The occurrence and ultrastructure of a mechanoreceptor in the human cricopharyngeus muscle.

An encapsulated nerve structure resembling the Golgi tendon organ was found in a human cricopharyngeus muscle near the cricoid cartilage where muscle fibers terminate. The capsule consisted of lamellated flattened cells with a basal lamina. Capsular cells separated the lumen into small compartments which contained myelinated and/or nonmyelinated nerve fibers. Nonmyelinated nerve fibers were also found in the interlamellar spaces of the capsular cells. Some nonmyelinated nerve fibers were dilated and contained abundant mitochondria, being partly surrounded by a Schwann cell sheath and embedded in collagen bundles. These features indicate that the nerve structure is a mechanoreceptor similar to the Golgi tendon organ. Its location and structure indicate that it is placed to detect the tension of the cricopharyngeus muscle.

Activation of Golgi tendon organs by asynchronous contractions of motor units in cat leg muscles.

Discharges from individual tendon organs of peroneus tertius muscle activated by the isometric contractions of single motor units were recorded in anaesthetized cats. Pairs of motor units acting on the same tendon organ were stimulated asynchronously at frequencies eliciting unfused contractions. Tendon organ responses to such contractions did not display a linear relation between discharge frequency and contractile force recorded at the muscle tendon. In several instances, one of the motor units exerted a predominant influence on the response of the tendon organ, even though this unit did not produce the strongest activation of the receptor when stimulated on its own.

[Functional properties of the Golgi tendon organs]. / Propriétés fonctionnelles des organes tendineux de Golgi.

Golgi tendon organs are encapsulated mechanoreceptors present at the myo-tendinous and myo-aponeurotic junctions of mammalian skeletal muscles. Within the tendon organ capsule, the terminal branches of a large diameter afferent fibre, called Ib fibre, are intertwined with collagen bundles in continuity with tendon or aponeurosis at one end. The other end is connected with a fascicle of 5-25 muscle fibres, contributed by several motor units. The contraction of these fibres, exerting strain on the collagenous bundle and causing deformation of sensory terminals, is the adequate stimulus of the tendon organ. For this stimulus, the tendon organ has a very low threshold, so that a single fibre twitch can elicit a discharge from the receptor. A tendon organ can thus signal the contraction of a single one of the 10-15 motor units which contribute fibres to the fascicle connected with the receptor. The number of tendon organs present in a muscle, taken together with the fact that a given motor unit can activate several tendon organs, strongly suggests that the contraction of every motor unit in this muscle is monitored by at least one tendon organ. The exact nature of the information provided by tendon organs to the central nervous system remains an open question because no simple relation could be established between the discharge frequency of a receptor and the contractile forces of its activating motor units. It is known, however, that, due to their dynamic sensitivity, tendon organs are efficient in signaling rapid variations of contractile force. The dynamic parameters of muscle contraction prevail in the information carried by afferent discharges from tendons organs.

Stretch sensitivity of Golgi tendon organs in fatigued gastrocnemius muscle.

Alterations in Golgi tendon organs' (Ib afferent receptors) stretch sensitivity were assessed in fatigued gastrocnemius muscle of cats. Muscle fatigue to 60-50% peak tension was induced in 13 cats by electrical stimulation of L7 and S1 ventral roots. Forty-three group Ib afferent fibers were recorded in L7 or S1 dorsal root filaments before and during fatigue. Fiber activity was assessed by applying to the Achilles tendon a 5-mm ramp stretch at 5 or 25-30 mm X s-1. Group Ib afferent responses to both velocities of stretch were either completely abolished or depressed over several seconds compared to pre-fatigue firing frequencies. When responding, longer latencies (measured at 5 mm X s-1 only) to firing onset occurred during fatigue. Increasing the rate of stretch (250 mm X s-1) to produce a 5-mm stretch approximately equivalent to twitch tension time revealed a depression more selective to static stretch sensitivity. Post-excitation depression of the Ib receptor potential appears to be one possible mechanism. These acute adaptations in Ib afferent discharge to muscle stretch were opposite in direction to those recently reported in fatigued muscle for group Ia and group II muscle spindle afferents. Functional implications of these results are discussed.

Force-sensitive interneurons in the spinal cord of the cat.

The input-output properties of interneurons mediating spinal reflexes were investigated by extracellularly recording the response of interneurons to excitation from muscle receptors in the ankle extensor muscles of decerebrated, spinal cats. A population ofinterneurons in the intermediate region ofthe spinal cord is potently excited by increases in muscle force. Unlike the discharge of Golgi tendon organs, which accurately encodes moment-to-moment variations in the force of a single muscle, the discharge of these interneurons depends in a dynamic and usually nonlinear way on the force in several muscles. Powerful input from unidentified mechanoreceptors in muscle, presumably free nerve endings, is at least partly responsible for these properties. These force-sensitive interneurons are more likely to mediate clasp knife-type inhibition than simple negative force feedback.

