Anatomy & Physiology I and II

Anatomy & Physiology Study Guide • Preganglionic fibers leave the brain as components of cranial nerves III, VII, IX, and X. Those leaving the sacral segments form pelvic nerves. Stimulation of parasympathetic neurons leads to the release of the neurotransmitter ACh: • All parasympathetic preganglionic and postganglionic fibers release ACh. The effects are short-lived because ACh is inactivated by acetylcholinesterase (AChE) and by tissue cholinesterase. • Postsynaptic membranes have two types of ACh receptors. The stimulation of muscarinic receptors produces a longer-lasting effect than the stimulation of nicotinic receptors. • The parasympathetic division innervates areas serviced by cranial nerves and organs in the thoracic and abdominopelvic cavities. The sympathetic and parasympathetic divisions interact, creating dual innervation: • The sympathetic division has widespread influence on visceral and somatic structures. • The parasympathetic division innervates only visceral structures that are serviced by cranial nerves or lying within the abdominopelvic cavity. Organs with dual innervation receive input from both divisions. • In body cavities, the parasympathetic and sympathetic nerves intermingle to form a series of characteristic autonomic plexuses (nerve networks): the cardiac, pulmonary, esophageal, celiac, inferior mesenteric, and hypogastric plexuses. • Important physiological and functional differences exist between the sympathetic and parasympathetic divisions. • Even when stimuli are absent, autonomic motor neurons show a resting level of activation, which is the autonomic tone. Visceral reflexes play a role in the integration and control of autonomic functions: • Visceral reflex arcs perform the simplest function of the ANS and can be either long reflexes (with interneurons) or short reflexes (bypassing the CNS). • Parasympathetic reflexes govern respiration, cardiovascular functions, and other visceral activities. • Levels of activity in the sympathetic and parasympathetic divisions of the ANS are controlled by centers in the brain stem that regulate specific visceral functions. • The SNS and ANS are organized in a parallel way. Integration occurs at the level of the brain stem and higher centers.

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