A team of Harvard researchers found that muscle fibers have retrograde influence on motoneurons.
Athletes generally refer to muscle fibers as "fast twitch" and "slow twitch" depending on muscle contraction speed. From slowest to fastest, the muscles are designated type I, IIA, IIX, IIB. Studies have shown that each motor neuron is homogenous for the muscle fibers it operates. Thus, a slow motoneuron would operate a bundle of slow twitch motor muscle fibers.
Muscle fibers can be converted to a different muscle fiber type by attachment to a alternate motoneuron. For example, a slow twitch muscle fiber can be converted to a fast twitch muscle fiber via attachment to a fast motoneuron. The question then becomes can this mechanism operate in reverse? In other words, can muscle fibers cause the motoneurons to change from fast to slow and vice versa?
This is the question a team of Harvard researchers sought to address (Retrograde influence of muscle fibers on their innervation revealed by a novel marker for slow motor neurons, 2010, Joe V. Chakkalakal, et al.).
The researchers introduced a novel marker of slow motoneurons, SV2A. Using immunofluorescence staining the researchers could mark slow motoneurons. Fast twitch muscle fibers were converted to slow twitch muscle fibers using the transcriptional cofactor PGC-1alpha (discussed in this post on smoking). It was found that an increase in slow twitch muscle fibers led to an increase in slow motoneurons. The possible mechanisms by which an increase in slow-twitch muscle fibers (via PGC-1alpha) increases slow motoneurons synaptic connections is shown below.
The increase in slow motoneurons demonstrates that the mechanism increasing slow synaptic connections is the conversion of motoneurons from fast to slow. Motoneurons ability to undergo conversion suggests that they have some postnatal plasticity. The authors suggest that there may be a combination of prenatal and postnatal determinants of motoneuron type.
The physiology of motoneurons and their relationship with muscle fibers may have interesting ramifications on the way we look at training. Evidently more than muscle fiber type and capillary proliferation are at play during training.