Architecture and consequent physiological properties of the semitendinosus muscle in domestic goats

Morphological and physiological analyses confirm that the semitendinosus muscle of goats contains two separate compartments in series, each with distinct innervation. These compartments of the muscle are in turn composed of short fibers (approximately four fibers in series in the proximal compartment and seven to eight fibers in the distal compartment) which overlap each other for more than 30% of their length, with much of the overlapping portions consisting of slender tails that terminate at one-tenth of the midfiber diameter. Groups of fibers are associated into relatively narrow bands that run end-to-end in each compartment. The data suggest that the maximum length of muscle fibers may be limited; even the fibers of parallel-fibered muscles may not scale with the dimension of the animal.

Conduction studies in peripheral cat nerve using implanted electrodes: II. The effects of prolonged constriction on regeneration of crushed nerve fibers

Arrays of chronically implanted electrodes were used to examine the time course of elongation and maturation of peripheral nerve fibers in the cat after crush of the tibial nerve in the proximal calf. Regeneration after crush alone was compared with crush 5 mm proximal to a tight constriction of the nerve. Regeneration was monitored by the progression of excitability along the electrode arrays on the tibial and plantar nerves. The sensitivity was sufficient to record the averaged activity in single nerve fibers allowing detection of the earliest regeneration. The diameters of the fastest regenerating fibers were estimated from the conduction velocity proximal to the site of crush. Both after crush alone, and after crush constriction, small myelinated fibers regenerated in front of large fibers. The rate of elongation after crush alone was 3.2 mm/day, whereas it was slower (P less than 0.02) distal to crush + constriction (2.2 mm/day). In both lesions, the extrapolated delay to onset of regeneration was 8 days. In observations up to 300 days after crush, maturation was delayed or impaired by the constriction, and the compound nerve action potential had a smaller amplitude and a dispersed shape. Transverse sections of nerves after crush + constriction showed a diminished number of large and an increased number of small fibers compared with crush alone, possibly due to persistent branching of regenerated fibers. After both crush alone and crush + constriction, regenerated fibers had similar g ratios, suggesting that myelination developed fully in fibers of diminished diameters.

Conduction studies in peripheral cat nerve using implanted electrodes: I. Methods and findings in controls

Silicone rubber cuff and patch electrodes with multiple contacts were implanted along the sciatic-tibial-plantar nerves in cat for repeated studies of conduction properties of normal peripheral nerve over periods of time. The contacts were used in various combinations for precise localization of changes in conduction velocities and excitability along the extent of normal nerves. In this paper the particular characteristics and limitations associated with cuff-electrode recordings of neural activity are discussed. The nerve action potential was recorded using a tripolar configuration with a central lead flanked by two shunted leads at symmetrical distances. This configuration records the spatial derivative of the action potential and rejects potentials from sources outside the cuff. The voltage changes are restricted by the silicone cuff, and the dynamic range is therefore very high, allowing detection from single myelinated fibers to whole nerve responses. The electrodes are well suited for following the development of regeneration and degeneration following experimental lesions.

Cross-correlation of EMG reveals widespread synchronization of motor units during some slow movements in intact cats

It is commonly assumed that the motor units comprising a single mammalian muscle will be recruited asynchronously at subtetanic firing rates to produce smoothly modulated force output. However, electromyograms from certain neck muscles, recorded by implanted bipolar "patch" electrodes having large contacts, often exhibited a rhythmic clustering of spike activity whose patterns suggested that motor-unit firing was synchronized both within and across muscles. We have developed a computerized processing system that digitizes EMG activity and calculates auto- and cross-correlation products of selected segments. The presence or absence of synchronization caused by neural mechanisms can be identified and differentiated from that due to the rhythmicity of the behavior itself (e.g. shaking) or due to cross-talk, according to the shapes of the resultant correlograms. These methods have so far been applied to the study of hindlimb and neck muscle EMG during various natural motor behaviors, but they provide a general, quantitative tool for the study of an important aspect of motor control that may be overlooked by conventional sampling and smoothing techniques.

