Cat Medial Gastrocnemius Research Articles

Axotomy-like changes in cat motoneuron electrical properties elicited by botulinum toxin depend on the complete elimination of neuromuscular transmission

The electrical properties of cat medial gastrocnemius (MG) spinal motoneurons were studied 14-21 d following injection of type A botulinum toxin (BTX) into the MG muscle. Treated MG muscles were atrophic, displayed pronounced fibrillation activity, and were markedly but not completely paralyzed. MG motoneuron electrical properties from animals with the highest MG muscle-twitch forces (greater than 20 gm) appeared normal, while motoneuron properties from animals with the lowest MG muscle-twitch forces (less than 10 gm) exhibited axotomy-like changes, though these changes were less pronounced than after axotomy itself. No changes in the axonal conduction velocity were observed, however. Motoneuron connectivity with MG muscle fibers was determined following intracellular stimulation of MG motoneurons by averaging EMG signals from 3 or 4 pairs of recording electrodes inserted into the BTX-treated MG muscles. Normal electrical properties were observed among motoneurons in which detectable EMG activity linked to the intracellular stimulation pulse was observed. The level of this connectivity, however, indicated that a relatively small number of muscle fibers were activated by individual motoneuron action potentials. Axotomy-like changes of electrical properties were observed in MG motoneurons that could not be associated with detectable EMG activity in the BTX-treated MG muscle following repeated trials of intracellular stimulation. These results indicate that the existence of effective neuromuscular transmission at a small number of motor terminals is sufficient to prevent the appearance of axotomy-like changes in motoneuron electrical properties, and that the absence of such transmission at all motor terminals is associated with the appearance of axotomy-like changes. The results suggest that the effects of axotomy itself on motoneuron properties may be based upon the loss or elimination of a potent interaction between muscle and motoneurons normally mediated by neuromuscular transmission.

Enzymatic responses of cat medial gastrocnemius fibers to chronic inactivity

The role of neuromuscular activity in maintaining the normal enzyme heterogeneity found in a predominantly fast mixed muscle was studied. Enzymatic profiles of single fibers in the adult cat medial gastrocnemius (MG) were examined after almost complete elimination of neuromuscular activity for 6 mo. Inactivity was achieved by spinal cord isolation (SI), i.e., spinal transection at T12-T13 and L7-S1 combined with bilateral dorsal rhizotomy between the two transection sites. Cross-sectional area and succinate dehydrogenase (SDH) and alpha-glycerophosphate dehydrogenase (GPD) activities were determined in a population of fibers identified in frozen serial cross sections. Each fiber was categorized as light or dark on the basis of its staining characteristics for qualitative myosin adenosinetriphosphatase (ATPase), alkaline preincubation, and its reaction to fast and slow myosin heavy chain (MHC) antibodies. SI resulted in a conversion of nearly all light (approximately 36% in the control) to dark ATPase fibers. Virtually all MG fibers in the SI cats reacted with the fast MHC antibody, whereas very few fibers reacted with slow MHC antibody. On the basis of fiber cross-sectional area, it was estimated that the MG atrophied by approximately 10% after SI. Compared with the mean of the dark and light ATPase fibers in control (weighted by the percent fiber type distribution), mean SDH activity was significantly lower (approximately 70%) and mean GPD activity was significantly higher (approximately 120%) in the SI cats. These data indicate that prolonged electrical silence of a mixed fast hindlimb extensor results in virtually all fibers expressing fast MHC as well as oxidative and glycolytic enzyme profiles normally observed in fast glycolytic fibers.(ABSTRACT TRUNCATED AT 250 WORDS)

Mechanical output of the cat medial gastrocnemius: In-situ vs in-vivo characteristics during locomotion & jumping

Mechanical output of the cat medial gastrocnemius: In-situ vs in-vivo characteristics during locomotion & jumping

Enzymatic plasticity of medial gastrocnemius fibers in the adult chronic spinal cat

