Ia EPSPs Research Articles

Ia EPSPs in rat spinal motoneurons are potentiated after a 5-week whole body vibration.

Whole body vibration (WBV) is often applied as an alternative method for strength training or to prevent muscle force decrease. In this study, we evaluated the influence of WBV on Ia monosynaptic input from muscle spindles because the tonic vibration reflex is responsible for the enhancement of muscle activity observed after WBV. The aim was to investigate whether repeated activation of muscle spindles during WBV may result in altered synaptic excitation of motoneurons. WBV was performed on adult male Wistar rats, 5 days/wk, for 5 wk, and each daily session consisted of four 30-s runs of vibration at 50 Hz. Fast-type medial gastrocnemius motoneurons were investigated intracellularly in deeply anesthetized animals in the experimental (n = 7, 34 motoneurons) and control (n = 7, 32 motoneurons) groups. Monosynaptic Ia excitatory postsynaptic potentials (EPSPs) were evoked by electrical stimulation of afferent fibers from the synergistic lateral gastrocnemius and soleus muscles. Data were analyzed using a mixed linear model. The central latencies of EPSPs were 0.45-0.9 ms with no differences in the mean values between the analyzed groups (P = 0.291). WBV induced an increase of the mean EPSP amplitude by 28% (P = 0.025), correlated with the resting membrane potential and input resistance, and a shortening of the mean EPSP rise time by 11% (P = 0.012). The potentiation of synaptic excitation of motoneurons was not accompanied by changes of passive membrane properties, pointing to synaptic plasticity. This indicates that WBV may support rehabilitation or training processes aimed at increasing muscle strength on the basis of increased motoneuronal drive.NEW & NOTEWORTHY The study provides new information on neuronal plasticity following repeatedly exerted mechanical loading. We demonstrate in electrophysiological experiments on rat lumbar motoneurons that low-volume whole body vibration applied systematically for 5 wk potentiates synaptic excitation from primary muscle afferents. The adaptive changes are expressed by higher amplitudes and shorter rise times of monosynaptic EPSPs evoked in motoneurons of the vibrated group compared with the control.

Recruitment gain of spinal motor neuron pools in cat and human.

The output from a motor nucleus is determined by the synaptic input to the motor neurons and their intrinsic properties. Here, we explore whether the source of synaptic inputs to the motor neurons (cats) and the age or post-stroke conditions (humans) may change the recruitment gain of the motor neuron pool. In cats, the size of Ia EPSPs in triceps surae motor neurons (input) and monosynaptic reflexes (MSRs; output) was recorded in the soleus and medial gastrocnemius motor nerves following graded stimulation of dorsal roots. The MSR was plotted against the EPSP thereby obtaining a measure of the recruitment gain. Conditioning stimulation of sural and peroneal cutaneous afferents caused significant increase in the recruitment gain of the medial gastrocnemius, but not the soleus motor neuron pool. In humans, the discharge probability of individual soleus motor units (input) and soleus H-reflexes (output) was performed. With graded stimulation of the tibial nerve, the gain of the motor neuron pool was assessed as the slope of the relation between probability of firing and the reflex size. The gain in young subjects was higher than in elderly subjects. The gain in post-stroke survivors was higher than in age-matched neurologically intact subjects. These findings provide experimental evidence that recruitment gain of a motor neuron pool contributes to the regulation of movement at the final output stage from the spinal cord and should be considered when interpreting changes in reflex excitability in relation to movement or injuries of the nervous system.

Are Human Digit Muscles Devoid of Recurrent Inhibition?

Are Human Digit Muscles Devoid of Recurrent Inhibition?

