Collateral pyramidal influences on the corticorubrospinal system.

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Abstract
Repetitive antidromic activation of the cat''s pyramidal tract (PT) inhibited ipsilateral corticorubral (CR) cells. Antidromic pyramidal intensities and IPSP [inhibitory postsynaptic potential] amplitudes were linearly related. Activation of the pyramid also elicited EPSPs [excitatory postsynaptic potential] and IPSPs in identified rubrospinal (RN) cells of the ipsilateral red nucleus. IPSPs were preceded by presynaptic ''f-potentials, pnd EPSPs by "s-potentials, shown by field-track analysis to be generated by fibers running through the medullary and peduncular pyramids to terminations in or near the red nucleus. Conduction velocities of f and "s"-fibers were 50 m/sec. and 15 m/sec. Relative latencies of the potentials showed EPSPs to be generated monosynaptically and IPSPs di- or polysynaptically. The pyramids were confirmed as the tract of origin by persistence of pre- and postsynaptic potentials after brain transections histologically confirmed to exclude all but ipsilateral pyramidal axon collaterals branching between planes A-0 and A-10. PT-cells inhibit, and sometimes excite, nearby CR-cells through recurrent axon collaterals in motorsensory cortex. Furthermore, large, phasic, PT-cells inhibit; and small, tonic PT-cells excite RN-cells through midbrain axon collaterals. Cortical recurrent interactions may sharpen cortical input-output functions and may influence the balance of cortical local input from muscle and other senses. The subcortical collateral interactions may influence the balance between cerebral and cerebellar control of the spinal final common path.

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