Three forms of the scratch reflex in the spinal turtle: Central generation of motor patterns

1. A turtle with a complete transection of the spinal cord, termed a spinal turtle, exhibits three types or 'forms' of the scratch reflex: the rostral scratch, pocket scratch, and caudal scratch. Each scratch form is elicited by tactile stimulation of a site on the body surface innervated by afferents entering the spinal cord caudal to the transection. 2. We recorded electromyographic (EMG) potentials from the hindlimb during each of the three forms of the scratch in the spinal turtle. Common to all scratch forms is the rhythmic alternation of the activity of the hip protractor muscle (VP-HP) and hip retractor muscle (HR-KF). Each form of the scratch displays a characteristic timing of the activity of the knee extensor muscle (FT-KE) with respect to the cycle of activity of the hip muscles VP-HP and HR-KF. In a rostral scratch, activation of FT-KE occurs during the latter portion of VP-HP activation. In a pocket scratch, activation of FT-KE occurs during HR-KF activation. In a caudal scratch, activation of FT-KE occurs after the cessation of HR-KF activation. The timing characteristics of these muscle activity patterns correspond to the timing characteristics of changes in the angles of the knee joint and the hip joint obtained with movement analyses. 3. We recorded electroneurographic (ENG) potentials from peripheral nerves of the hindlimb during each of the three forms of the 'fictive' scratch in the spinal turtle immobilized with neuromuscular blockade. Common to all forms of the fictive scratch is the rhythmic alternation of the activity of hip protractor motor neurons (VP-HP) and hip retractor motor neurons (HR-KF). Each form displays a characteristic timing of the activity of knee extensor motor neurons (FT-KE) with respect to the cycle of VP-HP and HR-KF motor neuron activity. The timing characteristics of these motor neuron activity patterns are similar to the timing characteristics of the muscle activity patterns obtained in the preparation with movement. 4. The motor pattern for each scratch form is generated centrally within the spinal cord. In the spinal immobilized preparation, neuromuscular blockade prevents both limb movement and phasic sensory input, and complete spinal transection isolates the cord from supraspinal input. Each scratch motor pattern produced in this preparation is an excellent replica of the motor pattern produced in the spinal preparation in which the limbs are free to move. A neural mechanism for motor pattern selection also resides within the turtle spinal cord because, under these conditions, the cord responds to stimulation of a given site with the appropriate scratch motor pattern. 5. The set of spinal neurons that generates each scratch motor pattern is termed the central pattern generator (CPG) for that form of the scratch. Future experiments with recordings from spinal cord interneurons are necessary to discriminate whether 1) the same set of interneurons is utilized by all three scratch CPGs, and synaptic connectivity is modified to produce each distinct motor pattern, 2) some interneurons belong only to one CPG and other interneurons are shared among CPGs, or 3) the CPG for each scratch form comprises a distinct and separate population of interneurons. 6. Stimulation of a site within a transition zone, a narrow region that separates the boundaries of two receptive fields, elicits motor patterns characteristic of either a single form of scratch or a blend of both scratch forms. A blend is exhibited as either 1) a switch between forms in which one or more cycles of a given form follows smoothly after one or more cycles of the other form, or 2) a hybrid that combines characteristics of the two scratch forms into each of several successive cycles. These blends of motor patterns are revealed by EMG recordings in the spinal preparation with limb movement and ENG recordings in the spinal immobilized preparation. We used limb-movement analyses in the previous paper to describe similar blends. 7. Blends of scratch motor patterns are generated centrally within the turtle spinal cord. The central production of blends indicates that there is communication between the interneurons controlling different forms of the scratch.