Fusimotor reflexes in triceps surae elicited by natural stimulation of muscle afferents from the cat ipsilateral hind limb

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Abstract
Experiments were performed in 41 cats anesthetized with chloralose to investigate whether activity in stretch-sensitive muscle receptors may cause reflex effects in fusimotor neurons. Activity in fusimotor neurons was studied indirectly by recording from primary and secondary muscle spindle afferents of the triceps surae muscle. The mean rate of firing of the afferents as well as either dynamic index (during ramp extension) or modulation (during sinusoidal extension) was determined. This was done under control conditions, with the posterior biceps-semitendinosus muscles relaxed, and under test conditions with the same muscles extended. All together, 71 primary afferents were studied quantitatively. Pure or predominantly dynamic effects were observed in 21, pure or predominantly static effects in 9 and no statistically significant effects in 40 of the units. Among 7 secondary afferents studied, 2 showed weak fusimotor activation; the other 5 were not influenced. Electrical stimulation of the posterior biceps-semitendinosus or medial gastrocnemius nerves at group II strength caused dynamic fusimotor reflexes on a number of occasions. The reflex effects observed were recorded in spinalized preparations. The reflex effects were not accompanied by any detectable emg [electromyogram] activity in triceps, as judged from surface emg recordings. The reflex effects observed were tentatively ascribed to activation of .gamma.-motoneurons, but a contribution from .beta.-motoneurons could not wholly be excluded. The effects observed were apparently caused by activation of muscle spindle secondary endings.