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Neurone firing behaviour is a result of complex interaction between synaptic inputs and cellular intrinsic properties. Intriguing firing behaviour, delayed spiking, was shown in some neurones, in particular, in cat neocortical neurones and rat pyramidal hippocampal neurones. In contrast, the similar spiking mode was not reported for animal spinal motoneurones. In the present study, an attempt was made to look for possible evidence of delayed spiking in human motoneurones firing within the low-frequency, sub-primary range, characteristic for voluntary muscle contractions and postural tasks. Forty-seven firing motor units (MUs) were analyzed in ten experiments on three muscles (the flexor carpi ulnaris, the tibialis anterior, and the abductor pollicis brevis) in four healthy humans. Single MUs were activated by gentle voluntary muscle contractions. MU peri-stimulus time histograms, durations of inter-spike intervals, and motoneurone excitability changes within a target interspike interval were analyzed. It was found that during testing the firing motoneurone excitability by small, transient excitatory Ia afferent volley, depending firstly on volley timing within a target interspike interval and excitatory volley strength, the same motoneurone displayed either the direct short-latency response (the H-reflex) or the delayed response (with prolonged and variable latency). Thus, the findings, for the first time, provide evidence for a possibility of two modes of spiking in firing motoneurones. Methods of the estimation of delayed responses and their possible functional significance are discussed. It is emphasized that, for understanding of this issue, the integration of data from studies on experimental animals and humans is desirable.

This study investigated whether modulation of corticospinal-motoneuronal excitability and/or synaptic transmission of the Ia afferent spinal reflex contributes to decreases in voluntary activation and muscular force after an acute bout of prolonged static muscle stretching. Fifteen men performed five 60-s constant-torque stretches (15-s rest intervals; total duration 5 min) of the plantar flexors on an isokinetic dynamometer and a nonstretching control condition in random order on 2 separate days. Maximum isometric plantar flexor torque and triceps surae muscle electromyographic activity (normalized to M wave; EMG/M) were simultaneously recorded immediately before and after each condition. Motor-evoked potentials (using transcranial magnetic stimulation) and H-reflexes were recorded from soleus during EMG-controlled submaximal contractions (23.4 ± 6.9% EMG maximum). No changes were detected in the control condition. After stretching, however, peak torque (mean ± SD; -14.3 ± 7.0%) and soleus EMG/M (-17.8 ± 6.2%) decreased, and these changes were highly correlated (

Objective: To investigate the effect of short-term vibration frequencies on muscle force generation capabilities. Method: Six healthy participants were recruited for this study and only their dominant leg was tested. The subjects were tested under five conditions of vibration frequencies with constant amplitude: 0 Hz (no vibration), 30 Hz, 60 Hz, and 90 Hz, and the vibration amplitude was 10 mm for all frequency conditions. The vibration was applied to the rectus femoris (RF). The subjects were then instructed to maintain a steadystate isometric knee joint torque (100 Nm) for the first 6 s. After the steady-state torque production, the subjects were required to produce isometric knee joint torque by leg extension as hard as possible with a start signal within the next 3 s. The vibration was applied for ~4 s starting from 1 s before initiation of the change in the steady-state knee joint torque. Results: The results showed that the maximum voluntary torque (MVT) of the knee joint increased with the vibration frequencies. On average, the MVTs were 756.47 Nm for 0 Hz (no vibration) and 809.61 Nm for 90 Hz. There was a significant positive correlation (r = 0.71) between the MVTs and integrated electromyograms (iEMGs). Further, the co-contraction indices (CCIs) were computed, which represent the ratio of the iEMGs of the antagonist muscle to the iEMGs of all involved muscles. There was a significant negative correlation (r = 0.62) between the CCIs and MVTs, which was accompanied by a significant positive correlation (r = 0.69) between the iEMGs of the vibrated muscle (RF). There was no significant correlation between the MVTs and iEMGs of the antagonist muscle. Conclusion: The results of this study suggest that the short-term vibration on the muscle increases the level of muscle activation possibly owing to the increased Ia afferent activities, which enhances the muscle force generation capability.

Stretches of relaxed cat and rat muscle elicit similar history-dependent muscle spindle Ia firing rates that resemble history-dependent forces seen in single activated muscle fibers (Nichols and Cope, 2004). During stretch of relaxed cat muscle, whole musculotendon forces exhibit history-dependence that mirror history-dependent muscle spindle firing rates, where both muscle force and muscle spindle firing rates are elevated in the first stretch in a series of stretch-shorten cycles (Blum et al., 2017). By contrast, rat musculotendon are only mildly history-dependent and do not mirror history-dependent muscle spindle firing rates in the same way (Haftel et al., 2004). We hypothesized that history-dependent muscle spindle firing rates elicited in stretch of relaxed rat muscle would mirror history-dependent muscle fiber forces, which are masked by noncontractile tissue at the level of whole musculotendon force. We removed noncontractile tissue force contributions from the recorded musculotendon force using an exponentially-elastic tissue model. We then show that the remaining estimated muscle fiber force resembles history-dependent muscle spindle firing rates recorded simultaneously. These forces also resemble history-dependent forces recorded in stretch of single activated fibers and attributed to muscle cross-bridge mechanisms (Campbell and Moss, 2000). Our results suggest that history-dependent muscle spindle firing in both rats and cats arise from stretch of cross-bridges in muscle fibers.

Stretches of relaxed cat and rat muscle elicit similar history-dependent muscle spindle Ia firing rates that resemble history-dependent forces seen in single activated muscle fibers ( Nichols and Cope, 2004). Owing to thixotropy, whole musculotendon forces and muscle spindle firing rates are history dependent during stretch of relaxed cat muscle, where both muscle force and muscle spindle firing rates are elevated in the first stretch in a series of stretch–shorten cycles ( Blum et al., 2017). By contrast, rat musculotendon exhibits only mild thixotropy, such that the measured forces when stretched cannot explain history-dependent muscle spindle firing rates in the same way ( Haftel et al., 2004). We hypothesized that history-dependent muscle spindle firing rates elicited in stretch of relaxed rat muscle mirror history-dependent muscle fiber forces, which are masked at the level of whole musculotendon force by extracellular tissue force. We removed estimated extracellular tissue force contributions from recorded musculotendon force using an exponentially elastic tissue model. We then showed that the remaining estimated muscle fiber force resembles history-dependent muscle spindle firing rates recorded simultaneously. These forces also resemble history-dependent forces recorded in stretch of single activated fibers that are attributed to muscle cross-bridge mechanisms ( Campbell and Moss, 2000). Our results suggest that history-dependent muscle spindle firing in both rats and cats arise from history-dependent forces owing to thixotropy in muscle fibers.

