Autonomic Nervous System
The nervous system can be structurally divided into the central (CNS) and peripheral nervous systems (PNS). It is further divided into two functional systems, the somatic nervous system and the autonomic nervous system (ANS). The ANS is also called the involuntary nervous system because it affects the activity of the internal organs (viscera) and smooth muscle. The ANS is divided into two parts based on the different functions they serve: the parasympathetic and sympathetic systems. These two systems work together and in opposing actions to maintain homeostasis of the body and its viscera.
The ANS is made up of sensory and motor nerve fibers as well as ganglia. Each part of the ANS consists of a two-neuron chain with the first neuron originating in the brain or spinal cord. These preganglionic neurons are named as such because their cell bodies are located within the CNS or before the ganglion of the ANS. The second neuron is located in a ganglion outside of the CNS, and thus it is referred to as a postganglionic neuron.
The parasympathetic system promotes normal homeostatic or vegetative functions of the body. The cell bodies of its preganglionic neurons are located in two separate regions in the brainstem and at the end of the spinal cord. Specifically, these neurons are located in the medulla oblongata in the nuclei of cranial nerves III, VII, IX, and X, and in the sacral region of the spinal cord. These preganglionic neurons send their axons out toward the target organ where the postganglionic cell bodies are located. Generally, postganglionic neurons are located in ganglia very close to or within the target organ.
The sympathetic system is part of the emergency response system that speeds up the heart, increases blood pressure, and generally diverts blood away from the organs and out to the extremities. These actions are referred to as the “fight-or-flight response.” The preganglionic cell bodies for the sympathetic system are found in one area, and the preganglionic cell bodies in the parasympathetic systems are found above and below these neurons. Thus, the preganglionic neurons of the sympathetic system are located in the lateral horn of the spinal cord, specifically at the levels of T1 through L3. The cell bodies of the postganglionic neurons are located in two regions. The ganglia that are next to the main branches of the abdominal aorta are called the prevertebral ganglia. These axons run to the organs located in the abdominal and pelvic regions. The other ganglia are part of the sympathetic trunk and run from the skull to the coccyx. These are called the paravertebral ganglia, which have axons running to the eye, nose, mouth, respiratory tract, and heart.
The axons of the preganglionic sympathetic neurons exit the spinal cord by passing through the ventral root and separate from the spinal nerve to form the white ramus communicans. They then join a distinct chain of sympathetic ganglia called the vertebral or paravertebral ganglia that are arranged along either side of the vertebral column. At the level of the paravertebral ganglia, there are several options for the axons to exit. They can synapse with the postganglionic neuron, ascend or descend in the sympathetic trunk, then synapse with a postganglionic neuron at a different level, pass through the trunk and synapse with a prevertebral ganglia, or pass through without synapsing on anything until they reach the organ. This latter option is specific for the adrenal gland, which makes and secretes hormones to regulate the fight-or-flight response.
The postganglionic sympathetic axons from the paravertebral ganglia leave as gray rami communicans and join the spinal nerves on their way out to the body. These axons synapse with postganglionic neurons at the most superior paravertebral ganglia, then travel with blood vessels to the head and neck, or they travel to organs in the thorax, such as the heart, lungs, and esophagus. From the prevertebral ganglia, these postganglionic sympathetic axons travel with blood vessels out to the abdominal and pelvic organs. They terminate and act on glands, blood vessels, and smooth muscle.
The efferent nervous activity of the ANS is regulated by autonomic reflexes. In many of the reflexes, sensory information is transmitted to control centers in the CNS. These include the hypothalamus and the brainstem and are involved in the overall homeostatic regulation of the body. Most of the sensory input arriving from the abdominal and thoracic viscera is transmitted to the brainstem by the afferent fibers of the vagus nerve (cranial nerve X). Other cranial nerves also bring a rich supply of sensory input to the hypothalamus and brainstem. By integrating these signals, the hypothalamus and brainstem are able to monitor and regulate important body processes including blood pressure, respiration, blood/oxygen/CO2 levels, body temperature, heart rate, hunger, and thirst. The hypothalamus and brainstem are controlled by higher centers in the brain like the cerebral cortex and limbic systems, so integration of body processes is monitored at multiple levels of the CNS.
See also: Central Nervous System; Cranial Nerves; Peripheral Nervous System
Moore, Keith L., Anne M. R. Agur, & Arthur F. Dalley (Eds.). (2010). Essential clinical anatomy (4th ed.). Baltimore, MD: Williams and Wilkins.