Diversification of pectoral control through motor pool extension.

Flexible control of pectoral appendages enables motor behaviors of vastly different strength, speed, and amplitude, as in a human playing the piano or throwing a ball. Such control necessitates a fine-tuned, coordinated activation of motoneurons, which is facilitated by spatially ordered motoneuron pools in mammals. While differently sized neurons are known to contribute to different strengths of pectoral movements, it remains unclear how these pectoral motor pools are organized in less complex pectoral systems as those of teleost fish. We show how pectoral motor control can be extended to increase the speed- and amplitude-range of motor behaviors by investigating anatomical and physiological features of pectoral motoneurons and the motor pools they form in freshwater hatchet fish, well-known for their pectoral aerial escape response. Through the differentiation of one motor pool, the pectoral motor network of hatchet fish acquired additional flexibility to enable specific control of vastly different amplitudes, velocities, and strengths. Similar neuronal organization patterns have been described for controlling fast, intermediate, and slow axial muscles in zebrafish and in tetrapod motor systems controlling pectoral limbs. We show that hatchet fish share organizational principles of their pectoral motor pools with those found in other motor networks in both teleosts and tetrapods. Our data thus suggest that principles of spatial and physiological differentiation of motor pools associated with different pectoral muscles and behaviors might be deeply homologous between actinopterygian and sarcopterygian vertebrates.