Final answer:
When startled by a loud noise at night, the sympathetic nervous system is activated, initiating the fight or flight response. This system increases heart rate and blood pressure, preparing the body to face or flee from potential threats. The correct option in the final answer is 1.
Step-by-step explanation:
Hearing a loud and unexpected noise in the night activates the sympathetic nervous system. This part of the autonomic nervous system is responsible for preparing the body for rapid response to perceived threats, a reaction commonly referred to as "fight or flight." When a loud noise startles you in the night, your body springs into action under the influence of the sympathetic nervous system. This system triggers a cascade of physiological responses such as accelerating your heart rate and increasing blood pressure, which are all geared towards preparing you to either confront the danger or run away from it.
The parasympathetic nervous system, by contrast, is responsible for the body's rest and digest functions, helping to calm and restore the body to a state of equilibrium after a stressor has passed. The somatic nervous system involves voluntary control over body movements through skeletal muscles and the processing of sensory information, which is different from the fight or flight response induced by the sympathetic nervous system. The enteric nervous system, another component of the peripheral nervous system, governs the function of the gastrointestinal tract.
Thus, the correct option in the final answer to which system is activated by a loud and unexpected noise in the night is the sympathetic nervous system.