The metabolic plasticity of single fibers in adult cat medial gastrocnemius (MG) 6 mo after complete spinal cord transection (Sp) at T12-T13 was studied. Some Sp cats were trained to weight support (Sp-WS) 30 min/day beginning 1 mo posttransection. Cross-sectional area, succinate dehydrogenase (SDH), alpha-glycerophosphate dehydrogenase (GPD), and myofibrillar adenosinetriphosphatase (ATPase) activities were determined in fibers identified in frozen serial sections. Fibers were categorized as light or dark based on myosin ATPase staining, alkaline preincubation. The percentage of dark ATPase fibers was higher in Sp and Sp-WS (approximately 85%) than in control (approximately 60%). All dark ATPase fibers reacted positively to a fast myosin heavy chain monoclonal antibody. In both spinal groups, a higher percentage of dark ATPase fibers reacted to both fast and slow myosin heavy chain antibodies than in controls. Neither Sp nor Sp-WS cats showed fiber atrophy. Compared with control, SDH activity was decreased in both fiber types of Sp cats. Daily weight-support training ameliorated this adaptation. There were no differences among the three groups in mean GPD and ATPase activities for either fiber type. There was a slight tendency, however, for spinal cats to have higher GPD and ATPase activities (independent of type) than control, probably reflecting the larger proportion of dark ATPase fibers in these cats. These observations indicate that 6 mo after spinalization in adult cats, some of the fibers of a fast muscle became "faster" and developed oxidative and glycolytic enzyme profiles that normally are exhibited in fast fatigable motor units.(ABSTRACT TRUNCATED AT 250 WORDS)

Analysis of effective synaptic currents generated by homonymous Ia afferent fibers in motoneurons of the cat.

1. We have developed a technique to measure the total amount of current from a synaptic input system that reaches the soma of a motoneuron under steady-state conditions. We refer to this quantity as the effective synaptic current (IN) because only that fraction of the synaptic current that actually reaches the soma and initial segment of the cell affects its recruitment threshold and firing frequency. 2. The advantage of this technique for analysis of synaptic inputs in comparison to the standard measurements of synaptic potentials is apparent from Ohm's law. Steady-state synaptic potentials recorded at the soma of a cell are the product of IN and input resistance (RN), which is determined by intrinsic cellular properties such as cell size and membrane resistivity. Measuring IN avoids the confounding effect of RN on the amplitudes of synaptic potentials and thus provides a more direct assessment of the magnitude of a synaptic input. 3. Steady-state synaptic inputs were generated in cat medial gastrocnemius (MG) motoneurons by using tendon vibration to activate homonymous Ia afferents. We found that the magnitude of the Ia effective synaptic current (Ia IN) was not the same in all MG cells. Instead, Ia IN covaried with RN (r = 0.64; P less than 0.001), being about twice as large on average in motoneurons with high RN values as in those with low RN values. Ia IN was also correlated with motoneuron rheobase, afterhyperpolarization duration, and axonal conduction velocity. 4. A comparison of transient Ia EPSPs with steady-state Ia EPSPs (Ia EPSPSS) evoked in the same cells suggested that the effective synaptic current that produces the transient Ia EPSP was also greater in motoneurons with high RN values than in those with low RN values. 5. The factors responsible for the Ia IN-RN covariance are uncertain. However, our finding greater values of Ia IN in high RN motoneurons is consistent with other evidence suggesting that Ia boutons on these motoneurons have a higher probability for neurotransmitter release than those on low RN motoneurons (19). 6. The neural mechanisms underlying orderly recruitment are discussed. The effect of the Ia input is to produce an approximately twofold expansion of the differences in motoneuron recruitment thresholds that are generated by intrinsic cellular properties. It is suggested that the higher efficacy of Ia input in low-threshold motoneurons confers particular importance on this input system in the control of vernier movements (7).

Ultrasound transit time gives direct measurement of muscle fibre length in vivo

The movement occurring in whole muscle during contraction is divided between the muscle fibres, the tendon and the longitudinal change in length due to a change in pennation angle. The relative importance of each of these components varies between muscles and with how they are being used. To date it has not been possible to measure each of these movements. By suturing piezoelectric crystals to each end of a small group of muscle fibres in cat medial gastrocnemius muscle, the transit time of ultrasound between the crystals was measured. Assuming a constant velocity of sound in muscle of 1580 m/s, the transit time was used to calculate the muscle fibre length. In “isometric” tetanic contractions the muscle fibres shortened by up to 25% at the expense of the tendons. Lengthening the muscle by 4 mm between each two tetanii, produced less than 4 mm stretch of the muscle fibres as some of the movement was taken up by a change in pennation angle. Except at long lengths, when passive tensions were present, none of this movement between tetanii could be accounted for by stretch of the tendon.

Changes in contractile speed of cat motor units during activity.

