Intracellular autogenetic and synergistic effects of muscular contraction on flexor motoneurones

1. Intracellular records have been taken from cat motoneurones innervating flexor muscles of the hind limb. Contractions of the ankle flexors tibialis anterior and extensor digitorum longus were elicited by stimulation of the peripheral end of the cut L 7 ventral root and the reflex effects of these contractions were recorded in silent and repetitively firing motoneurones.2. Contraction usually produces a hyperpolarizing response inside flexor motoneurones. This hyperpolarization is tension-sensitive in the sense that when, at constant muscle extension, the strength of the contraction is increased, the magnitude of the inhibitory response is augmented.3. Increasing the resting length of the muscles, while using a stimulus of constant strength to the ventral root, causes this inhibitory response to increase in some cells. More often, however, the hyperpolarization caused by contraction is gradually reduced in duration and/or amplitude as the muscles are extended.4. Even with the muscles slackened, so that they develop no tension at their ends, contraction usually produces prominent hyperpolarization of the motoneurones.5. By passing polarizing currents or injecting chloride ions through the intracellular micro-electrode, the hyperpolarizing potentials produced by contraction of the slack and extended muscles are shown to be, at least in part, genuinely post-synaptic inhibitory events.6. When the neurone is fired repetitively by injected current, the ;silent period' in contraction corresponds to the hyperpolarization of the post-synaptic membrane.7. Monosynaptic testing of the flexor motoneurone pool has been used to confirm the essential features of the intracellularly recorded activity.8. Acutely spinalizing the animal increases the magnitude of the inhibitory responses caused by contraction.9. Recordings from dorsal root fibres show that Golgi tendon organs of the ankle flexors are very sensitive to contraction and are indeed often activated by the internal forces developed in a contracting slack muscle.10. A number of muscle spindles of the ankle flexors are activated by stimulation of the ventral root at a strength submaximal or just maximal for the alpha-motor fibres, despite the simultaneous unloading effect of the contracting extrafusal fibres. This spindle activation, which occurs mainly during the phase of tension development in contraction, is favoured by an increased extension of the muscle. Attempts were made to establish whether it is due to alpha-motor innervation of the receptors or to some mechanical interaction between the intra- and extrafusal muscle fibres.11. The possible central and peripheral causes of the changes in motoneurone excitability produced by flexor muscle contraction are discussed. It is suggested that tendon organs of flexor muscles strongly inhibit flexor motoneurones and that alpha-motor innervation of muscle spindles is likely to play a more prominent role in flexors than in extensor muscles.