PROGRESSIVE DECREASE IN HETERONYMOUS MONOSYNAPTIC LA FACILITATION WITH HUMAN AGING

To evaluate functional change in the spinal reflex pathway with ageing , we studied heteronymous Ia facilitation from the quadriceps to soleu s muscle in 30 normal volunteers (aged 24-68 years). The size of the t est H-reflex of the soleus muscle was adjusted to 25% that of the maxi mal M-response. The conditioning stimulus was adjusted to 1.5-fold the motor threshold to stimulate all the Ia fibres in the femoral nerve. Facilitation was quantified as the slope of the very early part of fac ilitation, within 0.8 ms of onset. This procedure enabled us to evalua te the extent of monosynaptic Ia facilitation without contamination by other effects. The extent of facilitation decreased linearly with age . This decrease in facilitation could reflect a decrease in the number of Ia fibres and in their conduction velocities, and an increase in p resynaptic inhibition on Ia terminals. The increase in presynaptic inh ibition may be an adaptive phenomenon in the ageing of the neuromuscul ar system or, alternatively, a deteriorating process with decreasing f lexible supraspinal modulation.