MUSCLE SPINDLES AND THEIR MOTOR CONTROL

Topographic studies relating distribution of Ia- and gamma-fibres in spinal cord and position of muscle spindles in cat tibialis anterior muscle

The segmental distribution of 115 Ia- and 115 paired gamma-fibres of the tibialis anterior muscle was studied in anaesthetized cats. All Ia-fibres recorded were found in the lumbar segments L6 and L7, from caudal L6 to middle L7. The paired gamma-axons were also mainly found in these parts of the spinal cord, only 7 gamma-fibres were localized in caudal L7. A total of 70% of all fibres was found in L7. Of the fibres constituting 'muscle spindle units' of the tibialis anterior 92.2% enter the same segment (a 'muscle spindle unit' is here defined as a muscle spindle with its Ia-fibres and one gamma-fibre innervating it). More than that, 88% of the afferent and efferent fibres of muscle spindle units were found in the same part of the segment. For the first time, the position of the muscle spindles was related to the location of their Ia- and gamma-fibres in the spinal cord. In general, the muscle spindles located in the proximal muscle region project to the more cranial part of the spinal cord and the muscle spindles localized distally in the muscle project to the more caudal part of the spinal cord. The topographic pattern of the muscle spindle units is discussed with respect to the topographically arranged monosynaptic reflex loop.

Fastigial unit activity during voluntary movement in primates

To examine the proposition that fastigial n. of cerebellum provides a fast feedback pathway to suprasegmental structures for spinal information regarding movement. Macaca irus were trained to make flexion and/or extension voluntary wrist movements. Fastigial neuron activity was then correlated with force, velocity, handle position and with shoulder or forearm muscle activity. From 200 units (75% participating), our data establish that fastigial neurons are uniformly recruited after movement onset, processing force-velocity information. Units were found specifically correlated with force or velocity alone. Fastigial activity is strategically placed to provide a 'correction' signal path for motor performance.

Form and distribution of sensory terminals in cat hindlimb muscle spindles

The sensory innervation of cat hindlimb muscle spindles was studied by reconstruction, electron microscopy, and examination of teased, silver preparations to ascertain the form of the terminals and their distribution to bag1(b1), bag2(b2), and chain (c) muscle fibres. Reconstructions were made of two primary endings, one secondary ending, and the branching of four primary and six secondary axons. For the silver analysis spindles were teased from 14 different hindlimb muscles, the largest samples being from tenuissimus, peroneus brevis, p. longus, p. tertius, superficial lumbrical, extensor digitorum longus, and soleus. Among 310 spindles examined, 40 lacked ab1fibre. These were all portions of tandem spindles in which theb2fibre was continuous from one capsule, where it was accompanied byb1andcfibres, to another, in which it was accompanied bycfibres only. These have been designated ‘b2cspindle units’ as distinct from ‘b1b2cspindle units’.

Is the functional stretch response servo controlled or preprogrammed?

Recent findings in humans and primates suggest that the long latency electromyographic (EMG) responses, evoked by opposing sudden limb displacements, might be mediated via a transcortical pathway. However, it is not clear whether these "late" responses are servo controlled or preprogrammed. We have addressed these questions in two interrelated experimental series. Firstly, the late EMG responses (termed the FSR) evoked in the ankle muscles of 8-12 normal human subjects were mapped as a function of displacement amplitude and velocity. Secondly, the FSR evoked by sustained ramp displacements (lasting 500 msec) were compared with those elicited by transient pulse displacements (lasting 60 msec) having entirely different amplitude and velocity profiles. The findings demonstrated a general lack of a systematic relationship between the characteristics of the initial component of the FSR and displacement parameters, although exceptions were noted in individual subjects. Furthermore, no marked difference was found in either the latency or the rise time of the FSRs evoked by ramp and pulse displacements. The similarity of the FSRs in spite of widely different input patterns is not consistent with the behaviour of a servo response. However, such a finding is in agreement with the concept of a preprogrammed pattern of activity which, once triggered, has to run its full course without the possibility of modification. It is therefore concluded that, at least the initial part of the FSR is generated by the triggered release of a preformulated pattern of intended movement relatively independent of the specific pattern of limb displacement, rather than acting in the manner of a servo mechanism.

