LESSONS 18 19 THE ANS SYMPATHETIC AND PARASYMPATHETIC DIVSIONS Introduction to the ANS and Higher Order Functions Somatic Nervous System SNS o Operates under conscious control o Seldom affects long term survival o SNS controls skeletal muscles Autonomic Nervous System ANS o Operates without conscious instruction o ANS controls visceral effectors smooth cardiac muscles glands adipocytes o Coordinates system functions Cardiovascular respiratory digestive urinary reproductive Organization of the ANS Integrative centers o For autonomic activity located in hypothalamus o Neurons comparable to upper motor neurons in SNS Visceral motor neurons o Located in brain stem and spinal cord are known as preganglionic neurons o Preganglionic fibers Axons of preganglionic neurons leave CNS and synapse on ganglionic neurons located in a cluster ganglion outside the CNS o Autonomic ganglia Contain many ganglionic neurons Axons of ganglionic neurons postganglionic fibers innervate visceral effectors Such as cardiac muscle smooth muscle glands and adipose tissue The Autonomic Nervous System Operates largely outside our awareness Has two divisions o Sympathetic division Increases alertness metabolic rate and muscular abilities Kicks in only during exertion stress or emergency o Parasympathetic division Fight or flight Reduces metabolic rate and promotes digestion In control during resting conditions Rest and digest Sympathetic and Parasympathetic Division roles Most often these two divisions have opposing effects o If the sympathetic division causes excitation the parasympathetic causes inhibition The two divisions may also work independently o Only one division innervates some structures The two divisions may work together with each controlling one stage of a complex process o Males sexual physiology erection parasympathetic while ejaculation sympathetic Sympathetic Division Preganglionic fibers thoracic and superior lumbar thoracolumbar division synapse in ganglia near spinal cord Preganglionic fibers are short Postganglionic fibers are long except at adrenal medullae Prepares body for crisis producing a fight or flight response o Stimulates tissue metabolism o Increases alertness Seven Responses to Increased Sympathetic Activity Heightened mental alertness Increased metabolic rate Energy reserves activated Reduced digestive and urinary functions Increased respiratory rate and respiratory passageways dilate Increased heart rate and blood pressure Sweat glands activated Parasympathetic division Preganglionic fibers originate in brain stem and sacral segments of spinal cord craniosacral division Synapse in ganglia close to or within target organs Preganglionic fibers are long Postganglionic fibers are short Parasympathetic division stimulates visceral activity Conserves energy and promotes sedentary activities Five Responses to Increased Parasympathetic Activity Decreased metabolic rate Decreased heart rate and blood pressure Urination and defecation stimulation Increased secretion by salivary and digestive glands Increased motility and blood flow in digestive tract Enteric Nervous System ENS Third division of ANS influenced by both the sympathetic and parasympathetic division Extensive network in digestive tract walls Complex visceral reflexes coordinated locally does not require instructions Roughly 100 million neurons approx as many as in spinal cord fairly from CNS complex The Sympathetic Division Preganglionic neurons located between segments T1 and L2 of spinal cord Ganglionic neurons in ganglia near vertebrae column Cell bodies of preganglionic neurons in lateral gray horns of spinal cord Axons enter ventral roots of segments Ganglionic Neurons Found in three locations o Sympathetic chain ganglia Are on both sides of vertebral column Preganglionic neurons located in T1 L2 Control effectors In body wall Inside thoracic cavity In head In limbs o Collateral ganglia Are anterior to vertebral bodies Contain ganglionic neurons that innervate tissues and organs in abdominopelvic cavity o Adrenal Medullae Suprarenal medullae Very short axons When stimulated release neurotransmitters into bloodstream not at synapse Function as hormones to affect target cells throughout body Releases the hormone norepinephrine NE and epinephrine E into bloodstream Organization and Anatomy of the Sympathetic Division Ventral roots of spinal segments T1 L2 contain sympathetic preganglionic fibers o Give rise to myelinated white ramus o Carry myelinated preganglionic fibers into sympathetic chain ganglion o May synapse at collateral ganglia or in adrenal medullae Sympathetic Chain Ganglia Preganglionic fibers o One preganglionic fiber synapses on many ganglionic neurons o Fibers interconnected sympathetic chain ganglia o Each ganglion innervates particular body segment s Postganglionic fibers control visceral effectors o In body wall head neck or limbs Postganglionic fibers innervate effectors o Sweat glands of skin o Smooth muscles in superficial blood vessels Postganglionic fibers innervating structures in thoracic cavity form bundles called sympathetic nerves Only spinal nerves T1 L2 have white myelinated rami Every spinal nerve has gray unmyelinated ramus that carries sympathetic postganglionic fibers for distribution in body wall distributes signal for whole body response Collateral Ganglia Receive sympathetic innervation via sympathetic preganglionic fibers as splanchnic nerves does not form synapse in sympathetic chain Postganglionic fibers leave collateral ganglia o Extend through abdominopelvic o Innervate variety of visceral tissues cavity and organs Release of stored energy reserves Reduction of blood flow and energy by organs not vital to short term survival Ex Digestive tract Adrenal Medullae Preganglionic fibers entering adrenal gland proceed to center adrenal medulla adrenal cortex is on the outside It is modified sympathetic ganglion Preganglionic fibers synapse onto neuroendocrine cells o Specialized neurons secrete hormones epinephrine E 75 80 and norepinephrine NE into bloodstream Bloodstream carries neurotransmitters neurohormones through body Causing changes in metabolic activities of different cells including cells not innervated by sympathetic postganglionic fibers Effects last longer but slower to act o Hormones continue to diffuse out of bloodstream Sympathetic Activation Sympathetic Activation activates the entire sympathetic division during crisis Change activities of tissue and organs by o
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