The Autonomic Nervous System (ANS) - Geneeskunde 1

# The Autonomic Nervous System (ANS) The ANS has sympathetic, parasympathetic, and enteric divisions. - Output from the central nervous system (CNS) travels along two anatomically and functionally distinct pathways: - The **somatic motor neurons**, which innervate striated skeletal muscle....

# The Autonomic Nervous System (ANS) The ANS has sympathetic, parasympathetic, and enteric divisions. - Output from the central nervous system (CNS) travels along two anatomically and functionally distinct pathways: - The **somatic motor neurons**, which innervate striated skeletal muscle. - The **autonomic motor neurons**, which innervate smooth muscle, cardiac muscle, secretory epithelia, and glands. All viscera are richly supplied by efferent axons from the ANS that constantly adjust organ function. The autonomic nervous system (from the Greek for "self-governing", functioning independently of the will) was first defined by Langley in 1898 as including the local nervous system of the gut and the efferent neurons innervating glands and involuntary muscle. Thus, this definition of the ANS includes only efferent neurons and enteric neurons. Since that time, it has become clear that the efferent ANS cannot easily be dissociated from visceral afferents as well as from those parts of the CNS that control the output to the ANS and those that receive interoceptive input. This larger visceral control system monitors afferents from the viscera and the rest of the body, compares this input with current and anticipated needs, and controls output to the body's organ systems. The ANS has three divisions: - **Sympathetic** - **Parasympathetic** - **Enteric** The sympathetic and parasympathetic divisions of the ANS are the two major efferent pathways controlling targets other than skeletal muscle. Each innervates target tissue by a two-synapse pathway. - The cell bodies of the first neurons lie within the CNS. - These **preganglionic neurons** are found in columns of cells in the brainstem and spinal cord and send axons out of the CNS to make synapses with **postganglionic neurons** in peripheral ganglia interposed between the CNS and their target cells. - Axons from these postganglionic neurons then project to their targets. The sympathetic and parasympathetic divisions can act independently of each other. However, in general, they work synergistically to control visceral activity and often act in opposite ways, like an accelerator and brake to regulate visceral function. An increase in output of the sympathetic division occurs under conditions such as stress, anxiety, physical activity, fear, or excitement, whereas parasympathetic output increases during sedentary activity, eating, or other “vegetative” behavior. The **enteric division of the ANS** is a collection of afferent neurons, interneurons, and motor neurons that form networks of neurons called plexuses (from the Latin “to braid”) that surround the gastrointestinal (GI) tract. It can function as a separate and independent nervous system, but it is normally controlled by the CNS through sympathetic and parasympathetic fibers.