Motor systems I: Lower systems

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1_24. Diagram illustrating the position of the Ia inhibitory interneuron (Ia IN) as an integrating node in a number of spinal circuits. Excitatory neurons and synaptic terminals are indicated by open profiles, inhibitory by closed profiles. FRA, flexor reflex afferents; MN, motorneuron; Ia afferent, sensory nerve from muscle spindle.