A study of stretch and vibration reflexes of the cat by intracellular recording from motoneurones.

1. Intracellular records have been made from alpha motoneurones innervating the triceps surae muscles of the cat. The post-synaptic responses to afferent inputs from this muscle group have been studied during high-frequency vibration and during maintained stretches applied to their tendons of insertion.2. A maintained stretch of the muscles caused a maintained depolarization of the motoneurones. The greater the extension of the muscles, the greater was the depolarization.3. High frequency vibration of the muscles also caused a maintained depolarization of the motoneurone. As the frequency of vibration was increased, so the depolarization increased. The depolarization fluctuated in amplitude at the frequency of vibration.4. The amplitude of the depolarizations caused by maintained stretch and vibration were in general greater in the motoneurones which innervated slow twitch motor units than in those which innervated fast twitch motor units. The former motoneurones were also more sensitive to changes in the amplitude of stretch or to the frequency of vibration than were those innervating fast twitch motor units.5. Those motoneurones responding with relatively large depolarizations to maintained stretch also responded to vibration with large depolarizations. This suggests that the same population of neurones respond to both stretch and vibration.6. When the muscles were stretched and vibrated at the same time, the phasic response to the dynamic phase of stretching was largely occluded, whereas the response to the maintained component of stretch was not occluded. The implications of these results are discussed.