11.1: Introduction to the Autonomic Nervous System (ANS)

Learning Objectives

By the end of this section you should be able to:

• Name and describe the divisions of the autonomic nervous system
• Descrone the components of the divisions of the autonomic nervous system

The nervous system can be divided into two functional parts: the somatic nervous system and the autonomic nervous system (ANS). The major differences between the two systems are evident in the responses that each produces. The somatic nervous system causes contraction of skeletal muscles. The autonomic nervous system controls cardiac and smooth muscle, as well as glandular tissue. The somatic nervous system is associated with voluntary responses (though many can happen without conscious awareness, like breathing), and the autonomic nervous system is associated with involuntary responses, such as those related to homeostasis.

The autonomic nervous system regulates many of the internal organs through a balance of two aspects, or divisions. In addition to the endocrine system, the autonomic nervous system is instrumental in homeostatic mechanisms in the body. The two divisions of the autonomic nervous system are the sympathetic division and the parasympathetic division. The sympathetic system is associated with the fight-or-flight response, and parasympathetic activity is referred to by the epithet of rest and digest. Homeostasis is the balance between the two systems. At each target effector, dual innervation determines activity. For example, the heart receives connections from both the sympathetic and parasympathetic divisions. One causes heart rate to increase, whereas the other causes heart rate to decrease.

Interactive Link

Watch this video to learn more about adrenaline and the fight-or-flight response. When someone is said to have a rush of adrenaline, the image of bungee jumpers or skydivers usually comes to mind. But adrenaline, also known as epinephrine, is an important chemical in coordinating the body’s fight-or-flight response. In this video, you look inside the physiology of the fight-or-flight response, as envisioned for a firefighter. His body’s reaction is the result of the sympathetic division of the autonomic nervous system causing system-wide changes as it prepares for extreme responses. What two changes does adrenaline bring about to help the skeletal muscle response?

Neurons of the Autonomic Nervous System

The autonomic nervous system is composed of a chain of two lower motor neurons. The cell body of the first of the two ANS motor neurons is located in the brainstem or spinal cord and is called a preganglionic neuron. The axon of the preganglionic neuron extends outside of the CNS through cranial or spinal nerves. These axons project to an autonomic ganglion of the peripheral nervous system. The preganglionic axons release acetylcholine (ACh) to excite the postganglionic neuron. The cell body of the postganglionic neuron resides within the autonomic ganglion and its axon extends to an effector or target organ (cardiac muscle, smooth muscle, or gland). Postganglionic neurons have axons that release either acetylcholine (ACh) or norepinephrine (NE) to either excite or inhibit an effector, depending on the type of receptors present on the effector.

For example, a general pathway of communication in the autonomic nervous system would begin at the preganglionic neuron in the CNS. The axons of the preganglionic neuron extend out from the CNS and synapse onto a postganglionic neuron in the PNS. The preganglionic neuron releases acetylcholine (a neurotransmitter) that stimulate the postganglionic neuron. The axons of the postganglionic neuron synapse onto the effector or target organ. The neurotransmitter release by the postganglionic neuron to the target organ will then activate the target organ's autonomic response. The type of neurotransmitter released at each synapse between the postganglionic neuron and the target organ depends on the division of the ANS and the receptors present at the target organ. These will be discussed in the upcoming sections on the ANS.

Figure \(\PageIndex{1}\) The Autonomic Nervous System: In the autonomic nervous system, a preganglionic neuron of the CNS synapses with a postganglionic neuron of the PNS. The postganglionic neuron, in turn, acts on a target organ. Autonomic responses are mediated by the sympathetic and the parasympathetic systems, which are antagonistic to one another. The sympathetic system activates the “fight or flight” response, while the parasympathetic system activates the “rest and digest” response. (Image and caption credit: "Autonomic Nervous System" by OpenStax is licensed under CC BY 4.0)