Identification of muscle spindle afferents during in vivo recordings in man

Microneurography: how it started and how it works

In the first section, this historical review describes endeavors to develop the method for recording normal nerve impulse traffic in humans, designated microneurography. The method was developed at the Department of Clinical Neurophysiology of the Academic Hospital in Uppsala, Sweden. Microneurography involves the impalement of a peripheral nerve with a tungsten needle electrode. Electrode position is adjusted by hand until the activity of interest is discriminated. Nothing similar had previously been tried in animal preparations, and thus the large number of preceding studies that recorded afferent activity in other mammals did not offer pertinent methodological guidance. For 2 years, the two scientists involved in the research impaled their own nerves with electrodes to test various kinds of needles and explore different neural systems, all the while carefully watching for signs of nerve damage. Temporary paresthesiae were common, whereas enduring sequelae never followed. Single-unit impulse trains could be discriminated, even those originating from unmyelinated fibers. An explanation for the discrimination of unitary impulses using a coarse electrode is inferred based on the electrical characteristics of the electrode placed in the flesh and the impulse shapes, as discussed in the second section of this paper. Microneurography and the microstimulation of single afferents, combined with psychophysical methods and behavioral tests, have generated new knowledge particularly regarding four neural systems, namely the proprioceptive system, the cutaneous mechanoreceptive system, the cutaneous nociceptive system, and the sympathetic efferent system to skin structures and muscular blood vessels. Examples of achievements based on microneurography are presented in the final section.

A purposely designed neural signal amplifier for short interval stimulation and recording microneurography using a common electrode

'Common Electrode' microneurography (i.e. stimulating a nerve and recording return afferent neural activity through the same microelectrode facilitates investigation of linkages between sensory and motor nervous systems in humans. Currently there is no commercial product designed specifically to conduct common electrode microneurography experiments. However, such experiments would advance investigations in several key areas including spinal injury research. In this paper, we report on the successful production and testing (on a human subject) of an integrated amplifier built specifically for this purpose. The amplifier was built using commercially available components to allow for both easy and economical manufacture. In particular, we report on the design requirements and outline our chosen design solutions. The amplifier handles low-level neural signals amidst large 50 Hz interference, with protection against potentially high stimulation voltages of over 100 V dc, with minimal cross-coupling of rapid stimulus pulses onto the high gain amplifier's input, and a short 'blocking' time between stimulation and recording. The amplifier also includes necessary filters, selectable gains and internal stimulator triggering circuits to provide a simple, integrated solution for common electrode operation on human subjects.

Directional tuning of human forearm muscle afferents during voluntary wrist movements

1. Single unit activity was recorded with the microneurography technique from sixteen spindle afferents and one Golgi tendon organ afferent originating from the forearm extensor muscles. Impulse rates were studied while subjects performed unobstructed aiming movements at the wrist in eight different directions 45 deg apart. In addition, similar imposed movements were performed while the subject was instructed to remain relaxed. Movement amplitudes were about 5 deg and the speed 10-30 deg x s(-1). Joint movements were translated to movements of a cursor on a monitor to provide visual feedback. 2. Individual spindle afferents modulated their activity over a number of targets, i.e. were broadly tuned, during these aiming movements. The preferred direction for a spindle afferent was the same during both passive and active movements, indicating that the fusimotor effects associated with active contractions had little or no effect on the direction of tuning. 3. The direction of tuning of individual spindle afferents could be predicted from the biomechanically inferred length changes of the parent muscle. Thus spindle afferents responded as stretch receptors, i.e. impulse rates increased with lengthening and decreased with shortening, in active as well as passive movements. 4. Spindles from muscles, which continuously counteracted gravity exhibited a stretch response and directional tuning during the phase of movement alone whereas their position sensitivity was poor. In contrast, spindle afferents from the muscles that had no or minimal antigravity role were directionally tuned during both the dynamic and the static phase of the aiming task and their position sensitivity was substantially higher. 5. In spite of the limited data base from three extensor muscles it could be demonstrated that wrist joint position was remarkably well encoded in the ensemble muscle spindle data. In some cases the ensemble muscle spindle data encoded the instantaneous trajectory of movement as well.