Postactivation depression (PActD) of Ia afferent excitatory postsynaptic potentials (EPSPs) in spinal motoneurons results in a long-lasting depression of the stretch reflex. This phenomenon (PActD) is of clinical interest as it has been shown to be reduced in a number of spastic disorders. Using in vivo intracellular recordings of Ia EPSPs in adult mice, we demonstrate that PActD in adult (100–220 days old) C57BL/6J mice is both qualitatively and quantitatively similar to that which has been observed in larger animals with respect to both the magnitude (with ∼20% depression of EPSPs at 0.5 ms after a train of stimuli) and the time course (returning to almost normal amplitudes by 5 ms after the train). This validates the use of mouse models to study PActD. Changes in such excitatory inputs to spinal motoneurons may have important implications for hyperreflexia and/or glutamate-induced excitotoxicity in the neurodegenerative disease amyotrophic lateral sclerosis (ALS). With the use of the G127X SOD1 mutant mouse, an ALS model with a prolonged asymptomatic phase and fulminant symptom onset, we observed that PActD is significantly reduced at both presymptomatic (16% depression) and symptomatic (17.3% depression) time points compared with aged-matched controls (22.4% depression). The PActD reduction was not markedly altered by symptom onset. Comparing these PActD changes at the EPSP with the known effect of the depression on the monosynaptic reflex, we conclude that this is likely to have a much larger effect on the reflex itself (a 20–40% difference). Nevertheless, it should also be accounted that in aged (580 day old) C57BL/6J mice there was also a reduction in PActD although, aging is not usually associated with spasticity.

Muscle kinematics and kinetics are nonlinearly encoded by proprioceptors, and the changes in muscle length and velocity are integrated into Ia afferent. Besides, proprioceptive signals from multiple muscles are probably mixed in afferent pathways, which all lead to difficulties in proprioceptive recognition for the cerebellum. In this study, the artificial neural networks, whose organizations are biologically based on the spinocerebellar tract and cerebellum, are utilized to decode the proprioceptive signals. Consistent with the controversy of the proprioceptive division in the dorsal spinocerebellar tract, the spinocerebellar tract networks incorporated two distinct inferences, (1) the centralized networks, which mixed Ia, II, and Ib and processed them together; (2) the decentralized networks, which processed Ia, II, and Ib afferents separately. The cerebellar networks were based on the Marr–Albus model to recognize the kinematic states. The networks were trained by a specific movement, and the trained networks were subsequently required to predict kinematic states of six movements. The results demonstrated that the centralized networks, which were more consistent with the physiological findings in recent years, had better recognition accuracy than the decentralized networks, and the networks were still effective even when proprioceptive afferents from multiple muscles were integrated.

The human soleus stretch reflex is comprised of several components with different latencies typically termed the SLR, MLR and in some cases LLR component. The SLR is suggested to be mediated by Gr Ia afferent arising from activation of spindle afferents, the MLR is predominantly due to gr Ib and II feedback while LLR likely contains a cortical component. Here we report data on the changes in both the SLR and MLR following a well-established conditioning protocol. Participants attended 30 sessions where the initial six served as a baseline measure of reflex size and the following 24 sessions were devoted to training the participants to either increase or decrease the SLR component. Following successful up-conditioning of the SLR, the MLR remained unchanged. However the SLR and MLR both decreased with down-conditioning. This suggests that the MLR is not exclusively Gr Ib or II based but has some Ia afferent input.

The reflex motor responses of 16 symmetric leg muscles evoked by transcutaneous electric stimulation of spinal nerves at the L2-L3 or L3-L4 level were studied in subjects with signs of lumbar nerve compression. The evoked responses agreed with Ia afferent activation through monosynaptic neuronal circuits. Their monosynaptic nature was demonstrated by amplitude changes in response to a conditioning stimulus and continuous tendon vibration. Multisegmental monosynaptic responses (MMRs) of large distal flexors containing large numbers of slow motor units were subjected to the greatest modifications against a background of lumbar nerve compression.

In humans, an inhibitory via Ia afferent pathway from the medial gastrocnemius (MG) to the soleus (SOL) motoneuron pool has been suggested. Herein, we examined the relation between MG fascicle length changes and the SOL H-reflex modulation during passive knee movement. Twelve subjects performed static and passive (5 degrees s(-1)) knee movement tasks with the ankle immobilized using an isokinetic dynamometer in sitting posture. The maximal H- and M-waves were measured at four target angles (20 degrees, 40 degrees, 60 degrees, and 80 degrees flexion from full knee extension). The MG fascicles length and velocity were measured using a B-mode ultrasonic apparatus. Results demonstrated that the SOL Hmax/Mmax; i.e., ratio of the maximal H- to M-waves, was attenuated with increasing MG fascicle length in static tasks. The SOL Hmax/Mmax at 20 degrees was significantly attenuated compared with 60 degrees and 80 degrees with increasing MG fascicle length and lengthening velocity in passive knee extension. However, no significant differences in the SOL Hmax/Mmax were found across the target angles in the passive knee flexion task. In conclusion, as muscle spindles increase their discharge with lengthening fascicle velocity, but keep silent when fascicles shorten, our data suggest that lengthening the MG facilitates an inhibitory Ia pathway from MG to SOL, and modulates SOL motoneuron activity during movements.

Sudden stretch of active muscle typically results in two characteristic electromyographic responses: the short latency M1 and the long latency M2. The M1 response originates from the monosynaptic Ia afferent reflex pathway. The M2 response is less well understood and is likely a compound response to different afferent inputs mediated by spinal and transcortical pathways. In this study the possible contribution of the Ia afferent pathway to the M2 response was investigated. A mechanism was hypothesized in which the M1 response synchronizes the motoneurons, and therewith their refractory periods. Stretch perturbation experiments were performed on the wrist and results were compared with a computational model of a pool of motoneurons receiving tonic and Ia afferent input. The simulations showed the same stretch amplitude, velocity, and duration-dependent characteristics on the M2 as found experimentally. It was concluded that the stretch duration effect of the M2 likely originates from the proposed Ia afferent mediated mechanism.

