Influence of sex hormones on the neuromuscular junction

Motor units in mammalian skeletal muscle are classified based on metabolic and functional properties. The neuromuscular junctions at slow and fast fiber types display remarkable differences in structure and synaptic efficacy. Androgen receptors are present in both motoneurons and muscle fibers, although expression may vary during development. The effects of anabolic-androgenic steroids have been well-characterized in sexually dimorphic muscles. Anabolic-androgenic steroids are clearly necessary for the survival of motoneurons of sexually dimorphic muscles such as the rat bulbocavernosus muscle. Exogenous treatment with anabolic-androgenic steroids can lead to improved synaptic efficacy at neuromuscular junctions by facilitating synaptic vesicle cycling, increasing neurotransmitter synthesis and/or increasing ACh receptor expression. Concurrently, anabolic-androgenic steroids can lead to muscle fiber hypertrophy, which if anything would decrease efficacy of neuromuscular transmission. All of these effects show motor unit type specificity and are enhanced by concomitant resistance training in non-sexually dimorphic muscles.