Video Transcript
The sympathetic nervous system can
be activated by physical stress, for instance, the threat of an attack by a
predator. Which of the following responses
would be initiated by the sympathetic nervous system in this situation? (A) The circular muscles of the
iris contract to constrict the pupil. (B) Bronchoconstriction reduces
ventilation in the lungs. (C) Blood vessels supplying the
skeletal muscles dilate to increase blood flow. (D) The heart rate decreases to
reduce the flow of blood around the body.
To identify the correct answer
choice, let’s review what we know about the sympathetic nervous system and its
functions in the body. The sympathetic nervous system is
one subdivision of the autonomic nervous system. You may recall that the autonomic
nervous system is the part of the peripheral nervous system that controls all of the
involuntary, subconscious responses in the body. The autonomic nervous system is
further subdivided into two systems based on the level of activity needed for a
particular environmental context.
The two subdivisions of the
autonomic nervous system are the sympathetic and parasympathetic nervous
systems. The parasympathetic nervous system
takes over when the body is resting, relaxing, or feeding. The goal of the parasympathetic
nervous system is to decrease activity of the body. The responses of the
parasympathetic nervous system are commonly called the rest-and-digest
responses. The rest-and-digest responses cause
numerous changes in the body, including decreased heart rate, constriction of the
pupils, bronchoconstriction in the lungs, and decreased blood glucose levels.
The sympathetic nervous system
takes over in times of stress when the body is preparing for action. The goal of the sympathetic nervous
system is to increase activity of the body. The responses of the sympathetic
nervous system are commonly called the fight-or-flight responses and are triggered,
for instance, by the threat of an attack by a predator. Some examples of the
fight-or-flight responses include an increased heart rate, dilation of the pupils,
bronchodilation in the lungs to increase ventilation, and increased blood flow to
the muscles via vessel dilation.
Now that we’ve recalled the
functions of the parasympathetic and sympathetic nervous systems, we can eliminate
the incorrect answer choices. Any answer choice that decreases
activity of the body would be carried out by the parasympathetic nervous system. So we can eliminate those choices
as incorrect. Any answer choice that increases
activity of the body would be carried out by the sympathetic nervous system and
would be the correct answer choice.
Now that we have this information,
we can answer the question. In times of stress, the responses
initiated by the sympathetic nervous system would be that the blood vessels in the
skeletal muscles dilate to increase blood flow.