The central nervous system employs different strategies to execute specific motor tasks. Because afferent feedback during shortening and lengthening muscle contractions differs, the neural strategy underlying these tasks may be quite distinct. Cortical drive may be adjusted or afferent input regulated. The exact mechanisms are not clear. Here, we examine the control of synaptic transmission across the Ia synapse during shortening and lengthening muscle contractions. Subjects were instructed to maintain isolated activity in a single tibialis anterior (TA) motor unit while muscle length was varied from flexion to extension and back. At a fixed interval after a firing of the active motor unit, a single electrical stimulus was applied to the common peroneal nerve to activate Ia afferents from the TA muscle. We investigated the stimulus-induced change in firing probability of 19 individual low-threshold TA motor units during shortening and lengthening contractions. Any change in firing probability depends on both pre- and postsynaptic mechanisms. In this experiment, motoneuron firing rate was similar during both contraction types. There was no difference in the firing probability between shortening and lengthening contractions (0.23 ± 0.03 and 0.20 ± 0.02, respectively). We suggest that there is no contraction type-specific control of Ia input to the motoneurons during shortening and lengthening muscle contractions. Cortical adjustments may have occurred.

The quadriceps femoris (QF) muscle group plays an essential role in human movement, such as standing, walking and running. The ability to maintain a steady force during physical activity of the human lower limb is important for mobility, postural control and balance. Although prolonged mechanical vibration of the muscle-tendon unit can moderate the efficacy of synaptic input from Ia afferent onto the α-motor neuron pathway, the effect of prolonged tendon vibration on fluctuations of knee extensor force has received little attention. The purpose of the present study was to examine the effects of prolonged patellar tendon vibration on the force steadiness of the QF muscle. Nine healthy men performed a submaximal force-matching task involving isometric knee extension before and after patellar tendon vibration or quiet seated rest (n = 7, control condition) for 30 min. The target force was 2.5, 10 and 30 % of maximal voluntary contraction (MVC). Surface electromyography (EMG) of the four QF synergists was recorded and normalized to EMG amplitude during the MVC. The knee extension force and the EMG amplitude of vastus medialis during the MVC were significantly reduced after the vibration, but did not significantly decrease in the control condition. Fluctuations of force and normalized EMG of individual QF muscles at each submaximal force level did not significantly change after the vibration. We conclude that prolonged patellar tendon vibration does not influence the force steadiness of the QF muscle during an isometric force-matching task.

The Hoffmann reflex (H-reflex) is dependently modulated during isometric and anisometric muscle actions. However, the manner of the H-reflex modulation during dynamic muscle movements in relation to ageing is less stated in the literature. This study was designed to investigate the effects of ageing on soleus (SOL) H-reflex modulation during dynamic muscle actions. Twenty young (24±4 years of age) and 20 older adults (73±5 years of age) voluntarily participated in the study. The SOL H-reflex was measured during passive and active shortening and lengthening muscle actions in a sitting position. The older group showed a lower ratio of the maximal amplitude of H-reflex to M-wave (SOL Hmax/Mmax) during the passive lengthening than that during the passive shortening (shortening: 0.40±0.22 vs. lengthening: 0.15±0.10, P＜0.05), whereas the SOL Hmax/Mmax ratio of the young group was significantly higher during the shortening than that during the lengthening contractions at maximal effort (shortening: 0.51±0.26 vs. lengthening: 0.37±0.18, P＜0.05). These results suggested different modulations of group Ia afferent inputs to the SOL motoneurons during passive and active dynamic muscle actions between young and older adults.

Transmission in the corticospinal and Ia pathways to soleus motoneurons was investigated in healthy human subjects during bicycling. Soleus H reflexes and motor evoked potentials (MEPs) after transcranial magnetic stimulation (TMS) were modulated similarly during the crank cycle being large during downstroke [concomitant with soleus background electromyographic (EMG) activity] and small during upstroke. Tibialis anterior MEPs were in contrast large during upstroke and small during downstroke. The soleus H reflexes and MEPs were also recorded during tonic plantarflexion at a similar ankle joint position, corresponding ankle angle, and matched background EMG activity as during the different phases of bicycling. Relative to their size during tonic plantarflexion, the MEPs were found to be facilitated in the early part of downstroke during bicycling, whereas the H reflexes were depressed in the late part of downstroke. The intensity of TMS was decreased below MEP threshold and used to condition the soleus H reflex. At short intervals (conditioning-test intervals of −3 to −1 ms), TMS produced a facilitation of the H reflex that is in all likelihood caused by activation of the fast monosynaptic corticospinal pathway. This facilitation was significantly larger in the early part of downstroke during bicycling than during tonic plantarflexion. This suggests that the increased MEP during downstroke was caused by changes in transmission in the fast monosynaptic corticospinal pathway. To investigate whether the depression of H reflexes in the late part of downstroke was caused by increased presynaptic inhibition of Ia afferents, the soleus H reflex was conditioned by stimulation of the femoral nerve. At a short interval (conditioning-test interval: −7 to −5 ms), the femoral nerve stimulation produced a facilitation of the H reflex that is mediated by the heteronymous monosynaptic Ia pathway from the femoral nerve to soleus motoneurons. Within the initial 0.5 ms after its onset, the size of this facilitation depends on the level of presynaptic inhibition of the Ia afferents, which mediate the facilitation. The size of the facilitation was strongly depressed in the late part of downstroke, compared with the early part of downstroke, suggesting that increased presynaptic inhibition was indeed responsible for the depression of the H reflex. These findings suggest that there is a selectively increased transmission in the fast monosynaptic corticospinal pathway to soleus motoneurons in early downstroke during bicycling. It would seem likely that one cause of this is increased excitability of the involved cortical neurons. The increased presynaptic inhibition of Ia afferents in late downstroke may be of importance for depression of stretch reflex activity before and during upstroke.

