Analysis of the forelimb crossed extension reflex in thalamic cats during stepping.

Forelimb crossed extension reflexes were examined in 22 thalamic cats. These reflexes were elicited either by backward passive movement or by repetitive electrical stimulation of cutaneous and joint afferent nerves in the contralateral forelimb. Single stimulation of the superficial radial nerve evoked two types of reflex responses--early (ER) and late (LR)--from the triceps brachii muscle on the contralateral side. The latencies were about 7 and 16-25 ms, corresponding to the propriospinal (PSR) and spino-bulbo-spinal (SBS) reflexes of the ipsilateral flexor, respectively. Repetitive stimulation of the superficial radial nerve evoked the LR but not the ER. The crossed extension reflex and LR were abolished by lesions of the dorsolateral funiculus of the cervical cord on the side opposite to the recording. The tonic EMG activity, crossed extension reflex and LR in the extensor on the side of lesions were abolished by lesions of the ventrolateral funiculus of the cervical cord. During forelimb stepping, the amplitudes of both ER and LR fluctuated depending on the phase of the step cycle. The ER appeared during a narrow period in the early phase of the stance, whereas the LR was observed during a wide period from the middle of the swing to the middle of the stance. Both responses were absent from the middle of the stance to the middle of the swing. These observations suggest that forelimb crossed extension reflexes involve both spinal and supraspinal (SBS) loop mechanisms, and that these are utilized during stepping, with the latter mechanism in particular playing an important part in the extension phase of the forelimb forward movement.