In its simplest form, selection of action direction can be expressed by escape behavior ( 17, 18), which will lead to survival only if its directionality has been successfully implemented. The ability to move the whole body, or parts thereof, to the left or right is an essential action selection ( 14 ⇓– 16). The circuits that implement such rapid behaviors need to be wired in a way to ensure accuracy, speed, and reliability of the motor action, and a precise control over its directionality. The processing carried out by these circuits, however, may not be adequate for situations where accurate and near instantaneous selection of action direction is required, such as during escape from a predator ( 10 ⇓ ⇓– 13). Different circuits in the brain, especially the basal ganglia, control the selection and initiation of purposive actions, or sequences of actions, in a goal-directed manner ( 1, 7 ⇓– 9). Therefore, an optimized system for selection of action direction can have an immediate impact on survival when facing a hazardous situation, such as a predator attack. Movements need to be precisely directed for an action to achieve its intended goal. Vertebrates are endowed with a panoply of motor behaviors that are selected and executed as circumstances demand, allowing animals to thrive in a competitive environment ( 1 ⇓ ⇓ ⇓ ⇓– 6). Thus, in vertebrates, local spinal circuits can implement directionality of urgent motor actions vital for survival. Unilateral ablation of cholinergic V2a interneurons eliminated escape command propagation. The information transfer within this circuit relies on fast and reliable axo-axonic synaptic connections, bypassing soma and dendrites. These interneurons amplify brainstem-initiated escape commands and rapidly deliver the excitation via a feedforward circuit to all fast motor neurons and commissural interneurons to direct the escape maneuver. A central component of this circuit is a unique class of segmentally repeating cholinergic V2a interneurons expressing the transcription factor Chx10.
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Here we reveal a specialized spinal circuit for the execution of escape direction in adult zebrafish. However, urgent tasks, such as defensive escape, require an immediate implementation of the directionality of escape trajectory, necessitating local circuits.
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In vertebrates, action selection often involves higher cognition entailing an evaluative process.