Step, swim, and scratch motor patterns in the turtle

The turtle generates a variety of coordinated hindlimb movements, including different forms of locomotion and scratching. The intact turtle produces forward step, forward swim, and backpaddle. Following spinal cord transection, rostral, pocket, and caudal scratches can be evoked by mechanical stimulation of the shell. Comparisons of the kinematics and motor patterns of these six behaviors provide insights regarding neuronal mechanisms underlying their production. All six behaviors were characterized by alternating hip flexion and extension and by an event during which force was exerted against a substrate. The portion of the cycle occupied by hip flexion or extension movement varied across behaviors. Hip extension occupied well over half the cycle period in the forward step and the caudal scratch. The cycle was split into approximately half hip flexion and half hip extension for the forward swim, the backpaddle, and the rostral scratch. Hip flexion occupied over half the cycle in the pocket scratch. The swim and scratch forms had curvilinear, crescent-shaped toe trajectories and a single burst of monoarticular knee extensor activity during each cycle. The forward step had a linear toe trajectory and two bursts of knee extensor activity during each cycle, one during swing and one during stance. Timing of monoarticular knee extensor onset was similar for: the forward swim, the rostral scratch, and the swing phase burst of forward step; the pocket scratch and the stance phase burst of forward step; and the backpaddle and the caudal scratch. Amplitudes of muscle activity varied among the six behaviors; high amplitudes of activity were associated with events during which force was exerted against a substrate. These times of force exertion were: stance phase in the forward step, powerstroke in the forward swim and the backpaddle, and rubs of the limb against the shell in the scratch forms. The six behaviors studied represent a range of parameter values, as evidenced by relative durations of hip flexion to hip extension, knee extensor phasing, and electromyogram (EMG) amplitudes. This range of behaviors could be produced by assembling different combinations of neurons from a common pool, with all six behaviors likely sharing some basic circuitry. The extent of shared circuitry may be greater between behaviors with similar timing, e.g., backpaddle and caudal scratch.