Effect of fusimotor stimulation on ia discharge during shortening of cat soleus muscle at different speeds

1. In barbiturate-anaesthetized cats, the L7 and S1 dorsal and ventral roots were dissected to isolate functionally single afferents identified as primary endings of soleus muscle spindles, and motor filaments which exerted a fusimotor action on the afferents with limited action on extrafusal muscle. Up to seven filaments, with an action on a given primary ending, could be isolated and each was classified as exerting either a predominantly dynamic or static action.2. Combined stimulation of these filaments, at rates up to 200 impulses/s could maintain afferent firing during muscle shortenings at speeds up to 200 mm/s.3. Fusimotor stimulation could also maintain afferent firing at a target frequency of 100 impulses/s during muscle shortenings up to 200 mm/s. The timing, in relation to the onset of shortening, and the rates of fusimotor stimulation were found to be critical in achieving the target frequency.4. Sinusoidal modulation of the frequency of fusimotor stimulation was used to study the conditions required to achieve constant afferent firing in the face of imposed sinusoidal length changes.5. For given depths of modulation, the phase advance of fusimotor stimulation needed to produce minimum modulation of afferent firing (best compensation) increased with increasing frequency of the sinusoids. The compensation deteriorated with an increase in the frequency of the sinusoids and a change in the mean muscle lengths, although in some cases it could be restored by adjustments to the depth of modulation of fusimotor rate. This suggests that for movements of varying speeds and amplitudes, settings which are appropriate for shortening at a given velocity and mean muscle length, do not apply if either of these two variables are altered.6. These findings demonstrate that the fusimotor system is potentially capable of eliciting constant afferent firing as envisaged in the ;servo-assistance' hypothesis (Matthews, 1964, 1972; Stein, 1974). This, and the fact that constant afferent firing is not seen during normal unobstructed shortenings at velocities greater than 0.2 resting length/s (Prochazka, 1981), are used to argue that it is by choice rather than necessity that ;servo-assistance' (as defined above) is not employed during normal movements. However, servo-assistance of a different form (involving modulated spindle afferent feed-back from both agonists and antagonists) remains a viable alternative.

Conditioning and recovery of functions lost by brain damage

The muscle spindle is a receptor apparatus in striated muscle that is necessary for voluntary muscle movement. Paralysis victims lose the feedback of this receptor and cannot initiate motor responses. Four hemiplegic patients were trained to use auditory feedback from electromyographic recordings to initiate activity in paretic limbs. With the addition of visual feedback and the use of the auditory signal as a reinforcing stimulus, the patients' muscle activity was then shaped to approximate normal movement.

Autonomic innervation of receptors and muscle fibres in cat skeletal muscle

Cat hindlimb muscles, deprived of their somatic innervation, have been examined with fluorescence and electron microscopy and in teased, silver preparations; normal diaphragm muscles have been examined with electron microscopy only. An autonomic innervation was found to be supplied to both intra- and extrafusal muscle fibres. It is not present in all muscle spindles and is not supplied at all to tendon organs. Fluorescence microscopy revealed a noradrenergic innervation distributed to extrafusal muscle fibres and some spindles. On the basis of the vesicle content of varicosities the extrafusal innervation was identified as noradrenergic (32 axons traced), and the spindle innervation as involving noradrenergic, cholinergic and non-adrenergic axons (14 traced). Some of the noradrenergic axons that innervate spindles and extrafusal muscle fibres are branches of axons that also innervate blood vessels. We cannot say whether there are any noradrenergic axons that are exclusively distributed to intra- or extrafusal muscle fibres. The varicosities themselves may be in neuroeffective association with striated muscle fibres only, or with both striated fibres and the smooth muscle cells in the walls of blood vessels. The functional implications of this direct autonomic innervation of muscle spindles and skeletal muscle fibres are discussed and past work on the subject is evaluated.

Wundt's views on sensations of innervation: a reevaluation

It is sometimes stated that Wundt believed in the primacy of 'sensations of innervation' in the control of eye and limb position, to the exclusion of afferent feedback. Wundt's own statements on the subject are traced through the six editions of the Grundzüge: he believed that sensations of innervation were a contributory factor along with peripheral feedback; but in the fourth edition he dropped the term and subsumed them under 'central sensations of senses', because increased neurophysiological knowledge made a literal interpretation of the original term impossible. In the fifth edition he developed a more precise model of central processes, described as 'sensory costimulation', and an additional idea of 'reproduced' or conditioned sensations. He mentions clinical evidence on the perception of eye and limb position of weight by paralysed and other subjects. This evidence is discussed in relation to modern theories. It is concluded that Wundt's later theories have some similarities with modern "hybrid' theories of efference-contingent afference.

