Static sensitivity of tendon organs to tetanic contraction of in-series motor units in feline peroneus tertius muscle

The structure and response properties of Golgi tendon organs in control and hypodynamia–hypokinesia rats

The Golgi tendon organs (GTOs) are encapsulated mechano-receptors that, in normal conditions, monitor via Ib afferent fibers the contractile force. A 14-day period of hypodynamia, absence of weight bearing and hypokinesia, and reduction of motor activity (HH) is known to induce changes in postural muscles such as the soleus. At present, there is no data available regarding the Ib afferent feedback in normal rats (CONT group) and in rats after a hypodynamia-hypokinesia (HH group) period. Consequently, the aim of our study was to determine the HH effects on the morphological (histochemistry on gross morphology) and electrophysiological properties of the GTOs in rat soleus muscle. In the histological study, nine CONT and nineteen HH GTOs of the soleus muscle were identified. The results demonstrated that HH GTOs were morphologically similar to the CONT GTOs. Regarding the electrophysiological study, a L2-L6 laminectomy was performed under deep anesthesia (sodium pentobarbital, 60 mg kg(-1)). Responses in single Ib fibers from the L5 dorsal root to the isometric twitch and tetanic fused contractions of "in-series" motor units (MUs) were recorded. Twenty-three and twenty-eight GTO/MU pairs were studied in the CONT and HH groups, respectively. In the HH group, the Ib afferent response exhibited a decrease in dynamic peak for the high stimulation frequencies and an increase in static sensitivity for all stimulation frequencies. Our results suggest that after an HH period, the GTOs continue to fulfil their mechano-sensory function to signal the contractile force but with a higher static sensitivity.

Two groups of Golgi tendon organs in cat tibial anterior muscle identified from the discharge frequency recorded under a ramp-and-hold stretch

Twenty Golgi tendon organs (GTOs) from the tibial anterior muscle of the cat were investigated under a ramp-and-hold stretch of the passive muscle. The stretch rate was varied between 1 and 100 mm/s, the stretch amplitude between 0.1 and 7 mm, the prestretch of the muscle between 0 and 12 mm. The action potential sequences of the GTOs were recorded, and discharge patterns derived from them. The basic discharge frequencies, namely the initial frequency, the peak dynamic discharge, the maximum static value, and the final static value, were read from each discharge pattern. The tension of the muscle was determined at the same points in time at which one of the basic discharge frequencies was read from a discharge pattern. The static and dynamic properties of the GTOs were determined from the basic discharge frequencies. Two groups of GTOs were identified. Four GTOs discharged with an initial frequency and at the same time had static properties of small magnitude. Sixteen GTOs showed no initial activity and had static properties of large magnitude. The two groups of GTOs did not differ in their dynamic properties. The number of alpha-fibers activating a single GTO was determined from a further 11 GTOs. Eight GTOs without initial activity were activated by a mean number of 9.7 alpha-fibers. Three GTOs discharging with initial activity were activated by a mean number of 15.3 alpha-fibers. The two mean values were significantly different (p=0. 02). The identification of two groups of GTOs is explained by the GTOs being positioned differently within the muscle and its tendon.

Proportions of slow-twitch and fast-twitch extrafusal fibers in receptive fields of tendon organs in chicken leg muscles

Golgi tendon organs are mechanoreceptors that monitor the contractile force produced by motor units. Receptors are most responsive to contractions of extrafusal muscle fibers that terminate closest to them and on them. Three anterior and four posterior chicken leg muscles were examined. Proportions of immunohistochemically identified slow-twitch extrafusal fibers and fast-twitch extrafusal fibers were calculated for 374 tendon organ receptive fields. Tendon organs were observed in muscle regions occupied either by slow-twitch fibers or fast-twitch fibers only, but most were found in regions that contained both slow-twitch and fast-twitch extrafusal fibers. The frequency with which each fiber type occurred near tendon organs approached the frequency with which it occurred in more inclusive regions. In receptive fields with mixed fiber populations, fast-twitch fibers were the predominant type, especially in the anterior leg muscles. Distribution patterns of extrafusal fiber types adjacent to and farther removed from tendon organs suggest that afferent discharges from tendon organs are by and large unbiased measures of the contractile activity of the extrafusal fiber population of the muscle portion in which the tendon organs are located. In mixed muscle regions, slow-twitch fibers and fast-twitch fibers attach on given tendon organs, enabling them to monitor forces produced by slow motor units and by fast motor units. Most tendon organs are situated in mixed extrafusal fiber fields with high fast-twitch fiber content, indicating that in chicken leg muscles sensory feedback from tendon organs is largely one from fast motor units.

The static sensitivity of tendon organs during recovery from nerve injury

An investigation has been carried out into the physiological properties of tendon organs and their interactions with motor units following two types of nerve injury: nerve crush and nerve transection followed by suture repair. Recovery from nerve crush was very successful: 6 weeks after the injury 60% of the tendon organ-motor unit interactions (n = 62) evoked normal or near-normal patterns of afferent discharge but with reduced firing rates. After 10 weeks of recovery 81% of the interactions (n = 43) were normal. The main abnormality observed was a phasic-only pattern of discharge. The overall reductions in firing rate during early recovery may be attributable to the lower contractile forces generated by the reinnervated muscle units, while the phasic-only responses may also represent immaturity of the transduction mechanism of the regenerated afferent axons. Following nerve transection the quality of recovery was much lower and a range of abnormal, as well as normal patterns of response were observed. For all the afferents studied, both types of response were recorded, suggesting that although there may be changes in the sensitivity of the afferents to muscle contraction, the abnormal responses more probably reflect changes in the form of the mechanical input rather than abnormalities of the transduction process.

The dynamic response of Golgi tendon organs to tetanic contraction of in-series motor units

Golgi tendon organs (TOs) display a dynamic response related to the rate of tension development by the motor units (MUs) which activate each TO. When several MUs are activated together the response increases non-linearly with respect to tension. The dynamic response has been re-examined by recording the responses of TOs, in the adult cat, to tetanic, isometric contractions of one or more MUs and has been found to comprise two components: for each MU-TO pair the instantaneous frequency between the first two impulses of the response was linearly related to the rate of rise of the tension, the slope of the relationship being greatest for the slow units. Similarly, when activating MUs were stimulated together, the relationships between frequency and the initial rate of tension rise were linear. When the relationships were expressed in terms of relative tension rate the organs were found to be most strongly activated by the IIb muscle fibres, the responsiveness at the level of the whole muscle being reduced by unloading effects. The later component of the response showed an exponential relationship between discharge frequency and rate of tension development, the slope of the relationship between frequency and the logarithm of tension rate being independent of MU size. Although absolute tension increases during a contraction, this did not appear to impinge on the response which was dominated by the dynamic parameters of the contraction.