ANAT 1607 The Autonomic Nervous System

ANAT 1607 The Autonomic Nervous System - The Autonomic...

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The Autonomic Nervous System THE AUTONOMIC NERVOUS SYSTEM The autonomic nervous system innervates visceral organs including the vascular and glandular systems. (Remember that in the neurological sense the term visceral refers to cardiac muscle, smooth muscle, and glands). Since it generally acts below the level of consciousness, it is said to be involuntary and functions to maintain homeostasis of the internal environment, e.g. blood pressure, body temperature. Functionally, the autonomic system consists of all the central and peripheral nerve cell fibers that influence visceral activity. The central nervous system pathways influencing visceral function are beyond the scope of our present consideration and will be covered later in the Neurobiology course. In the peripheral nervous system the afferent as well as the efferent visceral nerves should be included in the autonomic nervous system. By traditional definition, however, the autonomic system is purely efferent (GVE). Recognize, that the visceral afferent fibers, although not included in the autonomic system by traditional definition, are of great functional importance since they comprise the sensory limb of the visceral reflex system. The autonomic system has two major parts: the sympathetic and the parasympathetic. The autonomic nervous system always has two motor neurons in the circuit which transmits impulses form the brain or spinal cord to the peripheral effector organ. The cell bodies of the first, or proximal, neurons are located in the brain or spinal cord (the CNS), and their axons synapse in the autonomic ganglia of the peripheral nervous system. These are preganglionic neurons and their axons are preganglionic fibers. The second, or distal, neurons are located entirely in the peripheral nervous system. These are postganglionic neurons and their cell bodies are located in the autonomic ganglia. There are postganglionic fibers which innervate smooth muscle, cardiac muscle, and glands. PREGANGLIONIC COMPONENTS. The cell bodies of preganglionic sympathetic neurons are located in the twelve thoracic and upper two lumbar spinal cord segments. The cell bodies of the preganglionic parasympathetic neurons are in the brain stem and in the second, third, and fourth sacral segments of the spinal cord. Thus, in reference to the locations of preganglionic cell bodies, the sympathetic and parasympathetic systems may be referred to as thoracolumbar and craniosacral respectively. POSTGANGLIONIC COMPONENTS. The cell bodies of postganglionic neurons are found in ganglia located at various distances form the central nervous system. The ganglia of the autonomic nervous system are: 1. Prevertebral (collateral) ganglia lie in front of the vertebral column and are actually located within plexuses closely related to branches of the abdominal aorta.
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2. Paravertebral ganglia occur within the sympathetic trunks lying against the vertebral column and produce enlargements or "beads" in the trunk. 3. Terminal ganglia usually are intimately associated with the viscera they serve, often
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ANAT 1607 The Autonomic Nervous System - The Autonomic...

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