Now study this diagram showing how the body responds to stressful stimuli. Notice that short-term stress follows one signaling pathway that begins in the brain, travels out the spinal cord, and directly to the adrenal glands. Many organs of the body will receive direct stress response messages using this pathway as well. When the adrenal glands receive the signal from this pathway, they release a stress hormone that signals many systems in the body to prepare to either run away from the stressor (such as a predator or an oncoming vehicle, for example), or fight. The part of the nervous system that activates this response to acute stress is called the sympathetic nervous system. After the stressful stimulus has passed, the parasympathetic (nicknamed the “rest and digest” response) calms the nervous system and restores the normal function of the body systems, maintaining homeostasis.

The Sympathetic vs. Parasympathetic Nervous Systems

These two divisions of the nervous system counteract each other to allow the body to receive the resources it needs to respond to a life-threatening situation (more glucose to the brain to enhance thinking ability, more oxygen and sugars to the muscles to run, etc.), and then return to a relaxed state. As you saw in the diagram, the sympathetic response begins when a stressful situation is detected by your sensory nerves, which make up the peripheral nervous system, or PNS. The sensory nerve endings can deliver this message directly to the brain through cranial nerves, which gather the information we need to give us the senses of smell, sight, hearing, and taste. Stress stimuli can also be detected by peripheral nerve endings throughout the body and delivered to the brain via the spinal cord. The brain and the spinal cord make up the central nervous system, or CNS.

View this diagram to review the anatomy of the sympathetic and parasympathetic nervous systems.

Notice that the “rest and digest” (parasympathetic) message is delivered through cranial nerves that originate in the brain stem, and sacral nerves that originate in the sacrum at the end of the spinal cord. The “fight or flight” (sympathetic) response originates from the spinal cord. These nerve impulses are delivered by the CNS to the effector organs responsible for reacting to the situation by either simulating the organ to take action, or calming (inhibiting) the organ to return homeostasis. In this situation, the nervous system sends a signal to the endocrine system to handle acute, or short-term stress.

When the body is faced with long-term, or chronic stress, the endocrine system predominantly responds by releasing hormones such as cortisol. You will learn more about cortisol in the next module.

Read Understanding the Stress Response in Harvard Health Publications, published by the Harvard Medical School.

From your reading address the following:

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