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1H06 Anatomy & Physiology 1H06: Autonomic Nervous System.docx

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McMaster University
Health Sciences
Alexander Ball

Autonomic Nervous System: contributes to homeostasis by responding to subconscious visceral sensations and exciting or inhibiting smooth muscle, cardiac muscle, and glands.  Operates via reflex arcs  Includes sensory neurons, integrating centers in CNS, autonomic motor neurons, and enteric division  Usually operates without conscious control Comparison between Somatic and Autonomic Nervous Systems Somatic:  Includes both sensory and motor neurons  Sensory neurons convey input from receptors for somatic senses (thermal, pain) and from receptors for special senses (sight, hearing, taste)  Somatic neurons innervate skeletal muscles, producing both reflexive and voluntary movements Autonomic:  Interoceptors – sensory receptors located in blood vessels, visceral organs, muscles, and nervous system that monitor conditions in internal environment  Ex. Chemoreceptors that monitor blood carbon dioxide level  Autonomic motor neurons regulate visceral activities by either increasing or decreasing ongoing activities in effector tissues (cardiac, smooth, glands) Comparison between Somatic and Autonomic Motor Neurons Somatic Motor Neuron Autonomic Motor Neuron  Extends from CNS to skeletal  Consist of two motor neurons in muscle fibers in motor unit series (one following the other)  Release only acetylcholine  Release either Ach or norepinephrine Autonomic: First neuron has cell body in CNS; its myelinated axon extends from CNS to autonomic ganglion (collection of neuronal cell bodies in PNS). The cell body of the second neuron (postganglionic neuronr) is also in same autonomic ganglion; its unmyelinated axon extends directly from ganglion to effector (smooth, cardiac, gland)  Output of ANS has sympathetic divison and parasympathetic division.  Most organs have dual innervation (receive impulses from both sympathetic and parasympathetic neurons)  Sympathetic = fight or flight  Parasympathetic = rest-and-digest Anatomy of Autonomic Motor Pathways Preganglionic neuron  Cell body is in brain or spinal cord  Axon exits CNS as part of cranial or spinal nerve  Axon is myelinated B fiber that usually extends to autonomic ganglion and synapses with postganglionic neuron  In sympathetic division, have cell bodies in lateral horns of gray matter in 12 thoracic segments and first two lumbar of spinal cord o Sympathetic division also called thoracolumbar division, axons of sympathetic preganglionic neurons are called thoracolumbar outflow.  In parasympathetic division, have cell bodies in nuclei of four cranial nerves in brain stem and lateral gray matter of second through fourth sacral segments of spinal cord o Parasympathetic division also called craniosacral division, and axons called craniosacral outflow. Autonomic Ganglia  Two major groups: sympathetic ganglia and parasympathetic ganglia Sympathetic ganglia:  Sites of synapses between sympathetic preganglionic and postganglionic neurons  Two types: sympathetic trunk ganglia and prevertebral ganglia Sympathetic trunk: extend from base of skull to coccyx  Postganglionic axons from sympathetic trunk ganglia primarily innervate organs above diaphragm (head, neck)  Names: superior, middle, inferior cervical ganglia Prevertebral ganglia: lies anterior to vertebral column and close to large abdominal arteries  Innervate organs below diaphragm  5 major: celiac, superior mesenteric, inferior mesenteric, aorticorenal, renal Parasympathetic Ganglia:  most located within wall of visceral organ  Terminal in head have names: ciliary, pterygopalatine, submandibular, otic Postganglionic neuron  Lies entirely outside CNS in PNS  Cell body and dendrites located in autonomic ganglion and forms synapses with one or more preganglionic axons  Axon is unmyelinated C fiber that terminates in visceral effector Preganglionic neurons convey nerve impulses from CNS to autonomic ganglia, and postganglionic neurons relay impulses from autonomic ganglia to visceral effectors. Axons of sympathetic preganglionic neurons connect with postganglionic neurons in 4 ways: 1. Axon may synapse with postganglionic neurons in ganglion it first reaches 2. Axon may ascend or descend to higher or lower ganglion before synapsing with postganglionic neurons 3. Axon may continue without synapsing through sympathetic trunk ganglion to end at prevertebral ganglion and synapse with postganglionic neurons there 4. May pass without synapsing, through sympathetic trunk ganglion and prevertebral ganglion and then extend to chromaffin cells of adrenal medullae. Autonomic Plexuses  Axons of both sympathetic and parasympathetic neurons form t
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