Neural circuits for speed change in swimming fish

Publisher Summary This chapter discusses the morphology and activity of fish myotomal muscles. The swimming movements of fishes are of two basic patterns: (1) steady, continuous movement in which the body is bent periodically into a propulsive wave that passes backward at a velocity that is close to the swimming speed and (2) rapid, transient movement—such as accelerations and fast turns—involving large-amplitude movements. The forward velocity of a swimming fish depends on its tail-beat frequency and on the cycle period of the segmental motor output. This can be determined by the generating circuits of the spinal cord in response to sensory and descending inputs. Steady movements are the product of a single yet highly modifiable motor program in which swimming speed is changed not only by changing the cycle length of regular contraction but also by recruiting the white muscle system for faster and stronger contractions. This new recruitment results from activation of neurons projecting to the spinal cord from brainstem structures. A quite distinct program—using separate pathways, motoneurones, and muscle fibers—is implemented for very rapid, transient movements. These two programs operate independently in many fishes, but in higher teleost, they can be switched to achieve wide ranges of speed.