Motor Unit Responses in the Lateral Rectus Muscle of the Cat: Intracellular Current Injection of Abducens Nucleus Neurons

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Abstract
Single motor units of the cat''s lateral rectus muscle were activated with short intracellular current pulses delivered through an intracellular micropipette penetrating a single motoneuron, and the mechanical responses in the muscle were recorded. Lateral rectus unit responses fell within the same range as those obtained earlier with extracellular stimulation of inferior oblique units, confirming that in the latter case single and not multiple motor units were studied. Fast and slow contracting muscle units were connected to fast conducting, large size motoneurons. The organization of motor unit recruitment in eye movements would thus seem different from the recruitment pattern in limb muscles. Intracellular stimulation of abducens nucleus interneurons did not initiate contractions at the ipsilateral lateral rectus muscle. These interneurons did not appear to have excitatory inputs to the abducens motoneurons, but may be involved in relaying information to the other extraocular muscle nuclei or to other brain centers.