Some effects of fast and slow motor fibres on muscle spindles of the frog

SUMMARY1. Stretch receptor discharges were recorded in the lizard Tiliqua nigrolutea. Responses were identified as coming from muscle spindles by their 'in parallel' behaviour during a twitch.2. Recordings from spindles in the iliofibularis muscle which contains many single innervated fibres revealed spindles which discharged on the rising phase of the muscle contraction. This 'in series' response increased in frequency and duration at high resting tensions on the muscle and was attributed to a specific intrafusal contraction.3. The 'in series' discharge could be induced by stimulation of only one small filament of the muscle nerve. This suggested a focal intrafusal motor innervation.4. Responses from spindles in the semitendinosus muscle, which contains many multiple innervated fibres provided examples of an 'in series' response which could be produced by stimulation of several muscle nerve filaments. This was interpreted as being an example of multiple intrafusal motor innervation.5. Spindles with short or elongated sensory regions were subjected to 'ramp' stretches. The presence of distinct phasic and tonic responses from the two types was confirmed and related to the behaviour of mammalian spindles.

Section: Discussionmentioning

confidence: 99%

An electrophysiological analysis of responses from lizard muscle spindles

Proske¹

1969

“…In frog spindles motor supply is by branches from axons innervating extrafusal motor units (Katz, 1949), these being from either small or large axons innervating tonic or twitch motor units respectively (Gray, 1957;Matthews & Westbury, 1965). There is no clear evidence of polyneuronal innervation of single intrafusal fibres, the frog spindle usually consisting of several intrafusal fibres as in mammals.…”

Section: Frogmentioning

confidence: 99%

Innervation of extrafusal and intrafusal fibres in snake muscle

Cliff¹,

Ridge²

1973

1. Intrafusal fibres of snake receive motor supply from branches of axons innervating extrafusal motor units. By intramuscular stimulation of motor units by single shocks, and critical curarization of the muscle, we have identified at least some of the motor units contributing motor supply to individual intrafusal fibres. Intrafusal fibre activation was observed by visual examination of the contracting intrafusal fibre, and by recording the resulting spindle afferent discharge. 2. The main finding is that in some cases the motor supply to one intrafusal fibre comes from more than one motor unit. The contributing motor units may be either dissimilar twitch units, or twitch and tonic units. Thus some of the intrafusal fibres studied showed polyneuronal motor innervation of heterogeneous origin. 3. In critically curarized muscle, the time course of a spindle afferent discharge, following single‐shock stimulation of a motor unit contributing motor supply to the intrafusal fibre, showed little variation with the type of motor unit being stimulated. 4. The response of each spindle to a standard stretch was recorded. There was no correlation between dynamic index and type of motor unit or units contributing motor supply. However, the method limits the value of negative findings, and this is discussed. 5. The contraction times and tensions of a sample of motor unit isometric twitches are described.

“…The preparation was mounted in a 'Perspex' bath with two compartments (Matthews & Westbury, 1965). The smaller compartment contained Ringer solution (Adrian, 1956) and allowed the muscle to be mounted for the application of movements.…”

Section: Methodsmentioning

confidence: 99%

Adaptation of the discharge of frog muscle spindles following a stretch

Brokensha¹,

Westbury²

1974

Self Cite

SUMMARY1. Stretching a frog muscle spindle evoked a discharge ofaction potentials in its sensory axon. As the rate of this discharge decreased during the adaptation that followed the dynamic phase of a stretch, the variability of the interspike intervals of the impulse train increased.2. Adaptation occurred in two phases. At first the impulse train was almost regular and adapted rapidly, but later this gave way to a phase of slower adaptation where the variability of the discharge was much increased. In the second phase ofadaptation the interspike intervals increased in length less than half as quickly as in the first phase.3. When the rate of adaptation changed from the more rapid to the slower phase there was often an abrupt change in the character of the discharge and the relationship between the mean interspike interval and the variability changed. The interspike interval at which this change-over occurred was relatively constant in records of the discharge from one afferent fibre even though stretches of different amplitude were employed, though it differed from one afferent fibre to another.4. These features of the discharge during adaptation suggest that the two sections of the impulse trains were derived from different spike generators by a process of probabilistic mixing.