Muscle spindle activity in the affected upper limb after a unilateral stroke

Weakness, loss of dexterity and exaggerated reflex responses to proprioceptive and cutaneous stimuli are typical features of hemiparetic stroke. Since the extent to which altered fusimotor drive contributes to these deficits has not been established, this study was designed to assess fusimotor function in stroke patients by comparing three aspects of muscle spindle afferent behaviour (background discharge rate, responses to reflex inputs and responses to voluntary contractions) in 11 subjects affected by recent cerebrovascular lesions, with those in 18 healthy volunteers. The mean background discharge rates of muscle spindle afferents in the radial nerve when subjects attempted to relax the recorded limb completely were 6.6 +/- 5.3 Hz (n = 26) in patients and 6.4 +/- 6.1 Hz (n = 76) in control subjects. The variability of discharge rate of active afferents was also similar (0.12 +/- 0.07 and 0.09 +/- 0. 10, respectively). Reflex activation of fusimotor neurons was assessed using trains of electrical stimuli to the superficial radial nerve or to the palm of the hand, and using natural skin stimuli. Neither type of cutaneous stimulation affected muscle spindle afferent discharge in the absence of an EMG response. During deliberate voluntary contractions muscle spindle discharge rates were enhanced similarly in both the control and patient groups, indicating that volitional drives could access fusimotor neurons in the patients. Qualitatively, spindle behaviour was similar in patients and control subjects. These findings suggest that fusimotor function is not disturbed any more or less than skeletomotor function in hemiparetic patients and it is concluded that fusimotor dysfunction probably contributes little to their deficit.

Discharge of human muscle spindle afferents innervating ankle dorsiflexors during target isometric contractions

1. There are discrepancies in the literature about the reproducibility of forces at which human muscle spindle afferents accelerate their discharge during isometric voluntary contractions. The aim of this study was to determine for single muscle spindle afferents both the reproducibility of the 'acceleration threshold' and the factors contributing to variability of 'acceleration threshold'. 2. Microneurographic recordings were made from muscle spindle afferents innervating tibialis anterior while subjects performed isometric ankle dorsiflexions. Subjects matched the force of their contractions with a visually displayed 'ramp-and-hold' template. Template parameters were determined by the force of maximal isometric ankle dorsiflexion (MVC), and expressed as per cent MVC. The required 'ramp' rate and 'hold' force was adjusted between trials (range, 0.5-5% MVCs-1 and 0.5-20% MVC, respectively). The duration of the hold phase was 4 s and, following each contraction, stretch was applied transversely to the tendon to minimize the influence of any 'after-effects' on spindle afferent responses in subsequent contractions. 3. For each contraction, the force at which the rate of muscle spindle discharge increased was defined as the 'acceleration threshold'. Of twenty-six muscle spindle afferents innervating tibialis anterior, all but two increased their discharge in the test contractions. In 90% of contractions, acceleration thresholds were less than 3.2% MVC (range, 0.01-11.9% MVC). 4. Individual muscle spindle afferents increased their discharge at similar but not identical forces in repeated contractions. There was a positive correlation between the rate of contraction and the acceleration threshold (P < 0.001), but the strength of the target contraction had no effect on the threshold, and there was no trend for thresholds to change over time. 5. The results suggest, first, that most muscle spindle endings in the human pretibial muscles receive a significant increase in fusimotor drive during relatively weak isometric efforts and secondly, that when fusimotor after-effects are controlled, much of the residual variability in 'acceleration threshold' for any one spindle in repeated contractions is due to extrafusal factors, particularly variability in contraction rate.

