Adaptation of the discharge of frog muscle spindles following a stretch

Abstract

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.

Effects of combining static and dynamic fusimotor stimulation on the response of the muscle spindle primary ending to sinusoidal stretching.

“…An alternative type of explanation, which we find the more attractive, is that the occlusion arises from a switching, by virtue of their relative discharge frequencies, between pace-makers, thus, between the sites, determining the stream of impulses which is finally discharged along the main afferent axon (cf. Crowe & Matthews, 1964a;Lennerstrand, 1968;Brokensha & Westbury, 1974); the particular pacemaker with the highest instantaneous frequency is then the only one whose behaviour is momentarily open to observation in the present type of experiment. But the observed asymmetry can then only be explained in terms of further assumptions on the number, site and properties of the various pace-makers involved, and two alone would appear insufficient.…”

Interaction of impulse activities originating from individual Golgi tendon organs innervated by branches of a single axon.

“…Multiple sites of impulse initiation have been suggested for myelinated terminal branches of an axon innervating a frog spindle (Katz, 1950;Ito & Vernon, 1975) and for those supplying a lizard tendon organ (Proske & Gregory, 1976). Brokensha & Westbury (1974) have suggested probabilistic mixing of impulse discharge originating from individual impulse initiation sites in terminal branches of a frog spindle to account for the two distinct phases of discharge adaptation observed during a maintained stretch.…”

Static and dynamic fusimotor action on the response of IA fibres to low frequency sinusoidal stretching of widely ranging amplitude

“…In addition, stretching of itself might change the mechanical properties of the system by inducing extra contractile activity of certain intrafusal muscle fibres, whether by a stretch evoked depolarization of their membrane (Boyd, 1976) or by a 'stretch activation' of the contractile process at the myofilament level as described for certain insect muscles (Pringle, 1972). Finally, there seems every possibility that the afferent terminal may have more than one pacemaker site for impulse initiation (Crowe & Matthews, 1964a, b;Brokensha & Westbury, 1974), and that FUSIMOTOR ACTION ON SINUSOIDAL RESPONSE 833 these could respond differently at various phases of the stretching cycle, when their competitive interaction could produce a variety of patterns of response and so contribute to the paradox. Thus for the moment the matter must remain open, but some of the main points are developed in more detail below.…”