Rigidity Produced by Deafferentation

Posture, locomotion and evoked activity was investigated clinically and electromyographically in cats with intradurally (6 cats) or extradurally (7 cats) deafferented hind legs. Within 3–4 weeks after the deafferentation an overaction of extensor muscles developed characterized by an extension posture of the deafferented extremity when the cats were sitting, standing, walking or jumping. Reciprocal innervation was disturbed during stepping. Only intradurally operated cats showed rigidity as evidenced by increased resistance to passive flexion of the limb and persistent electrical activity in the extensor muscles when investigated in a standardized position (sling). The hind limb of extradurally operated cats hung loosely without resistance to passive flexion and without spontaneous electrical activity. Crossed extensor reflexes were elicited in intradurally deafferented cats only. Damage to the spinal cord by the intradural operation was evidenced by a disturbance in bladder function as well as by the anatomic findings on the operated side: tissue defects, signs of degeneration in the lateral funiculi and a decrease in interneurones. Similarly as in an asphyxiated cord (Gelfan and Tarlow 1959) the rigidity in intradurally operated cats might be due to hypersensitivity of partially denervated motoneurones. A slight reduction of synaptic connections to motoneurones is likely to occur in extradurally deafferented cords as well. This might explain that also in these cats the deafferented hind leg was often in the posture of extension.