The chronic spinalized cat: a model for neuromuscular plasticity.
The ability of the hindlimbs to produce weight-supported locomotion on a motorized treadmill was assessed in cats spinalized (T12) at 2 or 12 weeks of age. The animals were assigned at random to nonexercised (NE) or exercised (E) groups, the latter being trained to walk on the treadmill for 30 minutes, five-times weekly. During the four-month recovery period, spontaneous clonus at 8 to 12 Hz and hyperactive cutaneous reflexes developed in all animals. Age of spinalization had an effect on the recovery of locomotion, as kittens in both the 2E and 2NE groups rated significantly higher than the 12E and 12NE cats. Treadmill training was not critical to the recovery process in the 2-week transected kittens, since both 2E and 2NE cats exhibited stepping with adequate weight support of the hindquarters. However, none of the 12NE cats developed weight-supported stepping, while some of the 12E cats did. Electromyographic recordings of ankle muscles from cats demonstrating the best locomotion revealed that progressive recruitment of the soleus and gastrocnemius muscles was normal despite significant changes in the fiber composition of these extensors. Kinematic analyses of these same animals revealed that gait abnormalities, such as the absence of a yield phase during stance and uncoupling of knee and ankle actions, were more common in the 2NE and 12E animals as compared to the 2E animals. It was concluded that although the spinal cord contains the basic pattern-generating circuitry essential for ambulatory locomotion, the development or recovery of this behavior was influenced by the animal's age at the time of cord transection, and to a less obvious extent by subsequent training.