Motor unit and neuromuscular junction remodeling with aging.

The neuromuscular system is one of the largest and most vital organ systems of the body. The function and mass of the neuromuscular system gradually deteriorate during the natural process of aging. The neuromuscular system is comprised of individual motor units, each of which features a single motor neuron and all the muscle fibers it innervates. Motor units also demonstrate age-related remodeling such as reduced number, muscle fiber atrophy, but an increased number of fibers per motor unit. Enabling communication between motor neurons and the muscle fibers they innervate is a specialized synapse known as the neuromuscular junction. Aging, too, elicits remodeling of this synapse joining motor nerve terminal endings with a small ( < 0.1%) area of the muscle fiber's surface called the endplate. Aged neuromuscular junctions exhibit elevations in pre-synaptic nerve terminal branching, and in the post-synaptic distribution of receptor sites for neurotransmitter. This anatomical remodeling is coupled with age-related neurophysiological alterations including increased quantal content, with a more rapid rundown of endplate potential strength during continuous stimulation of the pre-synaptic neuron. Moreover, there is a growing body of evidence indicating that aging impacts the capacity of the NMJ to adapt to increased, as well as decreased physical activity. Because of the marked increase in the number of people considered to be aged in industrialized countries, it is essential to expand our understanding of the influence of aging on the neuromuscular system, its constituent motor units, and the neuromuscular junctions which allow neural cells and muscle fibers to effectively work together.