Experiments were conducted to measure the extent of contractile changes during phasic activity of different motor units. Motor units of cat medial gastrocnemius were isolated and classified by their mechanical properties as fast and fatigable (FF), fast and fatigue resistant (FR), or slow (S). Single stimuli interpolated between stimulus trains of the fatigue test produced twitches whose shapes were measured at different times during this test. After 30 seconds of fatigue testing, twitch contraction times of 33% of FF units fell into the slow range, i.e., 5 of 17 units had become slower than the fastest slow units. Mean twitch contraction time of FF units increased by 11.8 msec, whereas that of S units decreased by 16.2 msec. We conclude that the mechanical properties of rested motor units change markedly with use and are a poor index for determining the contractile speed of active muscles.

Cat triceps surae motor nuclei are organized topologically

A “compartment nucleus” consists of a group of spinal motoneurons that supply the individual primary branches of a muscle nerve. Each primary muscle nerve branch innervates a specific subvolume or compartment of a muscle. The spatial and size distributions of motoneurons supplying primary nerve branches of the cat medial gastrocnemius muscle were examined to: (i) determine whether or not the medial gastrocnemius motor nucleus is organized in the same manner as that of the lateral gastrocnemius; (ii) determine the type of organization which collectively governs the triceps surae (lateral and medial gastrocnemius, soleus) motor nuclei. Individual primary branches of the medial gastrocnemius nerve were exposed to a horseradish peroxidase solution to retrogradely label their cells of origin. Each compartment nucleus was compared with the distribution of the entire medial gastrocnemius motor nucleus. These data were also compared with data on the spatial and size distribution of motoneurons in the lateral gastrocnemius motor nucleus. Results indicate rostrocaudal overlap in the distribution of motoneurons in each compartment nucleus. However, a rostral to caudal topographic arrangement and preferential size distribution for each compartment nucleus exist for proximal to distal muscle compartments. These and other results indicate that the motor nuclei of the entire triceps surae muscle group are organized by a topologic type map.

Correlation analysis of muscle receptor discharge during active contractions of the cat medial gastrocnemius muscle.

The spike trains of afferent fibers innervating muscle spindles and Golgi tendon organs in the medial gastrocnemius muscle were recorded during spontaneous contractions in either decerebrate cats or decapitate cats treated with L-dopa. For each afferent fiber, the approximate location of its receptor within the muscle was determined. Cross-correlation histograms were compiled from the simultaneously recorded spike trains of pairs of afferent fibers (Ia, Ib, spindle II) to determine if the degree of temporal correlation in their discharge was related to the mutual proximity of the receptors they innervated within the muscle. The frequency of occurrence and degree of correlated activity between pairs of muscle afferents, regardless of receptor type, was much greater in the decerebrate preparations than in the decapitate-L-dopa preparations. However, in all cases, the extent of correlated activity appeared to be unrelated to the relative locations of the receptors. The results suggest that the degree to which the discharge patterns of muscle receptors display temporal correlations and thereby potentially reinforce the "sensory partitioning" (5) of their parent muscle is strongly dependent on the type of preparation used, and thus by inference, dependent on the central state of the animal.

Specific tension measurements in single soleus and medial gastrocnemius muscle fibers of the cat

Direct measurements of the sizes of and forces produced by single fibers of the cat soleus and medial gastrocnemius muscles were made to determine whether or not different fiber types have characteristically distinct specific tensions. Single fibers (5-mm lengths), whose sarcolemmas had been chemically removed using a 5-mM EGTA “skinning” solution, were attached to a photodiode force transducer. Each single fiber was first placed in “relaxing” solution (22 ± 1°C, pH 7.0, pCa 8), its sarcomere length set at 2.7 μm using its laser diffraction pattern, and its diameter measured using the calibrated graticule of a microscope eyepiece (±2 μm). Subsequently, each fiber was transferred to an activating bathing solution ( pCa 3.6) in which the fiber produced its maximum tension. The specific tension values for single soleus muscle fibers displayed a threefold range (1.19 to 3.53 kg/cm 2) with a mean value of 2.30 ± 0.61 (SD) kg/cm 2 ( N = 42). The medial gastrocnemius fibers studied had a fourfold range in specific tensions (1.05 to 4.47 kg/cm 2) and a mean value of 2.42 ± 0.61 kg/cm 2 ( N = 104). Many medial gastrocnemius fibers ( N = 64) were type-identified using a standard actomyosin ATPase histochemical assay. Type I medial gastrocnemius fibers had mean specific tension values of 2.45 ± 0.47 kg/cm 2 ( N = 18), whereas, type II single fibers had mean specific tension values of 2.43 ± 0.67 kg/cm 2 ( N = 46). Our results suggest that there is no significant difference between the specific tensions of the different muscle fiber types within the cat medial gastrocnemius muscle.

