually evoked eye movements, burst-tonic fibers discharged in strict relation to horizontal eye movements, exhibiting a burst of activity prior to on-direction saccades and steady firing related to horizontal eye position during fixations. 2) Vertical vestibular plus eye-position fibers (n = 14). Vertical fibers discharged in relation to vertical head velocity in the absence of eye movements and in relation to vertical eye position in the absence of head movement, and paused with saccades in any direction. 3. The quantitative similarity of horizontal burst-tonic fiber discharge to that of ipsilateral medial rectus oculomotoneurons suggests that burst-tonic fibers provide the major excitatory synaptic drive to medial rectus motoneurons during conjugate eye movements of vestibular or visual origin. 4. The discharge pattern of vertical vestibular plus eye-position fibers is significantly different from that of oculomotoneurons, suggesting that additional neural processing of vertical fiber information must occur in the mesencephalon. 5. The functional dichotomy of horizontal and vertical MLF fibers and their contrasting discharge patterns provide new evidence for the anatomic and functional separation of horizontal and vertical eye movement mechanisms in the pons and mesencephalon, respectively.

Electrophysiological identification of neurons in locus coeruleus

In 17 urethane-anesthetized rats 35 neurons, histologically verified as being situated in the locus coeruleus, were driven antidromically (latency, 44 msec) by electrical stimulation of the supracallosal bundle. Neurons of the locus coeruleus were also activated antidromically by stimulation of sites along the dorsal noradrenergic bundle in the midbrain (8-msec latency) and the hypothalamus (12-msec latency), and by stimulation of sites in the olfactory bulb (latency, 39 msec). Conduction velocity from these sites to the locus coeruleus was estimated to be 0.4 to 0.6 m/sec. Refractory periods of fibers in the dorsal noradrenergic bundle were determined at twice threshold and in the supracallosal bundle at intensities just above threshold; refractory periods ranging from 4 to 20 msec were observed. Because neurons both in and near the locus coeruleus were antidromically activated by stimulation of the dorsal noradrenergic bundle whereas stimulation of the supracallosal bundle antidromically activated only neurons in the locus coeruleus, stimulation of the dorsal noradrenergic bundle could not be used to identify locus coeruleus neurons. It is concluded that a subpopulation of neurons in the locus coeruleus can be identified by their slow, steady firing rate (2.6 per second) and long-latency antidromic response to stimulation of the supracallosal bundle. The electrophysiological properties of locus coeruleus neurons are considered in relation to neuroanatomical and functional studies of the locus coeruleus.

Simulation of phase-dependent pattern changes to perturbations of regular firing in crayfish stretch receptor

A comparison was made of responses of a repetitively firing neuron to perturbations imposed in an ongoing steady firing pattern with those predicted by a model.termed the ‘active pacemaker model’ and by a simpler ‘integrate-and-fire’ model. The active pacemaker model simulates the kinetics of processes such as active response generation and electrogenic sodium pumping which govern the membrane potential trajectory between a reset-point following an impulse and threshold for the next impulse. Responses of the crayfish stretch receptor neuron (MRO) and of both models were studied for both abrupt steps and square pulses of current, both hyperpolarizing and depolarizing, as a function of phase of the start of the stimulus in the normal interspike interval. Physiological results are consistent with a substantially weaker effect of a perturbation delivered earlier in the interspike interval as compared to the same one given later. The active pacemaker model successfully simulates several of the prominent results of the physiology (though by no means all), while the linear integrate-and-fire model is less successful.

Electrical properties of frog motoneurons in the in situ spinal cord.

Electrical properties of the spinal motoneurons of Rana temporaria and R. esculenta were investigated in the in situ spinal cord at 20-22 degrees C by means of intracellular recording and current injection. Input resistance values depended on the method of measurement in a given cell but were generally inversely related to axon conduction velocity. The membrane-potential response to a subthreshold current pulse was composed of at least two exponentials with mean time constants of 2.5 and 20 ms. The membrance potential reached by the peak of a spike depended on the mode of spike initiation and membrane potential. Preceding a suprathreshold depolarization by a hyperpolarizing pulse could delay and eliminate spike initiation, similar to effects reported in certain invertebrate neurons. Antidromic invasion frequently failed in motoneurons of normal resting potential. Antidromic spike components (m,IS, SD) were similar to those of cat motoneurons. The delayed depolarization and the long afterhyperpolarization following an antidromic spike had many properties in common with the analogous afterpotentials of cat motoneurons. The reversal potential of the short afterhyperpolarization occurring immediately after the spike varied with resting potential and could not be used to determine potassium equilibrium potential. Sustained rhythmic firing could be evoked by continuous synaptic drive or long pulses of injected current. The plot of firing rate versus current strength had a substantial linear region. Both steady firing and adaptation properties varied markedly with motoneuron input resistance.

Atmospheric penetration of exhaust plumes under rarefied flow conditions

The interaction of a low-density atmosphere with the exhaust gases produced by the steady firing of a rocket motor moving at hypersonic speed is studied using the kinetic theory of gases. The Krook collision model is employed in conjunction with a simple representation of the exhaust gas distribution to derive analytical and numerical solutions for the atmospheric gas density as it penetrates the rocket plume. The atmosphere gas is composed of an initially unscattered beam, which is attenuated as it collides with the much denser exhaust plume, and a scattered component, which is convected away from the motor by the expanding jet. These processes are illustrated by the computed results for both an axially symmetric and a three-dimensional flow.