In human peripheral nerves, physiological evidence has been presented for a number of biophysical differences between cutaneous afferents and alpha motor axons. The differences in strength-duration properties for cutaneous afferents and motor axons in the median nerve have been attributed to greater expression of a persistent Na(+) conductance (I(Na,P)) on cutaneous afferents. However, it is unclear whether the biophysical properties of human group Ia afferents differ from those of cutaneous afferents. The present studies were undertaken to determine whether the properties of human group Ia afferents can be studied indirectly using 'threshold tracking' to measure the excitability changes in the H reflex, and to determine whether the excitability of group Ia afferents differs from that of cutaneous afferents. The strength-duration properties of the soleus H reflex and soleus motor axons were measured at rest and during sustained voluntary contractions. Similar experiments were performed on the median nerve at the wrist to study the strength-duration properties of cutaneous afferents, alpha motor axons and H reflex of the thenar muscles. In addition, the technique of 'latent addition' was used to determine whether there was a difference in a low-threshold conductance on soleus Ia afferent and motor axons. The present findings indicate that the strength-duration time constant (tau(SD)) for the H reflex is longer than that for alpha motor axons, but similar to that for cutaneous afferents. There were no differences in tau(SD) for the soleus H reflex at rest and during contractions, suggesting that tau(SD) for the H reflex is largely unaffected by changes in synaptic or motoneurone properties. Finally, the difference in latent addition suggests that the longer tau(SD) of the soleus H reflex may indeed be due to greater activity of a persistent Na(+) conductance on Ia afferents than on soleus alpha motor axons.

The purpose of the present study was to examine the possible inhibitory effect of passive plantar flexor muscle stretching on the motoneuron facilitatory system. Achilles tendon vibration (70 Hz) and triceps surae electrical stimulation (20 Hz) were imposed simultaneously in 11 subjects to elicit contraction through reflexive pathways in two experiments. In experiment 1, a vibration-stimulation protocol was implemented with the ankle joint plantar flexed (+10°), neutral (0°), and dorsiflexed (−10°). In experiment 2, the vibration-stimulation protocol was performed twice before (control), then immediately, 5, 10, and 15 min after a 5-min intermittent muscle stretch protocol. Plantar flexor torque and medial and lateral gastrocnemius and soleus (EMGSol) EMG amplitudes measured during and after (i.e., self-sustained motor unit firing) the vibration protocol were used as an indicator of this facilitatory pathway. In experiment 1, vibration torque, self-sustained torque and EMGSolwere higher with the ankle at −10° compared with 0° and +10°, suggesting that this method is valid to assess motoneuronal facilitation. In experiment 2, torque during vibration was reduced by ∼60% immediately after stretch and remained depressed by ∼35% at 5 min after stretch ( P < 0.05). Self-sustained torque was also reduced by ∼65% immediately after stretch ( P < 0.05) but recovered by 5 min. Similarly, medial gastrocnemius EMG during vibration was reduced by ∼40% immediately after stretch ( P < 0.05), and EMGSolduring the self-sustained torque period was reduced by 44% immediately after stretch ( P < 0.05). In conclusion, passive stretch negatively affected the motoneuronal amplification for at least 5 min, suggesting that motoneuron disfacilitation is a possible mechanism influencing the stretch-induced torque loss.

The amplitude of the Hoffmann (H)-reflex in the soleus (Sol) muscle is known to be suppressed during passive stepping compared with during passive standing. The reduction of the H-reflex is not due to load-related afferent inputs, but rather to movement-related afferent inputs from the lower limbs. To elucidate the underlying neural mechanisms of this inhibition, we investigated the effects of the stepping velocity on the Sol H-reflex during robot-assisted passive stepping in 11 healthy subjects. The Sol H-reflexes were recorded during passive standing and stepping at five stepping velocities (stride frequencies: 14, 21, 28, 35, and 42 min(-1)) in the air. The Sol H-reflexes were significantly inhibited during passive stepping as compared with during passive standing, and reduced in size as the stepping velocity increased. These results indicate that the extent of H-reflex suppression increases with increasing movement-related afferent inputs from the lower limbs during passive stepping. The velocity dependence suggests that the Ia afferent inputs from lower-limb muscles around the hip and knee joints are most probably related to this inhibition.

Aim Following the electrical stimulation of mixed nerve during voluntary muscle contraction, temporarily suppressed EMG activation called mixt nerve silent period (MNSP). MNSP is interrupted by several bursts; first one is segmental reflex (SR) and others known as a long-loop reflexes(LLR). Vibration has been shown suppress the segmental H-reflex amplitude by increasing Ia afferent discharge. In this study, our aim was to provide insight into SR characteristics and the effects of vibration on other parameters. Method Eleven healthy volunteer (5 women, 33.36 ± 5.35 years) included the study. Vibration was applied for 2 min at 100 Hz with repositioning amplitude of 1 mm on right forearm. Median nerve stimulated at wrist during thumb abduction and MNSP recorded at baseline, during and after the vibration application. Latencies of SR, LLR and MNSP and amplitudes of SR and LLR compared for each session. Results Average SR and LLR amplitudes, MNSP durations before, during and after vibration were found as (1093.36 ± 606.16/877.22 ± 390.37/863.0 ± 604.51), (561.17 ± 353.8/1363.92 ± 1242.78/1100.0 ± 901.65) and (108.6 ± 12.41/113.55 ± 21.35/100.47 ± 18.03) respectively. MNSP durations, SR and LLR latencies were not significantly different between sessions. While SR amplitude tended to decrease, LLRA tended to increase. Conclusion Present study demonstrated that short-therm vibration is not effective on MNSP parameters. Our results may support that SR share similar characteristics with HR.

Muscle spindle discharge in the cat is sensitive to the mechanical activity of anatomically close motor units (e.g. Binder & Stuart, 1980; Conway, Halliday & Rosenberg, 1993). In man similar have been demonstrated by electrical stimulation of skeletomotor axons (McKeon & Burke, 1983). However, studies of voluntary contractions in man have failed to reveal effects of this nature when single afferent firing was correlated with gross surface EMG in leg muscles (Gandevia, Burke & McKeon, 1986). Here we examine this problem in the case of finger extensor muscles during low force voluntary isometric contractions.

