The body's biological stress response systems are activated by being exposed to traumatic events or a series of chronic traumatic events, such as child maltreatment. When stress activation occurs, it ends up having behavioral and emotional effects that resemble specific PTSS symptoms. Within one's biological stress response system, different systems collaborate to protect the body from environmental threats and push metabolic resources to take on a "fight or flight" reaction. The traumatic events experienced have stressors connected to them that are handled by sensory systems by the thalamus in the brain. This eventually activates the amygdala, a central aspect of the brain that detects fear and anxiety. Cortisol levels are increased as fear signals are transferred to neurons within the prefrontal cortex, hypothalamus, and hippocampus. Activity also increases in the locus coeruleus and sympathetic nervous system. In addition to this, there are also changes in the heart rate, metabolic rate, blood pressure, and alertness.