Researchers here make an observation of this change, but the proximate causes remain to be established. The root causes are presumably the same as for the rest of aging - the accumulation of cellular and molecular damage, and evolved reactions to that damage.
[Researchers] report the first evidence that "set points" in the nervous system are not inalterably determined during development but instead can be reset with age. They observed a change in set point that resulted in significantly diminished motor function in aging fruit flies. "The body has a set point for temperature (98.6 degrees), a set point for salt level in the blood, and other homeostatic (steady-state) set points that are important for maintaining stable functions throughout life. Evidence also points to the existence of set points in the nervous system, but it has never been observed that they change, until now."
[The team] recorded changes in the neuromuscular junction synapses of aging fruit flies. These synapses are spaces where neurons exchange electrical signals to enable motor functions such as walking and smiling. "We observed a change in the synapse, indicating that the homeostatic mechanism had adjusted to maintain a new set point in the older animal." The change was nearly 200 percent, and the researchers predicted that it would leave muscles more vulnerable to exhaustion.
Aside from impairing movement in aging animals, a new functional set point in neuromuscular junctions could put the synapse at risk for developing neurodegeneration - the hallmark of disorders such as Alzheimer's and Parkinson's diseases. It appears that a similar change could lead to effects on learning and memory in old age. An understanding of this phenomenon would be invaluable and could lead to development of novel therapies for those issues, as well.