A multiple-contact EMG recording array for mapping single muscle unit territories

The glycogen-depletion technique has become a well-established method for determining histologically the cross-sectional distribution of a single muscle unit. A major drawback of this method is its low yield of one depleted unit per experiment. Furthermore, this technique is particularly unsuited for determining the longitudinal distribution of single muscle units in long, broad muscles because of the formidable serial sectioning job that would be required. Our alternative, electrophysiological method utilizes a multiple-contact, two-dimensional EMG recording array to map efficiently the cross-sectional and longitudinal distributions of numerous single muscle units in anatomically diverse muscles. Additionally, architectural information on muscle fiber lengths, end-plate locations, motor subunit (MSU) arrangements and muscle conduction velocities can be determined.

Cat hindlimb motoneurons during locomotion. II. Normal activity patterns

Activity patterns were recorded from 51 motoneurons in the fifth lumbar ventral root of cats walking on a motorized treadmill at a range of speeds between 0.1 and 1.3 m/s. The muscle of destination of recorded motoneurons was identified by spike-triggered averaging of EMG recordings from each of the anterior thigh muscles. Forty-three motoneurons projected to one of the quadriceps (vastus medialis, vastus lateralis, vastus intermedius, or rectus femoris) or sartorius (anterior or medial) muscles of the anterior thigh. Anterior thigh motoneurons always discharged a single burst of action potentials per step cycle, even in multifunctional muscles (e.g., sartorius anterior) that exhibited more than one burst of EMG activity per step cycle. The instantaneous firing rates of most motoneurons were lowest upon recruitment and increased progressively during a burst, as long as the EMG was still increasing. Firing rates peaked midway through each burst and tended to decline toward the end of the burst. The initial, mean, and peak firing rates of single motoneurons typically increased for faster walking speeds. At any given walking speed, early recruited motoneurons typically reached higher firing rates than late recruited motoneurons. In contrast to decerebrated cats, initial doublets at the beginning of bursts were seen only rarely. In the 4/51 motoneurons that showed initial doublets, both the instantaneous frequency of the doublet and the probability of starting a burst with a doublet decreased for faster walking speeds. The modulations in firing rate of every motoneuron were found to be closely correlated to the smoothed electromyogram of its target muscle. For 32 identified motoneurons, the unit's instantaneous frequencygram was scaled linearly by computer to the rectified smoothed EMG recorded from each of the anterior thigh muscles. The covariance between unitary frequencygram and muscle EMG was computed for each muscle. Typically, the EMG profile of the target muscle accounted for 0.88-0.96 of the variance in unitary firing rate. The EMG profiles of the other anterior thigh muscles, when tested in the same way, usually accounted only for a significantly smaller fraction of the variance. Brief amplitude fluctuations observed in the EMG envelopes were usually also reflected in the individual motoneuron frequencygrams. To further demonstrate the relationship between unitary frequencygrams and EMG, EMG envelopes recorded during walking were used as templates to generate depolarizing currents that were applied intracellularly to lumbar motoneurons in an acute spinal preparation.(ABSTRACT TRUNCATED AT 400 WORDS)

Cat hindlimb motoneurons during locomotion. I. Destination, axonal conduction velocity, and recruitment threshold