Normal Distribution of VGLUT1 Synapses on Spinal Motoneuron Dendrites and Their Reorganization after Nerve Injury

Peripheral nerve injury induces permanent alterations in spinal cord circuitries that are not reversed by regeneration. Nerve injury provokes the loss of many proprioceptive IA afferent synapses (VGLUT1-IR boutons) from motoneurons, the reduction of IA EPSPs in motoneurons, and the disappearance of stretch reflexes. After motor and sensory axons successfully reinnervate muscle, lost IA VGLUT1 synapses are not re-established and the stretch reflex does not recover; however, electrically evoked EPSPs do recover. The reasons why remaining IA synapses can evoke EPSPs on motoneurons, but fail to transmit useful stretch signals are unknown. To better understand changes in the organization of VGLUT1 IA synapses that might influence their input strength, we analyzed their distribution over the entire dendritic arbor of motoneurons before and after nerve injury. Adult rats underwent complete tibial nerve transection followed by microsurgical reattachment and 1 year later motoneurons were intracellularly recorded and filled with neurobiotin to map the distribution of VGLUT1 synapses along their dendrites. We found in control motoneurons an average of 911 VGLUT1 synapses; ~62% of them were lost after injury. In controls, VGLUT1 synapses were focused to proximal dendrites where they were grouped in tight clusters. After injury, most synaptic loses occurred in the proximal dendrites and remaining synapses were declustered, smaller, and uniformly distributed throughout the dendritic arbor. We conclude that this loss and reorganization renders IA afferent synapses incompetent for efficient motoneuron synaptic depolarization in response to natural stretch, while still capable of eliciting EPSPs when synchronously fired by electrical volleys.

Enhanced Transmission at a Spinal Synapse TriggeredIn Vivoby an Injury Signal Independent of Altered Synaptic Activity

Peripheral nerve crush initiates a robust increase in transmission strength at spinal synapses made by axotomized group IA primary sensory neurons. To study the injury signal that initiates synaptic enhancement in vivo, we designed experiments to manipulate the enlargement of EPSPs produced in spinal motoneurons (MNs) by IA afferents 3 d after nerve crush in anesthetized adult rats. If nerve crush initiates IA EPSP enlargement as proposed by reducing impulse-evoked transmission in crushed IA afferents, then restoring synaptic activity should eliminate enlargement. Daily electrical stimulation of the nerve proximal to the crush site did, in fact, eliminate enlargement but was, surprisingly, just as effective when the action potentials evoked in crushed afferents were prevented from propagating into the spinal cord. Consistent with its independence from altered synaptic activity, we found that IA EPSP enlargement was also eliminated by colchicine blockade of axon transport in the crushed nerve. Together with the observation that colchicine treatment of intact nerves had no short-term effect on IA EPSPs, we conclude that enhancement of IA-MN transmission is initiated by some yet to be identified positive injury signal generated independent of altered synaptic activity. The results establish a new set of criteria that constrain candidate signaling molecules in vivo to ones that develop quickly at the peripheral injury site, move centrally by axon transport, and initiate enhanced transmission at the central synapses of crushed primary sensory afferents through a mechanism that can be modulated by action potential activity restricted to the axons of crushed afferents.

Erratum

Acta PhysiologicaVolume 189, Issue 4 p. 385-385 Free Access Erratum This article corrects the following: Spinal cord plasticity in acquisition and maintenance of motor skills J. R. Wolpaw, Volume 189Issue 2Acta Physiologica pages: 155-169 First Published online: January 23, 2007 First published: 14 March 2007 https://doi.org/10.1111/j.1748-1716.2007.01677.xAboutSectionsPDF ToolsRequest permissionExport citationAdd to favoritesTrack citation ShareShare Give accessShare full text accessShare full-text accessPlease review our Terms and Conditions of Use and check box below to share full-text version of article.I have read and accept the Wiley Online Library Terms and Conditions of UseShareable LinkUse the link below to share a full-text version of this article with your friends and colleagues. Learn more.Copy URL Share a linkShare onFacebookTwitterLinked InRedditWechat In Wolpaw (2007) the following errors were published: On page 159, the legend to Figure 2 was published incorrectly: Figure 2 Operant conditioning of the spinal stretch reflex (SSR)/H-reflex pathway and the associated spinal and supraspinal plasticity. (a) Triceps surae motor neurones on the conditioned side of H-reflex down-conditioning (HR) monkeys have more positive firing thresholds and slightly smaller Ia EPSPs. These two findings… The text was incorrect and should have read as follows: Figure 2 Operant conditioning of the spinal stretch reflex (SSR)/H-reflex pathway and the associated spinal and supraspinal plasticity. (a) Triceps surae motor neurones on the conditioned side of H-reflex down-conditioned (HR↓) monkeys have more positive firing thresholds and slightly smaller Ia EPSPs. These two findings… On page 161, the first sentence of the second paragraph was published incorrectly: During development, the CST to provide the normal adult pattern of largely contralateral innervation (Eyre et al. 2001, Eyre 2003, ten Donkelaar et al. 2004, Martin 2005). The text was incorrect and should have read as follows: During development the CST changes to provide the normal adult pattern of largely contralateral innervation (Eyre et al. 2001, Eyre 2003, Donkelaar et al. 2004, Martin 2005). On page 163, Figure 5 was labelled incorrectly. Figure 5 should have been published as follows: We apologize for the errors. Reference Wolpaw, J.R. 2007. Spinal cord plasticity in acquisition and maintenance of motor skills. Acta Physiol 189, 155– 169. Wiley Online LibraryCASPubMedWeb of Science®Google Scholar Volume189, Issue4April 2007Pages 385-385 ReferencesRelatedInformation

