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The Autonomic Nervous System Chapter 14 Introduction The stability of our internal environment depends largely on the autonomic nervous system Autonomic nervous system(ANS) receives signals from visceral organs The ANS makes adjustments as necessary to ensure optical support for body system Overview of the ANS The autonomic nervous system differs in – Effectors – Efferent pathways – Target organ Effectors of ANS The somatic nervous system stimulates skeletal muscles The ANS innervates cardiac and smooth muscles and glands Efferent Pathways and Ganglia In the somatic system – Cell bodies are within the central nervous system – Axons extend to the muscles they serve – Somatic motor fibers are thick, heavily myelinated Type A fibers that conduct impulses rapidly Efferent Pathways and Ganglia In the autonomic nervous system – The motor unit is a two neuron chain – The cell body of the first neuron, the preganglionic neuron, resides in the brain or spinal cord – Its axon, the preganglionic axon, synapses with the second motor neuron, the postganglionic neuron, in an autonomic ganglion outside the central nervous system – The postganglionic axon then extends to the effector organ Efferent Pathways and Ganglia Axons of most preganglionic neurons run from the CNS to synapse in a peripheral autonomic ganglion with a postganglionic neuron Efferent Pathways and Ganglia Axons of postganglionic neurons run from the ganglion to the effectors (cardiac and smooth muscle fibers and glands) Efferent Pathways and Ganglia Preganglionic axons are lightly myelinated thin fibers Postganglionic axons are even thinner and are unmyelinated Conduction though the autonomic chain is slower than through the somatic motor Many pre and postganglionic fibers are incorporated into spinal or cranial nerves for most of their course Efferent Pathways and Ganglia Autonomic ganglion are motor ganglia containing the cell bodies of motor neurons They are sites of synapse and information transmission from pre to postganglionic neurons The presence of intrinsic ganglionic cells, analogous to interneurons, suggests that certain intergrative functions may occur there Neurotransmitter Effects All somatic motor neurons release acetylcholine at their synapses with skeletal muscle fibers The effect is always excitatory, and if stimulation reaches threshold, the skeletal muscle contacts Neurotransmitter Effects Neurotransmitters released onto visceral effector organs by postganglionic autonomic fibers include – Norepinephrine secreted by most sympathetic fibers – Acetylcholine released by parasympathetic fibers Depending on the receptors present on the target organ its response to these neurotransmitters may be either excitation or inhibition Divisions of ANS There are two division of the ANS – Parasympathetic – Sympathetic Generally the two divisions serve the same visceral organs but cause essentially opposite effects If one division stimulates certain smooth muscle to contract or a gland to secrete, the other division inhibits that action Through this process of duel innervation the two systems counterbalance each other Divisions of ANS The sympathetic part mobilizes the body during extreme situations (such as fear, exercise or rage) The parasympathetic division allows us to unwind as it performs maintenance activities and conserves body energy Role of Parasympathetic Division The parasympathetic division is most effective in non-stressful situations This division is chiefly concerned with keeping body energy use as low as possible, even as it directs body processes such as digestion and elimination Resting and digesting division Role of Sympathetic Division Its activity is evident when we are excited or find ourselves in emergency or threatening situations (frightened) Pounding heart, rapid deep breathing, cold sweaty skin and dilated eyes are signs Also changes in brain wave patterns Its function is to provide the optimal conditions for an appropriate response to some threat (run / see / think)