Final answer:
When Shayla is startled by a garden snake, the amygdala in her brain is responsible for her fear response, as this region regulates emotions and initiates the fight-or-flight response.
Step-by-step explanation:
When Shayla is startled by a garden snake, a spike of activity is probably occurring in the amygdala in her brain. The amygdala is known for regulating emotions such as fear and aggression. It is part of the limbic system, which includes the hippocampus and parts of the thalamus and hypothalamus, and it plays a key role in processing emotional responses and memory function. In response to fear or stress, the amygdala can also influence the hypothalamus to initiate the fight-or-flight response through the release of stress hormones. Shayla's startled reaction is thus a direct consequence of her amygdala's response to the sudden encounter with the snake.