Comment: a schema for the interactions between motor programs and sensory input.

In this overview we utilize and extend a model, originally developed for "command interneuron" control of the generation of motor programs, to discuss the roles sensory inputs play in movement control. To provide a conceptual framework, we present a modular schematic of the motor control and sensory processing apparatus of an hypothetical nervous system. In the schematic, "subroutines" (basic units of motor programs) are seen as "playing out" through switching and sequencing networks to "driver neurons." The "driver neurons" then activate motoneurons to execute the programmed movements. Five modes of interaction between motor programs and sensory input are considered using examples from invertebrate and vertebrate neuronal circuitry. These modes of interaction occur at the following locations: (1) the "program selector," to initiate a motor program; (2) the "motor subroutine directory," advancing the program to the next subroutine; (3) the "driver neurons" and motoneurons, where the "gain" of subroutine instructions can be modulated; (4) the "motor programmer," which monitors programs in progress and provides for program development and updating; and (5) the "driver neurons" themselves, which control sensory processing by "selecting" the appropriate sensory inputs for the program in progress. Mode 5 is illustrated in more detail through a consideration of the modification of stretch receptor input by "extensor" and "flexor" command interneurones in the circuitry controlling postural movements of the crayfish abdomen.