Dendritic voltage dependent conductances increase the excitatory synaptic response and its postsynaptic inhibition in a reconstructed α-motoneuron: A computer model

Mammalian spinal monosynaptic Ia EPSP and its postsynaptic reciprocal depolarizing inhibition was studied using NEURON simulator in a morphologically and physiologically characterised α-motoneuron. Low g Na and g K located at the dendrites greatly enhanced EPSP A+N maximal amplitude and its reciprocal inhibition. A small area of active dendrite is sufficient for obtaining the maximal increase in excitatory response for each g Na. EPSP peak is monotonically augmented by raising g Na, whereas inhibition is enhanced until a specific g Na and a further increase diminishes the inhibitory effect. Computer simulations performed at four additional α-motoneurons yielded similar results.

Depression of group Ia monosynaptic EPSPs in cat hindlimb motoneurones during fictive locomotion.

The effects of fictive locomotion on monosynaptic EPSPs recorded in motoneurones and extracellular field potentials recorded in the ventral horn were examined during brainstem-evoked fictive locomotion in decerebrate cats. Composite homonymous and heteronymous EPSPs and field potentials were evoked by group I intensity (<= 2T) stimulation of ipsilateral hindlimb muscle nerves. Ninety-one of the 98 monosynaptic EPSPs were reduced in amplitude during locomotion (mean depression of the 91 was to 66 % of control values); seven increased in amplitude (to a mean of 121 % of control). Twenty-one of the 22 field potentials were depressed during locomotion (mean depression to 72 % of control). All but 14 Ia EPSPs were smaller during both the flexion and extension phases of locomotion than during control. In 35 % of the cases there was < 5 % difference between the amplitudes of the EPSPs evoked during the flexion and extension phases. In 27 % of the cases EPSPs evoked during flexion were larger than those evoked during extension. The remaining 38 % of EPSPs were larger during extension. There was no relation between either the magnitude of EPSP depression or the locomotor phase in which maximum EPSP depression occurred and whether an EPSP was recorded in a flexor or extensor motoneurone. The mean recovery time of both EPSP and field potential amplitudes following the end of a bout of locomotion was approximately 2 min (range, < 10 to > 300 s). Motoneurone membrane resistance decreased during fictive locomotion (to a mean of 61 % of control, n = 22). Because these decreases were only weakly correlated to EPSP depression (r 2 = 0.31) they are unlikely to fully account for this depression. The depression of monosynaptic EPSPs and group I field potentials during locomotion is consistent with the hypothesis that during fictive locomotion there is a tonic presynaptic regulation of synaptic transmission from group Ia afferents to motoneurones and interneurones. Such a reduction in neurotransmitter release would decrease group Ia monosynaptic reflex excitation during locomotion. This reduction may contribute to the tonic depression of stretch reflexes occurring in the decerebrate cat during locomotion.

Monosynaptic Ia projections from intrinsic hand muscles to forearm motoneurones in humans.

