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BIOB32H3 (79)


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University of Toronto Scarborough
Biological Sciences
Kenneth Welch

Autonomic Nervous System (ANS) Autonomic Nervous System (ANS) • The 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 • Have viscera as most of their effectors ANS Versus Somatic Nervous System (SNS) • The ANS differs from the SNS in the following three areas • Effectors • Efferent pathways • Target organ responses Effectors • The effectors of the SNS are skeletal muscles • The effectors of the ANS are cardiac muscle, smooth muscle, and glands Efferent Pathways • Heavily myelinated axons of the somatic motor neurons extend from the CNS to the effector • Axons of the ANS are a two-neuron chain • The preganglionic (first) neuron with a lightly myelinated axon • The gangionic (second) neuron that extends to an effector organ Neurotransmitter Effects • All somatic motor neurons release ACh, which has an excitatory effect • In the ANS: • Preganglionic fibers release ACh • Postganglionic fibers release norepinephrine or ACh and the effect is either stimulatory or inhibitory • ANS effect on the target organ is dependent upon the neurotransmitter released and the receptor type of the effector Divisions of the ANS • The two divisions of the ANS are the sympathetic and parasympathetic • The sympathetic mobilizes the body during extreme situations • The parasympathetic performs maintenance activities and conserves body energy • The two divisions counterbalance each other’s activity Role of the Parasympathetic Division • Concerned with keeping body energy use low • Involves the D activities – digestion, defecation, and diuresis • Its activity is illustrated in a person who relaxes after a meal • Blood pressure, heart rate, and respiratory rates are low • Gastrointestinal tract activity is high • The skin is warm and the pupils are constricted Role of the Sympathetic Division • The sympathetic division is the “fight-or-flight” system • Involves E activities – exercise, excitement, emergency, and embarrassment • Promotes adjustments during exercise – blood flow to organs is reduced, flow to muscles is increased • Its activity is illustrated by a person who is threatened • Heart rate increases, and breathing is rapid and deep • The skin is cold and sweaty, and the pupils dilate Sympathetic Outflow • Is from nerves T t1rough L 2 • Sympathetic neurons produce the lateral horns of the spinal cord • Preganglionic fibers pass through the white rami communicantes and synapse in the paravertebral ganglia • Fibers from T -5 f2rm splanchnic nerves and synapse in collateral ganglia • Postganglionic fibers innervate the numerous organs of the body Sympathetic Trunks and Pathways • Preganglionic fibers pass through white rami communicantes and enter paravertebral ganglia • The paravertebral ganglia form part of the sympathetic chain • Typically there are 23 ganglia – 3 cervical, 11 thoracic, 4 lumbar, 4 sacral, and 1 coccygeal • A pregangiolonic fiber follows one of three pathways upon entering the paravertebral ganglia: • Synapse with the ganglionic neuron within the same ganglion • Ascend or descend the sympathetic chain to synapse in another chain ganglion • Pass through the chain ganglion and emerge without synapsing Pathways with Synapses in a Chain Ganglion • Postganglionic axons enter the ventral rami via the gray rami communicantes • These fibers innervate sweat glands and arrector pili muscles • Rami communicantes are associated only with the sympathetic division Pathways to the Head • Preganglionic fibers emerge from T -T and synapse in the superior cervical ganglion 1 4 • These fibers: • Serve the skin and blood vessels of the head • Stimulate dilator muscles of the iris • Inhibit nasal and salivary glands Pathways to the Thorax • Preganglionic fibers emerge from T -T a1d 6ynapse in the cervical chain ganglia • Postganglionic fibers emerge from the middle and inferior cervical ganglia and enter nerves C 4C 8 • These fibers innervate the heart via the cardiac plexus, as well as innervating, the thyroid and the skin • Other T 1T p6eganglionic fibers synapse in the nearest chain ganglia • Postganglionic fibers directly serve the heart, aorta, lungs, and esophagus Pathways with Synapses in a Collateral Ganglion • These fibers (T -5 )2leave the sympathetic chain without synapsing • They form thoracic, lumbar, and sacral splanchnic nerves • Their ganglia include the celiac, the superior and inferior mesenterics, and the hypogastric Pathways to the Abdomen • Sympathetic nerves innervating the abdomen have preganglionic fibers from T -L 5 2 • They travel through the thoracic splanchnic nerves and synapse at the celiac and superior mesenteric ganglia • Postganglionic fibers serve the stomach, intestines, liver, spleen, and kidneys Pathways to the Pelvis • Preganglionic fibers originate from T -L10 2 • Most travel via the lumbar and sacral splanchnic nerves to the inferior mesenteric and hypogastric ganglia • Postganglionic fibers serve the distal half of the large intestine, the urinary bladder, and the reproductive organs Pathways with Synapses in the Adrenal Medulla • Fibers of the thoracic splanchnic nerve pass directly to the adrenal medulla • Upon stimulation, medullary cells secrete norepinephrine and epinephrine into the blood Visceral Reflexes • Visceral reflexes have the same elements as somatic reflexes • They are always polysynaptic pathways • Afferent fibers are found in spinal and autonomic nerves Referred Pain • Pain arising from the viscera but is perceived as somatic in origin • This may be due to the fact that visceral pain afferents travel along the same pathways as somatic pain fibers Neurotransmitters and Receptors • Acetylcholine (ACh) and norepinephrine (NE) are the two major neurotransmitters of the ANS • ACh is released by all preganglionic axons and all parasympathetic postganglionic axons • Chol
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