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The function and significance of muscle spindles and golgi tendon organs in the neural control of skeletal muscle. The authors discuss their anatomical arrangement, sensitivity to muscle length changes and force development, and the role they play in feedback control. The document also touches upon recent research on the effects of vibration on muscle spindles and the importance of studying groups of synergistic muscles.
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JOHN N. HOWELL, Ph.D. Ohio University College of Osteopathic Medicine Athens, Ohio MARC D. BINDER, Ph.D. University of Washington School of Medicine Seattle, Washington
Emory University Atlanta, Georgia GERALD E. LOEB, M.D. National Institutes of Health Bethe&, Maryland
The purpose ofthis report is to share with the readershia of JAOA. hiahliahts of a
search Conference of the AOA in March
National Institutes of Health, who also delivered the keynote address, Dr. Man: D. Binder of the University of Wash-
Emory University. Dr. John N. Howell of the Ohio University College of Os- teopathic Medicine, program chairman
ator for the symposium. In introducing the symposium, Dr. Howell pointed out that the traditional importance of somatic dysfunction and its treatment in osteopathic practice has led to a natural interest within osteopathic medicine in the aroblem of control of skel- etal muscle fun'ction. In 1974, Korr pub- lished an important article entitled, "Roprioceptors and somatic dysfunc-
ings of osteopathic practitioners to what was then knowh about the physiology of the proprioceptive organs within skeletal muscle. Specifically, he postulated that
ceptor system might contribute to the etiology of somatic dyefunction and to its treatment through osteopathic manip- ulative treatment. Korr's hypothesis has been widely accepted within osteopathic medicine because it provided a mecha- nism that appeared to be consistent with
However, as Korr pointed out in a recent article,2 his h*thesis still awaits test- ing in the laboratory. Since the publica- tion of Ko~r'shypothesis, additional
function of muscle proprioceptors. The goal of the symposium was to summarize this newer information so that it might be available for integration into thinking about the pathophysiology and treatment of somatic dysfunction. Dr. Binder led off wi* a summary of the traditional ideas about how muscle epindles and Golgi tendon organs func- tion and then focused on newer informa- tion about t h e tendon organs. On
anatomical grounds, it is clear that the
trafusal muscle fibers and are thus suited to sensing length changes of the muscle. The tendon organs, on the other hand, are located in series with the muscle fibers and are thus suited to detecting force gen- erated by the muscle fibers. Spindles have been known for some time to be sensitive to very small perturbetions in muscle length, producing a reflex activation, or s h o r t e n i n g , of t h e muscle b e i n g stretched. In contrast, tendon organs were thought to respond only to high lev- els of force and to provide protection
by inhibiting the homonymous muscle (i.e., muscle of origin of the reflex). Recent work has shown this idea about tendon organ function to be unlikely.3. Each tendon organ provides a point of insertion for 3 to 25 muscle fibers.6 Ex- perimental stimulation of a single motor unit, one or more fibers of which insert on a given tendon organ, will stimulate that
afferent neurons to the spinal cord. Stim- ulation of a motor unit which hasno fibers inserting on a particular tendon organ mmetimes silencesthat tendon or'gan if it has been previously activated by sus- tained stretch of the muscle. This pre- sumably results from unloading of the tendon organ by virtue of contraction of adjacent muscle fibers. Each tendon organ appears to provide insertion for fi- bers from motor units producing a wide range of forces. Thus, each organ may sample muscle activity over the full range of force development, from the small motor units that are the first to be re- cruited to the large units that are re- cruited only during maximal con- tractions. This means that the tendon
conditions of high force development but
muscle force. Determination of the effect of Golgi ten- don activity on neurons within the spinal cord has been difficult because it has not been possible to selectively stimulate the tendon organs or their Ib afferent fibers without at the same time stimulating the muscle spindles or their Ia sensory fibrs. Recently it has been shown that applica- tion of prolonged, high-frequency, low- amplitude vibration to muscles, which is an effective stimulus to the muscle spin- dles: can cause a transient elevation of the electrical thresholds of the Ia fibers. Under these conditions the Ib fibers can
