Monosynaptic excitation of motoneurones from secondary endings of muscle spindles.

Abstract

SINCE Lloyd's original work on the myotatic unit more than 30 yr ago1, it has been assumed that the monosynaptic excitation of motoneurones from receptor nerve fibres derives entirely from the Group Ia afferents, which terminate as primary endings in the muscle spindles. Group Ib afferents from tendon organs are thought to exert their inhibitory actions2 over di- or trisynaptic pathways3, while Group II and III fibres, which include those from the secondary endings of muscle spindles, form part of the flexor reflex afferent (FRA) system4, the synaptic actions of which are thought also to be mediated polysynaptically5. Whereas the monosynaptic excitation of motoneurones by Group Ia fibres has been directly confirmed in terms of unitary synaptic potentials evoked by single, stretch excited primary endings6, knowledge of the synaptic actions of fibres innervating secondary endings is known only indirectly from electrical stimulation of Group II fibres and is not without controversy7.