Mammalian Ia afferents have a tendency to fire action potentials phase-locked to periodic stimuli, when certain static gamma axons are stimulated at constant frequency. This driving is thought to be mediated by chain fibres, which, in contrast to other intrafusal fibres, have fast contraction properties, comparable to type II extrafusal fibres. Intrafusal contractions are mechanically transmitted to the sensory ending, which transduces the movement into a receptor current. The ensuing oscillatory changes in receptor potential which accompany these rapid chain fibre contractions then cause the generation of one spike during each contraction cycle. If stimulation frequencies are high, or the bag2 fibre is coactivated, the afferent may fire at subharmonic frequencies, giving 1:2 or 1:3 driving. Also, very irregular spike intervals can be observed, which gives the impression of an erratic firing response. Changes in the initial muscle length often cause transitions between subharmonic, 1:1 driving, and irregular firing (Boyd, Murphy & Mann, 1985; Banks, 1991).

Recent work has shown that the monosynaptic pathway of the SSR can be operantly conditioned, and that a significant part of the plasticity responsible for the behavioral change resides in the spinal cord. The most likely sites of this activity-driven plasticity are the synapse of the Ia afferent neuron on the motoneuron and/or the motoneuron itself. Because the SSR pathway is the simplest and most accessible stimulus-response pathway in the vertebrate CNS, it may provide a valuable experimental model for elucidating activity-driven CNS changes responsible for learning.

The complexity and inaccessibility of the vertebrate CNS impede the localization and description of memory traces and the definition of the processes that create them. Recent work has shown that the spinal stretch reflex (SSR), which is produced by a monosynaptic twoneuron pathway, can be operantly conditioned, and that memory traces responsible for this behavioral change reside in the spinal cord. The probable locations are the terminal of the Ia afferent neuron on the motoneuron and/or the motoneuron itself. Because it modifies a simple well-defined and accessible pathway, SSR conditioning may be a valuable experimental model for studying vertebrate memory.

In addition to reduced nerve conduction velocity, diabetic neuropathic patients often exhibit a reduction in the amplitude of the compound muscle action potential elicited by stimulation of the la-afferent-mediated reflex pathway (Hoffman or H wave) that can contribute to diminished or absent tendon reflexes. In contrast to nerve conduction velocity deficits, changes in H-wave amplitudes have not been reproduced in diabetic animal models. Using electrophysiological techniques developed for repeated recordings in individual animals, we report H-wave deficits in streptozotocin (STZ)-treated insulin-dependent diabetic rats. After 4 weeks of diabetes induced by STZ treatment, a 47% reduction in the H-wave amplitude was demonstrated by recording compound muscle action potentials in foot muscles after stimulation of la afferents. Interestingly, we also demonstrate that the H-wave amplitude gradually recovers to a 26% deficit after 12 weeks of experimental diabetes. The recovery of the H wave in STZ-treated rats distinguishes this deficit mechanistically from other STZ-induced electrophysiological changes and may model a similar recovery of the H wave reported in diabetic patients.

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).

The amplitude of monosynaptic EPSPs produced by the action of single semitendinosus (ST) Ia afferent fibers in ST motoneurons display little or no increase in amplitude within hours of a spinal cord transection performed at the thirteenth thoracic segment. This finding is in marked contrast to results obtained at the medial gastrocnemius (MG) Ia fiber-motoneuron synapse, where EPSPs have been shown to be enlarged, even in preparations with normal ST EPSPs. The increase in EPSP amplitude reported previously at the MG Ia-motoneurons synapse is selective and therefore is probably not a direct consequence of injury to the isolated segment of the spinal cord.

The purpose of this paper is to review research on the reflex action of group II afferent fibers originating in the muscle spindle. Although the reflex actions of the Ia afferent fibers from muscle spindles are well defined, the reflex action of the group II afferent fibers is not clearly understood. Experiments on animals whose spinal cord had been transected gave rise to the idea that group II afferent fibers facilitate flexor muscles and inhibit extensor muscles regardless of the origin of the fibers. This concept has been accepted by neurophysiologists and has even been incorporated into therapeutic exercise procedures by physical therapists. However, accumulating evidence is presented that challenges this classical concept. Physical therapists, when trying to incorporate the reflexes evoked by the group II afferent fibers into treatment, must become better aware of the complex reflex actions of which these fibers are capable.

In cats anesthetized with allobarbitone-urethane, Mn 2+ and Co 2+ (and occasionally La 3+ ) were released extracellularly from micropipettes while recordings were made of monosynaptic excitatory postsynaptic potentials evoked in lumbosacral motoneurones by Ia afferent stimulation. These excitatory postsynaptic potentials consistently showed a marked depression in their rate of rise (by an average of 44%) and an increase in half-amplitude duration (by an average of 50%). There was a less pronounced reduction in peak amplitude and increase in time-to-peak. After applications of Mn 2+ or Co 2+ , presynaptic potentials recorded in motoneurones showed no sign of any depression but the synaptic delay was clearly increased. It is concluded that the monosynaptic excitatory postsynaptic potential evoked by Ia afferents in cat motoneurones is probably mediated by chemical transmission.

1. Electrical potentials in the cat lumbosacral spinal cord evoked by the action of single medial gastrocnemius Ia afferent fibres were recorded using low impedance, bevelled micropipette electrodes and the spike triggered averaging technique. 2. Axonal potentials from the Ia fibres recorded extracellularly appeared as brief triphasic predominantly negative potentials. 3. Terminal potentials recorded in regions of Ia afferent termination appeared as brief diphasic positive-negative waves, often with additional wavelets. 4. Focal synaptic potentials, recorded extracellularly in regions of the medial gastrocnemius Ia afferent termination, appeared as slow (about 10 msec duration) negative potentials following terminal potentials. 5. Excitatory post-synaptic potentials, recorded intracellularly in Ia target cells of the medial gastrocnemius, appeared as slow (about 10 msec duration) positive potentials following terminal potentials. 6. Analysis of the temporal progression of these potentials through the spinal cord allowed calculations of the Ia conduction velocity in the dorsal funiculus stem axon (50-60 m/sec), in major collateral branches (8-19 m/sec) and in terminal branches (0.2-1.0 m/sec). 7. The number of major collateral branches (nine or fewer) and their spacing along the spinal cord (1071 micron mean value) were determined by analysing the extent of the triceps surae motoneurone column. 8. The structural and functional properties of medial gastrocnemius Ia afferent fibres are discussed in relation to recent single fibre anatomical data and the present single fibre electrophysiological data.