Anomolous path taken by branch of medial gastrocnemius nerve

An anomolous branch of the cat medial gastrocnemius nerve was found to travel with the combined lateral gastrocnemius and soleus nerves in the popliteal fossa before innervating muscle fibers in the medial gastrocnemius. The innervation pattern was confirmed by EMG recording, elicitation of contraction in the medial gastrocnemius by stimulating the branch, isolation of a single motor unit innervating the medial gastrocnemius whose axon traveled in the branch, and glycogen depletion of medial gastrocnemius muscle fibers by stimulating the branch. Awareness of such anatomic variation is important, for exmple, in interpreting studies of synaptic connections and of mechanical properties during reinnervation of muscle.

Synchronous afferent discharge from a passive muscle of the cat: significance for interpreting spike‐triggered averages.

Evidence is presented for the existence of synchrony between the spike trains of muscle afferents of the passive cat medial gastrocnemius muscle held at fixed length. Synchrony between the spike trains of a population of muscle afferents was quantified by means of a synchronization index (Is), derived from spike-triggered averages of the muscle-nerve neurogram and the rectified neurogram. A previously used test based solely upon the neurogram average (Watt, Stauffer, Taylor, Reinking & Stuart, 1976) is shown to be invalid. The differences between experimentally derived Is values and theoretical Is values derived for the condition of asynchrony were compared to estimated confidence limits for those differences. This comparison revealed that twenty-two of fifty-three muscle-afferent spike trains whose rectified averages satisfied certain conditions for interpreting the Is were synchronized with the discharge of other afferents. The form of the rectified averages of another eight afferents suggested that these afferents might also have been synchronized. Synchrony in the discharge of muscle afferents was found in three experiments in which the neurogram was recorded from a single nerve branch to medial gastrocnemius, as well as in the data of experiments in which the whole muscle nerve was used. The degree of synchrony was similar for Ia, spindle group II and Ib afferents. The magnitude of the synchrony found in these experiments was judged by comparison to the results of analog simulations and the increase in Is values resulting from the application of small, quick stretches to the medial gastrocnemius muscle. The degree of synchrony found on average was approximately equivalent to that of a single spike occurring once for every four discharges of the reference spike train. Simulations were performed to determine the distortion of monosynaptic excitatory post-synaptic potentials (e.p.s.p.s) obtained by spike-triggered averaging which would be produced by synchrony between the spike trains of Ia and spindle group II afferents of the magnitude found in this study. These simulations indicate that the apparent amplitude would be increased by approximately 4 microV on average. Both the 10-90% rise time and the half-width would increase, the effects being greater for smaller e.p.s.p.s. Consequently, the synchrony found in this study is of most concern in the study of small post-synaptic potentials, such as those due to spindle group II afferents.

Membrane electrical properties and prediction of motor-unit type of medial gastrocnemius motoneurons in the cat.

Membrane electrical properties [time constant, action potential afterhyperpolarization (AHP), rheobase, input resistance, and axonal conduction velocity] were measured in motoneurons of cat medial gastrocnemius (MG) motor units. Motor units were classified on the basis of their mechanical responses as fast twitch, fast fatiguing (FF); fast twitch with intermediate fatigue resistance (FI); fast twitch, fatigue resistant (FR); or slow twitch, fatigue resistant (S; 11, 22). Motoneuron membrane time constant, estimated from the voltage response at the onset or termination of long (50-100 ms) current pulses and corrected for voltage-response nonlinearities (32), was found to differ significantly among the major motor-unit types, increasing in the order FF less than FR less than S. Afterhyperpolarization magnitude, half-decay time, and duration were all significantly greater for the fast (FF + FI + FR) versus the slow (S) motor units. The AHP half-decay time was correlated with muscle unit twitch time over the entire motoneuron population and within the type S motor-unit population. There was no significant correlation between twitch time and AHP half-decay time among the types FF and FR motor-unit populations. In agreement with previous studies, we found a significant difference in both rheobase and input resistance among the major motor-unit types, with rheobase increasing in the order S less than FR less than FF and input resistance decreasing in that order (S greater than FR greater than FF). The differences in input resistance were present both before and after correcting for voltage-response nonlinearities (32). Also in agreement with previous studies, the mean axonal conduction velocity was significantly faster among the fast (FF + FI + and FR) compared with the slow (S) motor units. These data were used to examine the properties alone to determine motor-unit type, which has traditionally been defined on the basis of the muscle unit's mechanical properties (11, 22). We used a discriminant analysis program to classify 73 mechanically typed motor units for which we had measures of rheobase, input resistance, membrane time constant, and AHP half-decay time. This model was able to properly classify 71 of the 73 motor units of this data set, indicating that the motor units of this data set could be grouped into three categories representing the three major motor-unit types (FF, FR, and S) on the basis of their rheobase, input resistance, membrane time constant, and AHP half-decay time.(ABSTRACT TRUNCATED AT 400 WORDS)

