Aged muscles (bottom) fall apart and signal poorly.

Muscle activity is driven by spikes of calcium release. Weisleder et al. (page 639) find that aged muscles, when compared with young muscles, have reduced Ca2+ release following stimulation. The dampened response may be caused by a failure to maintain robust membranous compartments that can respond to an input with a single output.

Previous work from the group showed that Ca2+ sparks can be induced in skeletal muscle by osmotic shock and that the sparks resemble those produced during exercise. The group has now found that, compared with young tissue, aged muscle releases fewer of these Ca2+ sparks. The poor response may be a result of the poor state of the sarcoplasmic reticulum (SR) network in aged fibers. This network stores Ca2+ and releases it in response to incoming nerve signals, but in aged fibers the SR was fragmented. Therefore, a significant portion of the stored Ca2+ was unavailable for release during normal action potential–triggered skeletal muscle contraction.

Degradation of the SR may result from a lack of a synaptophysin protein, mitsugumin-29 (MG29), which the group found was expressed at low levels in aged relative to young muscles. Muscle from young mg29-null animals showed a disruption in membrane structure and a weak response to stimulation that was similar to that seen in aged muscle. Weisleder et al. speculate that the putative membrane fusion function of MG29 may help maintain the structure of the SR.