Fine flexible wire microelectrodes chronically implanted in the fifth lumbar ventral root (L5 VR) of 17 cats rendered stable records of the natural discharge patterns of 164 individual axons during locomotion on a treadmill. Fifty-one out of 164 axons were identified as motoneurons projecting to the anterior thigh muscle group. For these axons, the centrifugal propagation of action potentials was demonstrated by the technique of spike-triggered averaging using signals recorded from cuff electrodes implanted around the femoral nerve. The axonal conduction velocity was measured from the femoral nerve cuff records. For 43/51 motoneurons, the corresponding target muscle was identified by spike-triggered averaging of signals recorded from bipolar EMG electrodes implanted in each of the anterior thigh muscles: vastus intermedius, medialis and lateralis, sartorius anterior and medialis, and rectus femoris. For 32/51 motoneurons, the recruitment threshold during locomotion was determined from the mean value of the rectified digitally smoothed EMG of the target muscle measured at the time when the motoneuron fired its first spike for each step. The recruitment threshold of every motoneuron was relatively constant for a given speed of walking, but for some units there were small systematic variations as a function of treadmill speed (range: 0.1-1.3 m/s). Recruitment thresholds were standardized with respect to the mean value of peak EMG activity of the target muscle during 16 s of walking at 0.5 m/s. For 28/51 motoneurons recorded in nine cats, recruitment thresholds (range: 3-93% of peak target muscle EMG) were linearly correlated (r = 0.51, P less than 0.02) to axonal conduction velocities (range: 57-117 m/s). In addition, for seven recorded pairs of motoneurons that projected to the same muscle in the same cat, the recruitment thresholds were ordered by relative conduction velocities. Taken together, these results are consistent with the notion that, in normal cat locomotion up to a medium trot, anterior thigh motoneurons are progressively recruited in an orderly fashion.

Cat hindlimb motoneurons during locomotion. III. Functional segregation in sartorius

Cat sartorius has two distinct anatomical portions, anterior (SA-a) and medial (SA-m). SA-a acts to extend the knee and also to flex the hip. SA-m acts to flex both the knee and the hip. The objective of this study was to investigate how a "single motoneuron pool" is used to control at least three separate functions mediated by the two anatomical portions of one muscle. Discharge patterns of single motoneurons projecting to the sartorius muscle were recorded using floating microelectrodes implanted in the L5 ventral root of cats. The electromyographic activity generated by the anterior and medial portions of sartorius was recorded with chronically implanted electrodes. The muscle portion innervated by each motoneuron was determined by spike-triggered averaging of the EMGs during walking on a motorized treadmill. During normal locomotion, SA-a exhibited two bursts of EMG activity per step cycle, one during the stance phase and one during the late swing phase. In contrast, every recorded motoneuron projecting to SA-a discharged a single burst of action potentials per step cycle. Some SA-a motoneurons discharged only during the stance phase, whereas other motoneurons discharged only during the late swing phase. In all cases, the instantaneous frequencygram of the motoneuron was well fit by the rectified smoothed EMG envelope generated by SA-a during the appropriate phase of the step cycle. During normal locomotion, SA-m exhibited a single burst of EMG activity per step cycle, during the swing phase. The temporal characteristics of the EMG bursts recorded from SA-m differed from the swing-phase EMG bursts generated by SA-a.(ABSTRACT TRUNCATED AT 250 WORDS)

Cat hindlimb motoneurons during locomotion. IV. Participation in cutaneous reflexes

The responses of 11 individual motoneurons, the muscle to which each projected, plus all other muscles in the anterior thigh of the cat, were recorded following single non-noxious electrical stimuli to cutaneous nerves while the intact animal walked on a treadmill. The various excitatory and/or inhibitory responses were qualitatively similar for stimuli within the range 1.1-10 times threshold for group I fibers in the stimulated nerve (usually saphenous). Monarticular knee extensor muscles in the vastus group and their motoneurons were usually inhibited in the period 10- to 25-ms poststimulus. The faster contracting vastus medialis and lateralis muscles tended to have an excitatory rebound at approximately 25- to 40-ms poststimulus that was confined to the stance phase of the step cycle when these muscles were normally active. Biarticular hip flexor muscles rectus femoris and both the anterior and medial parts of sartorius and their motoneurons all had similar bimodal excitatory responses, including an early period 3- to 18-ms poststimulus and a later period 20- to 35-ms poststimulus. The short-latency excitatory responses appeared to be proportional to the normal recruitment of the muscles in the step cycle, whereas the long-latency responses tended to be phase advanced with respect to normal recruitment. Motoneurons projecting to muscles with two excitatory peaks tended to have similar excitatory responses at both latencies and occasionally responded at both latencies to a single stimulus.

