Motor Unit Recruitment and Proprioceptive Feedback Decrease the Common Drive

It has been documented that concurrently active motor units fire under the control of a common drive. That is, the firing rates show high correlation with near-zero time lag. This degree of correlation has been found to vary among muscles and among contractions performed at different force levels in the same muscle. This study provides an explanation indicating that motor units recruited during a contraction cause an increase in the variation (SD) and a decrease in the degree (amplitude) of the correlation of the firing rates. The degree of correlation is lower in muscles having greater spindle density. This effect appears to be mediated by the proprioceptive feedback from the spindles and possibly the Golgi tendon organs. Muscle spindles in particular respond to the mechanical excitation of the nonfused muscle fibers and provide a discordant excitation to the homonymous motoneurons, resulting in a decrease in the correlation of the firing rates of motor units. The implication of this work is that the decreased correlation of the firing rates in some muscles is not necessarily an indication of a decreased common drive from the CNS, but rather an inhibitory influence of the proprioceptive feedback from the peripheral nervous system. This explanation is useful for understanding various manifestations of the common drive reported in the literature.

Download Full-text

Ensemble discharge from Golgi tendon organs of cat peroneus tertius muscle

Journal of Neurophysiology ◽

10.1152/jn.1990.64.3.813 ◽

1990 ◽

Vol 64 (3) ◽

pp. 813-821 ◽

Cited By ~ 22

Author(s):

G. Horcholle-Bossavit ◽

L. Jami ◽

J. Petit ◽

R. Vejsada ◽

D. Zytnicki

Keyword(s):

Motor Unit ◽

Motor Units ◽

Good Correspondence ◽

Golgi Tendon Organs ◽

Afferent Fibers ◽

Slow Type ◽

Tendon Organs ◽

Fast Twitch ◽

Force Profiles ◽

Tendon Organ

1. The responses of individual tendon organs of the cat peroneus tertius muscle to motor-unit contractions were recorded in anesthetized cats during experiments in which all the Ib-afferent fibers from the muscle had been prepared for recording in dorsal root filaments. This was possible because the cat peroneus tertius only contains a relatively small complement of approximately 10 tendon organs. 2. Motor units of different physiological types were tested for their effects on the whole population of tendon organs in the muscle. Effects of unfused tetanic contractions were tested under both isometric and anisometric conditions. Each motor unit activated at least one tendon organ, and each tendon organ was activated by at least one motor unit. Individual slow-type units were found to act on a single or two receptors, whereas a fast-type unit could activate up to six tendon organs. 3. In one experiment, the effects of 8 motor units on 10 tendon organs were examined. One fast-twitch, fatigue resistant (FR)-type unit acted on six tendon organs, of which four were also activated by another FR unit. The contraction of each unit, on its own, elicited a range of individual responses, from weak to strong. The discharge frequencies of individual responses displayed no clear relation with the strength of contraction, nor did they accurately represent the shape of force profiles. But when all the discharges were pooled, a fairly good correspondence appeared between variations of contractile force and variations of averaged discharge frequencies.(ABSTRACT TRUNCATED AT 250 WORDS)

Download Full-text

Site of impulse initiation in tendon organs of cat soleus muscle

Journal of Neurophysiology ◽

10.1152/jn.1985.54.6.1383 ◽

1985 ◽

Vol 54 (6) ◽

pp. 1383-1395 ◽

Cited By ~ 17

Author(s):

J. E. Gregory ◽

D. L. Morgan ◽

U. Proske

Keyword(s):

Motor Unit ◽

Soleus Muscle ◽

Motor Units ◽

Impulse Generator ◽

Golgi Tendon Organs ◽

Cross Adaptation ◽

Tendon Organs ◽

The Right ◽

Tendon Organ ◽

Stimulation Of

A continuing controversy surrounds the question of whether Golgi tendon organs are examples of receptors in which impulses may be generated at more than one site. This paper reports a systematic examination of a number of models incorporating single or multiple impulse generators and of the compatibility of their predictions with experimental observations. Two phenomena, in particular, that must be accounted for are nonlinear summation and cross-adaptation. When two motor units each with a direct effect on the tendon organ are stimulated together, the rate of discharge is greater than either individual rate but is less than their sum. In cross-adaptation a conditioning response elicited by one motor unit contraction produces adaptation of the discharge associated with stimulation of a second motor unit. A model with a central impulse generator can be modified to account for nonlinear summation by postulating a nonlinear transformation in the generator current-to-impulse rate conversion. Experiments measuring summation of responses to stimulation of three inputs produced results that did not support this model. Another variation of the model, which had a nonlinearity in the tension-to-current step and cross-connections (mechanical or neural) between tendon strands stressed by contracting muscle fibers, was able to account for the observations. A second model that provided the right predictions was a multiple impulse generator with cross-connections. Which of the two models best fits the experimental observations can be decided by comparing the calculated summation coefficients and cross-adaptation coefficients. A central impulse generator predicts a negative correlation, the multiple impulse generator a positive correlation. All of the observations were made using tendon organs of cat soleus muscle. Responses were recorded to stimulation of filaments of ventral root. In a comparison between 20 pairs of responses from six tendon organs the correlation between summation and cross-adaptation coefficients was found to be significantly positive. We conclude that the tendon organ model that accurately predicts all of the experimental observations incorporates multiple generators.

