Reflex pathways from group II muscle afferents

Summary

A hypothesis is forwarded regarding the role of secondary spindle afferents and the FRA (flexor reflex afferents) in motor control. The hypothesis is based on evidence (cf. Lundberg et al. 1987a, b) summarized in 9 introductory paragraphs. Group II excitation. It is postulated that subsets of excitatory group II interneurones (transmitting disynaptic group II excitation to motoneurones) may be used by the brain to mediate motor commands. It is assumed that the brain selects subsets of interneurones with convergence of secondary afferents from muscles whose activity is required for the movement. During movements depending on coactivation of static γ-motoneurones impulses in secondary afferents may servo-control transmission to α-motoneurones at an interneuronal level. The large group II unitary EPSPs in interneurones are taken to indicate that, given an adequate interneuronal excitability, impulses in single secondary afferents may fire the interneurone and produce EPSPs in motoneurones; interneuronal transmission would then be equivalent to that in a monosynaptic pathway but with impulses from different muscles combining into one line. It is postulated that impulses in the FRA are evoked by the active movements and that the role of the multisensory convergence from the FRA onto the group II interneurones is to provide the high background excitability which allows the secondary spindle afferents to operate as outlined above. The working hypothesis is put forward that a movement governed by the excitatory group II interneurones is initiated by descending activation of these interneurones, but is maintained in a later phase by the combined effect of FRA activity evoked by the movement and by spindle secondaries activated by descending activation of static γ-motoneurones. As in the original “follow up length servo” hypothesis (Rossi 1927; Merton 1953), we assume that a movement at least in a certain phase can be governed from the brain solely or mainly via static γ-motoneurones. However, our hypothesis implies that the excitatory group II reflex connexions have a strength which does not allow transmission to motoneurones at rest and that the increase in the gain of transmission during an active movement is supplied by the movement itself. Group II inhibition. It is suggested that the inhibitory reflex pathways like the excitatory ones have subsets of interneurones with limited group II convergence. When higher centres utilize a subset of excitatory group II interneurones to evoke a given movement, they may mobilize inhibitory subsets to inhibit muscles not required in the movement. Inhibition may be reciprocal of extensors during flexor activation (the spinal pattern), of flexors during extensor activation or of flexors and extensors in more complex movements involving cocontraction of other flexors and extensors. It is postulated that group II inhibition depends on conjoint activation from spindle afferents and other sources (descending and/or the FRA) so that inhibition may be coupled to group II excitation of other motoneurones. Such a coupling would correspond to the “α-γ-linkage in reciprocal Ia inhibition” (Lundberg 1970) and is denoted “α-γ-linkage in lateral group II inhibition”. FRA and other reflex pathways. Results are summarized showing that the FRA evoke convergent excitation in interneurones not only in group II reflex pathways but also in other reflex pathways like the reciprocal Ia inhibitory, the nonreciprocal group I inhibitory and probably also in specialized reflex pathways from cutaneous afferents. It is inferred that facilitation of reflex transmission by impulses in the FRA evoked by the active movement may be a general principle. In this way reflex transmission to α-motoneurones may be weak at rest and not disturb passive movements but have a high gain when the reflexes are required to regulate active movement.