Heteronymous Ia excitatory projections from intrinsic hand muscles to human forearm motoneurones (MNs) were investigated. Changes in firing probability of single motor units (MUs) in the flexor carpi radialis (FCR), flexor carpi ulnaris (FCU), flexor digitorum superficialis (FDS), extensor carpi radialis (ECR), extensor carpi ulnaris (ECU) and extensor digitorum communis (EDC) were studied after electrical stimuli were applied to the median and ulnar nerve at wrist level and to the corresponding homonymous nerve at elbow level. Homonymous facilitation, occurring at the same latency as the H reflex, and therefore attributed to monosynaptic Ia EPSPs, was found in all the sampled units. In many MUs an early facilitation was also evoked by heteronymous low-threshold afferents from intrinsic hand muscles. The low threshold (between 0.5 and 0.6 times motor threshold (MT)) and the inability of a pure cutaneous stimulation to reproduce this effect indicate that it is due to stimulation of group I muscle afferents. Evidence for a similar central delay (monosynaptic) in heteronymous as in homonymous pathways was accepted when the difference in latencies of the homonymous and heteronymous peaks did not differ from the estimated supplementary afferent conduction time from wrist to elbow level by more than 0.5 ms (conduction velocity in the fastest Ia afferents between wrist and elbow levels being equal to 69 m s-1). A statistically significant heteronymous monosynaptic Ia excitation from intrinsic hand muscles supplied by both median and ulnar nerves was found in MUs belonging to all forearm motor nuclei tested (although not in ECU MUs after ulnar stimulation). It was, however, more often found in flexors than in extensors, in wrist than in finger muscles and in muscles operating in the radial than in the ulnar side. It is argued that the connections of Ia afferents from intrinsic hand muscles to forearm MNs, which are stronger and more widely distributed than in the cat, might be used to provide a support to the hand during manipulatory movements.

Monosynaptic Ia pathways at the cat shoulder.

1. The study aimed to describe in cat forelimb and shoulder motoneurones the convergence and projection patterns from large muscle spindle afferents (Ia). In 11 chloralose-anaesthetized cats maximum Ia EPSPs evoked by electrical stimulation of ipsilateral forelimb nerves were obtained in 309 intracellularly recorded alpha-motoneurones. 2. Groups of motor nuclei displayed similar Ia patterns. As in the distal forelimb they were often interconnected by bidirectional pathways, which were used to combine Ia synergistic groups. Three such groups are described at the shoulder. 3. The first group was composed of the main flexors of the scapulo-humeral joint. Regular disto-proximal Ia excitation from elbow extensors (and median afferents) indicates a coupling of flexion in the scapulo-humeral joint to the angular position of the elbow. 4. The second group comprised the outward rotators of the humerus with differentiated Ia convergence onto the different group members. The patterns of Ia excitation received and sent by the group members demonstrate that the outward rotators are incorporated in versatile synergisms and may occupy a central position in steering forelimb movements. 5. The third group was formed by the spinatus muscle and the subscapularis. This arrangement is suggested by the common convergence onto them from the elbow extensors and flexors. The pattern may serve to guide and keep the humeral head in the joint capsule. 6. The Ia synergistic groups receive Ia convergence from muscles acting at distant joints and also project to distant muscles. This is discussed as part of an extended pattern of Ia connections along the forelimb. In this way the shoulder muscles would be incorporated in flexor and extensor oriented synergisms which are needed to co-ordinate the muscular activation along the multijoint forelimb during locomotion. When the shoulder Ia pathways are compared with those in the distal forelimb, organization of the Ia system apparently follows a few basic principles which have adapted to the mechanical situation at the particular joints and their mechanical interaction.

Tonic presynaptic reduction of monosynaptic Ia EPSPs during fictive locomotion.

Tonic presynaptic reduction of monosynaptic Ia EPSPs during fictive locomotion.

Comparison of peripheral Ia and corticomotoneuronal composite EPSPs in human motoneurons

Comparison of peripheral Ia and corticomotoneuronal composite EPSPs in human motoneurons

Short-latency subliminal effects of transcranial magnetic stimulation on forearm motoneurones.