Linear and non-linear summation of α-motoneuron potential changes elicited by contractions of homonymous motor units in cat medial gastrocnemius

An analysis is made of linear and non-linear summation of the motor unit (MU) tension and motoneuron (MN) membrane potential (PSP) trajectories evoked by stimulation of MUs as previously reported. In most cases, summation of both averaged responses was linear. Seldom, PSPs summed non-linearly, then usually coinciding with non-linear summation of the responsible MU twitches. The results suggest that the afferent pathways mediating the mechanical events in muscle to MNs behave rather linearly, at least in the small-signal range of interest here.

Wiring diagrams of functional connectivity in monosynaptic reflex arcs of the spinal cord

The direct functional connections between Ia and group II spindle afferent fibers from the cat medial gastrocnemius muscle and their homonymous motoneurons were examined in 10 acute experiments. Trains of stretch-evoked impulses from as many as 20 undivided sensory fibers were recorded simultaneously from 5 dorsal root filaments, as well as the corresponding excitatory postsynaptic potentials (EPSPs) they elicited in 10–20 motoneurons. Spike-triggered averaging [13] of these tape-recorded signals revealed the functional connections (or non-connections) between each Ia or group II afferent fiber and each motoneuron. Wiring diagrams constructed from these data indicate that the probability of a functional connection between an afferent fiber and a motoneuron decreases with the size of either and with the distance between the entry point of the afferent fiber and the motoneuron.

Topographic factors in distribution of homonymous group Ia-afferent input to cat medial gastrocnemius motoneurons.

Experiments were performed to determine whether the topographic relationships between muscle spindles and their surrounding extrafusal fibers are preserved in the pattern of homonymous, monosynaptic connections from Ia-afferents to motoneurons. The medial gastrocnemius (MG) muscle of adult cats was chosen as a model system because previous studies have shown that its muscle nerve divides into several branches, each of which innervates a distinct muscle compartment (20, 27, 28, 38), and that the Ia-afferent fibers innervating muscle spindles within a compartment are found in the same nerve branch (27, 28). Thus, we could make intracellular recordings from MG motoneurons, determine which intramuscular compartment they innervated, and then compare the synaptic input they received from Ia-afferents innervating the same compartment with that which they received from Ia-afferents innervating different compartments. Our results indicate that homonymous Ia-afferent input is "topographically weighted" within the MG motor nucleus such that afferents innervating a given intramuscular compartment exert relatively greater synaptic effects in motoneurons that project to the same compartment than in other homonymous motoneurons. The degree of topographic weighting was quite variable in the different experiments but appeared more prominently in experiments in which a high proportion of the motoneurons studied were characterized by high-rheobase values and low input resistances. This suggests that topographic factors may exert more influence on the distribution of Ia-afferent input to large motoneurons than to small motoneurons. In the DISCUSSION, the extent of topographic weighting within the homonymous motoneuron pool is compared with weighting across synergist motoneuron pools, and alternative models of topographic weighting are proposed and evaluated.

Correlation of myoelectric activity and muscle force during selected cat treadmill locomotion

The purpose of this investigation was to associate the force produced by the cat medial gastrocnemius (MG) during unrestrained treadmill locomotion with the corresponding myoelectric signals (MES's). An intervention analysis based on an autoregressive-integrated moving average (ARIMA) process was performed on records obtained at treadmill speeds of 0.67 ms −1 and 2.24 ms −1. Results indicate that the pattern of MG myoelectric activity during a single step cycle may be divided into two parts. The primary burst ( E 1 burst) of activity occurs before foot contact and represents an energy build up which is related to a major part of the MG force monitored at the tendon. During stance, a second burst of activity is depicted by an almost critically damped second order system, and is responsible for the residual tension observed. Thus in vivo forces can be linked to MES's provided that phase differences between electrical and mechanical responses are taken into account.