Distribution and innervation of short, interdigitated muscle fibers in parallel-fibered muscles of the cat hindlimb

The cat hindlimb contains several long, biarticular strap muscles composed of parallel muscle fascicles that attach to short tendons. Three of these muscles--sartorius, tenuissimus, and semitendinosus--were studied by dissecting individual gold-stained fibers and determining the surface distribution of acetylcholinesterase-stained end-plate zones. In each muscle, fascicles were composed of muscle fibers that ran only part of the fascicle length and tapered to end as fine strands that interdigitated with other tapering fibers within the muscle mass. Most muscle fibers measured 2-3 cm in length. Fascicles of muscle fibers were crossed by short transverse bands of endplates (1 mm wide by 1-5 mm long) that were spaced at fairly regular intervals from the origin to the insertion of the muscle. The endplate pattern suggested that the fiber fascicles were organized into multiple longitudinal strips. In the sartorius, the temporospatial distribution of electromyographic (EMG) activity evoked by stimulating fine, longitudinal branches of the parent nerve confirmed that each strip was selectively innervated by a small subset of the motor axons. These axons appeared to distribute their endings throughout the entire length of the fascicles, providing for synchronous activation of their in-series fibers.

Activity of spindle afferents from cat anterior thigh muscles. II. Effects of fusimotor blockade

Chronically implanted electrodes and nerve cuff catheters were used to record the activity of individual muscle spindle afferents during treadmill walking as low doses of lidocaine were infused around the femoral nerve to progressively block gamma motoneuron activity. Both primary and secondary endings from both the monarticular knee extensors and the biarticular hip/knee muscles of the anterior thigh showed large decreases in afferent activity, usually well before changes in the electromyographic activity, force output, or length and velocity were seen in the parent muscles. This decline in the proprioceptive signal feeding back onto the spinal cord, which we presume to have involved most of the spindles supplied by the femoral nerve, did not cause noticeable irregularities or instability of the walking gait. At the peak of the fusimotor blockade, spindle afferents from knee extensor muscles lost about half of their usually brisk activity during the near-isometric contraction of the stance phase. Significant decreases in the response to passive stretch during the flexion phase also occurred. At the peak of the fusimotor blockade, spindle afferents from the biarticular muscles lost all of their activity during the rapidly shortening swing phase and about half of their activity during the rapidly lengthening stance phase. For both monarticular and biarticular muscle spindles, the activity decreases in stance and swing phase often occurred at distinctly different stages of the progressive fusimotor blockade, indicating several different sources of fusimotor control. From these data, we infer that the sensitivity of most spindle afferents is substantially influenced by fusimotor activity during phases of both extrafusal activity and extrafusal silence. At least some of this influence appears to come from fusimotor neurons whose recruitment is independent of the extrafusal recruitment. The extent and type of fusimotor effects on spindle afferent sensitivity (dynamic or static) appear to be specialized for the mechanical events that tend to occur during those phases.

Activity of spindle afferents from cat anterior thigh muscles. I. Identification and patterns during normal locomotion

The naturally occurring activity patterns of anterior thigh muscle spindle afferents were recorded during unrestrained treadmill locomotion by means of floating microelectrodes chronically implanted in the fifth lumbar dorsal root ganglion. Conduction velocity of units from primary and secondary endings was determined by spike-triggered averaging of the signals from a chronically implanted nerve cuff. Activity from knee extensor muscle spindles generally occurred during periods of muscle lengthening, but was often greater for small stretches when the muscle was active (during stance phase of walking) than for larger stretches when the muscle was passive (swing phase), indicating fusimotor enhancement of spindle sensitivity in phase with extrafusal muscle recruitment. Activity from spindles in biarticular muscles acting across the knee and hip was more variable and complex than that seen in the pure knee extensors, and frequently included activity during rapid muscle shortening (swing phase) indicative of strong static fusimotor input. Changes in speed of gait caused changes in the range and velocity of muscle length excursions monitored by chronically implanted length gauges, but such changes were accompanied by only modest changes in spindle afferent activity, suggesting concurrent and compensatory changes in fusimotor influence on spindles. Activity from spindle secondary endings was generally lower, more regular, and less velocity dependent than that from primary endings, consistent with their lack of input from the dynamic fusimotor apparatus. The activity of all spindle afferents studied was similarly well modulated during extrafusal activity of the parent muscles, regardless of the kinematic conditions of muscle length and velocity during which this muscle work occurred. This suggests that the fusimotor apparatus is well orchestrated to regulate the static and dynamic sensitivity of primary spindle afferents at levels appropriate to the anticipated motion.