Some Effects of Short-Term Adaptation on Incremental Responses of Auditory Neurons

Single units in the cochlear nuclei of anesthetized Mongolian gerbils were studied using best frequency tones. The units showed an onset peak and a decay to a steady firing rate in response to 150-msec tone bursts. As in our previous study [SMITH, R. L., and ZWISLOCKI, J. J. (1971). J. Acoust. Soc. Amer. 50, 1520–1525], 300-msec tone bursts served as pedestals upon which intensity increments were placed. The time delay from pedestal onset to increment onset was fixed at 150 msec. Total onset firing rates have been determined in response to positive and negative increments. The “adapted” response curves so obtained are approximately parallel to the “unadapted” ones measured at the pedestal onset. They are shifted along the stimulus intensity scale relative to the unadapted curves, and the shift increases with the pedestal intensity. This pattern holds for intensity levels of the pedestal up to 15 dB re the threshold of the units. At higher levels, where the pedestal responses show saturation effects, the slope and the maximum firing rate of the incremental response curves decrease.

Intensity characteristics of the noctuid acoustic receptor.

Spiking activity of the more sensitive acoustic receptor is described as a function of stimulus intensity. The form of the intensity characteristic depends strongly on stimulus duration. For very brief stimuli, the integral of stimulus power over stimulus duration determines the effectiveness. No response saturation is observed. With longer stimuli (50 msec), a steady firing rate is elicited. The response extends from the spontaneous rate of 20-40 spikes/sec to a saturated firing rate of nearly 700 spikes/sec. The characteristic is monotonic over more than 50 db in stimulus intensity. With very long stimuli (10 sec), the characteristics are nonmonotonic. Firing rates late in the stimulus decrease in response to an increase in stimulus intensity. The non-monotonic characteristics are attributed to intensity-related changes in response adaptation.

Response of Auditory Neurons to Intensity Increments

Single units in the cochlear nuclei of anesthetized Mongolian gerbils were studied. Stimuli were tones at unit's best frequency and response firing rates were determined from PST histograms. The units under consideration typically showed an onset peak and decay to steady firing rate in response to 300-msec tone bursts. The tone bursts served as pedestals upon which step intensity increments were placed. In response to the increments, units showed a jump in firing rate. The time of occurrence of the increment was varied relative to the onset of the pedestal. The maximum firing rate reached in response to the increment was found to be relatively independent of the time of occurrence of the increment. Thus, both the absolute and the relative change in firing rate caused by the increment increased as a function of time relative to the onset of the pedestal. The effect occurred over a wide range of increment and pedestal intensities and could not be attributed to saturation of firing rate. It may be related to the psychophysical ability to detect more easily intensity increments during, rather than at, the onset of tone pedestals.

Repetitive impulses generated in fast and slow pyramidal tract cells by intracellularly applied current steps.

Responses to current steps were recorded from pyramidal tract (PT) cells of the cat by means of intracellular microelectrodes. PT cells with resting potentials from -60 to -80 mV set up a well sustained repetitive discharge during stimulation. When comparing fast and slow PT cells, quantitative differences were found between them in the pattern of repetitive firing. Thus, (1) the rheobase is lower in slow PT cells (mean and S.D.; 0.53±0.63 nA) than in fast cells (1.57±1.11 nA). (2) Following stimulation with a current step twice rheobase the latency and the successive interspike intervals are shorter in fast PT cells than in slow cells. (3) The interspike interval distribution shows a greater irregularity in fast PT cells than in slow cells. At firing rates around 30 impulses/sec the coefficient of variation has a mean value of 0.243 for fast PT cells and 0.085 for slow cells. (4) Fast PT cells show a greater decrease of firing rate during the initial 300 msec of current stimulation (adaptation) than do slow cells. The mean value of this initial decrease is 1.85 times the later steady firing rate in fast PT cells and 0.56 times in slow cells. (5) The slope constant of the firing rate-current relationship is larger in fast PT cells, being five times or more than in slow cells. These characteristics of firing pattern are termed “kinetic” and “tonic” for fast and slow PT cells respectively, and their functional meanings are discussed in comparison with other neural organs.

The role of spontaneous firing of the trigeminal motoneuron.

1. In the trigeminal motor nucleus, the motoneuron responded to light pressure applied to a restricted site of the masseter muscle.2. The process of firing was differentiated into three phases. One was the phase with initial burst discharges at the onset of pressure application to the muscle, the second was the steady firing phase, and the third was the silent phase after removal of the pressure stimulus.3. The activity of a motoneuron was depressed by light pressure applied about 20mm anterior from the focus in the muscle. It is reasonable to suppose that a motoneuron shows a prominent increase in discharge if pressure is applied to an area comparable to that of the spindle capsule, and that this motoneuron stops discharging by the slack of the muscle spindle when pressure is applied to some area distant from the site of the spindle capsule.4. Time of recovery of motoneuron activity from both the excitation and the depression by pressure was completely proportional to the magnitude of applied pressure.5. The area of the muscle which could fire a motoneuron by pressure was mainly located on the anterior site of the masseter muscle.