The H-reflex technique has been used to evaluate the time-course of the effects evoked by transcranial clockwise magnetic stimuli in flexor or extensor carpi radialis motoneurones. In six subjects, magnetic stimulation was applied over the scalp in the focus for the motor response of those muscles. At intensities below motor threshold, a facilitation of the H-reflex started at a conditioning-test interval of -4 ms (i.e. when the magnetic stimulus lagged the test stimulus by 4 ms), reached a peak at about -2 ms and rapidly decayed. At about -1 ms, the decay attained a local minimum, which in three subjects had values indicating the presence of an inhibition. Thereafter, a second facilitatory phase peaked at about +1 ms. By matching the time course with the latency of the cortical muscle action potential (CMAP) evoked by suprathreshold magnetic stimulation, it is inferred that the motoneuronal discharge coincides with the second peak of facilitation and is preceded by 3-4 ms of subliminal excitation. This early effect could be brought to threshold by convergence of a subliminal Ia EPSP, leading to a reduction of the CMAP latency. The early excitatory effects reported above are as fast as those described as following transcranial electrical stimulation, and should likewise be considered as monosynaptic.

Pattern of monosynaptic heteronymous Ia connections in the human lower limb.

Changes in the firing probability of single motor units in response to electrical stimulation of muscle nerves were used to derive the projections of muscle spindle Ia afferents to the motoneurones of various leg and thigh muscles. Discharges of units in soleus, gastrocnemius medialis, peroneus brevis, tibialis anterior, quadriceps, biceps femoris and semitendinosus were investigated after stimulation of inferior soleus, gastrocnemius medialis, superficial peroneal, deep peroneal and femoral nerves. Homonymous facilitation, occurring at the same latency as the H reflex and therefore attributed to monosynaptic Ia EPSPs, was found in virtually all the sampled units. In many motor nuclei an early facilitation was also evoked by heteronymous low-threshold afferents. The heteronymous facilitation was considered to be mediated through a monosynaptic pathway when the difference between the central latencies of heteronymous and homonymous peaks was not more than 0.2 ms. The heteronymous Ia connections were widely distributed. In particular, monosynaptic coupling between muscles operating at different joints appears to be the rule in humans, though it is rare between ankle and knee muscles in the cat and the baboon.

An analysis of synaptic transmission to motoneurones in the cat spinal cord using a new selective receptor blocker.

We have investigated the role of alpha-amino-3-hydroxy-5-methyl-4-isoxazolepropionate (AMPA) receptors on synaptic transmission in vivo between Ia primary afferents and cat spinal motoneurones using a selective non-N-methyl-D-aspartate (non-NMDA) receptor antagonist, GYKI 52466. Both microionophoretic and intravenous application of GYKI 52466 depressed the excitatory post-synaptic potential (Ia EPSP) in a dose-dependent manner, without any apparent effect on membrane conductance or resting potential of the motoneurone. GYKI 52466 reduced selectively alpha-amino-3-hydroxy-5-methyl-4-isoxazolepropionate (AMPA)- but not N-methyl-D-aspartate (NMDA)-induced depolarizations. Our results suggest that a large part of the Ia EPSP is mediated by AMPA receptors. The participation of other excitatory amino-acid receptors in the Ia EPSP is also discussed.

Quantal analysis of synaptic potentials in neurons of the central nervous system.

This review concentrates on the results of quantal analysis at central synapses, particularly where the morphology and the electrophysiology of the same synaptic connection has been investigated. Results from quantal analysis at peripheral synapses are drawn on to help resolve some issues. The major topics covered in this review are the properties of the quantal synaptic potential and the junctional mechanisms involved in its generation

Pattern of monosynaptic Ia connections in the cat forelimb.

