The tonic neck reflex is an important postural reflex that helps maintain balance and orientation in response to head position [1][2]. When the head is turned to one side, the reflex causes extension of the limbs on the side the head is turned towards, and flexion on the opposite side [3][4]. For example, turning the head left causes extension of the left arm and leg, and flexion of the right arm and leg.

This reflex is mediated by proprioceptive inputs from the neck signaling to vestibular nuclei, which modulate antigravity muscle tone via projections to limb motor neurons [2][5]. The reflex emerges around 1-2 months of age, peaks at 4-5 months, and integrates with volitional movement by 6-7 months [1][6]. It helps infants maintain balance when changing head position, and persists at diminished levels in adults to aid posture [2]. Exaggeration of the reflex can occur with corticospinal tract lesions [4][5]. Overall, the tonic neck reflex demonstrates the importance of head position sensing for motor control and coordination.