Activity of spindle afferents from cat anterior thigh muscles. III. Effects of external stimuli

Chronically implanted electrodes were used to record the activity of identified single muscle spindle afferents in awake cats during responses to various types of manual and electrical stimulation. During vigorous cyclical responses such as shaking and scratching, spindle afferents generally maintained at least some activity during both lengthening and shortening of the parent muscle, indicating that the programs for these movements include both extra- and intrafusal recruitment. During noncyclical responses such as ipsilateral limb withdrawal and crossed-extension, spindle activity was modest and poorly correlated with extrafusal activity. Weak cutaneous nerve shocks during walking elicited complex excitatory and inhibitory phase-dependent reflexes in the various muscles studied but caused relatively little change in spindle afferent activity, indicating a lack of correlation between alpha and gamma motoneuron activity. A primary and a secondary afferent from sartorius muscle were recorded simultaneously during walking cycles that were perturbed by electrically induced twitches of the antagonist hamstring muscles; both demonstrated highly sensitive, short latency responses to the resulting skeletal motion, consistent with their previously suggested roles in detecting small brief mechanical perturbations. The degree to which fusimotor responses were correlated with extrafusal responses to somatosensory perturbations was highly dependent on the specific nature of the stimulus and the response. Fusimotor reprogramming of the spindle sensitivity appears to be a feature of cyclical movements that are presumably under proprioceptive control, whereas brief perturbations within the context of a particular motor program may be ignored by the fusimotor system.

The distal hindlimb musculature of the cat. Cutaneous reflexes during locomotion

In order to better understand the organization of the locomotor control system, we examined the temporal patterns of distal hindlimb muscle responses to brief electrical stimulation of cutaneous nerves during walking on a treadmill. Electromyographic recordings were made from twelve muscles; stimuli were applied individually to three nerves at random times throughout the step cycle. A new graphical technique was developed to assist detailed examination of the time course and gating of complex reflex patterns. The short latency reflexes were of two primary types: inhibition of extensors and excitation of flexors; these responses were only evident during locomotor phases in which the respective motoneuron pools were active. Longer-latency response components were gated in a similar but not identical manner, suggesting some independence from the basic locomotory influence on the motoneuronal pool. The phase-dependent gating of reflexes appeared to be consistent with a functional role for reflex responses during locomotion. The reflex responses of muscles with complex anatomical actions were often correspondingly complex.

Motoneurone task groups: coping with kinematic heterogeneity

Physiological principles of motor control have generally been based on linear servocontrol theories in which transducers of force, length and velocity are used to provide feedback to the motor actuators. Within the past few years, recordings of the activity of motoneurones and proprioceptors during normal motor behaviour have indicated a diversity that is not consistent with any one theory of motor control. This paper examines the heterogeneity of kinematic conditions under which muscles are called on to perform, and attempts to correlate this with the effects of various fusimotor states on the activity of the muscle spindle afferents, the major sensory signal source in most feedback control schemes. The concept of ‘task groups’ is proposed, in which functional groups of alpha and gamma motoneurones and spindle afferents are programmed to achieve optimal control over relatively restricted but more linear parts of their operating curves. Such a functional compartmentalization of the motor apparatus is thus consistent with several different theories of servocontrol, although it remains unclear whether such conceptual mechanisms are actually embodied in the highly complex neural circuitry present in the spinal cord and higher motor centres.

