Journal of Neurophysiology, Vol 53, Issue 6 1567-1581, Copyright © 1985 by APS
Differential changes in the end-plate architecture of functionally diverse muscles during aging
J. L. Rosenheimer and D. O. Smith
Architectural changes at the neuromuscular junctions of three functionally
diverse muscles, the diaphragm, extensor digitorum longus (EDL), and
soleus, were studied in rats from adulthood (10 mo) through senescence (31
mo). End-plate structure was examined using a silver-gold impregnation of
the axon and its terminal arborization. The mean number of terminal
branches per end plate, including those that appeared to exhibit signs of
sprouting or degeneration, were examined as a function of age. Nerve
terminal branch length, the number of major myelinated axon branches per
end plate, and end-plate area were also examined. To ascertain trends in
the data, smoothing of mean values for time-series analysis was performed.
Both fast-twitch diaphragm and EDL muscles revealed more pronounced
fluctuations in nerve terminal branch number during aging than did the
slow-twitch soleus. Smoothed trends of the mean number of nerve terminal
branches per end plate show that nerve terminal proliferation increased
with age at the diaphragm neuromuscular junction but decreased at the
hindlimb junctions. Age-related changes in average terminal branch number
progressed gradually in all muscles until 25 mo of age, after which we
observed a precipitous increase at 28 mo and subsequent decline at 31 mo.
Trends of change indicate that although sprouting within the diaphragm was
maintained during aging, both EDL and soleus experienced a general decline
in sprouting. Degeneration generally decreased at all nerve-muscle
junctions with aging, although the decline began at a later age in the
diaphragm. Analysis of mean data values indicates that in both the EDL and
soleus, sprouting and degeneration attained minimum values at 25 mo of age.
This was followed by a significant increase at 28 mo and then a decline by
31 mo of age. In general, sprouting fluctuated more severely than
degeneration. We conclude that despite the variability from one age to the
next, age-related trends of change are evident in nerve terminal branch
number. The direction of these changes appears to be influenced by the
degree to which muscle activity is maintained during aging. However,
regardless of functional status, all three muscles experienced dramatic
changes in end-plate morphology after 25 mo of age.
