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Autonomic Nervous System ( ANS) Part 1 Dr Taha Sadig Ahmed Physiology Department , College of Medicine , King Saud University , Riyadh Nervous System (I) Central Nervous System (II) Peripheral Nervous System (I) Central Nervous System (CNS) Consists of Brain & Spinal Cord Brain (1)Forebrain: Cerebrum, Diencephalon (Thalamus, Metathalamus, Epithalamus, Hypothalamus and subthalamus (2)Mid brain (3) Hind brain: Pons, Medulla , Cerebellum B/Spinal cord • (II) Peripheral Nervous System: Consists of cranial nerves (arising from brain) and spinal nerves (arising from spinal cord) The Nervous System can also be classified into : (I) Somatic ( voluntary) Nervous System : For organs • under voluntary control (namely skeletal muscles) (II) Autonomic (involuntary,visceral) Nervous System: • Not under voluntary control. • It regulates internal organs’ visceral functions and homeostasis • Effectors include cardiac muscles , smooth muscles and glands • The Autonomic Nervous System ( ANS) helps to adjust & maintain the internal environment ( homeostasis); and helps the body to adapt to to changes in the external environment , ANS is controlled activated by centers located in the spinal cord, brain stem, hypothalamus and also cerebral cortex especially the limbic cortex The hypothalamus is very important in this regard : Posterior hypothalamus controls Sympathetic NS , & Anterior hypothalamus controls Parasympathetic NS Parasympathetic NS Sympathetic NS • Parasympathetic system activities is related to the relaxed state , rest and anabolism • It promotes vegetative functions ( nutritive , body-building , restorative functions & tissue repair ) • Increased secretion of endocrine & exocrine glands . 5 COMPARISON OF AUTONOMIC AND SOMATIC SYSTEMS Somatic system – One motor neuron extends from the CNS to skeletal muscle – Axons are well myelinated – Conduct impulses rapidly THE AUTONOMIC NERVOUS SYSTEM Sympathetic and parasympathetic systems are consists of myelinated pre-ganglionic fibers which make synaptic connections with un-myelinated postganglionic fibers and then innervate the effector organ These synapses usually occur in clusters called ganglia. COMPARISON OF AUTONOMIC AND SOMATIC MOTOR SYSTEMS Autonomic nervous system – Chain of two motor neurons in series • Preganglionic neuron ( have myelinated axons) • Postganglionic neuron ( have unmyelinated , thin axons ) – Conduction is slower than in Somatic due to presence of thin , un-myelinated postganglionic axons Pre-ganglionic Post-ganglionic Ganglion The Autonomic Outflow consists 2 Neurons neurons arranged in series: • (A) Preganglionic nerves: (1) Are myelinated (2) In case of sympathetic NS arise from spinal cord In case of Parasympathetic NS arise from Brainstem + spinal cord •.(B) Postganglionic nerves : arise from ganglia . (1) Are unmywelinated (2) arise from ganglia . 9 COMPARISON OF AUTONOMIC AND SOMATIC MOTOR SYSTEMS THE AUTONOMIC NERVOUS SYSTEM 2 neurons in the efferent pathway. 1st neuron has its cell body in gray matter of brain or spinal cord. Preganglionic neuron. • Synapses with 2nd neuron within an autonomic ganglion. Postganglionic neuron. • Autonomic ganglion has axon which extends to synapse with target tissue. THE AUTONOMIC NERVOUS SYSTEM Sympathetic and parasympathetic systems are consists of myelinated pre-ganglionic fibers which make synaptic connections with un-myelinated postganglionic fibers and then innervate the effector organ These synapses usually occur in clusters called ganglia. Preganglionic neuron: Cell body in brain or spinal cord Axon is myelinated type fiber that extends to autonomic ganglion Postganglionic neuron: Cell body lies outside the CNS in an autonomic ganglion Axon is unmyelinated type fiber that terminates in a visceral effector DIFFERENCES IN SYMPATHETIC AND PARASYMPATHETIC DIVISIONS • Length of postganglionic fibers – Sympathetic long postganglionic fibers – Parasympathetic short postganglionic fibers – Therefore , in sympathetic Preganglionic fibers are shorter than Postganglionic ones – And in parasympathetic Preganglionic fibers are longer than Postganglionic ones . • Branching of axons – Sympathetic axons – highly branched – Parasympathetic axons – few branches DIFFERENCES IN SYMPATHETIC AND PARASYMPATHETIC DIVISIONS Subdivision of ANS Location of soma ( cellbody ) of preganglionic nerve Sympathetic Thoracolumbar Alongside Segments of vertebral spinal cord column ; ACh is transmitter Parasympath etic Cranium ( in side skull , in Brain ) and Sacral Segments of Spinal Cord Location of Ganglia & Transmitter at Ganglia Chemical secreted by Postganglionic Fiber General Function Norepinephrine; Fight or except flight (1) Vasodilator postganglionic sympathetic fibers in Skeletal Muscle (2) Postganglionic sympathetic fibers to Sweat Glands On or near Acetylcholine the effector organ; ACh is transmitter Conservation of body energy Preganglionic Sympathetic nerves exit the CNS from the thoracic + lumbar segments of the spinal cord Therefore , the sympathetic system is also called : “ Thoraco-lumbar Outflow “ Preganglionic Parasympathetic nerves exit the CNS from the Cranium ( skull ) +sacral segments of the spinal cord Therefore , the Parasympathetic system is called : “ Craniosacral Outflow “ Hence , in the Sympathtic system , Preganglionic fibers are myelinated and shorter than the unmyelinated Postganglionic fibers In Parasympathetic system , Preganglionic fibers are also myelinated but longer than the unmyelinated Postganglionic fibers . Origin of ANS fibers Sympathetic: Originate in the thoracic and lumbar regions of the spinal cord (Thoracolumbar) Parasympathetic: Originate from cranial nerves (3rd, 7th, 9th, 10th), and sacral spinal nerves (craniosacral) In the Parasympathetic NS , there is little divergence the ratio of pre- to postgaglionic fibers is 1/3 . This little divergence implies more specific , localized actions . On the other hand , in the Sympathetic NS there is greater divergence . The ratio (pre/post) being = 1/10 ; This divergence implies diffuse ( generalized ) actions . 20 THE AUTONOMIC NERVOUS SYSTEM