Effects of age on the spinal stretch reflex

The spinal stretch reflex consists of a relatively simple neuronal network. The la afferent fiber of the muscle spindle communicates to the alpha moto neuron via a single synapse. This basic pathway has been studied extensivel y over the past century, yet considerable information continues to emerge c oncerning the manner in which this pathway adapts to aging. It is well acce pted that the amplitude of the spinal stretch reflex declines with normal a ging, and it is intuitively agreed that these changes have a detrimental im pact on the motor output of aging individuals. Understanding the changes ob served in the spinal stretch reflex pathway due to aging requires a recogni tion of the changes that can occur in each component of this spinal network . This review will address these changes by following the spinal stretch re flex from initiation to completion. The components that result in the senso ry input to the motoneuron will be covered first, followed by a review of t he physiological changes that can occur to the motoneuron soma that can aff ect the processing of the sensory input. The output of the motoneuron encom passes the remaining components from the motor axon itself, to the neuromus cular junction, and then to the characteristic changes in the muscle. Final ly, the functional effect that these changes have on the reflex as a fundam ental motor behavior will be addressed, especially in terms of its impact o n posture and balance.