Download Full-text

Effects of muscle shortening on the responses of cat tendon organs to unfused contractions

Journal of Neurophysiology ◽

10.1152/jn.1988.59.5.1510 ◽

1988 ◽

Vol 59 (5) ◽

pp. 1510-1523 ◽

Cited By ~ 94

Author(s):

G. Horcholle-Bossavit ◽

L. Jami ◽

J. Petit ◽

R. Vejsada ◽

D. Zytnicki

Keyword(s):

Motor Units ◽

Tension Development ◽

Golgi Tendon Organs ◽

Dynamic Sensitivity ◽

Single Motor Units ◽

Muscle Shortening ◽

Length Changes ◽

Tendon Organs ◽

Tendon Organ ◽

Shortening Contractions

1. The discharges from individual Golgi tendon organs of peroneus tertius and brevis muscles were recorded in anesthetized cats. Responses to unfused isometric contractions of single motor units and combinations of motor units were compared with responses to contractions eliciting muscle shortening (i.e., shortening contractions). 2. In 75% of the examined instances, the effect of muscle shortening during unfused contractions was a slight decrease in tendon organ activation, in keeping with the reduction of contractile tension recorded at the muscle tendon. In other instances there was either no change in tendon organ response or, in less than 10% of instances, a slight increase For two motor units eliciting similar activation of a given tendon organ under isometric conditions, the effect of shortening contraction was not necessarily the same. 3. The reductions observed in tendon organ discharges upon muscle shortening were less than proportional to the reductions of contractile tension and difficult to correlate with the properties of motor units, as determined under isometric conditions. The present observations suggest three main reasons for this lack of relation. 4. The first reason depended on the properties of motor units, in that the relation between length changes and tension changes was not the same for all units. Two motor units developing similar isometric tensions did not necessarily produce the same degree of muscle shortening. Some units produced relatively significant shortening without much loss of tension. 5. Second, the dynamic sensitivity of tendon organs is known to exert a major influence on their responses to isometric unfused contractions, accounting for 1:1 driving of discharge during tension oscillations and high frequency bursts upon abrupt increase of tension. Although less tension was produced and the rate of tension development was slower in shortening contractions, similar manifestations of the dynamic sensitivity of tendon organs were observed. In such cases, the responses of tendon organs were the same whether or not the muscle shortened during contraction. 6. Third, when several motor units were stimulated in combination, the unloading influences of in-parallel units were facilitated by muscle shortening so that unloading effects, which were hardly visible under isometric conditions became evident during shortening contractions.

Download Full-text

Tendon organs of cat medial gastrocnemius: responses to active and passive forces as a function of muscle length

Journal of Neurophysiology ◽

10.1152/jn.1975.38.5.1217 ◽

1975 ◽

Vol 38 (5) ◽

pp. 1217-1231 ◽

Cited By ~ 35

Author(s):

J. A. Stephens ◽

R. M. Reinking ◽

D. G. Stuart

Keyword(s):

