The autonomic nervous system is the part of the nervous system that controls body functions you do not usually think about, such as heart rate, digestion, breathing rate, pupil size, and sweating. It helps keep internal conditions stable while the outside world changes. This stability is called homeostasis, and it is essential for cells to work properly.
The autonomic nervous system is active every moment, whether you are resting, exercising, eating, or responding to stress.
The autonomic nervous system has two main divisions: the sympathetic division and the parasympathetic division. The sympathetic division prepares the body for action by increasing heart rate, opening airways, and redirecting blood toward muscles. The parasympathetic division supports rest, digestion, energy storage, and recovery by slowing the heart and stimulating digestive organs.
Many organs receive signals from both divisions, and the balance between them adjusts organ activity to match the body's needs.
Key Facts
- The autonomic nervous system controls involuntary effectors: cardiac muscle, smooth muscle, and glands.
- Sympathetic division: fight or flight responses increase alertness, heart rate, breathing airflow, and blood glucose.
- Parasympathetic division: rest and digest responses slow the heart, stimulate digestion, and support energy storage.
- Heart rate change can be estimated by ΔHR = HRafter - HRbefore.
- Cardiac output is calculated by CO = HR × SV, where HR is heart rate and SV is stroke volume.
- Homeostasis depends on negative feedback, where a change triggers responses that reduce the original disturbance.
Vocabulary
- Autonomic nervous system
- The division of the peripheral nervous system that regulates involuntary body functions such as heart rate, digestion, pupil size, and gland secretion.
- Sympathetic division
- The autonomic division that prepares the body for action, stress, or danger by increasing energy availability and alertness.
- Parasympathetic division
- The autonomic division that promotes rest, digestion, recovery, and conservation of energy.
- Homeostasis
- The maintenance of stable internal conditions, such as temperature, blood pressure, and blood glucose, despite external changes.
- Neurotransmitter
- A chemical messenger released by neurons that carries signals across a synapse to another cell.
Common Mistakes to Avoid
- Thinking the sympathetic division is always bad, which is wrong because it is a normal survival system that helps the body respond to exercise, danger, and sudden demands.
- Thinking the parasympathetic division turns the body off, which is wrong because it actively controls digestion, gland secretion, and recovery processes.
- Assuming every organ responds the same way to autonomic signals, which is wrong because different organs have different receptors and can produce different effects from the same neurotransmitter.
- Confusing voluntary movement with autonomic control, which is wrong because skeletal muscle movement is mainly controlled by the somatic nervous system, not the autonomic nervous system.
Practice Questions
- 1 A student's resting heart rate is 70 beats per minute. During a sympathetic response, it rises to 118 beats per minute. What is the change in heart rate?
- 2 A person has a heart rate of 90 beats per minute and a stroke volume of 70 mL per beat. Using CO = HR × SV, calculate the cardiac output in mL per minute.
- 3 A person eats a large meal and then sits quietly. Explain which autonomic division should become more active and describe two effects it would have on the body.