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3. In one spindle Procion Yellow was injected into two intrafusal muscle fibres activated by the same static y axon; they were identified as a bag2 and a chain fibre.4. Nine intrafusal muscle fibres innervated by dynamic y axons were identified as seven bag, fibres, one bag2 fibre, and one long chain fibre.5. In one spindle two bag fibres were injected, one activated by a dynamic y axon, the other by a static y axon; the former proved to be a bag1 fibre, the latter a bag, fibre.6. Stimulation of static y axons elicited junctional potentials in seven bag2 fibres and one damaged chain fibre, and action potentials in one bag2 and four chain fibres. In the whole sample of impaled intrafusal muscle fibres (identified and unidentified) activated by static axons, junctional potentials were recorded from twenty-three (62.2 %), and action potentials from fourteen (37v8 %). Stimulation of dynamic y axons always elicited junctional potentials.7. In a number of instances it was possible to examine the ultrastructure of motor endings belonging to the stimulated y axon. The myoneural junctions of trail endings supplied by static y axons to bag2 and chain fibres were both smooth and folded; the deepest and most regular folding occurred on chain fibres. The terminals of P2 plates supplied to bag1 fibres by dynamic y axons had smooth myoneural junctions.

SUMMARY1. The superposition of records of 'instantaneous' frequency of discharge of spindle primary endings after stimulation of single fusimotor fibres leads to the construction of graphs called 'frequencygrams'.2. The superposition is made on the screen of a storage oscilloscope, the time base of which is synchronized with the stimulus. It is essential that the stimulus be delivered at all possible intervals with respect to the impulse of the resting discharge which precedes it.3. Frequencygrams obtained by stimulating some static fusimotor fibres by single stimuli display a response with a fast rising phase, a pause, and a slower decreasing phase.4. Frequencygrams may give some information on the time course of the contraction of intrafusal muscle fibres elicited by stimulating their motor axons.

6. Most of the convergent movements of sarcomeres observed in bag1 fibres occurred in a region of the pole that is ultrastructurally distinct from the region where most of the motor endings were located. The possible relevance of this to the production of contractions in the bag1 fibre is discussed. 7. Convergent movement foci in bag2 fibres produced by the stimulation of static axons occurred largely within the same regions of the pole as the motor endings were located, though, whereas foci were observed in both intra-and extracapsular regions, most of the endings were intracapsular.

SUMMARY1. Muscle spindles with an intact blood supply and uninterrupted connexions with ventral and dorsal spinal roots (Bessou & Pages, 1967 have been prepared in cat's tenuissimus muscles with the aim of cinephotographically recording intrafusal movements induced by the stimulation ofsingle static or dynamic y axons; the time course ofthese movements and the morphological kind of activated intrafusal muscle fibres have been established.2. Displacements of spindle guiding marks in the equatorial region elicited by stimulating single static y axons are 4-20 times greater in amplitude than the ones elicited by stimulating dynamic y axons at the same frequency.3. The dynamic y axons induced a contraction only in nuclear bag fibres which, in addition, never received any static y innervation. The static y axons evoked contractions either in nuclear bag fibres alone, or in nuclear chain fibres alone, or in both types of intrafusal fibres. Two thirds of static y axons supplied nuclear bag fibres. For various reasons, one half only of static y axons innervating nuclear bag fibres could be shown to simultaneously innervate nuclear chain fibres. Consequently, about one third of static y axons supplied both nuclear bag fibres and nuclear chain fibres, but it is highly probable that this latter figure is an underestimate. One third of static y axons produced contraction in nuclear chain fibres only. In this work, the distribution of fusimotor axons has been established in only one muscle spindle of the cluster of muscle spindles that each fusimotor axon is generally innervating.4. Generally speaking, a static y axon elicits contraction of several intrafusal fibres whereas a dynamic y axon innervates only one intrafusal fibre and frequently only one pole of the fibre. P. BESSOU AND B. PAGES 5. One third of static y axons evoked contractions in nuclear chain fibres that seemed to involve the whole pole. The other static y axons and all dynamic y axons produced, in the intrafusal fibres that they supplied, one or several foci of localized contractions.6. The nuclear chain fibres contract and relax faster than nuclear bag fibres. The contractions of nuclear bag fibres supplied by static y axons are stronger and faster than those of nuclear bag fibres innervated by dynamic y axons. Nearly all nuclear bag fibres innervated by static y axons, like the nuclear chain fibres, show transient contractions at each pulse of a stimulation at low frequency (2-20/sec).7. The results are discussed taking into account the available anatomical and physiological data on the muscle spindle. Their consequences with regard to intrafusal working are briefly considered.