1. The individual EPSPs evoked by the action of single Ia fibers from cat triceps surae (MG, LG, SOL) were recorded in homonymous and heteronymous motoneurons innervating these same three muscles. 2. In general, Ia fibers projected to a greater percentage of homonymous than heteronymous motoneurons. One class of Ia afferent evoked EPSPs in virtually all homonymous motoneurons; the other had a substantially lower projection frequency. Possible difficulties introduced by the limited resolution of the averaging technique are discussed. 3. Individual EPSPs were larger on the average if evoked a) in SOL rather than in MG or LG motoneurons, b) by LG rather than by MG or SOL afferent fibers, or c) in homonymous rather than in heteronymous motoneurons. The mean EPSP was larger in homonymous than in heteronymous motoneurons because the largest EPSPs (greater than 150 muV) were found mainly in homonymous motoneurons. 4. Rise times of EPSPs were only slightly shorter in homonymous than in heteronymous motoneurons, suggesting that other factors besides relative location of Ia terminals account for the observed EPSP amplitude differences. Rise times in SOL motoneurons were longer than those in MG or LG. 5. LG afferent fibers tended to produce larger EPSPs in rostral than in caudal LG motoneurons, and MG afferents produced larger EPSPs in caudal than in rostral MG motoneurons. These spatial effects were in accord with the more rostral entry of LG than MG Ia afferents into the spinal cord. The differential projection of SOL afferents to MG and SOL motoneurons which overlap spatially in the spinal cord suggests a species specificity in addition to a location specificity.

The excitability of a firing human motoneurones was tested by Ia afferent excitatory volleys. An H reflex was evoked by stimulation of the tibial nerve while the subjects maintained the background rhythmic firing of single MU of the soleus and of the medialis gastrocnemius during weak voluntary contraction. The mean background firing rate of MUs ranged from 3.6 to 9.4 imp/sec. In order to estimate the ‘effectiveness’ of an afferent volley, PSTHs were constructed. The firing index was used as a measure of motoneurone excitability. It has been shown that the ‘effectiveness’ of an afferent volley decreases with an increase in background firing rate of a MU. Such a dependence was deduced from changes in current motoneurone excitability within an interspike interval. The results indicate that during natural muscle contraction at a low firing rate of a motoneurone, the discharge rate is one of the factors affecting the excitability of a firing motoneurone and, hence, its input-output relations. The method of testing changes in human motoneurone excitability within an interspike interval may prove useful in studies on interaction of volleys arriving at a motoneurone via different excitatory inputs.

The effects of lidocaine (1--20 microgram/ml) on afferent discharge patterns of primary endings of muscle spindles in cat tenuissimus muscle were investigated. Discharge from the endings, recorded in Ia afferent axons, was evoked by ramp stretch of the muscle, stimulation of single static or dynamic fusimotor axons or by a combination of stretch and fusimotor stimulation. Spontaneous discharge of the endings at the initial length of the muscles was reduced by 2--5 microgram/ml and abolished by 10--15 microgram/ml lidocaine. The static but not the dynamic discharge elicited by muscle stretch was blocked by concentrations of 10--15 microgram/ml. The same concentrations abolished static and dynamic fusimotor influences on primary ending discharge. However, in one experiment where the spindle was microscopically observed, fusimotor stimulation still resulted in contraction of the intrafusal muscle even when fusimotor stimulation failed to elicit changes in discharge response patterns of the primary endings. These findings indicate that lidocaine interferes with the encoding mechanism prior to block of impulse conduction in either the fusimotor or afferent axons.

Summary The conduction velocities of the α motor fibres and the IA sensitive fibres of the ulnar nerve have been studied in a group of full term newborn babies and during the neonatal period. At birth the average conduction velocity of the α motor fibres is 25,7 m/sec (σ = 2,4 m/sec) that of the IA afferent fibres is 35 m/sec (σ = 3,2 m/sec). The post natal evolution of the conduction velocity as a function of age has been established for both types of fibres. The time required for nervous conduction between the olecranon process and the wrist diminishes clearly in the course of the first two years because of a discordance between the augmentations of the lengtht and of the diameter of the nerve fibers. The evolution of this conduction time as a function of the age and of the length of the forearm, has been established for the period between birth and the age of six years. The Hoffmann reflex in the inferior limb is present in all infants a birth ; its latency is 14,2 msec ; the ratioR = Hmax/Mmax × 100 = 55,75 p. 100 is identical to that found in the adult. A Hoffmann reflex exists in 66 p. 100 of the population studied in the ulnar nerve. Its latency is 11,1 msec ; R = Hmax/Mmax is equal to 30 p. 100. This reflex disappears progressively during the development.

Although GABA and piperidine-4-sulphonate depolarize Ia afferent terminations in the lumbar spinal cord by activation of bicuculline-sensitive GABA receptors, no evidence was obtained for abicuculline-sensitive alteration by either GABAmimetic of the excitability of vestibulospinal terminations. This suggests that the terminal arborizations of vestibulospinal fibers, unlike their cell bodies. are devoid of GABA receptors having properties similar to those on either central neuron cell bodies, primary afferent cell bodies or their central terminations.