Unit identification, sampling bias and technical issues in microneurographic recordings from muscle spindle afferents

Recordings have been made directly from human muscle spindle afferents for some 3 decades, and these have allowed assessment of fusimotor function in human subjects during normal motor acts and in patients with motor disturbances. However, inferences about fusimotor function are indirect, valid only if identification of the axon as of muscle spindle origin is secure and all extrafusal influences on the spindle have been controlled. As is discussed, the identification of spindle afferents and their classification into Group Ia and Group II are more problematic than in the cat, but these problems are probably relatively minor given the insight into normal function that can come from appropriately designed experiments in awake cooperative human subjects.

The methodology and scope of human microneurography

Microneurography was introduced in 1967 and has developed into an invaluable tool for investigating human somatosensory, motor and cardiovascular physiology and pathophysiology. It involves percutaneous insertion of a metal microelectrode into fascicles of limb and facial nerves. This review covers the procedures and equipment necessary for microneurography and provides a current circuit for a preamplifier. Evidence is presented that (i) most recordings from myelinated axons involve an effective penetration of the myelin by the electrode; (ii) based on physiological criteria, microstimulation through the electrode can be used to activate single axons although the probability of this is relatively low and (iii) despite 'micro' lesions caused by the electrode insertion into the nerve and its fascicles, the morbidity with the procedure is acceptably low.

Morphology of action potentials recorded from human nerves using microneurography

This study investigated the morphology of action potentials and the frequency of occurrence of the various waveforms encountered when using microneurography to record single-unit muscle afferent activity in humans. With 75% of the afferents recorded in this study (55 of 73 afferents), action potentials had a double-peaked morphology. For action potentials with an initial, positive double-peaked morphology, the relevant afferent conducts impulses past the microelectrode, with the second peak representing current fluctuations at the node of Ranvier proximal to the electrode. Accordingly, in the majority of recordings, the afferent is capable of conducting impulses to the spinal cord. The mean interpeak interval for these double-peaked units was 168 microseconds (range 90-310 microseconds). This represents marked prolongation of conduction time across the impaled internode. When the interpeak interval was relatively short (90-120 microseconds), the double-peaked morphology could be recognized only if the low-pass filter was high (> or = 10 kHz). The probability of recording a double-peaked unit was the same whether the recording was acquired early or late in a 3-h experiment. Conduction block developed in 6 of 73 single units during the recordings. These findings indicate that the majority of isolated single afferents and, indeed, the majority of afferents within the relevant fascicle are capable of transmitting impulses across the recording site, even though conduction across the impaled internode is slow. Conduction block due to direct injury or pressure is relatively uncommon.

Increased resting discharge of human spindle afferents following voluntary contractions

1. The aim of this study was to assess the incidence of lasting alterations in discharge rate of muscle spindle afferents innervating human ankle and toe dorsiflexor muscles following isometric contractions. 2. The subjects performed controlled isometric ankle dorsiflexions maintained for approximately 5 s. During the contraction the discharge of all but one spindle afferent increased above the precontraction level. After complete relaxation, there was prolonged enhancement of the discharge rate of nineteen of fifty-five muscle spindle afferents and none of three Golgi tendon organ afferents. Ten of the nineteen spindle afferents had been silent prior to the contraction. For the population of fifty-five spindle afferents, the mean 'postcontraction' discharge rate was 65% higher than the mean precontraction discharge rate, with the mean rate increasing from 2.3 to 3.9 Hz (P < 0.001). The mean duration of the enhanced postcontraction discharge was 52 s (range, 8-240 s). 3. Stretch applied to the tendon of the receptor-bearing muscle in twelve of fourteen spindle afferents with an enhanced postcontraction discharge rate eliminated or reduced the enhanced discharge rate. 4. The high incidence of an enhanced spindle discharge after voluntary contraction (35% of spindle afferents) suggests that muscle 'history' should be taken into account when interpreting changes in spindle discharge rates. The enhanced discharge rates following contraction probably reflect a long-lasting effect of the contraction-associated increase in fusimotor drive on intrafusal stiffness, rather than the persistence of fusimotor drive following relaxation.