Evolving views on the internal operation and functional role of the muscle spindle

Images

Ferromagnetic coupling to muscle receptors as a basis for geomagnetic field sensitivity in animals

Over the past decade several investigators have provided convincing evidence that the orientation of pigeons and other birds during homing and migrational activities is significantly affected by Earth-strength (less than or equal to 0.5 G) magnetic fields. The presumed mediator of such effects would be a highly sensitive magnetoreceptor which the birds would normally use to extract navigational information from the geomagnetic field. The recently reported measurement of magnet remanence in honeybees and in homing pigeons has stimulated interest in the possibility that the magnetically sensitive structure may be constructed from permanently magnetic material. Here we report the detection of permanently magnetic material in the neck musculature of pigeons (Columba livia) and migratory white-crowned sparrows (Zonotrichia leucophrys). We propose that a magnetic field detector might involve the coupling of magnetic particles to a sensitive muscle receptor such as a spindle. A detection mechanism of this kind could account for the difficulties encountered in conditioning immobile homing pigeons to magnetic field changes and for the puzzling requirement of movement in other behavioural experiments involving pigeons and magnetic fields.

Rat isolated phrenic nerve-diaphragm preparation for pharmacological study of muscle spindle afferent activity: effect of oxotremorine

1. Muscle spindle afferent discharges exhibiting an approximately linear length-frequency relation could be recorded from the phrenic nerve in the isolated phrenic nerve-diaphragm preparation of the rat. 2. Muscle spindle afferent discharges could be identified by their characteristic "spindle pause" during muscle contraction and by their response to succinylcholine. 3. Cholinergic influence on spontaneous and stretch-induced afferent discharges was indicated by the augmentation produced by physostigmine and acetylcholine. (+)-Tubocurarine, but not atropine, prevented this augmentation indicating the presence of curariform cholinoceptors in muscle spindles. 4. Acetylcholine did not appear to be involved in the genesis of spindle afferent discharges as incubation with hemicholinium-3 and (+)-tubocurarine failed to affect the rate of spontaneous and stretch-induced spindle discharges. 5. Oxotremorine markedly increased the rate of spontaneous and stretch-induced spindle afferent discharges and this effect was prevented in the presence of hemicholinium-3 and (+)-tubocurarine. 6. These results with oxotremorine are of interest in connection with the observation that muscle spindle afferents and hyperactive in Parkinsonian patients.

Electrophysiological recordings in peripheral nerve surgery: a review

The use of electrophysiological techniques as an adjunct in the surgical repair of peripheral nerve injuries can improve the quality of the repair and thus the degree of functional recovery. These techniques provide objective information needed to assess the degree of injury and can assist the surgeon in making proper decisions regarding treatment of peripheral nerve lesions. The purposes of this paper are to review: (1) the current theories of the functions of peripheral nerves and their and organs, (2) the development of electrophysiological techniques, (3) the principles involved in their use, and (4) the present and possible future applications of these techniques in peripheral nerve surgery and to outline them.

A possible hybrid mechanism for modification of visual direction associated with eye movements - the paralyzed-eye experiment reconsidered

Previous work on the paralyzed-eye experiment concerning visual direction can be interpreted by a hybrid mechanism: outflow control of gamma motor fibers determines whether or not muscle spindles respond to changes in muscle length.

Reflex control of the jaw muscles by stimuli from receptors in the periodontal membrane

In the opinion of a number of investigators, the only function of the mechano-receptors in the periodontal membrane is to inhibit closure of the jaws via a reflex pathway when a predetermined pressure on the teeth has been exceeded. In our opinion, the receptor system in the periodontal membrane may also play a role in a complex regulatory system which controls the movements of the lower jaw during chewing. In order to test this hypothesis a study was made of the changes occurring in the contraction pattern of three masticatory muscles of the cat, when the cuspids in the upper jaw were displaced in various directions. In all the experimental animals used, electromyographic changes in the three muscles investigated, could be recorded as a result of displacement of the cuspids. The results obtained suggest, at least in the case of the temporal and pterygoid muscles, that there are specific reflexogenic zones in the periodontal membrane, These results support the previously suggested hypothesis that a number of the pressure-sensitive receptors in the periodontal membrane play a role in a regulatory system which controls the movements of the lower jaw--as during chewing--by exerting a reflex influence on the degree of contraction of the masticatory muscles. In the discussion this hypothesis is worked out in more detail and the extent to which the results fit the chewing pattern of the cat is examined. In the conclusion, the author asks whether the finding of a regulatory system in the cat may also be applied to man.

Exteroceptive suppression and motor control of the masseter and temporalis muscles in normal man

Single electrical stimuli to the gums and mucosa inside the mouth elicit two successive exteroceptive suppressions (ES1 and ES2) in the voluntary electromyogram of the masseter and temporalis muscles in normal man. The same afferent axons appear to be involved in the two effects, as indicated by the intensity function, the electrical excitability, the afferent conduction velocity and the lack of differential effect of Xylocaine infiltration of the inferior alveolar nerve. Two similar phases of inhibition involve the monosynaptic masseter reflex and the synchronized electromyographical spikes induced by jaw vibration. Exteroceptive suppression is thought to be mediated, in the brain stem, by both an oligosynaptic (ES1) and a multisynaptic (ES2) mechanism.