Meeting Abstract
Signals are relayed from an animal’s nervous system to its muscles via neurotransmitters, chemicals that are released from a motoneuron, travel across the neuromuscular junction (NMJ), and interact with receptors in the muscle fibers. NMJs are a critical component of muscular contraction, and thus animal behavior. But, does NMJ size vary in association with the behavioral use of a muscle? Here, we test the hypothesis that species that use a muscle frequently evolved larger NMJs in that muscle. We studied 10 Anolis lizard species and their use of two muscles: the ceratohyoid (CH), which controls the movement of the dewlap, a throat fan used in courtship and aggressive displays; and the retractor penis magnus (RPM), the muscle that controls movement of the hemipenes during copulation. We collected field observational data for each species to determine the frequency of dewlap display and copulation. Then, we dissected the CH and RPM muscles from each species and stained them for acetylcholinesterase, an enzyme that breaks down the neurotransmitter acetylcholine and is only found in the NMJ, to measure the cross-sectional area and major and minor axes of the NMJs. Larger measurements indicate a broader area of connection between the motoneuron and muscle fiber. Our results show high variation in NMJ size in both the CH and RPM muscles among species, and this variation is not associated with lizard body size or the size of the structures these muscles control. The variation in the RPM muscle, however, is positively associated with the frequency of copulation, and NMJs in the CH (a muscle used every few minutes) are much larger than NMJs in the RPM (a muscle used every few days). This study is among the first of its kind to use a comparative approach to analyze NMJ size across species.