[Functional dependence of single muscle receptors on muscle activity]. / Zavisimost' funktsii odinochnykh myschechnykh retseptorov ot myshechnoi deiatel'nosti.

In experiments on chloraloso-urethane anesthetized cats changes in spontaneous and induced spike activity of single muscle spindles and Golgi's receptors following a direct and/or indirect electrical stimulation of the muscle were studied. It was found that contractile activity of the plantar (phase) and heel (tonic) muscles decreased the spike activity of both muscle spindle and Golgi's receptors, the decrease in the phase muscle spindle activity being more considerable than in the tonic one.

[Electromyographic silent period in the masseter and temporal muscles]. / Remarques sur la période silencieuse électromyographique des masseters et des temporaux antérieurs.

For some years, many authors have studied the silent periods (S.P.) in masticatory human muscles, during voluntary clenching, biting or mastication as well as during electrical or mechanical stimulation of various parts of the mouth. This S.P. appears as a flattening in the electromyograph tracing taken from masseter and temporalis anterior muscles simultaneously. After an analysis of the methods and results of previous authors, we give and discuss our result, obtained from records on masseter and temporalis anterior muscles, on right and left sides, on healthy subjects. They tapped first their teeth, then masticated pea-nuts, pieces of apple and soft bread. We obtained 571 S.P. after recording 1 745 cycles, and in contrary to the opinion of most of the authors, we were not able to obtain simultaneous S.P. in all four muscles at every cycle, except on 26 occasions. To conclude, we discuss our observations and suggest pathways for the inhibiting reflex of masticatory muscles during functions.

Analysis of muscle receptor connections by spike-triggered averaging. 1. Spindle primary and tendon organ afferents.

1. The synaptic connections of 44 single identified muscle spindle Ia afferents and of 21 Golgi tendon organ (Ib) afferents from medial gastrocnemius (MG) were studied in 46 cats by the spike-triggered averaging of synaptic noise in 803 motoneurons of various types. 2. The well-known monosynaptic Ia excitatory connections were confirmed and their characteristics examined in 113 cells. The method was used at greater sensitivity than before and revealed that, in addition to the larger EPSPs of the order of 300 muV, there were many below the previously reported lower 17-muV limit. 3. By studying the Ia disynaptic inhibitory pathway with quick stretch-evoked Ia volleys and by spike-triggered averaging (STA), it was shown that the latter method can reveal disynaptic and possibly trisynaptic excitatory connections. This is believed to depend on having continuous activity in the relevant interneurons. 4. Latencies of individual connections showed broad distributions and arguments are advanced for setting working limits to mono- and disynaptic paths for Ia excitation and inhibition. Monosynaptic EPSP latency from cord entry was 0.4-1.1 ms and disynaptic inhibition was 1.2-2.4 ms. It was recognized that the boundaries are not rigid and monosynaptic Ia EPSPs may have latencies up to 1.5 ms. 5. Rise times of disynaptic PSPs were, on average, significantly longer than monosynaptic, but individual disynaptic responses could have values within the monosynaptic range. 6. A small diphasic wave shortly preceding the monosynaptic EPSPs was interpreted as a presynaptic spike. Its timing was consistent with this and, as such, permitted estimates to be made of central conduction time. 7. An early negative wave (latency less than or equal 1.1 ms) of small emplitude was sometimes detected in antagonist motoneurons when triggering from Ia afferents. It was found tha extracellular fields could be detected due to single Ia afferent excitations and efforts were made to see if the early negative wave could be explained by this. In a few cases there was evidence that a very short-latency IPSP might be occuring. This evidence and its implications are discussed with attention to the new factors which have to be considered in using the spike-triggered averaging method at very high sensitivity. 8. Ib effect were di- or trisynaptic. They were excitatory to 18% of synergists and to 28% of antagonists. They were inhibitory to 41% of synergists and to 19% of antagonists. The Ib IPSPs were larger than the EPSPs.

Mechanical transduction in the Golgi tendon organ: a hypothesis.

Morphological evidence, gained from light and electron microscopy, has shown that the unmyelinated terminal branches of the Ib afferent fiber innervating the Golgi tendon organ (GTO) lie within the spaces between braids of collagen. Based on empirical data it is proposed that force applied to a muscle's tendon will straighten these collagen braids and cause compressional deformation of the axon branches trapped between them. The mechanical events, which are presumed to occur within the GTO, appear to explain how it may function as a biological force transducer under static loading conditions. The mechanical principal described for the GTO may be a primitive and wide-spread biological mechanism employed by certain types of sensory receptors that function as position (and force) detectors.

[Discharge frequency of tendon organs evoked by the contraction of motor units in the cat]. / Etude de la fréquence de décharge des organes tendineux provoquée par la contraction d'unités motrices chez le Chat

Isolated contraction of several motor units can excite a single tendon organ. Although the rate of receptor discharge may be different for each motor unit, it is not related to the tension. During repetitive stimulation of a motor unit at increasing frequency the receptor discharge reaches a maximum at rates below fusion frequency for that motor unit.