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ANP1105 (101)
Chapter 14

ANP1105 Autonomic Nervous System Chapter 14.docx
ANP1105 Autonomic Nervous System Chapter 14.docx

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University of Ottawa
Anatomy and Physiology
Jacqueline Carnegie

Chapter 14 : The Autonomic Nervous • Autonomic Nervous System (ANS) : is the system od motor neurons that innervates smooth and cardiac muscles and glands. ANS is also called the involuntary nervous system (according to how it controls) or the general visceral motor system.(according to its location) • ANS : The nervous system comprises the brain and various types of nerves, including afferent nerves which carry sensory impulses from all parts of the body to the brain and efferent nerves through which "messages" are conducted from the brain to the muscles and all of the organs of the body • Visceral activity : Referring to the viscera, the internal organs of the body, specifically those within the chest (as the heart or lungs) or abdomen (as the liver, pancreas or intestines). • ANS consists of motor neurons that – Innervate smooth and cardiac muscle, and glands – Make adjustments to ensure optimal support for body activities – Operate via subconscious control • Somatic Nervous system : voluntary system that controls impulses conveyed to the skeletal muscles from the central nervous system • Difference of somatic nervous system and autonomic nervous system. Both have motor fibers but they differ in – Their effectors – Their efferent pathways and ganglia – Target organ responses to their neurotransmitters EFFECTORS The somatic nervous system stimulates the skeletal muscles. Autonomic nervous system innervates the cardiac and smooth muscles and galnds. EFFERENT PATHWAYS AND GANGLIA • Somatic nervous system : their motor neuron cell bodies are in the CNS. Axons extend in spinal or cranial nerves all the way to the skeletal muscles they activate. Somatic motor fibers : thick, heavily myelinated, group A fibers that conduct nerve impulses rapidly. They lack ganglia. • ANS pathway uses two-neuron chain 1. Preganglionic neuron (in CNS) (first neuron) : cell body resides in the brain or spinal cord, has a thin, lightly myelinated preganglionic axon, synapses with the second motor neuron. 2. Postganglionic neuron (ganglionic neuron) : second motor neuron. Its cell body is in an autonomic ganglion outside CNS, has nonmyelinated postganglionic axon that extends to effector organ • Conduction through autonomic efferent chain is slower than conduction in the somatic nervous system. NEUOROTRANSMITTER EFFECTS • Somatic nervous system – All somatic motor neurons release acetylcholine (ACh) at their synapses with the skeletal muscle fibers. – Effects always stimulatory/ excitatory. If stimulation reaches threshold, muscle fibers contract. • Autonomic nervous system – Autonomic postganglionic fibers release two neurotransmitters – Preganglionic fibers release Ach. (Parasympathetic) – Postganglionic fibers release norepinephrine (NE) at effectors. (sympathetic) – Effect is either stimulatory or inhibitory, depending on type of receptors However, somatic nervous system and autonomic nervous system overlap to perform bodily functions Ex: most of the body’s adaptations to changing internal and external conditions involve both skeletal muscles and visceral organs. Higher brain centers regulate and corordinate both motor activities and most spinal nerves contain both somatic and autonomic fibers. DIVIVIONS OF THE ANS (ANS does not work in isolations) • Sympathetic division : mobilizes the body during activity • Parasympathetic division : promote maintenance functions and conserve body energy. • Dual innervation – ~ All visceral organs served by both divisions, but cause opposite effects. • Dynamic antagonism between two divisions maintains homeostasis • Role of the Parasympathetic Division (Rest and Digest) • Promotes maintenance activities and conserves body energy – Directs Digestion, Diuresis, Defecation (D division) • As in person relaxing and reading after a meal – Blood pressure, heart rate, and respiratory rates are low – Gastrointestinal tract activity high – Pupils constricted; lenses accommodated for close vision • Role of the Sympathetic Division (fight or flight) • Mobilizes body during activity; "fight-or-flight" system. Temporarily damps the nonessential activities to give more energy to what is in need (ex: if you are running away from something digestion can wait) • Exercise, Excitement, Emergency, Embarrassment (E division) – Increased heart rate; dry mouth; cold, sweaty skin; dilated pupils – During vigorous physical activity – Shunts blood to skeletal muscles and heart – Dilates bronchioles (increase ventilation to deliver more oxygen to body cells) – Causes liver to release glucose – Constrict visceral (sometimes cutaneous) blood vessels ANS ANATOMY Anatomically the parasympathetic and sympathetic divisions differ in – Sites of origin: Parasympathetic fibers are craniosacral. Sympathetic fibers are thoracolumbar, they originate in the thoracic and lumbar regions of the spinal cord – Relative lengths of their fibers: Parasympathetic division has long preganglionic and short post ganglionic fibers. Sympathetic division has short preganglionic and long post ganglionic fibers. – Location of their ganglia: Most parasympathetic ganglia are located in the visceral effector organs. Sympathetic ganglia are close to the spinal cord. PARASYMPATHETIC (CRANIOSACRAL) DIVISION • They originate in the brain and spinal cord – Craniosacral. • Long preganglionic fibers from brain stem and sacrum – Extend from CNS almost to target organs – Axons synapse with postganglionic neurons located in terminal ganglia close to/within target organs – Short postganglionic fibers synapse with effectors Cranial Part of Parasympathetic Division • Preganglionic fibers run in the oculomotor, facial, glossopharyngeal and vagus cranial nerves. Their Cell bodies lie in associated motor cranial-nerve nuclei in brain stem • Oculomotor nerves (occulomotor III) – smooth muscle of eye (neurons in ciliary ganglia) • Facial nerves (VII) – stimulate large glands in head • Glossopharyngeal nerves (IX) – parotid salivary glands • Vagus nerves – neck and nerve plexuses for ~ all thoracic and abdominal viscera Sacral Part of Parasympathetic Division • Serves pelvic organs and distal half of large intestine • Arise from neurons in lateral gray matter of spinal cord segments S 2S 4 • Axons travel in ventral root of spinal nerves • Synapse with – Ganglia in pelvic floor – Intramural ganglia in walls of distal half of large intestine, urinary bladder, ureters, and reproductive organs SYMPATHETIC (THORACOLUMBAR) DIVISION • Preganglionic neurons arise in spinal cord segments T 1 L 2 – Numerous preganglionic sympathetic neurons in the grey matter of the spinal cord Form lateral horns : • Preganglionic fibers pass through white rami communicantes and enter sympathetic trunk (chain or paravertebral) ganglia • Sympathetic Trunks and Pathways Upon entering sympathetic trunk ganglion short preganglionic fiber may – Synapse with ganglionic neuron in same trunk ganglion – Ascend or descend sympathetic trunk to synaps
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