Efferent responses to twitch tests used in identifying human muscle afferents

The response of a muscle afferent to the mechanical stimulus produced by a twitch contraction of the receptor-bearing muscle is an important test for differentiating between muscle spindle endings and Golgi tendon organs. The present study demonstrates that active alpha-motor axons can behave in a similar manner to spindle afferents, presumably responding not to the mechanical event per se, but reflexly to the change in afferent discharge created by the mechanical event. alpha-Motor axons were unequivocally identified during microneurography using spike-triggered averages of EMG. Caution is required when the twitch test is utilized to assist in the classification of muscle afferents during an intentional or unintentional voluntary contraction.

Fusimotor reflexes in relaxed forearm muscles produced by cutaneous afferents from the human hand

1. This study was designed to determine whether cutaneous receptors in the hand exert reflex effects on fusimotor neurones innervating relaxed muscles. Recordings were made from fifty-four muscle spindle afferents in the radial nerve while the arm was held relaxed in a supporting frame. Cutaneous afferents were activated by trains of stimuli at non-noxious levels to the superficial radial nerve or to the palmar surface of the fingers. 2. For the population of muscle spindle afferents, the mean discharge rate was 7.1 +/- 6.4 Hz (range 0-24 Hz). Thirty-three per cent had no background discharge, and this occurred significantly more often in finger extensors than wrist extensors. 3. Trains of cutaneous stimuli produced no change in the discharge rates of the majority of spindle endings irrespective of whether the spindle afferent had a background discharge or was given one by muscle stretch. However, with two of forty afferents, the stimuli produced an increase in discharge at latencies of 135 and 155 ms. 4. With a further fourteen muscle spindle endings, the dynamic responses to stretch were measured 100-400 ms after the trains of cutaneous stimuli. For four spindle afferents there was a statistically significant change in the dynamic response to stretch occurring at conditioned-stretch intervals of 100-200 ms. For two afferents the dynamic response decreased by 17 and 26% and for two others it increased by about 24 and 37%. 5. While these results support the view that the level of background fusimotor drive is low in the relaxed state, they suggest that there is some dynamic fusimotor drive to completely relaxed muscles operating on the human hand, and that this drive can be altered reflexly by cutaneous afferent inputs from the hand.

Force and displacement-controlled tendon vibration in humans

This study investigated how the mechanical characteristics of tendon vibration influence the responses of human muscle receptors. In this study, we used a tendon vibrator in which the force, displacement and frequency of vibration were precisely controlled. The tendon vibrator could produce large amplitude displacements, so it was also used to impose ramp-and-hold stretches to the tendon to help classify muscle spindle afferents. In normal human subjects, we recorded microneurographically from single muscle afferents during tendon vibration to determine how afferent responses are influenced by the force and the displacement applied to the tendon and how these influences of force and displacement change with vibration frequency. Our results indicate that the sensitivity of muscle spindle afferents to tendon vibration is enhanced by increasing force and displacement and decreased by increasing frequency. It is concluded that, in order to predict the afferent response to vibration, the mechanical characteristics of tendon vibration must be controlled. Controlling the mechanical characteristics of tendon vibration and understanding the effect of vibration on afferent discharge will be useful for furthering our understanding of the peripheral control of movement.

Decline in spindle support to alpha-motoneurones during sustained voluntary contractions

