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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 (

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.

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.

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 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.

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.

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.

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.

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.

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.

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.

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.

THE monosynaptic reflex response evoked in the cat by a maximal volley in the group Ia afferent fibres from a synergic group of muscles has been measured as a fraction of total motoneurone pool of that muscle by Jefferson and Benson1 and found to range between 4.3 and 26.5 per cent. Apparently there has been no measurement of the activated fraction of the motoneurone pool with the analogous H-reflexes that are evoked in the gastrocnemius-soleus muscle of man by stimulation of the group Ia fibres in the popliteal nerve2–4.

1. Properties of medial gastrocnemius motoneurones in cats were examined with intracellular electrodes 1-77 days after partial transection of the medial gastrocnemius muscle nerve. 2. Three sequential stages of reaction were defined: (i) firstly, conduction velocity of axotomized motoneurones fell, but e.p.s.p. properties and Ia-motoneurone connectivity were normal; (ii) secondly, e.p.s.p.s became characterized by prolonged rise times and half-widths and by diminished amplitudes; (iii) finally, failures of connectivity were also seen, that is, Ia spindle afferent fibres connected with fewer axotomized than normal homonymous motoneurones. 3. The reason for the change of e.p.s.p. properties cannot be stated with certainty: among the factors may be (i) a change of motoneurone membrane properties and morphology; (ii) partial detachment of synaptic boutons before their total disconnexion, and (iii) detachment of Ia afferent branches terminating on the motoneurone soma before those on the dendrites. 4. The previous conclusion that alteration of e.p.s.p. profiles following motoneurone axotomy was due to total disconnexion of those Ia fibres making synapses on the soma and proximal dendrites ("somatic stripping") must be modified to account for the findings that e.p.s.p. changes are seen before connectivity changes.

THE aim of the present investigation has been to give, in cats, excess use to the synapses of some monosynaptic pathways through the spinal cord, keeping other monosynaptic pathways as controls. In an initial aseptic operation the nerves to some muscles of a synergic extensor group in one hind-limb have been severed and capped to prevent regeneration. Since the remaining muscles of such a synergic group have to substitute for the whole group in supporting the weight of the animal, it can be expected that there will be an excess of stress on them; consequently there will be an increased discharge from their stretch receptors along the group Ia afferent fibres, so giving an increased activation of the excitatory synapses on their motoneurones.