Conduction velocity changes along lumbar primary afferent fibers in cats

Though peripheral conduction velocity is widely used to characterize afferent fibers according to somatosensory modality, disagreement exists as to whether or not conduction velocity varies along such an axon's length. Therefore, in this experiment, conduction velocities were measured over very short axonal segments (7.5 to 15 mm) within the posterior tibial nerve, sciatic nerve, and L7 dorsal root, using the method of spike-triggered averaging of neurograms recorded from tripolar electrodes. The conduction velocities for several units were also determined using electrical stimulation, so that the accuracy of the two techniques could be compared. For most units, dorsal root and sciatic nerve conduction velocities were not significantly different; however, they were not tightly correlated. Tibial nerve conduction velocity averaged 86% of that within the sciatic nerve. Variations in sciatic nerve conduction velocity within adjacent axonal segments (9 mm in length) rarely exceeded experimental error. It appears that spike-triggered averaging of signals from tripolar electrodes separated 15 mm or more apart is a more accurate method for measuring conduction velocity than electrical stimulation, which was subject to several large errors.

Physiological characterization of motor unit properties in intact cats

Single motor units were isolated in intact cats, by microstimulation through chronically implanted microwires in the L5 ventral roots. Motor unit axonal and mechanical properties were obtained by stimulus-triggered averaging the signals from an implanted femoral nerve recording cuff and patellar tendon force transducer. All unit types were sampled with this technique, and it was also possible to stimulate in isolation an axon whose ventral root spike was recorded during treadmill locomotion. A new technique was described, spike-triggered microstimulation, for verifying the identity of a stimulated and a recorded axon.

Spatial cross-correlation. A proposed mechanism for acoustic pitch perception

We propose in this paper a new class of model processes for the extraction of spectral information from the neural representation of acoustic signals in mammals. We are concerned particularly with mechanisms for detecting the phase-locked activity of auditory neurons in response to frequencies and intensities of sound associated with speech perception. Recent psychophysical tests on deaf human subjects implanted with intracochlear stimulating electrodes as an auditory prosthesis have produced results which are in conflict with the predictions of the classical place-pitch and periodicity-pitch theories. In our model, the detection of synchronicity between two phase-locked signals derived from sources spaced a finite distance apart on the basilar membrane can be used to extract spectral information from the spatiotemporal pattern of basilar membrane motion. Computer simulations of this process suggest an optimal spacing of about 0.3-0.4 of the wavelength of the frequency to be detected. This interval is consistent with a number of psychophysical, neurophysiological, and anatomical observations, including the results of high resolution frequency-mapping of the anteroventral cochlear nucleus which are presented here. One particular version of this model, invoking the binaurally sensitive cells of the medial superior olive as the critical detecting elements, has properties which are useful in accounting for certain complex binaural psychophysical observations.

Single unit conduction velocities from averaged nerve cuff electrode records in freely moving cats

The conduction velocity of peripheral neurons recorded by wire microelectrodes implanted in intact, freely moving cats was determined on-line using the technique of spike-triggered averaging of nerve cuff electrode records described here. Axonal velocity was estimated from the conduction latency between two adjacent sets of tripolar recording electrodes inside a cuff, thereby avoiding uncertainties that could arise from differences in spike shape, variable conduction distance, or unknown stimulus utilization time. This method rendered conduction velocity values for individual afferent and efferent myelinated fibers ranging from 27 to 120 m/sec, estimated with an uncertainty of +/-5%. In addition, predictions from theoretical models relating extracellular potential amplitude, wavelength, and conduction velocity were confirmed experimentally for en passant records obtained from intact myelinated fibers.

Discharge patterns of hindlimb motoneurons during normal cat locomotion

Long-term recording from single lumbar motoneurons of intact cats revealed activation patterns fundamentally different from those seen in decerebrate preparations. In intact cats, motoneuron bursts showed marked rate modulation without initial doublets. Each unit's frequencygram generally resembled the envelope of the gross electromyogram simultaneously recorded from the corresponding muscle. Average and peak discharge rates increased for faster gaits. These findings suggest that, in cat locomotion, rate modulation is a more important contributor to force regulation than was previously thought.

