An electrophysiological analysis of responses from lizard muscle spindles.

1. 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.