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The Autonomic Nervous System Chapter 17 Introduction • Makes all routine adjustments in physiological systems. • Consists of visceral motor (efferent) neurons • The ANS pathway from the CNS to the effector always involves 2 neurons synapsing in an autonomic ganglion – Preganglionic (neuron #1) – cell body is in the CNS, axon extends to the ganglion outside the CNS – Postganglionic (neuron #2) – cell body is in the ganglion, axon extends to the visceral effector Nerve Fibers of the ANS • Preganglionic (neuron #1) – Always myelinated – Neurotransmitter is always ACh • Postganglionic (neuron #2) – Always nonmyelinated – Neurotransmitter is Ach or norepinephrine ANS Fibers to Smooth Muscle Figure 17-07 Human Anatomy, 3rd edition Prentice Hall, © 2001 Subdivisions of the ANS • Sympathetic division (thoracolumbar) – Cell bodies for all the neurons #1 reside in the thoracic and lumbar portions of the spinal cord. • T1 – L2 – Stimulates heart beat & tissue metabolism, increases alertness, prepares the body to deal with emergencies (“fight or flight” division) Subdivisions of the ANS Figure 17-02b Subdivisions of the ANS • Parasympathetic division (craniosacral) – Cell bodies reside in the brain stem (cranial nerves) or in the sacral portion of the spinal cord. – Slows the heart rate, inhibits senses, prepares the body for rest and relaxation; (“rest and digest” division). The Sympathetic Division Sympathetic Chain Ganglia – A chain of ganglia that run alongside the spinal cord – Extends on both sides of the vertebral column – Carries preganglionic fibers and cell bodies of postganglionic neurons Sympathetic Chain Ganglia Figure 17-05 Anatomy of the Sympathetic Chain • Rami communicates from the spinal nerves connect to the chain • Other nerves (splanchnic) project from the chain Routes of Preganglionic Axons • Cell bodies of neurons #1 lie in the lateral gray horns of the spinal cord • The axons of neurons #1 leave the spinal cord via the ventral root • These axons pass to the spinal nerve • Axons leave the spinal nerve via the white and gray branches (rami communicates) – Connect with the sympathetic chain ganglia Routes of Preganglionic Axons • There are 3 possible routes that sympathetic neurons #1 may follow • Possibility #1: synapses with the ganglion at that level – Neuron #2 leaves at that level via the gray ramus communicans, rejoins the same level spinal nerve Routes of Preganglionic Axons Figure 17-04a Routes of Preganglionic Axons • Possibility #2: neuron #1 goes up or down the chain and synapses at some other level. – Neuron #2 leaves at that level via the gray ramus communicans, rejoins the spinal nerve at that level. • Possibility #3: neuron #1 does not synapse in the chain but exits by a splanchnic nerve and synapses in a collateral ganglion. – Neuron #2 travels from that ganglion to its destination. Route Through Collateral Ganglia Figure 17-04b Collateral Ganglia • Location – anterior to the aorta in the abdominopelvic cavity – Celiac ganglion • Innervates upper abdominal viscera – Superior mesenteric • Innervates middle abdominal viscera – Inferior mesenteric • Innervates lower abdominal & pelvic organs The Adrenal Medulla • Only preganglionic neurons are in this pathway • Neuron #1 stimulates the medulla, • The medulla releases norepinephrine and epinephrine (adrenaline) to blood Route to Adrenal Medulla Figure 17-04c Adrenal Medulla Figure 17-06 Effects of Sympathetic Stimulation • Widespread – The sympathetic chain allows one preganglionic fiber to synapse with many postganglionic neurons • Enhanced & prolonged by the adrenal medulla Neurotransmitters • Preganglionic fibers release acetylcholine (Ach) – Cholinergic • Postganglionic fibers (most) release norepinephrine (NE) – Adrenergic • Adrenal medulla releases norepinephrine and epinephrine (adrenaline) Membrane Receptors & Sympathetic Function • 2 types of receptors in synapses – The same neurotransmitter can have different effects • Alpha receptors cause a rise in intracellular calcium – Respond more than beta to NE • Beta receptors cause changes in the metabolic activity of the target cells Summary of Sympathetic Division • Cell bodies are found in the thoracic and lumbar portions of the spinal cord • Preganglionic fibers are short, connect to the sympathetic chain, and synapse with long postganglionic fibers • Preganglionic fibers produce ACh, postganglionic fibers produce NE or Ach • “Fight or flight” division The Parasympathetic Division Organization of the PNS • Cell bodies are in the brain or in the gray matter of the spinal cord (sacral region) • Neurons #1 exit the cranial region through cranial nerves 3, 7, 9, & 10 • Neurons #1 exit the spinal cord through the sacral spinal nerves Subdivisions of the ANS Figure 17-02b Organization of the PNS • Neurons #1 are long and synapse with neurons #2 (short) in ganglia • Ganglia are found on near the visceral effector Anatomy of the PNS • The cranial nerve fibers involved are motor control smooth muscle & glands in the upper body – Cranial nerve #3 – lens & pupil – Cranial nerve #7 – lacrimal glands, submandibular & submaxillary glands (salivary) – Cranial nerve #9 – parotid gland (salivary) – Cranial nerve #10 - viscera of thorax & abdomen • Sacral nerves innervate the kidneys, colon, & sex organs Anatomy of the PNS Figure 17-09 General Functions of the PNS • Prepares the individual for rest and relaxation • “Rest & digest” division • Effects on various organs: – – – – – – Decreases heart rate Constricts bronchioles Increases salivation Increases motility of stomach Increases motility of colon Constricts pupils Neurotransmitter • Both preganglionic and postganglionic fibers release acetylcholine – Causes localized and short-term effects Summary of the Parasympathetic Division • Cell bodies are found in the brain and in the sacral region of the spinal cord • Preganglionic fibers are long and synapse with short postganglionic fibers on or near the target viscera • Both preganglionic and postganglionic fibers produce Ach • “Rest & digest” division Relationship Between the Sympathetic and Parasympathetic Divisions • Most organs receive dual innervation • Visceral organs are intrinsically excited – ANS either increase excitation or inhibit the activity – Eg. Sympathetic fibers increase heart rate, parasympathetic fibers decrease heart rate