Somatosensory unit input to the spinal cord during normal walking

Chronic recording techniques in freely walking cats have been used to sample unitary activity from most large myelinated afferent classes. Cutaneous mechanoreceptors are highly sensitive and generate regular activity patterns predictable from their modalities. Knee joint afferents can fire briskly midrange locomotory movements but appear to be influenced by factors other than joint angle. Golgi tendon organs generate activity consistent with sensitivity to active muscle tension. Muscle spindle afferents do not appear to conform to any single functional pattern for all muscles. It is suggested that degree and rate of stretch are sensed by spindles (possibly under dynamic fusimotor bias) in extensor muscles which normally undergo isometric or lengthening contractions whereas rapidly modulated static fusimotor activity is employed to preserve spindle activity during the rapidly shortening contractions of flexor muscles. Both patterns may be represented in different spindles of bifunctional, biarticular muscles such as rectus femoris and sartorius.

Modulation of ipsi- and contralateral reflex responses in unrestrained walking cats

1. The modulation of reflex responses in up to 10 simultaneously recorded hindlimb muscles was studied in unrestrained cats walking on a treadmill. Single electrical shocks of various strengths were applied to different skin areas of teh hindlimb at different times of the step cycle while the resulting EMG responses were sampled and analyzed. 2. Two excitatory response peaks (P1 and P2) at a latency of about 10 and 25 ms, respectively, were seen in all flexors examined (sartorius, semitendinosus, tibialis anterior, extensor digitorum longus). Stimulation of most skin areas was effective but responses were most easily obtained from stimuli applied to the foot or ankle. During the step cycle there was a marked modulation of the amplitudes of the responses, especially the P2 responses, which grew larger toward the end of stance when a maximum was reached, followed by a steady decline throughout swing. This pattern was very similar for various flexors, although these muscles differed considerably in their normal EMG activity pattern during walking. 3. Flexor responses were absent when the same stimuli were applied during the early stance phase. Instead, inhibition of the ongoing EMG activity was seen at a latency of 10 ms or less in all extensors examined (semimembranosus, quadriceps, soleus, gastrocnemius medialis, flexor digitorum longus). The inhibition was followed by a late excitatory peak (P3) at about 35-ms latency in all extensors except soleus. 4. Certain stimulation sites yielded exceptions to the above patterns. Stimulation of the skin area innervated by the sural nerve yielded larger and earlier MG excitatory responses as compared to stimulation of other skin areas. Activation of the plantar surface of the foot often failed to elicit P2 responses in the hip flexor sartorius, which showed inhibition instead. 5. In the hindlimb contralateral to the stimulus, excitatory responses occurred both in flexors and extensors at a latency of 20-25 ms. The pattern of modulation of these responses was similar to the ipsilateral modulation of P2 flexor and P3 extensor responses. Soleus failed to show a crossed response. 6. The data indicate that flexor and extensor responses differ both with respect to their latency and to their correlation with the ongoing EMG reactivity. It is concluded that these stimuli do not demonstrate reflex reversal in the strict sense in the normal walking cat but that there is modulation of transmission in a flexor excitatory and extensor inhibitory pathway, possibly by the flexor part of the spinal locomotor oscillator. In addition, there are some specialized flexor inhibitory and extensor excitatory pathways. The slow soleus muscle does not seem to be excited through these pathways.

A simple technique for determination of footfall patterns of animals during treadmill locomotion

This paper describes a direct on-line electronic method for determination of the quadrupedal footfall patterns of animals during locomotion. This technique involves the application of a 30 kHz AC carrier signal to a conductive treadmill belt. The time of contact of each paw on the belt is determined by recording the increased conduction of these carrier signals from the treadmill surface through the footpads to monopolar electrodes implanted in each limb. Since the carrier signal frequency is very high, filters may be employed to electronically separate this signal from electromyographic signals simultaneously obtained through the same electrodes. In comparison with the cinematographic methods typically used for analysis of footfall patterns, this technique is less time consuming, is just as accurate and also yields signals which can be used for on-line triggering applications.

