autonomic nervous system (redirected from Sympathetic fibers)

autonomic nervous system: see nervous system.

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autonomic nervous system

Pathways of the autonomic nervous system. Nerve impulses begin in motor neurons in the brain or …

(credit: © Merriam-Webster Inc.)

Part of the nervous system that is not under conscious control and that regulates the internal organs. It includes the sympathetic, parasympathetic, and enteric nervous systems. The first, which connects the internal organs to the brain via spinal nerves, responds to stress by increasing heart rate and blood flow to the muscles and decreasing blood flow to the skin. The second comprises the cranial nerves and the lower spinal nerves, which increase digestive secretions and slow the heartbeat. Both have sensory fibres that send feedback on the condition of internal organs to the central nervous system, information that helps maintain homeostasis. The third division, embedded in the walls of the stomach and intestines, controls digestive movement and secretions.

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autonomic nervous system [¦ȯd·ə¦näm·ik ′nər·vəs ‚sis·təm]

(neuroscience)

The visceral or involuntary division of the nervous system in vertebrates, which enervates glands, viscera, and smooth, cardiac, and some striated muscles.

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Autonomic nervous system

The part of the nervous system that innervates smooth and cardiac muscle and the glands, and regulates visceral processes including those associated with cardiovascular activity, digestion, metabolism, and thermoregulation. The autonomic nervous system functions primarily at a subconscious level. It is traditionally partitioned into the sympathetic system and the parasympathetic system, based on the region of the brain or spinal cord in which the autonomic nerves have their origin. The sympathetic system is defined by the autonomic fibers that exit thoracic and lumbar segments of the spinal cord. The parasympathetic system is defined by the autonomic fibers that either exit the brainstem via the cranial nerves or exit the sacral segments of the spinal cord. See Parasympathetic nervous system, Sympathetic nervous system

The defining features of the autonomic nervous system were initially limited to motor fibers innervating glands and smooth and cardiac muscle. This definition limited the autonomic nervous system to visceral efferent fibers and excluded the sensory fibers that accompany most visceral motor fibers. Although the definition is often expanded to include both peripheral and central structures (such as the hypothalamus), contemporary literature continues to define the autonomic nervous system solely as a motor system. However, from a functional perspective, the autonomic nervous system includes afferent pathways conveying information regarding the visceral organs and the brain areas (such as the medulla and the hypothalamus) that interpret the afferent feedback and exert control over the motor output back to the visceral organs. See Homeostasis