Final answer:
The superior colliculus in the midbrain is responsible for initiating the motor response to turn the head and eyes towards a sudden sound in the periphery.
Step-by-step explanation:
The midbrain nucleus responsible for orienting reactions, such as turning towards a sudden sound in the periphery, is the superior colliculus. This part of the brain integrates sensory information from the visual and auditory systems, as well as somatosensory inputs, to initiate the appropriate motor responses. When you hear a sudden sound, the superior colliculus works with the muscles to turn the head and eyes towards the source of the sound for a better assessment of the environment.