Firing Rate ◽

Muscle Length ◽

Motor Units ◽

Medial Gastrocnemius ◽

Passive Force ◽

Force Development ◽

Single Motor ◽

Golgi Tendon Organs ◽

Single Motor Units ◽

Tendon Organs

The responses of 13 Golgi tendon organs to graded force development of 29 motor units in medial gastrocnemius of the cat have been studied in five experiments. Of the 13 tendon organs, 11 were responsive to passive stretch within the physiological range of muscle length and 5 were "spontaneously" active at very short lengths where no passive tension could be recorded. The relationship between passive force and the firing rates of the various afferents ranged from a linear one to a power relation (Y = Axb + c) with b, a widely varying exponent. Results support the general conclusion that although many Ib afferents respond to passive force within the physiological range of muscle stretch, this form of stimulus is not a particularly effective one. The statis responses of Golgi tendon organs to active force development produced by single motor units was studied at different muscle lengths. In all cases the apparent sensitivity (change in firing rate per active force developed) decreased as muscle length approached Lo. The static responses of Golgi tendon organs to force developed by single motor units were also studied during fatiguing contractions. The data suggest a sigmoid relationship between force developed at the tendon and the Ib response. The collective response of all 13 tendon organs to active and passive forces at different muscle lengths was also examined. This analysis offered further support for the viewpoint that active motor unit contractions provide themost significant excitatory input to tendon organs and that changes in passive force during muscle stretch have comparatively little effect on the collective tendon organ response. The interaction between active and passive force inputs to the Golgi tendon organs is discussed in relation to the complicated nature of the relationship between forces measured at the tendon and those acting within the receptor capsule. When these complications were taken into account it was possible to explain the differences in responsiveness of a given tendon organ to active contraction of several motor units and to passive force in terms of a single force-firing rate curve for the receptor. It is concluded that changes in the force of contraction of single motor units result in relatively small changes in Ib afferent firing and that during normal muscle contractions, changes in the number of motor units acting on a single receptor must produce far more significant changes in firing rate than changes in the amount of force developed by any single unit. Changes in dynamic Ib sensitivity to single motor unit contractions are also shown to depend on length and in a similar way to the changes in static Ib sensitivity. During fatiguing contractions, a sigmoid relation was found between the dynamic Ib response and the rate of force development by single motor units.

Download Full-text

Responses of Golgi tendon organs to concurrently active motor units

Brain Research ◽

10.1016/0006-8993(86)90968-6 ◽

1986 ◽

Vol 375 (1) ◽

pp. 157-162 ◽

Cited By ~ 3

Author(s):

E.K. Stauffer ◽

R.A. Auriemma ◽

G.P. Moore

Keyword(s):

Motor Units ◽

Golgi Tendon Organs ◽

Active Motor ◽

Tendon Organs

Download Full-text

Voluntary control of motor units in human antagonist muscles: coactivation and reciprocal activation

Journal of Neurophysiology ◽

10.1152/jn.1987.58.3.525 ◽

1987 ◽

Vol 58 (3) ◽

pp. 525-542 ◽

Cited By ~ 220

Author(s):

C. J. De Luca ◽

B. Mambrito

Keyword(s):

Firing Rate ◽

Motor Units ◽

Interphalangeal Joint ◽

Time Shift ◽

Extensor Pollicis Longus ◽

Firing Rates ◽

Antagonist Muscles ◽

Control Scheme ◽

Common Drive ◽

Flexion And Extension

1. Myoelectric (ME) activity of several motor units was detected simultaneously from the human flexor pollicis longus and extensor pollicis longus muscles, the only two muscles that control the interphalangeal joint of the thumb. The ME signals were detected while the subjects produced isometric force outputs to track three different paradigms: triangular trajectories, random-force trajectories requiring both flexion and extension contractions, and net zero force resulting from stiffening the joint by voluntarily coactivating both muscles. 2. The ME signals were decomposed into their constituent motor-unit action potential trains. The firing rate behavior of the concurrently active motor units was studied using cross-correlation techniques. 3. During isometric contractions, the firing rates of motor units within a muscle were greatly cross-correlated with essentially zero time shift with respect to each other. This observation confirms our previous report of this behavior, which has been called common drive. Common drive was also found among the motor units of the agonist and antagonist muscles during voluntary coactivation to stiffen the interphalangeal joint. This observation suggests two interesting points: 1) that the common drive mechanism has a component of central origin, and 2) that the central nervous system may control the motoneuron pools of an agonist-antagonist muscle pair as if they were one pool when both are performing the same task. 4. During force reversals, the firing rates of motor units reverse in an orderly manner: earlier recruited motor units decrease their firing rate before later recruited motor units. This orderly reversal of firing rates is consistent with the concept of orderly recruitment and derecruitment. 5. A control scheme is suggested to explain the behavior of the motor units in both muscles during force reversal. It consists of centrally mediated reciprocally organized flexion and extension commands along with a common coactivation command to both muscles. This control scheme allows for coactivation and reciprocal activation of an agonist-antagonist set. 6. The agonist-antagonist pair was observed to generate a net force in two control modalities: proportional activation and reciprocal activation. In proportional activation, the agonist-antagonist set is coactivated during either of two states: when uncertainty exists in the required task or when a compensatory force contraction is perceived to be required.(ABSTRACT TRUNCATED AT 400 WORDS)

Download Full-text