Influence of chronic spinal cord stimulation upon differentiation of beta muscle fibers in a fast muscle (posterior latissimus dorsi) of the chick embryo.

The aim of this work was to study the influence upon differentiation of muscle fiber types of the multiple innervation induced in a fast muscle by chronic spinal cord stimulation. In previous work, we showed that low-frequency stimulation applied to the spinal cord of the chick embryo caused a distributed innervation of muscle fibers in the posterior latissimus dorsi (PLD). In normal development, some beta fibers differentiate within this muscle, the maximal number being attained by 14 to 15 days of embryonic development. Later, the numbers of beta fibers decreased with age. In spinal cord-stimulated embryos the beta muscle fibers within the PLD were stabilized and did not disappear. After the cessation of spinal cord stimulation, the number of beta fibers within the PLD muscle did not decrease. There are possible explanations of the influence of chronic spinal cord stimulation and muscular activity upon formation and persistence of beta fibers within a fast muscle.