An evaluation of length and force feedback to soleus muscles of decerebrate cats.
TL;DR: This work has attempted to determine whether or not this regulation of force by signals from tendon organs is significant in decerebrate cats.
Abstract: IT IS NOW WELL KNOWN that contraction of a muscle is reflexly excited by responses of its spindle receptors to stretch (32, 34) and is reflexly inhibited by responses of its Golgi tendon organs to contraction (10, 31). Many experimental techniques have been used to confirm these observations (7, 12, 23). Nevertheless, the actual importance of each of these reflexes in the gradation of contraction remains obscure because of the lack of an experimental approach which is capable of estimating quantitatively their respective influences (39). Formerly it was believed by many that tendon organs responded and inhibited contraction on1 .y when muscular forces became excessive. Recent studies (19, 26) have cast doubt on this hypothesis by demonstrating for these receptors a much lower threshold to must ular contract ion than was previ not ouslv t .h assure ought. This find .ing alone does a con tinuo us regulation of muscular force by signals from tendon organs since impulses must be transmitted through one or two interneurons before they may inhibit homonymous motoneurons (7, 31). Studies have shown that these Ib pathways transmit impulses more effectively in spinal than in decerebrate cats (8). It is therefore likely that the gain of this reflex pathway is not constant but, rather, is subject to control by signals from various regions of the nervous system. For example, Ib pathways are facilitated by signals transmitted from the red nucleus (IS). As a result, the relative importance of tendon organs in the regulation of contraction would depend on the particular state of the experimental animal. We have attempted to determine whether or not this regulation of force is significant in decerebrate cats.
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TL;DR: The sections in this article are: Methodological Considerations, General Summary and Epilogue, Ascending Pathways that Monitor Segmental Interneuronal Activity, and Evidence That Ascending FRA Pathways Monitor Activity in interneurons of Reflex Pathways.
Abstract: The sections in this article are:
1
Methodological Considerations
1.1
Selective Stimulation of Primary Afferents
1.2
Stimulation of Central Motor Systems
1.3
Methods for Investigation of Convergence at Interneuronal Level
2
Spinal Neuronal Circuits Used in Common by Segmental Afferents and Supraspinal Motor Centers
2.1
Recurrent Inhibition
2.2
Pathways From Ia-Afferents and Their Control by γ-Motoneurons
2.3
Reflex Pathways From Group Ib Tendon Organ Afferents
2.4
Reflex Pathways From Cutaneous and Joint Afferents and From Groups II and III Muscle Afferents
2.5
Propriospinal Neurons
2.6
Presynaptic Inhibition of Transmission From Primary Afferents
3
Reticulospinal Inhibition of Segmental Reflex Transmission
3.1
Dorsal Reticulospinal System
3.2
Ventral Reticulospinal Pathways
3.3
Monoaminergic Reticulospinal Pathways
3.4
Decerebrate Preparation
4
Direct Projections of Descending Pathways to α-Motoneurons
5
Ascending Pathways that Monitor Segmental Interneuronal Activity
5.1
Evidence That Ascending FRA Pathways Monitor Activity in Interneurons of Reflex Pathways
5.2
Information Via Ascending Collaterals of Interneurons
5.3
Ventral Flexor Reflex Tracts
5.4
Ventral Spinocerebellar Tract
6
General Summary and Epilogue
6.1
General Summary
6.2
Epilogue
662 citations
TL;DR: It is confirmed that most subjects can suppress triggered reactions when the instruction calls for no intervention, leaving an unmodified reflex response, which implies the existence of and compensation for nonlinear muscle mechanical properties.
