Structural and functional asymmetries of myelinated branches in the frog muscle spindle

Ito, Fumio; Kanamori, Norio; Kuroda, Hideyo

doi:10.1113/jphysiol.1974.sp010662

Cited by 34 publications

(11 citation statements)

References 10 publications

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“…However, since there is some evidence for separate pathways via the different intrafusal muscle fibers, it follows that not a multiple but a single Ia encoder probably exists. This conclusion encouraged us to propose the following model of encoder action in mammalian Ia afferent fibers, which resembles one put forward earlier for frog muscle spindles (Ito et al 1974). First of all, one has to take into consideration that the entire tree of terminal branches is almost synchronously depolarized and unexcitable during an action potential.…”

Section: Discussionmentioning

confidence: 98%

“…On the one hand, the encoding of propagated impulses in frog muscle spindles has been assumed to be located only at the first node of Ranvier of the afferent stem axon (Ottoson 1976). On the other hand, a variable location of encoding has been discussed (Ito et al 1974) as well as the simultaneous presence of more than one distinct encoder located in the branches of the afferent terminal (Brokensha and Westbury 1974). In cat muscle spindle afferents evidence for initiation or propagation of all-or-none impulses not only in the stem fiber but also in the myelinated branches of the afferent terminals was recently provided (Quick et al 1980;Quick 1986).…”

Section: Introductionmentioning

confidence: 99%

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A study of the encoder properties of muscle-spindle primary afferent fibers by a random noise disturbance of the steady stretch response

1990

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Muscle spindle stretch responses (cat gastrocnemius muscles) were studied when both steady stretch and small near-threshold random stretch determined the Ia impulse sequence. Statistical properties of the inter-impulse-intervals gave some insight into the Ia encoder mechanism. Superimposed random stretch of mean velocities sigma vel below 5 mm s-1 did not change the mean discharge rate, but the width of the Ia interspike interval distribution was clearly increased. Raising the stretch velocity further (sigma vel greater than 5 mm s-1) led to an additional increase in the distribution width, finally reaching values of 0.6 for the coefficient of variation. The shapes of the impulse interval histograms changed from symmetrical to positively skewed ones. The 1st order serial correlation coefficient of the interval sequence shifted to slightly more negative values with increasing sigma vel; on the average, the r1,2-value varied between zero and -0.2. The data were discussed in relation to current ideas on the mechanism of impulse initiation in the Ia terminal ending. They provide evidence that a combination of multiple encoder sites located in the myelinated terminal branches and a separate pathway for large static and small-amplitude dynamic stretch is not very likely. A model is proposed as to how the whole tree of myelinated axons functions as a single encoder site.

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Section: Discussionmentioning

confidence: 98%

Section: Introductionmentioning

confidence: 99%

A study of the encoder properties of muscle-spindle primary afferent fibers by a random noise disturbance of the steady stretch response

1990

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“…The potentials recorded by the Vaseline gap were due to longitudinal current flow in the immediate extra-axonal space in the Vaseline embankment, as obtained with the paraffin gap method (Ito et al 1974). The resistance across the gap was usually found to range from 1 to 2 QM.…”

Section: Introductionmentioning

confidence: 89%

“…In this typical ramification, the terminal along the non-subdivided branch has been usually found to be the site of impulse initiation (cf. Ito et al 1974). …”

Section: Microscopic Observationsmentioning

confidence: 99%

Frog muscle spindles with unbranched myelinated afferent axons: the response to stretch and the length of the first myelinated segment.

Ito¹,

Komatsu²

1977

The Journal of Physiology

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SUMMARY1. Twenty-five muscle spindles innervated by unbranched myelinated axons in the capsule were isolated from sartorius muscles in young frogs (2.2-9.5 g in weight, 28-47 mm in rostro-caudal length).2. The lengths and the diameters of the first myelinated segments varied from 30 to 170 ,m and from 9 to 20 ,m respectively. There was no relationship between the lengths and the diameters.3. Dynamic and static components were analysed from discharge rates of the muscle spindles during ramp-and-hold stretches of 0-8 mm from different initial lengths. The values of the dynamic components to a certain stretch stimulation increased with shortening in the length of the first myelinated segment. The values of the static components were independent in length.4. The amplitudes of action potentials recorded from the spindle terminal decreased during the dynamic phase of the stretch. The ratio of amplitude decrease at the end of the dynamic phase versus the initial length depended upon the length of the first myelinated segment.5. These results suggest that the discharges during stretch may arise at the first node, though the spontaneous discharges may be generated at the terminal.

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“…The experiments of Ito (1970), Kuroda & Ito (1972) and of Ito, Kanamori & Kuroda (1974) have shown that several possible sites of impulse generation exist within the afferent terminals of the muscle spindle. A long-lasting, slowly adapting discharge was found which seemed to be initiated in the distal branches of the afferent axon, whereas a rapid discharge which adapted quickly could be evoked which appeared to be initiated at a more proximal site in the axon, perhaps at the initial point of branching at the spindle capsule.…”

Section: Discussionmentioning

confidence: 99%

Modification by previous afferent discharge of the adaptation of frog muscle spindles following an extension.

Brokensha¹,

Westbury²

1978

The Journal of Physiology

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SUMMARY1. Extension of a frog muscle spindle evoked a discharge of impulses in its sensory axon. The adaptation of the discharge after the dynamic phase of stretching occurred in two phases. At first the impulse train was almost regular and adapted quickly, but later this gave place to a phase of slower adaptation in which the variability of discharge was much increased.2. The discharge of action potentials by the muscle spindle depressed the response of the receptor to a subsequent extension. This was true whether they were elicited antidromically by afferent stimulation or orthodromically by longitudinal vibration. This depression had its most marked effect on the first phase of adaptation where it greatly increased the rate of adaptation. The second, slower, phase of adaptation was little influenced.3. The depression increased with the duration and with the frequency of afferent stimulation. It persisted for about 300 msec. 4. The results show that the properties of the spike generating mechanisms in the muscle spindle are modified by previous discharge and that this modification may influence the overall properties of the receptor.5. The fact that afferent stimulation has different effects on the two phases of adaptation is consistent with the suggestion that the impulse train evoked by extension is derived from more than one spike generator within the muscle spindle.

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Structural and functional asymmetries of myelinated branches in the frog muscle spindle

Cited by 34 publications

References 10 publications

A study of the encoder properties of muscle-spindle primary afferent fibers by a random noise disturbance of the steady stretch response

A study of the encoder properties of muscle-spindle primary afferent fibers by a random noise disturbance of the steady stretch response

Frog muscle spindles with unbranched myelinated afferent axons: the response to stretch and the length of the first myelinated segment.

Modification by previous afferent discharge of the adaptation of frog muscle spindles following an extension.

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