tral connections studied.8 Available evi- dence indicates that activation of the Ib sensory fibers, like the activation of cuta- neous aiTerents, can cause either excit- atory or inhibitory postsynaptic poten- tials to occur in motoneurons.s The site of integration involving the Ib input as well as inputs from other areas within the cen- tral nervous system appears not to be the motoneurons, but rather interneurons in laminae V and VI. Dr. Nichols traced the development of theories concerning the mechanical as- pects of the spindle-mediated stretch re- flex and went on to point out that most of our knowledge has been based on the ~ t u d yof single muscles. He stressed the need for extending the analysis of stretch reflexes to groups of synergistic muscles acting together upon a joint. A rather ex- treme view of the role of the stretch reflex system was proposed in 1953by Mertonlo who hypothesized that normal activation
gamma activation of the muscle spindles, and the resultant stimulation of Ia af- ferent~would in turn provide the excit- atory input to the alpha motor neurons and cause muscle contraction. This the- ory was referred to as the "follow-up length servo hypothesis." It stressed the feedback control of muscle length and the primacy of gamma activation for muscle contraction. It is now known, however, for many movements, including ballistic types and patterned activities of a re- petitive nature, simultaneous activation of both gamma and alpha motor neurons occurs, generally referred to as coactiva- tion.11 Because the larger alpha motor neurons conduct action potentials faster than the smaller gamma fibers, coactiva- tion implies that extrafusal muscle fibers must be activated before intrafusal fibers. Thus the initial phase of contraction oc- curs independently of any feedback con- tribution from the muscle spindle system. F'urthermore, it was pointed out that the Golgi tendon organs might also play an important role in feedback control, so that not only muscle length, but some
controlled.l The latter idea was extended further by Houk>3 who suggested that the param-
stiffness, definid as the ratio o f force , change^ to^ length change. Nichols and Houkl4 demonstrated that the linearity , of the spring-like behavior of muscles in the decerebrate cat preparation was lost when the proprioceptive feedback loop was i~terrupted,i.e. when the dorsal continued on page 6001117
Sept. 1986lJournalof AOAIvol. 86lno. 9
control, muscles demonstrated highly nonlinear properties which included a
when the active muscle was stretched. The linearization and maintenance of the spring-like properties of muscle and t h e enhancement of stiffness by the stretch receptors occurs in both the exten- mrs and flexore ofthe feline ankle, but not
flexors than in the anti-gravity extensors.
to the musclea; it is a function of the or- ganization of the neural control system.
tive reflex strength, and it has now been &own that monosynaptic reflex strength
ditioning.16 These experimental observa- tions emphasize the degree of control exerted by the descending influences within the central nervous system over
This comes as no surprise to clinicians
J e n h i k maneuver to enhance reflex activity in patients.
that demonstrated interactions between the flexors and extensors at the feline an- kle joint, muscles that function together at a joint and can thus be regarded as a myotatic unit. Although the extensors ex- hibit greater stretch reflex activity than
dingly powerful inhibitory effect on flexor activity. Stretch of the flexors, on the other hand, produces a strong inhibitory effect on the extensors. This may relate to the greater force capability and reflex
cat, making it necessary for these mus-
weaker antagonists to shorten during a tiexion movement. These observations il- lustrate how the action of two muscles actitlg on the same joint is coordinated, allowing them to act together as a unit.
.only between antagoniats about a single
tween mueeles acting on different joints within the same or even different limb,
Alex. The final speaker ofthe symposium was
prioceptor function, reminding us that the stretch receptors are not "wholly owned subsidiaries" of the muscles in which they wide. On the contrary, their signals go many places in the central
fluenced by signals originating in many loci. One experimental approach to these h u e s is to make measurements in intaet, conscious animals, rather than in the re: duced ~reoaratione(i.e.. aneathetized or
dication of the receptor mechanisms themsetm. One im~ortantauestionis whether or not the signals &nt to the
spinal cord from the muscle spindles in behaving animals correlate with length
able.