Abstract: 1. The stretch reflex in the elbow flexor musculature was studied in 23 human subjects. The subjects were required to establish an initial force equivalent to 10% maximum at a prescribed initial length; mechanical disturbances delivered at random times increased load force to 15% or reduced it to 5%. We measured arm force, displacement, and EMG (usually biceps); acceleration was calculated from displacement, and average responses from sets of 10 like trials. 2. Modification of the stretch reflex was studied by comparing average responses obtained with different instructions, but with the same disturbance. The usual introductions were "compensate for arm deflection" and "do not intervene voluntarily". The initial response did not depend on instruction; changes in response that depended on instruction began abruptly after a latent period which ranged from 70 to 320 ms (measured from force and acceleration), depending on conditions and subject. The latency became longer (10-50 ms) and more variable when the subject did not know the direction of disturbance in advance. This and other observations indicate that modifications of the stretch reflex are not produced by servo actions. They are produced by triggered reactions, which occur at both short and long latencies and which have properties resembling the movements produced in a reaction-time task. 3. We confirmed that most subjects can suppress triggered reactions when the instruction calls for no intervention, leaving an unmodified reflex response. This response consists of a compliant deflection of the arm in the direction of the disturbance. 4. The compensatory actions associated with unmodified stretch (and unloading) reflexes were assessed from EMG responses of biceps. During a 300-ms transient phase, EMG changes were notably asymmetric when responses to symmetric disturbances were compared. Increased force stretched biceps and produced a prominent increase in EMG, whereas decreased force allowed biceps to shorten and produced either an EMG decrease of smaller magnitude or an actual increase. These asymmetric reflex actions produced quite symmetric mechanical responses (arm displacements and forces), which implies the existence of and compensation for nonlinear muscle mechanical properties. This result is discussed in relation to the hypothesis that the function of the stretch reflex is to compensate for variations in muscle properties, thus maintaining stiffness. 5. Effective control of muscle length or joint position does not result from servo action by the stretch reflex. Errors in position are corrected only when triggered reactions are superimposed on the reflex response.
644 citations
TL;DR: The theory applies to movements across different distances, with different inertial loads, toward targets of different widths over a wide range of experimentally manipulated velocities and reconciles many apparent conflicts in the motor control literature.
Abstract: A theory is presented to explain how accurate, single-joint movements are controlled. The theory applies to movements across different distances, with different inertial loads, toward targets of different widths over a wide range of experimentally manipulated velocities. The theory is based on three propositions. (1) Movements are planned according to “strategies” of which there are at least two: a speed-insensitive (SI) and a speed-sensitive (SS) one. (2) These strategies can be equated with sets of rules for performing diverse movement tasks. The choice between SI and SS depends on whether movement speed and/or movement time (and hence appropriate muscle forces) must be constrained to meet task requirements. (3) The electromyogram can be interpreted as a low-pass filtered version of the controlling signal to the motoneuron pools. This controlling signal can be modelled as a rectangular excitation pulse in which modulation occurs in either pulse amplitude or pulse width. Movements to different distances and with loads are controlled by the SI strategy, which modulates pulse width. Movements in which speed must be explicitly regulated are controlled by the SS strategy, which modulates pulse amplitude. The distinction between the two movement strategies reconciles many apparent conflicts in the motor control literature.
531 citations
526 citations
TL;DR: This article reviews recent evidence indicating that the dominant opinion that stretch and unloading reflexes function to control the length of a muscle in opposition to changes in mechanical load is wrong and develops an alternative idea that neither muscle length nor force are regulated as individual vari ables, but that a property called stiffness is maintained relatively constant by skeletomotor reflexes.
Abstract: Since the early 1950s when Merton (44) first introduced his "follow-up servo hypothesis" of movement control, the dominant opinion has held that stretch and unloading reflexes function to control the length of a muscle in opposition to changes in mechanical load, thus providing load compensa tion. In this article I review recent evidence indicating that this notion is wrong, or at least incomplete, and proceed to develop an alternative idea (47) that neither muscle length nor force are regulated as individual vari ables, but that a property called stiffness (the ratio of force change to length change) is maintained relatively constant by skeletomotor reflexes. This leads to a discussion of the actions of descending motor commands on the segmental system, from which I conclude that these may be much less versatile than was formerly believed. The reader is referred to another review (55) and to Matthews' mono graph (42) for additional information.
433 citations
References
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TL;DR: The present paper is concerned with the central part of the motoneuron and the significance of its size in synaptic transmission and asks whether the cell bodies (and dendrites) connected with large and small motor fibers have different functional properties which can be recognized by their discharge characteristics.
