The activity of intrafusal muscle fibres during central stimulation in the cat

Summary

Results from a new method of studying fusimotor neuron activity are reviewed. The method involves observation of the movement of intrafusal muscle fibres in muscle spindles isolated from the tenuissimus muscle. The muscle is exteriorised with a pedicle containing its nerve and blood supply and the spinal roots are left intact. Observations were made in cats under barbiturate anaesthesia, during stimulation of the sensorimotor cortex and following decerebration and spinalisation. The results suggested that the central control of γ_d motoneurons influences muscle spindle afferents through dynamic bag₁ fibre activity, and that the central control of γ_s motoneurons influences spindle afferents through static bag₂ and nuclear chain activity. Unexpectedly the evidence further suggested a division of γ_s motoneurons into two groups under separate central control, exerting effects through predominantly static bag₂ fibre contraction or predominantly nuclear chain fibre contraction. The development of the method to give quantitative information on γ motoneuron firing rates by intracellular recording from intrafusal muscle fibres is described. Preliminary accounts are given of experiments on the spread of junctional potentials into the poles of dynamic bag₁ fibres and static bag₂ fibres, and also experiments employing intracellular recording from dynamic bag₁ fibres during γ_s axon stimulation to clarify whether the innervation of these fibres by γ_d motoneurons, and not γ_s motoneurons, is as exclusive as observation of the contraction of dynamic bag₁ fibres suggests.