1. To address whether the muscle spindle support to alpha-motoneurones is maintained during prolonged isometric voluntary contractions, the discharge of eighteen muscle spindle afferents, originating in the dorsiflexors of the ankle or toes, was recorded from the common peroneal nerve in eight subjects. Isometric contractions were generally sustained for 1 min, usually below 30% of the maximal voluntary dorsiflexion force. 2. Once the afferent had been identified, subjects were instructed to dorsiflex the foot slowly to recruit the spindle ending, to continue the ramp contraction until a predetermined target force was reached, and then to hold that force until requested to relax. 3. Five muscle spindle afferents maintained a constant discharge frequency during the hold phase of the isometric contraction. Following relaxation of the contraction two spindle afferents from tibialis anterior, exhibited a post-contraction discharge despite the absence of detectable electromyographic activity (EMG). 4. The discharge frequency of most of the spindle afferents (72%) declined progressively during the isometric contraction. The mean firing rates had declined to two-thirds of those at the onset of the contraction by 30 s, and to half after 1 min. The decline in spindle firing rate commenced during the ramp phase of the contraction and was statistically significant by 10 s, when force was held constant. The extent of the decline was greater for those units with the higher initial firing rates and for those units studied after many preceding contractions. 5. In the same contractions a progressive increase in EMG was required to maintain force and consequently the change in EMG was inversely related to the change in spindle discharge. While many mechanisms may contribute to the decline in spindle discharge during a sustained isometric contraction, it is argued that the result will be a progressive disfacilitation of alpha-motoneurones, which may contribute to the decline in motor unit firing rates during a sustained contraction.

Activation of fusimotor neurones by motor cortical stimulation in human subjects

1. Neural recordings were made from motor fascicles of the ulnar or radial nerves while the motor cortex was stimulated percutaneously using high-voltage electrical stimuli or transient magnetic pulses to determine whether human muscle spindle endings could be activated by such stimuli and, if so, whether this occurred before the recruitment of alpha-motoneurones. 2. In relaxed subjects, no evidence of muscle spindle activation could be detected in nine recordings of multiunit neural activity and four recordings from single spindle afferents using stimulus levels up to 600 V and 1.5 T. These levels produced a prominent twitch contraction of the intrinsic muscles of the hand and of forearm muscles. Passive stretch of the contracting muscle did not reveal a fusimotor action too weak to be detected under isometric circumstances. 3. With twenty-six single spindle afferents, the stimuli were delivered during a voluntary contraction of the receptor-bearing muscle. This served to 'focus' the effects of the stimulus on the relevant motoneurone pools and increased the probability that fusimotor neurones innervating the endings were active. 4. None of the twenty-six spindle afferents could be activated by stimuli subthreshold for alpha-motoneurones, even when the stimuli were delivered during passive stretch of the contracting muscle. With eighteen afferents, stimuli above threshold for alpha-motoneurones were delivered: twelve remained unaffected but the discharge of six altered. 5. Three afferents were activated at latencies of 35, 39 and 40 ms, respectively 16, 20 and 20 ms after the onset of the EMG potentials in the receptor-bearing muscles. This latency difference is too short to be attributable to activation of gamma-motoneurones: arguments are presented that the increase in spindle discharge could result from activation of beta-motoneurones. 6. The discharge of three afferents increased at latencies of 70, 75 and 85 ms, too early to be due to stretch on the falling phase of the twitch contraction of the receptor-bearing muscle. Responses at these latencies could involve activation of gamma- or beta-motoneurones. 7. These findings in human subjects suggest that transient stimulation of the motor cortex may effectively access fusimotor neurones.

Perceptual responses to microstimulation of single afferents innervating joints, muscles and skin of the human hand