Abstract: SINCE THE BEGINNINGS OF NEUROHISTOLOGY it has been recognized that neurons within the central nervous system vary widely in size, but the functional significance of this basic observation has never emerged from the realm of speculation. The largest cells have surface areas which are at least 100, perhaps 1,000, times greater than those of the smallest cells. Correspondingly, the diameters of axons in the central and peripheral portions of the nervous system range from less than .25 p. to more than 20 c-c. This broad spectrum of physical dimensions invites a search for functional correlates. This is one of a series of studies on the problem of size as it relates to spinal motoneurons. The preceding papers (21, 25) were concerned chiefly with the peripheral part of the motoneuron and the muscle fibers it innervates. They provided experimental evidence that the diameter of a motor nerve fiber is related to the number of muscle fibers it supplies. This finding seemed to make good sense: if a motor fiber innervates many muscle fibers and forms a large motor unit, it must have sufficient axonal substance to give off a large number of terminals. The present paper is concerned with the central part of the motoneuron and the significance of its size in synaptic transmission. It asks whether the cell bodies (and dendrites) connected with large and small motor fibers have different functional properties which can be recognized by their discharge characteristics. In order to investigate this problem one must be able to distinguish the signals of a large motoneuron from those of a small one. This may be done by recording their action potentials from thin filaments of lumbar ventral roots. As Gasser (8) demonstrated, the amplitudes of nerve impulses recorded externally from peripheral nerves are directly related to the diameters of their fibers. If it may be assumed that the diameters of axons are also related to the sizes of their cell bodies, as scattered histo-
2,232 citations
TL;DR: It was concluded tentatively that the size or surface area of a motoneuron determines its excitability and hence its responsiveness to stretch-evoked impulses and if this conclusion is correct, it may infer that size is a prime determinant of excitability throughout the nervous system.
Abstract: IN A RECENT PAPER (7) data were presented which indicate that during a gradually increasing stretch of an extensor muscle in a decerebrate cat the smallest alpha motoneurons of the muscle are the first to be discharged and that larger cells are recruited in order of increasing size. Some of the possible explanations for this relationship between size and order of recruitment were discussed briefly. It was concluded tentatively that the size or surface area of a motoneuron determines its excitability and hence its responsiveness to stretch-evoked impulses. If this conclusion is correct, we may infer that size is a prime determinant of excitability throughout the nervous system; if not, the size principle may apply only to motoneurons discharging in response to stretch. Fortunately, it is possible to test the proposed explanation experimentally. If size dictates excitability, the order in which a group of cells are recruited should be the same regardless of the source of the excitation and the particular circuits involved in mediating it. On the other hand, if the organization of the recruitment in input from , the stretch stretch receptors is responsible for the order of reflex, the order shoul .d change wi th d .ifferent types of excitatory input. Experiments were therefore designed in which the relative excitabilities of motoneurons were first determined by means of stretch reflexes and then retested by * means of crossed-extension reflexes, flexor reflexes, monosynaptic reflexes, and electrical “driving” from the muscle ner ve. The relative susceptibilities of neurons of different sizes to several varieties of inhibition were also investigated to ascertain whether inhibitibility bears some regular relation to cell size.
1,104 citations
512 citations
TL;DR: By supplying pulses to different subdivisions of the ventral nerve roots in rotation, it was possible to obtain smooth contractions of cat soleus with low rates of stimulation.
Abstract: 1. By supplying pulses to different subdivisions of the ventral nerve roots in rotation, it was possible to obtain smooth contractions of cat soleus with low rates of stimulation.
2. After contracting isometrically the muscle was subjected to constant velocity lengthening or shortening movements.
3. During shortening the tension always fell below the isometric value. The fall in tension was usually greatest when low rates of stimulation were used.
4. The effect of lengthening on tension depended on the rate of stimulation. At high rates of stimulation the tension during lengthening always rose above the isometric tension. At lower rates of stimulation (5-15 pulses/sec) the tension rose at the beginning of an extension, but decreased later in the movement to a level that was often less than the isometric tension corresponding to that muscle length. At these stimulus rates the tension during isometric contraction was usually higher than during a sustained movement in either direction.
5. At low rates of stimulation longitudinal vibratory movements of more than 0·1 mm also reduced the tension far below the isometric value, whereas the reduction was quite slight when the rate of stimulation was high.
6. The isometric tension during smooth contractions at low stimulus rates was remarkable in the following respects: it developed rather slowly, it was higher than the tension during or immediately after movements, and it was only slowly regained after movement had ceased.
7. The results are discussed in relation to the sliding filament theory of muscle contraction, which, with certain assumptions, provides an explanation for many of the findings.
464 citations
TL;DR: J. R. Wilkie striated muscle An analysis of the mechanical components in frog's This information is current as of March 19, 2008 and has been published is the official journal of The Journal of Physiology Online.
Abstract: J. Physiol. B. R. Jewell and D. R. Wilkie striated muscle An analysis of the mechanical components in frog's This information is current as of March 19, 2008 JournalsRights@oxon.blackwellpublishing.com reproduced without the permission of Blackwell Publishing: months after publication. No part of this article may be articles are free 12 The Journal of Physiology Online . http://jp.physoc.org/subscriptions/ go to: Physiology Online The Journal of continuously since 1878. To subscribe to The Physiological Society. It has been published is the official journal of The Journal of Physiology Online
458 citations