Neural control of limb movement: scratch reflex in the turtle
My research focuses upon the neuronal control of limb movement in the turtle. The nervous system must generate precise temporal sequences of muscular activation when an organism produces a coordinated movement. The sets of central nervous system neurons that coordinate these sequences are termed “central pattern generators.” We study the spinal cord pattern generators underlying the control of the scratch reflex in the turtle. The turtle is ideal for the study of the spinal cord since many vertebrae are rigidly attached to the dorsal carapace of the turtle’s shell. A turtle with a complete transection of the spinal cord, a “paraplegic” turtle, can produce goal-directed hindlimb movements. These movements are evoked by gentle tactile stimulation of the site on the shell or skin located posterior to the spinal transection. During this response, called the “scratch reflex,” the hindlimb reaches toward and rubs against the site that has received the tactile stimulus. The turtle uses three different motor strategies or “forms” in directing the hindlimb to rub against a site on the body surface that has received a sensory stimulus. A rostral scratch is used to rub sites in the middle of the body; a pocket scratch is used to rub sites in the pocket region just anterior to the hindlimb; a caudal scratch is used to rub sites near the tail. We have characterized the limb movements and muscle activation patterns during each of these forms of the scratch reflex.
We wish to understand the neuronal mechanisms responsible for the generation of the scratch reflex. We record the electrical activities of individual spinal cord neurons while the spinal cord is responding to a stimulus that elicits a scratch reflex motor output. We achieve the mechanical stability required to maintain the recording electrode within the spinal cord neuron by chemically preventing muscle activity. Spinal cord neurons respond to tactile stimulation of a site on the body surface by generating a motor output even though the turtle’s limb does not move. We monitor spinal cord motor output in this preparation by recording the electrical activities of peripheral nerves. The characteristic motor pattern for each form of the scratch recorded from nerves in the immobile preparation is an excellent replica of the pattern for each scratch form recorded from muscles in the preparation with movement. Microelectrodes placed in single motor neurons reveal that motor neurons are actively excited during some phases of the scratch and actively inhibited during other phases of the scratch. We currently are recording from the interneurons of the spinal cord to reveal directly how the spinal cord produces these behaviors. Our long-term goals are to understand the neuronal mechanisms underlying the spinal cord control of limb’s movement.