Autonomic Nervous System The autonomic nervous system regulates activities that are involuntary, or not under conscious control. For instance, when you start to run, the autonomic nervous system speeds up your heart rate and blood flow to the skeletal muscles, stimulates the sweat glands, and slows down the contractions of smooth muscles in the digestive system. You may not be aware of any of these activities, but all of them enable you to run faster and farther.

The autonomic nervous system consists of two equally important parts, the sympathetic nervous system and the parasympathetic nervous system. Why two systems? In general, the sympathetic and parasympathetic systems have opposite effects on each organ they influence. In the same way that a driver must be able to turn the steering wheel both left and right to keep a car on the road, the two systems produce a level of fine control that coordinates organs throughout the body.

For example, heart rate is increased by the sympathetic nervous system but decreased by the parasympathetic nervous system. In general, the sympathetic system prepares the body for intense activity. Its stimulation causes an increase in blood pressure, the release of energy-rich sugar into the blood, and shutting down of activities not related to the body's preparation to “fight or flee” in response to stress. In contrast, the parasympathetic system causes what might be called the “rest and digest” response. It lowers heart rate and blood pressure, activates digestion, and activates pathways that store food molecules in the tissues of the body.

31.3 Assessment

Review Key Concepts

1. 1. Review Describe the role of the sensory division.

   2. Explain Give three examples of stimuli that your sensory receptors are responding to right now.

   3. Infer Which type of sensory receptors most likely responds to a change in blood pressure that causes more force to be exerted on your blood vessels? Explain.

2. 1. Review Describe the function of the two parts of the motor division of the peripheral nervous system.

   2. Explain Is a reflex part of the central nervous system, the peripheral nervous system, or both?

   3. Apply Concepts Describe a situation in which you would expect your sympathetic nervous system to be more active than your parasympathetic nervous system.

Apply the Big idea

Structure and Function

3. Which part of the peripheral nervous system is involved in both innate behaviors and learned behaviors? Explain. (Hint: See Lesson 29.1.)

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