1. The behaviour of nuclear bag and nuclear chain intrafusal fibres in isolated cat muscle spindles with a blood supply, during stimulation of dynamic gamma axons, dynamic beta axons, or static gamma axons in ventral root filaments was observed and recorded on still and moving film. 2. Most spindles were controlled by one dynamic gamma axon (sometimes a beta axon) and three static gamma axons, one of which was often non-selective in distribution. A large majority of fusimotor axons controlled one pole of the spindle only. 3. Dynamic gamma and beta axons produced focal contraction in only one of the two nuclear bag fibres in any spindle and this fibre was never activated by static gamma axons. Maximal tetanic contraction was attained slowly and the primary sensory spiral on this fibre was stretched by a small amount only. This fibre has been named the 'dynamic nuclear bag fibre'. 4. Static gamma axons produced either: (a) focal contraction in the second of the two nuclear bag fibres only; (b) local contraction in the bundle of nuclear chain fibres only; or (c) contraction in one nuclear bag fibre and the nuclear chain fibres together. Maximum tetanic contraction of this nuclear bag fibre stretched its primary sensory spiral considerably and the time to plateau was relatively short. This fibre has been named the 'static nuclear bag fibre'. 5. 'Driving' of the Ia afferent discharge could always be produced by non-selective static gamma axons, frequently by static gamma axons controlling nuclear chain fibres alone, and was probably due to mechanical oscillation in nuclear chain fibres. It was never produced by dynamic gamma axons and on one occasion only by a static gamma axon controlling a nuclear bag fibre alone. 6. The conduction velocities of dynamic gamma and static gamma axons overlapped extensively, though dynamic gamma axons were absent from the lower end, and static gamma axons innervating nuclear chain fibres only were absent from the upper end, of the range of velocities. 7. The observations are correlated with spindle structure and histochemistry. Dynamic and static nuclear bag fibres are shown to correspond with 'bag1 fibres' and 'bag2 fibres', respectively (Ovalle & Smith, 1972). 8. The possible origin of the dynamic and static actions of fusimotor axons and the role of the dynamic and static intrafusal systems in motor control are discussed.

The depolarization of the sensory terminals of muscle spindle primary endings is studied in terms of a simplified core conductor model of the system. The terminals branching from a group Ia afferent fibre have different geometrical structures, depending on whether they innervate the nuclear bag or the nuclear chain intrafusal fibres of a muscle spindle.

1. Tonic activation of muscular activity was observed during forced vibration of the gastrocnemius and soleus muscles in the decerebrate cat. Such tonic activity was not observed in the tibialis anterior muscle.2. Intervals of both unitary EMG and motoneuronal spikes were shown to occur according to the principle of integer multiplication of vibratory cyclic time.3. Gradual recruitment of tonic vibration reflex (vibratory facilitation) was found to survive after the cessation of vibratory stimulation (postvibratory facilitation).4. Nonsequential interspikes revealed that gradual tonic recriutment is effected by the spikes with a shorter firing interval.5. Muscle activities during TVR may be effected by PTP with some excitatory frequency associated with that of the applied vibration.6. Postvibratory facilitation did not correlate with the vibratory frequency. The firing of such phases may be determined by the intrinsic repetition rate of the individual motoneuron.

The frequency transfer characteristics of the tonic stretch reflex have been studied using homonymous Ia afferent stimulation as input to the soleus motoneurone. By varying additionally the frequency of inhibitory volleys from either the Ia component of antagonist muscle nerve or the homonymous Ib afferents, the suppressive effect on the excitatory synaptic activity has been investigated for a wide variety of different phase relations.

JANKOWSKA, E., S. LUND and A. LUNDBERG. The effect of DOPA on the spinal cord. 4. Depolarization evoked in the central terminals of contralateral Ia afferent terminals by volleys in the jlexor rejlex afferents. Acta physiol. scand. 1966. 68. 337-341. After injection of DOPA in unanaesthetized spinal cats volleys in the flexor reflex afferents (FRA) evoke a late long lasting contralateral dorsal root potential. Measurements of excitability of contralateral afferent terminals show a corresponding depolarization of Ia, but not of Ib or cutaneous afferent terminals. Results are also presented showing that transmission from the FRA to contralateral Ia afferent terminals can be inhibited from the FRA. In previous papers in this series it has been shown that after an i.v. injection of 1-3,4-dihydroxyphenylalanine (DOPA) volleys in the flexor reflex afferents evoke a late ipsilateral dorsal root potential (DRP) which is caused by a selective depolarization in the central terminals of Ia afferents (Andtn et al. 1966a, b). In the present paper it will be shown that after DOPA volleys in the FRA produce similar effects on the contralateral side. The experimental procedures have been given in the previous papers.

In Chapter 11 the afferent fibres from the primary endings in muscle spindles (Group Ia axons) were discussed. It was shown that each Ia afferent fibre, near its entrance into the spinal cord, gives off collaterals at an average frequency of one every millimetre. The collaterals have three main areas of termination in the lumbosacral cord: in lamina VI, in lamina VII (to the region containing the Ia inhibitory interneurones) and in lamina IX (the motor nuclei).

The response of the primary sensory ending in an inactive spindle to a ramp and hold stretch exhibits five distinct components: an initial transient peak, thought to be due to stiction in intrafusal fibres, a ‘fast rise’ followed by a ‘slow rise’ reaching a peak at the end of the stretch, then a ‘fast fall’ and a ‘slow fall’ of adaptation when the spindle is held at the new length (Fig.1a). The ‘dynamic index’, or fall in Ia afferent frequency in 0.5sec immediately following the peak, and which is used to distinguish ‘dynamic’ from ‘static’ fusimotor axons (Matthews, 1972) comprises the fast fall and part of the slow fall. The dynamic bag1 intrafusal fibre (Db1), and the static bag2 fibre (Sb2) are responsible, respectively, for the dynamic and static fusimotor actions on the Ia afferent discharge (Boyd, 1981). This paper describes the action of axons selectively innervating the Db1 fibre or the Sb2 fibre, and the effect of changing the velocity of stretch, on the various phases of the group Ia and group II afferent responses.

When a subject is instructed to resist external load, stretch-induced reflex responses can be observed in the emg of the contracting muscles when the load is abruptly increased. As commonly known, these reflex responses are grouped into segments denoted M1, M2 and M3 according to the nomenclature of Tatton et al. (1975). The long latency components have attracted particular attention; they have been studied in normal as well as in pathologic states, such as in parkinsonism and chorea.

For several years we have studied the input-output relations of muscle spindles and tendon organs using random length perturbations as stimuli. While studying the effect of gamma stimulation on spindle afferent responses, we made the chance observation that although static gamma fibers altered the response of a spindle Ia afferent to random length stimuli, in response to specific length sequences with concurrent gamma stimulation, the receptor produced an unexpectedly large number of spikes in exactly the same temporal relation to the repeated “random” sequence. We report here on the data obtained in one such experiment in which a gamma fiber we had classified as “static” modified the Ia afferent response to muscle stretch.