A pulsed integrator for EMG analysis

A novel device is described which converts EMG signals to an output which is proportional to the area under the rectified curve in discrete time intervals. This is achieved by resetting a long time constant integrator at the desired rate and saving each integral in a sample/hold circuit for output during the subsequent integration period. This output is particularly useful for quantifying fast reflex events since the signal is integrated without leakage in each period and leaves no residual signal in subsequent periods.

Activity patterns in individual hindlimb primary and secondary muscle spindle afferents during normal movements in unrestrained cats

1. Chronically implanted microelectrode wires in the L7 and S1 dorsal root ganglia were used to record unit activity from cat hindlimb primary and secondary muscle spindle afferents. Units could be reliably recorded for several days, permitting comparison of their activity with homonymous muscle EMG and length during a variety of normal, unrestrained movements. 2. The general observation was that among both primary and secondary endings there was a broad range of different patterns of activity depending on the type of muscle involved and the type of movement performed. 3. During walking, the activity of a given spindle primary was usually consistent among similar step cycles. However, the activity was usually poorly correlated with absolute muscle length, apparently unrealted to velocity of muscle stretch, and could change markedly for similar movements performed under different conditions. 4. Spindle activity modulation not apparently related to muscle length changes was assumed to be influenced by fusimotor activity. In certain muscles, this presumption leads to the conclusion that gamma-motoneurons may be activated out of phase with homonymous alpha-motoneurons as well as by more conventional alpha-gamma-motoneuron coactivation. 5. Simultaneous recordings of two spindle primary afferents from extensor digitorum longus indicated that spindles within the same muscle may differ considerably with respect to this presumed gamma-motoneuron drive. 6. Spindle secondary endings appeared to be predominantly passive indicators of muscle length during walking, but could demonstrate apparently strong fusimotor modulation during other motor activities such as postural changes and paw shaking. 7. Both primary and secondary endings were observed to undergo very rapid modulation of firing rates in response to presumed reflexly induced intrafusal contractions. 8. It is suggested that the pattern of fusimotor control of spindles may be tailored to the specific muscle and task being performed, rather than necessarily dominated by rigid alpha-gamma coactivation.

Long-term unit recording from somatosensory neurons in the spinal ganglia of the freely walking cat

A new technique has been developed for stable, long-term recording from groups of individual primary afferent neurons in the freely walking cat. A number of fine, flexible wires are inserted into dorsal root ganglia via a small laminotomy in the lumbar spine. The cut end of each wire can record stable and separable action potentials from one to three dorsal root ganglion neurons; each unit has typically held for 1 to 4 days. A broad range of myelinated somatosensory afferent (conduction velocities of 30 to 120 meters per second) have been studies during locomotion. Most cutaneous and proprioceptive afferent studied have been sensitive monitors of complex combinations of step-cycle components, and their firing patterns would often have been difficult to predict from existing information.

Histological reaction to various conductive and dielectric films chronically implanted in the subdural space

Thirty different test patches of various thin film materials were chronically implanted in the subdural space of cats to determine their suitability as components for proposed neuroprosthetic devices. In particular, materials employed by the microelectronics industry were screened, and reactions were found to be quite dependent on specific formulations or surface preparations of otherwise similar materials. A nonspecific but severe complication of pressure necrosis under thin films that spontaneously roll and curl in vivo was noted.

Action currents, internodal potentials, and extracellular records of myelinated mammalian nerve fibers derived from node potentials

The potential distribution within the internodal axon of mammalian nerve fibers is derived by applying known node potential waveforms to the ends of an equivalent circuit model of the internode. The complete spatial/temporal profile of action potentials synthesized from the internodal profiles is used to compute the node current waveforn, and the extracellular action potential around fibers captured within a tubular electrode. For amphibia, the results agreed with empirical values. For mammals, the amplitude of the node currents plotted against conduction velocity was fitted by a straight line. The extracellular potential waveform depended on the location of the nodes within the tube. For tubes of length from 2 to 8 internodes, extracellular wave amplitude (mammals) was about one-third of the product of peak node current and tube resistance (center to ends). The extracellular potentials developed by longitudinal and radial currents in an anisotropic medium (fiber bundle) are compared.