Paul Stein

Tactile inputs excite neural networks in the spinal cord that produce patterns of motor output responsible for specific behaviors (Stein, 2005, 2008, 2010, 2018). We study the physiology of spinal networks that generate the scratch reflex. A turtle with a complete transection of the spinal cord exhibits a scratch reflex in response to tactile stimulation of a specific site on the body surface. During a scratch, a limb reaches toward and rubs against the stimulated site. We stimulate sites on the turtle shell to elicit three different types or "forms" of scratching. We have characterized the patterns of muscular activation during these three forms. After immobilization of the spinal turtle with a neuromuscular blocking agent, tactile stimulation of a specific site will elicit a pattern of motor neuron activity that is an excellent replica of the sequence of muscle activity recorded in the mobile preparation. This pattern, recorded from peripheral nerves of the immobilized animal, is termed "fictive" since it occurs in the absence of actual movements. We test hypotheses of modular spinal cord motor pattern generation. We study preparations that can produce either a flexor rhythm, an extensor rhythm, or rhythmic alternation between flexor and extensor activation. We use extracellular microsuction electrodes to obtain single-unit axonal recordings of interneurons. These recordings provide support for the concept of modular spinal cord organization (Stein and Daniels-McQueen, 2002, 2004; Stein, 2005, 2008, 2010, 2018; Stein et al., 2016).

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