Publisher Summary This chapter describes the mechanisms of transmitter release at Ia afferent terminations. Fluctuations in the amplitude of a synaptic potential usually indicate that the amount of transmitter released at the nerve terminal is fluctuating. Some details of transmitter release mechanisms can be established from an analysis of these fluctuations. The afferent connection consisted of three boutons on a trunk dendrite. The average excitatory postsynaptic potentials (EPSPs) recorded in the neuron is shown. The standard deviation time course is analyzed. The number of different components in the EPSP is equal to the number of boutons. In two other results of this type, the number of boutons has always been equal to or greater than the number of different components in the EPSP.

A reconstruction was made of the intramedullary trajectory of 23 physiologically identified Ia afferents from cat hind limb muscles (medial gastrocnemius, soleus, plantaris, flexor digitorum-hallucis longus, and hamstring). The afferents were stained by intra-axonally injected HRP. The axons of these afferents were traced over distances of 5.8 mm to 15.7 mm rostrocaudally. In the dorsal funiculus fibers from all the muscles showed a similar course and similarly bifurcated into an ascending and a descending branch. The mean diameters of stem axons, ascending branches, and descending branches were 6.6 micrometer, 5.8 micrometer, and 3.0 micrometer, respectively. Within the analyzed lengths of the spinal cord five to eleven collaterals were given off from the two branches. The distances between adjacent collaterals of the ascending and descending branches averaged 1200 micrometer and 790 micrometer, respectively. The collaterals as a rule passed through the medial half of the dorsal horn before they entered the deeper parts of the gray matter. The terminal distribution areas common to all Ia collaterals were: (1) the medial half of the base of the dorsal horn, mainly lamina VI: (2) lamina VII; and (3) lamina IX. The numbers of terminals were largest in lamina IX and smallest in lamina VII. The density of terminals in lamina IX was highest in the homonymous motor cell column. The terminal distribution areas of adjacent collaterals showed no overlap in the sagittal plane. Terminal branches carried one bouton terminal and up to six boutons en passage with an average of 1.8 terminals per terminal branch. Apparent axosomatic and axodendritic contacts were seen on small-sized and medium-sized neurons in laminae V-VI, medium-sized neurons in lamina VII, and large neurons in lamina IX. One motoneurons was contacted by an average of 3.3 terminals. In addition to the common features, Ia collaterals of various muscles of origin showed some differences in their trajectories in the ventral horn, and in their terminations in the gray matter.

The distribution of Ia terminals to alpha-motoneurons has been investigated under conditions where a) the Ia fiber innervates a foreign muscle and b) the motor axon innervates a foreign muscle. This distribution has been assessed by examining which motoneurons develop an EPSP in response to stimulation of a single Ia afferent fiber. Cross-union of the medial gastrocnemius and lateral gastrocnemius-soleus nerves has been performed in 5--8 day old kittens and the effect of this procedure on Ia connections to alpha-motoneurons has been investigated in the adult about one year later. No rearrangement of synaptic connections has been observed under these conditions. The connections from self-unioned Ia afferents and to self-unioned motoneurons are also normal. The individual EPSP's in these motoneurons with regenerated axons have normal rise times. The findings of normal Ia connections as well as normal EPSP rise times in alpha-motoneurons with regenerated motor axons indicates reversibility of the changes seen in axotomized motoneurons.

The spike-triggered averaging technique (26) was used to determine whether the synaptic input from medial gastrocnemius (MG) Ia-afferent fibers to homonymous motoneurons is "topographically weighted" (22) by means of differences in projection frequency, excitatory postsynaptic potential (EPSP) amplitude, or a combination of both factors. Motoneurons were classified as either "same branch" or "other branch," depending on whether a Ia-afferent fiber and motor axon were contained in the same or different intramuscular nerve branches. No difference was found in the projection frequency of Ia-afferents to the same branch and other branch motoneurons (95 versus 94%, respectively). The mean EPSP amplitude was larger in the same branch group of motoneurons (92 +/- 8 (SE) microV; n = V; n = 97) than in the other branch group (77 +/- 7 microV; n = 79). This difference was most striking in high-rheobase (greater than or equal to 10 nA) motoneurons, for which the mean EPSP amplitude in the same branch group was 82 +/- 12 microV (n = 48), whereas that in the other branch group was 52 +/- 5 microV (n = 37). In 60 cases it was possible to compare the EPSPs produced by a same branch afferent and an other branch afferent in the same motoneuron. The same branch afferent produced the larger EPSP in 73% (44/60) of the cases. Moreover, the mean ratio of the same branch to the other branch EPSP amplitudes was 1.7, which was both statistically significant and consistent with analogous results from our preceding study of aggregate EPSPs (22). Mean rise times and half-widths of EPSPs in the same branch group were not significantly different from those in the other branch group. Furthermore, no significant differences in rise times or half-widths between the two groups were evident when motoneurons were segregated according to their rheobase values. This suggests that the segregation of Ia-afferent and motor axons across the intramuscular nerve branches is not reflected in the locations of Ia terminals on the motoneuron somadendritic surface and that other factors must account for observed EPSP amplitude differences. Our data suggest that the topographic weighting of homonymous Ia-afferent input to cat MG motoneurons is mediated by a gradient of EPSP amplitude rather than by a gradient of Ia connectivity and also suggest that the effect is most prominent in high-rheobase motoneurons.

Publisher Summary This chapter discusses the neural mechanisms of voluntary movements and precentral motor area. The separation of motor and sensory areas is artificial. The spinal projections from the areas are important diagnostic characters for distinguishing areas. The relation of the multiple arborizations to different motoneuron species was determined by retrograde labeling of ulnar, median, and radial motoneurons by HRP injected into the nerves. The same corticospinal (CS) axons supplied at least four motoneuron pools, both ulnar and radial, with boutons de passage and boutons terminaux. Most synapses were presumed to be on distal dendrites, which were not filled with horseradish peroxidase (HRP). Some CS axons crossed the midline and arborized bilaterally. The general distribution was coextensive with that of the arborizations of the group Ia afferent axons. It was suggested that the divergence being revealed is in some ways a corollary of the convergence that had been demonstrated and that any repeatable results of cortical stimulation must be because of the existence of certain condensations or concentrations in these otherwise relatively diffuse neuronal networks.