1. Microneurographic techniques were used to isolate single afferent axons within cutaneous and motor fascicles of the median and ulnar nerves at the wrist in thirteen subjects. Of the sixty-five identified afferents, eleven innervated the interphalangeal and metacarpophalangeal joints, sixteen innervated muscle spindles, three innervated Golgi tendon organs and thirty-five supplied the glabrous skin of the hand. 2. Intrafascicular stimulation through the recording microelectrode, using trains of constant-voltage positive pulses (0.3-0.8 V, 0.1-0.2 ms, 1-100 Hz) or constant-current biphasic pulses (0.4-13.0 microA, 0.2 ms, 1-100 Hz), evoked specific sensations from sites associated with some afferent species but not others. 3. Microstimulation of eight of the eleven joint afferent sites (73%) evoked specific sensations. With four, subjects reported innocuous deep sensations referred to the relevant joint. With the other four, the subjects reported a sensation of joint displacement that partially reflected the responsiveness of the afferents to joint rotation. 4. Microstimulation of fourteen of the sixteen muscle spindle afferent sites (88%) generated no perceptions when the stimuli did not produce overt movement. However, subjects could correctly detect the slight movements generated when the stimuli excited the motor axons to the parent muscle. 5. With seven of the nine rapidly adapting (type RA or FAI) cutaneous afferents (88%) microstimulation evoked sensations of 'flutter-vibration', and with two of eight slowly adapting (type SAI) afferents (25%) it evoked sensations of 'sustained pressure'. Of the eighteen SAII afferents, which were classified as such by their responses to planar skin stretch, the majority (83%) generated no perceptions, confirming previous work, but three evoked sensations of movements or pressure. 6. The present results suggest a relatively secure transmission of joint afferent traffic to perceptual levels, and it is concluded that the human brain may be able to synthesize meaningful information on joint displacement on the basis of impulses in a single joint afferent. This could partly compensate for the low responsiveness of individual joint afferents within the physiological range of joint displacements. Although single muscle spindle afferents can adequately encode joint position and movement, the results suggest that the brain needs the information from more than one muscle spindle afferent to perceive changes in joint angle.

Responses to passive movement of receptors in joint, skin and muscle of the human hand

1. Microneurographic techniques were employed to record unitary activity from afferents associated with digital joints of six conscious human subjects. Of 120 single afferents sampled from the median and ulnar nerves at the wrist, eighteen (15%) were classified as joint afferents; the majority of the sample (72.5%) were of cutaneous origin, and 12.5% were from muscle spindles and tendon organs. 2. Of the eighteen joint afferents six were tonically active in the rest position of the hand. All except two were recruited or accelerated their background discharge during passive joint movement. Three tonically active afferents were responsive to passive movement throughout the physiological range. The majority of the afferents, including the other three tonically active units, responded only towards the limits of joint rotation. 3. As a group, the sample of joint afferents had a limited capacity to signal the direction of joint movement. Nine of the sixteen joint afferents sensitive to movement responded in two axes of angular displacement, and two responded in all three axes. In any one axis of rotation eight afferents were activated in both directions of movement. However, one afferent, associated with the interphalangeal joint of the thumb, responded uni-directionally throughout the physiological range of joint movement and was thereby capable of adequately encoding joint position and movement. 4. Twenty-one of twenty-nine slowly adapting and eleven of eighteen rapidly adapting cutaneous afferents tested were activated by joint movement, but only towards the limits of joint rotation; half of the thirty-two movement-sensitive afferents were bi-directionally responsive. Muscle spindle afferents responded to stresses applied to the joint only if the resulting passive movement stretched the parent muscle. 5. It is concluded that human joint afferents possess a very limited capacity to provide kinaesthetic information, and that this is likely to be of significance only when muscle spindle afferents cannot contribute to kinaesthesia.

Twitch contraction for identification of human muscle afferents

A classical test to differentiate between Golgi tendon organs and muscle spindles is the twitch contraction elicited by electrical stimulation. The possibility of producing maximal twitches in the finger extensor muscles using surface stimulation over the muscle belly was investigated as well as the feasibility of the test in microneurography experiments. Electrical stimuli were applied either over the muscle in the forearm or the radial nerve in the upper arm, while the resulting torque output at single metacarpophalangeal joints was measured. The relationship between current intensity and maximal contraction force was determined and stimulus response plots were constructed over a large range of current intensities. Stimulation of the radial nerve always yielded plots with a steep and monotonous rising limb up to a plateau. It was concluded that the plateau represented maximal twitch contractions. With transcutaneous stimulation over the muscle belly, the stimulus response plots were usually more complex. However, this could be explained by force transmission through the intertendinous connections on the dorsum of the hand and by antagonist activation. It was concluded that maximal twitch contractions can readily be elicited in the human extensor digitorum muscle with nonpainful transcutaneous electrical stimulations. Moreover, maximal twitches are compatible with single unit recording from muscle afferents in microneurography experiments.

