Clinical neurophysiology of long ascending and descending pathways

The corticospinal projection to the upper-limb motoneurones probably involves a disynaptic relay through a cervical interneuronal system. Changes in the disynaptic projection following stroke may contribute to functional recovery. A single transcranial stimulus evokes a complex corticospinal volley, and the latency of the MEP depends on the component triggering the motoneurone discharge, probably a D wave with strong magnetic stimuli, but not with weak. The potentiating effects of a voluntary contraction is predominantly on the spinal motoneurone pool, not the cerebral cortex. When mixed nerves in the lower limb are stimulated, the SEPs are normally generated by the muscle afferent component of the mixed volley, which reaches cortex by traversing the posterior spinocerebellar tract, relaying into the lemniscal system. However, with mixed nerves in the upper limbs the SEP is normally of cutaneous afferent origin.