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Lecture 15 W3005/4005y Darcy B. Kelley Review of muscles and of central pattern generators Review: The basic building block of movement is the motor unit, the motor neuron and all the muscle fibers that it innervates. Study of certain elementary reflexes (the stretch reflex, the crossed flexor/extensor reflex) reveals an intrinsic set of connections between interneurons and motor neurons in the spinal cord. Different kinds of sensory input (stretch via Ia fibers and the spindle system (muscle length), muscle tension via Ib fibers and the Golgi tendon organ) can activate different reflex pathways by accessing specific interneurons with characteristic connectivity patterns (Ia and Ib interneurons). Some inhibitory interneurons are activated by axon collaterals of motor neurons(Renshaw cells). Introduction to pattern generation: The interneurons and motor neurons of the spinal cord generate rhythmic activity that is independent of sensory input; they serve as central pattern generators. In locusts, Wilson observed rhythmic output from the spinal cord to wing muscles in the absence of any sensory input. Sherrington’s study of the scratch reflex demonstrated that stimulus intensity did not affect rhythmic activity of muscle groups. The rhythmic activity generated by CPGs is responsible for rhythmic motor patterns such as walking, flying and even singing and is seen in ventral roots disconnected from the muscles themselves (fictive walking etc). Even though sensory input does not create rhythmic patterns it can modify them. This rhythmic activity is due to cyclical changes in the firing patterns of spinal cord neurons; each limb or segment has a pair of independent pattern generator whose activity is co-ordinated with those of other limbs to produce locomotion. Rhythmic activity can also be set off by application of L-Dopa or by stimulation of supraspinal centers such as the mesencephalic locomotor region How do these circuits generate rhythmic patterns? • Models • Lamprey and tadpole swimming, case studies Supraspinal motor systems The execution of voluntary movement requires the co-ordinated activity of three supraspinal systems: the cerebellum, the basal ganglia and the motor cortices; the first of which is the subject of today's lecture. Roughly speaking, the motor cortices are involved in the planning of movement, the generation of motor programs that result in the execution of complex movements involving multiple muscle groups and multiple joints. The basal ganglia are required to maintain the flow of movement, to supress unwanted movements, to insure the rapid initiation of wanted movements and to maintain them during execution.