Static stretch sensitivity of Ia and II afferents in the cat's gastrocnemius.

The static discharge characteristics of deefferented Ia and II spindle afferents in the cat medial gastrocnemius muscle have been compared with respect to incidence and firing rate of spontaneously firing units, length threshold of nonspontaneous units, linearity of the response to stretch, and position sensitivity. Measurements of the stretch response were taken 20s after each step increase in length so as to obtain the nearly fully adapted rate. Under this condition the frequency-length relationship was quite linear, especially for group II units. The mean position sensitivity of group II fibers exceeded that of Ia fibers in the ratio of 1.5: 1.0. The incidence of spontaneously discharging units was about 20% for both types of unit. The relationships between some of these measures and axonal conduction rate were examined. No correlation of position sensitivity with axonal conduction velocity was detected for either afferent type, but the length thresholds of the group II afferents showed a significant trend for slower conducting, that is, smaller fibers to be less sensitive. There was some indication of a direct relationship between conduction velocity and length threshold for Ia fibers. The contradiction to the "size principle" and mechanisms contributing to the static discharge behavior of the spindle are discussed.

Basis of segmental motor control

The principles of organization of motor control at the segmental level are developed and discussed in this review. Consideration is given to the concepts of the motor unit, the muscle unit, and the motoneuron pool. Recent studies from our laboratories that have significance for hypotheses regarding segmental motor control are presented. These studies indicate that the critical factor controlling motor unit recruitment in heterogenous muscles is motor unit type. This results in motor unit recruitment in the order of increasing contraction strength and fatigability: slow twitch, fatigue-resistant first; fast twitch, fatigue-resistant second; fast twitch with intermediate fatigue resistance third; and fast twitch, fatigable units last. A recruitment model that incorporates this hypothesis is presented in which there is an orderly recruitment of motor units by type. This recruitment model, based on data from cat medial gastrocnemius motor units, closely approximates a theoretical optimal recruitment strategy and is consistent with actual medial gastrocnemius muscle forces generated during free movements in intact animals. (Neurosurgery 8:608-621, 1981)

The effect of long-term immobilization on the motor unit population of the cat medial gastrocnemius muscle

The properties of medial gastrocnemius and soleus muscles and of individual medial gastrocnemius motor units were evaluated after chronic (3–29 weeks) unilateral immobilization of ankle and knee joints in 10 adult female cats. Joint fixation markedly impaired the mobility of the pinned limb and, when prolonged (> 17 weeks; 8 animals), produced marked muscle atrophy and some shortening of whole muscle isometric twitch contraction times (mean contraction time of pinned medial gastrocnemius was 83.3 percent of the contralateral muscle). However, the immobilized muscles were not truly disused, since the patterns of medial gastrocnemius and soleus electromyographic activity during posture and locomotion were qualitatively normal in the 4 animals examined. Despite severe whole muscle atrophy, all but 3 of the 239 individual medial gastrocnemius motor units studied were readily classified into one of the 4 motor unit types (types FF, F(int), FR and S) found in the normal muscle. The histochemical mosaic of immobilized muscles was also the same as in contralateral unoperated muscles. When compared to data from normal cats, there was a highly significant loss of maximum twitch and tetanic force output in all unit types after long-term immobilization (17–29 weeks; N = 187), most marked among the fatigue-resistant unit groups (types FR and S), and mean isometric twitch contraction times were shorter than expected in all unit type groups. The most surprising finding in the present work was that the mean homonymous (medial gastrocnemius) and heteronymous (lateral gastrocnemius-soleus) group Ia excitatory postsynaptic potentials were significantly smaller than expected in all motor unit groups in both short-term and long-term immobilization. The mean axonal conduction velocity of medial gastrocnemius motor axons was unchanged from the normal sample. These results indicate that the essential features that define the motor unit types in normal medial gastrocnemius muscles are resistant to change despite long-term immobilization. We conclude that the motor units types found in normal muscles are robust and essentially unaltered despite severe muscle atrophy. The effects on group Ia excitatory postsynaptic potentials suggest that limb immobilization produces altered patterns of impulse traffic in group Ia afferents that induce significant decreases in synaptic efficacy, although it is unclear whether or not Ia traffic is increased or decreased by immobilization.