Evoked skin sympathetic nerve responses in man

Activity in human unmyelinated efferent nerve fibres was recorded from seven upper limb cutaneous nerve fascicles. The activity induced by contralateral nerve trunk stimulation or tone burst was averaged and could be compared providing the stimuli were delivered at random times and in a random sequence. The average evoked sympathetic nerve responses to nerve trunk stimulation and tone burst were identical in latency and duration.

Coupling between human muscle spindle endings and motor units assessed using spike-triggered averaging

The electromyographic activity (EMG) generated by voluntary contraction of a muscle was averaged using the potentials from 18 identified muscle spindle afferents as a trigger. In post-spike averages of 1000-10,000 sweeps, no evidence of reflex excitation of the homonymous motoneurone pool was detected. In pre-spike averages there was no evidence of a motor-unit EMG potential that was closely correlated to the trigger spike. A single spindle afferent has only a weak reflex effect on an active motoneurone pool and must be part of a synchronized volley to affect motoneurone discharge significantly. No evidence was found for spindle activation via beta-motoneurones in weak voluntary contractions.

The afferent volleys responsible for spinal proprioceptive reflexes in man

1. To define the neural volleys responsible for the Achilles tendon jerk and the H reflex, muscle afferent activity was recorded using micro-electrodes inserted percutaneously into appropriate fascicles of the tibial nerve in the popliteal fossa.2. The response of soleus muscle afferents to tendon percussion consisted of a dispersed volley, starting 3.5-7.0 ms after percussion, increasing to a peak over 6.5-11.0 ms, and lasting 25-30 ms, depending on the strength of percussion. Electrical stimuli to the sciatic nerve at a level adequate to evoke an H reflex but subthreshold for the M wave produced a more synchronized volley, the fastest fibres of which had conduction velocities of 62-67 m/s, and the slowest 36-45 m/s.3. The wave of acceleration produced by percussion subthreshold for the ankle jerk spread along the skin at over 150 m/s. Midway between the bellies of the gastrocnemii it consisted of a damped oscillation with four to five separate phases and maximum amplitude approximately one-twentieth of that recorded on the Achilles tendon.4. With ten primary spindle endings, tendon percussion subthreshold for the ankle jerk elicited two to five spike discharges per tap, the shortest interspike intervals being 4-7 ms. Tendon percussion elicited single discharges from two Golgi tendon organs, and altered the discharge pattern of a single secondary spindle ending. The degree of dispersion of the multi-unit muscle afferent volley can be explained by the pattern of discharge of primary spindle endings.5. Percussion on the Achilles tendon evoked crisp afferent volleys in recordings from nerve fascicles innervating flexor hallucis longus, tibialis posterior, the intrinsic muscles of the foot and the skin of the foot. Electrical stimuli delivered to the tibial nerve in the popliteal fossa at a level sufficient for the H reflex of soleus produced either a volley in muscle afferents from the intrinsic muscles of the foot or a volley in cutaneous afferents from the foot.6. For comparable stimuli in the two positions, the H reflex was inhibited but the Achilles tendon jerk enhanced when the ankle was dorsiflexed from 105 degrees to 90 degrees .7. The duration of the rise times of the excitatory post-synaptic potentials (e.p.s.p.s) produced in soleus motoneurones by electrical stimulation, and by tendon percussion subthreshold for the H reflex and the ankle jerk respectively, was estimated from post-stimulus time histograms of the discharge of voluntarily activated single motor units in soleus. The mean e.p.s.p. rise times were 1.9 ms for electrical stimulation and 6.6 ms for tendon percussion. There was evidence that the duration of the electrically evoked e.p.s.p. was curtailed by an inhibitory post-synaptic potential (i.p.s.p.) of only slightly longer latency than the e.p.s.p.8. The mechanically induced and electrically induced afferent volleys are not homogeneous volleys in group I a afferents from triceps surae. The afferent volleys differ in so many respects that it is probably invalid to compare the H reflex and tendon jerk as a measure of fusimotor activity. It is suggested that neither reflex can be considered a purely monosynaptic reflex.