out in his laboratom at NIH on conscious animals carrying surgically implanted muscle length and force transducers, EMG electrodes, electrodes for recording
nerve for recording nerve activity and for irrigation of the nerve ~ l t hlocal anes- thetic from an external syringe, and elec- trodes for stimulating the saphenous nerve and t h e nerve to t h e ham- strings.1618 The results showed that no simple correlation between Ia Aring and length or velocity exists. Spindles gener- ate signals over a variety of lengths whether the muscles are shortening or
eome time, there must be a fusimotor pro- gram causing contraction of the intra- fueal fibers in order to maintain spindle sensitivity and thus to keep information
gamma innervation to the intrafusal fi- bers, a gamma dynamic system largely associated with one type of nuclear bag fiber and a gamma static system associ- ated with a different type of nuclear bag fiber and with nuclear chain fibers.19 The nuclear bag fibers under gamma dynamic control appear to be responsible for the sensitivity of the primary afferent end- ings to velocity and acceleration, whereas
primarily for the static length sensitivity
Activity in the gamma system, both dy- namic and static, is such that afferent information from the stretch receptors continuesto flow into the central nervous system during patterned activity involv-
ing and lengthening. In order for this to occur, the activation patterns of the gamma fibers must be driven indepen- dently of alpha motor neurons.
from the siindltk duringvoluntary motor behavior tend to be restricted to the range of 50 to 200 pulses per second regardlean of the muscle activity occurring. The- oretical considerations20 suggest that fir- ing rates below this range do not occur because the rate of information transfer from spindleswould become limiting, i.e.,
mation during continuing motor func-
second, the signal to noiee ratio becomes
existence of multiple traneduction zones within the Ia neuron innervating the in- trafusal fiber. Normally the two trans- duction zones are electrically coupled,
the heart, so that the faster zone locks the other zone to its lurce. Under conditions of intense activati&, however, these zones
generates signals independently,produc- ing irregular interspike intervals and a (^) I noisy signal. Thus, the job of the gamma program appears to keep the spindle ad-
sory Ia fibers is kept within the appropri- ate range for optimal signaling. This / appears to occur in the cat during pre-
motion and even paw-shaking, but does not occur in response to single unexpected perturbations.1S
being sensed and regulated by the spindle system. The possibilities provided by the gamma system suggest that different pa- rameters can be regulated depending on the type of activity occurring in the gamma efferent system. In muscles whose length is being changed passively by the action of gravity or of other mus- cles, the gamma motoneurons may act alone to establish the mixture of static (length) and dynamic (velocity) influ- ences on spindle afferent sensitivity that
length changes. In active muscles, sim- ilar fusimotor control may be viewed as part of the servocontrol loops involving active alpha motoneurons that reaulate length, kiffness, or other features i f mo- tor output, depending on the motor task being performed. These variations are represented schematically in Figure 1. h e r a l interesting p i n t s arose during the discussion that followed the Dresenta- tions summarized above. ~ l t d o u ~ hwe tend to focus on the well-defined pro- prioceptive sense organs in muscle, namely the spindles and the tendon organs, free nerve endings are also found in muscles. Much of their function is not
are important in proprioception, andthey mav be ~articularlvim~ortantin the re- sponse & direct tierabutic manipula- tion of muscle, perhaps acting through interneurons that carry oligosynapticre- flexes and/or establish baseline levels of alpha motoneuron polarization. The idea that altered output of the gamma system plays a role in altering the sensitivity of the myotatic reflex has not yet found s u p port experimentally. For instance, the Jendrasaik maneuver, which is employed clinically in order to enhance the stretch reflex response, is not mediated by altera- tions in gamma output.21 It appears to result from changes in the excitability of the motor neurons themeelves in a way that is not related to activity in the gamma loop. However, this observation does not appear to exclude the possibility, suggested by Korr, that gamma loop changes occur in the kind of pathologic dysfunction that is Been clinically. Given
ftom the spindlesduring muscle activity,
question.
1. Kon; lrvin M.: Roprioceptors and somatic dysfunction. JAOA 74638-50, ~ a 75 r
2. Korr, Irvin M.: Somatic dysfunction, 0s- teopathic manipulative treatment, and the nervous system. A few facts, some theories, many questions. JAOA 86:109-14, Feb 86 3. Houk, J., and Henneman, E.: Responses of Golgi tendon organs to active contractions of the soleus muscle of the cat. J Neurophysiol 30:466-a, May 67
organ afferents and their relation to reflexes evoked from muscle spindle afferents. Prog Brain Res 50:29-36, g. Powers, R.K., and Binder, M.D.: Distribution of oligosynaptic group I input to the cat medial gastrocnemius motoneuron pool. J Neu- rophysiol 53:497-517, Feb 85
Research
Neumsci Abstr 10:129,