If you walk into a surprise party, your sympathetic nervous system (SNS) (one of two "parts" of the autonomic nervous; the second part is the parasympathetic nervous system (PSNS)) will "activate." Activating the SNS will cause multiple effects: increased heart rate, increased blood pressure, vasodilation, increased breath rate (respiration), increased perspiration, "shut down" the urinary system and GI system, blunt sexual arousal, pupil dilation, increase blood sugar to give the body energy to respond, activate the adrenal glands, and more. This makes sense because the sympathetic nervous system is commonly known as "fight or flight" response...the body and brain are automatically preparing you to respond to a threat.
Since the body and brain don't know the difference physiologically between surprise and fear, certain structures of the brain are involved: amygdala, hypothalamus (controls the pituitary gland), thalamus, brain stem are the main players, in addition to much of the limbic system, which some of these structures are a part of. Because the body is surprised, the brain responds with a surge of hormones and catecholamines, which are basically preparing the body for "fight or flight" (again, the SNS is activated). A couple of these endocrine responses include, more specifically, epinephrine (aka adrenaline), norepinephrine (aka noradrenaline), and cortisol, which have downstream effects on the brain and body, such as preparing the muscles to engage for "fight or flight", or the other effects described above.
All of this happens SUPER fast. Think about it critically and from an evolutionary perspective: we need a quick acting and fast responding nervous system to activate the adrenal glands, and at basically the same time, the endocrine system prepares and mobilizes the body for action - like running away or fighting. These are necessary body and brain responses to survive real danger and threats.
The overall pathway is sensing by the peripheral nervous system, which sends signals to the central nervous system via Afferent pathways. The signals received by the central nervous system are then sent Efferently as information to the glands and muscles to tell the body what to do - like run.
Once the brain and body register that the surprise is not actually a threat, the complement of the SNS kicks in: the parasympathetic system then does basically exactly the opposite to bring your body back to homeostasis: breathing slows, heart rate slows, perspiration decreases, there's vasoconstrictionetc...the opposite of everything I listed above.
Breakdown:
brain = central nervous system + peripheral nervous system
autonomic nervous system = part of the peripheral nervous system
autonomic nervous system = 2 branches: 1. sympathetic nervous system and 2. parasympathetic nervous system
Hope this helps!
