14: Autonomic Nervous System
The autonomic nervous system (ANS) controls cardiac and smooth muscle, as well as glandular tissue, to maintain homeostasis when the body is at rest or in an emergency.. It is associated with unconscious responses to regulate heart rate, blood pressure, body temperature, sweating and digestion. It can be divided into two divisions: the sympathetic divisions maintains homeostasis in conditions of "fight-or-flight" while the parasympathetic division maintains homeostasis when the body is at rest. In this chapter, you will look at the structures of the two divisions of the ANS, their reflexes and the control on the ANS exercised by the central nervous system.
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- 14.1: Introduction to the Autonomic Nervous System
- The autonomic nervous system is often associated with the “fight-or-flight response,” which refers to the preparation of the body to either run away from a threat or to stand and fight in the face of that threat. Besides the fight-or-flight response, there are the responses referred to as “rest and digest”. Much of the function of the autonomic system is based on the connections within an autonomic, or visceral, reflex.
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- 14.2: Divisions of the Autonomic Nervous System
- The ANS is composed of two divisions. The sympathetic division is responsible to maintain homeostasis in situations of "fight-or-flight". Sympathetic ganglia along the vertebral column or in the abdominal region receive preganglionic axons from autonomic motor neurons of the thoracic and lumbar region of the spinal cord. The parasympathetic division is responsible to maintain homeostasis when the body is at rest. Parasympathetic ganglia are located near or within the effector organs.
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- 14.3: Autonomic Synapses, Effects and Reflexes
- Postganglionic axons contain varicosities, swellings containing vesicles of neurotransmitters. The main signaling molecules of the ANS are acetylcholine, norepinephrine and epinephrine. The axons of the two divisions differ in the released neurotransmitters. Most organs are dually innervated and display autonomic tone. Autonomic reflexes are similar to somatic ones in their afferent branch but not efferent one. Autonomic reflexes can be long if they pass by the CNS or short if they don't.
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- 14.4: Central Control Autonomic Function
- Coordinating the balance between the two divisions of the ANS requires integration that begins with forebrain structures and continues into the brainstem and spinal cord. Inputs to the hypothalamus come from the optic nerve and the medial forebrain bundle while outputs follow the dorsal longitudinal fasciculus and the medial forebrain bundle. The amygdala influences the state of activity of the hypothalamus. The medulla oblongata contains nuclei that controls the cardiovascular system.