1. In anaesthetized cats intracellular records were obtained from antidromically identified motoneurones. The motor nuclei to the elbow extensor and flexor muscles and to the muscles innervated by the deep radial, ulnar and median nerves were investigated. The maximum Ia EPSPs from electrical stimulation of various peripheral nerves were measured. The characteristic convergence and projection patterns to each motor nucleus were established from pooled data. 2. The total aggregates of the Ia EPSPs between the different motor nuclei ranged from 3.5 to 11.7 mV. The smallest aggregates were found in the nuclei to the digit muscles. The ratio of the heteronymous versus homonymous EPSP amplitudes varied between 3.9 and 0.5. A general rule which would govern the distribution of the EPSP aggregates, such as a proximo-distal gradient, was not observed. 3. The Ia connections followed a complex but highly organized pattern. Bidirectional and unidirectional pathways were present. In many cases the convergence pattern of a motor nucleus included muscles acting at different joints. The connections of one nucleus were not necessarily restricted to one side of the limb, but could cross the radio-ulnar plane. 4. Muscles with similar actions onto the same joint were interconnected with bidirectional, rather balanced Ia pathways. Such relations were also present between close functional synergists and then often extended across several joints. The relations between the anatomical extensors of wrist and digits were graded according to the neighbourhood of these muscles. It is suggested that this reflects the graded mechanical synergism in the wrist action of these muscles. 5. A large number of unidirectional or strongly skewed bidirectional Ia pathways project from proximal to distal muscles. It is suggested that they may serve a readjustment of distal joints during changes in the position of proximal ones (e.g. stabilization of the position of the radio-ulnar plane during elbow extension in case of the undirectional projections onto supinator and abductor pollicis longus motoneurones). 6. The motor nuclei to some multifunctional muscles display a negative correlation between different heteronymous Ia inputs: motoneurones with a large input from one muscle show a significant tendency to receive a smaller input from another muscle and vice versa. This organization leads to subpopulations of neurones with different convergence patterns within the same motor nucleus. 7. Motor nuclei with bidirectional Ia relations between each other displayed similar convergence and projection patterns. They were combined into 'Ia synergistic groups.' One motor nucleus may belong to several groups.(ABSTRACT TRUNCATED AT 400 WORDS)

Ia reflexes and EPSPs in human soleus motor neurones.

The reflex responses of single motor units in the soleus muscle to electrical stimulation of the tibial nerve were recorded in human volunteers. A feature of the experiments was the stimulation paradigm used. In order to control the peri-stimulus firing rate, a computer triggered the stimulus isolator only when 2 interspike intervals of specified duration occurred in succession. In addition, the timing of the stimulus in relation to the preceding action potential was controlled in a manner similar to a conditioning/testing paradigm. The general pattern of response was an initial, "H-reflex" excitation at monosynaptic latency, followed first by a silent period due to the refractoriness of the motor neurone, then by other phases of reduced activity. When the stimulus intensity was increased, the intensity of the excitation and the duration of the silent period increased in parallel. When the pre-stimulus firing rate of the motor unit was varied, the amplitude of the H-reflex response. normalized to the number of stimulus trials, was similar at 6, 8 and 10 Hz, but was greater at 4 Hz in most units tested. These findings were consistent with a simple model of the events occurring at the cell membrane in this reflex which was proposed by Ashby and Zilm (1982a), although some modification of the model was necessary to account for the different response at 4 Hz. The improved stimulation paradigm enabled a direct estimate to be made of the amplitude and shape of the rising phase of the Ia EPSP in human motor neurones.

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).

On the cause of hyporeflexia in the Holmes-Adie syndrome.

Electrophysiologic studies were carried out on 11 patients with Holmes-Adie syndrome, 8 of whom had reduced or absent ankle jerks. Conduction velocities and evoked nerve and muscle compound action potentials in the peroneal, posterior tibial, and sural nerves were normal. The H reflex was absent (or virtually absent) in the patients with depressed reflexes. The amplitude of the composite Ia EPSP in single soleus motoneurons was estimated from changes in firing probability of voluntarily activated soleus motor units in response to stimulation of low threshold afferents in the tibial nerve. These amplitudes were used to test the afferent side of the reflex pathway. Composite group Ia EPSPs in Holmes-Aide patients with hyporeflexia were smaller than normal or absent, indicating that the areflexia in the Holmes-Aide syndrome is due to loss of large spindle afferents or reduced effectiveness of their monosynaptic connections to motoneurons.