Autonomic Nervous System Keri Muma Bio 6 Organization of the Nervous System Efferent Division Somatic Nervous System Voluntary control Effector = skeletal muscles Muscles must be excited by a motor neuron or they are inactive (On or off) One motor neuron extends from the CNS all the way to the effector muscle 1
Efferent Division Autonomic Nervous System Involuntary control Effectors = glands, smooth and cardiac muscles Two neuron chain between the CNS and the effector Preganglionic neuron Postganglionic neuron Anatomy of the Sympathetic System Origin sites Thoracolumbar - emerges from thoracic and lumbar regions T1- L2 Length of neurons Short preganglionic Long postganglionic Location of Sympathetic Ganglia Location of ganglia Close to vertebral column Paravertebral ganglia chain running parallel to the vertebral column, extends from C3 to S4 Collateral (prevertebral) ganglia lie anterior to the vertebral column 2
Path of Sympathetic Neurons Cell bodies are located in the lateral gray horns of spinal segments T1 L2 White ramus communicans pre neuron passes from the spinal nerve to the paravertebral ganglia Gray ramus communicans post neuron exits the paravertebral ganglia Path of Sympathetic Neurons Three different routes the preganglion neuron can take once it enters the paravertebral ganglia Anatomy of the Parasympathetic System Origin sites Craniosacral - emerges from brainstem and sacral regions Length of neurons Long preganglionic Short postganglionic 3
Location of Parasympathetic Ganglia Ganglia close to target organ Terminal ganglia very close to target organ Intramural ganglia within the walls of the target organ Structural Differences between ANS Divisions Division Sympathetic Parasympathetic Origin of Fibers Thoracolumbar region of the spinal cord Brain and sacral spinal cord Length of Fibers Short preganglionic and long postganglionic Long preganglionic and short postganglionic Location of Ganglia Close to the spinal cord Close to or in the visceral effector organs 4
Regulatory Functions of the ANS Cardiovascular activities Cardiac output, heart rate, blood pressure and distribution Body fluid chemistry ph, osmolarity, thirst, water content Pulmonary activities Breathing rate, bronchiole diameter, O2 and CO2 content Gastrointestinal activities Motility along digestive tract, mechanical and chemical digestion Visceral reflexes Micturation, defecation, sexual reflexes Stress Stimulates various hormones to cope with situation Autonomic Control Centers Functions of the ANS Sympathetic division fight or flight Enables body to cope rapidly during emergency situations Dominant when excited, frightened, or during exercise Increase heart rate and blood pressure Increase respiratory rate, dilates bronchioles Blood shunted to skeletal muscles, brain, and heart away from digestive organs and skin Dilates pupils Liver releases glucose to meet increased energy needs Increased cellular metabolism Initiates sweating to lower body temperature Increased RBC production and clotting ability Na+ absorption / K+ secretion, decreased urine production 5
Functions of the ANS Parasympathetic division rest and digest Dominant in non-stressful situations Conserves energy and directs maintenance activities such as digestion and excretion Blood shunted to visceral organs Constricts pupils Increased digestive glandular secretions and activity Respiratory and lacrimal secretions Blood pressure, heart rate, respiratory rates at low normal levels Divisions of the ANS Dual Innervation - most internal organs are innervated by both autonomic divisions Antagonistic control the divisions counterbalance each other by continuously making adjustments The ANS either further excites or inhibits the organs 6
Dual Innervation There are exceptions to dual innervation: Most blood vessels, sweat glands, and arrector pili muscles are controlled by sympathetic fibers only Sympathetic Tone: The sympathetic division controls blood pressure and keeps the blood vessels in a continual state of partial constriction Sympathetic Tone Increased sympathetic activity constricts blood vessels Decreased sympathetic activity dilates blood vessels Sympathetic Effects Some sympathetic metabolic effects are not reversed by the parasympathetic division Increases the metabolic rate of body cells Raises blood glucose levels Mobilizes fat as a food source Stimulates the reticular activating system (RAS) of the brain, increasing mental alertness 7
Sympathetic Effects The sympathetic nervous system triggers the release of epinephrine from the adrenal medulla Some preganglion fibers travel to the adrenal medulla where the synapse with hormone producing cells These cells release epinephrine into the blood stream ANS Neurotransmitters Acetylcholine (ACh) and norepinephrine (NE) are the two major neurotransmitters of the ANS Neurons that release ACh are cholinergic fibers Neurons that release NE are adrenergic fibers Cholinergic Fibers All parasympathetic pre and postganglionic neurons All sympathetic preganglionic neurons Sympathetic postganglionic that innervate sweat glands and arrector pili muscles 8
Adrenergic Fibers Most sympathetic postganglionic neurons (except sweat glands and arrector pili) Types of Receptors Cholinergic receptors bind Ach Nicotinic Muscarinic Adrenergic receptors binds NE and/or E Alpha Beta ANS Receptors Neurotransmitter effects can be excitatory or inhibitory depending upon the receptor type they bind to 9
Nicotinic Receptors Nicotinic receptors are found on: Motor end plates (somatic targets) All postganglionic neurons of both sympathetic and parasympathetic divisions The hormone-producing cells of the adrenal medulla The effect of ACh binding to nicotinic receptors is always excitatory Muscarinic Receptors Muscarinic receptors are found on all effectors stimulated by postganglionic cholinergic fibers Parasympathetic effectors Sweat glands and arrector pili The effect of ACh binding to muscarinic receptors: Can be either inhibitory or excitatory depending on the receptor subtype of the target organ Examples slows cardiac muscle: inhibitory smooth muscle of digestive: excitatory Adrenergic Receptors Alpha receptors greater sensitivity to NE α 1 binding of NE is stimulatory Constriction of blood vessels serving the skin and abdominal viscera α 2 binding of NE is inhibitory Inhibits insulin secretion from the pancreas 10
Adrenergic Receptors Beta Receptors β 1 found primarily in the heart, binding of NE and E is excitatory Increases cardiac output, release of renin from kidneys to increase BP β 2 binding of E is generally inhibitory Dilates coronary blood vessels and bronchioles in the lungs, relaxes digestive and urinary smooth muscle β 3 found in adipose tissue Stimulates lipolysis Localized vs. Wide Spread Effects Parasympathetic localized and short lived effect Preganglionic synapse with one or a few postganglionic ACH is quickly destroyed Sympathetic longer lasting and body wide mobilization Preganglionic synapse with many posts at different levels NE is inactivated slower than ACH NE and E are indirect neurotransmitters, using a second-messenger system Epinephrine is released into the blood and remains there until destroyed by the liver 11
Effects of Drugs on the ANS Drugs are designed to obtain an inhibitory or excitatory effect on a target organ by either blocking or initiating desired effects of neurotransmitters Sympathomimetic agents enhance sympathetic response Sympatholytic agents decrease sympathetic response Parasympathomimetic / Parasympatholytic Effects of Drugs Atropine blocks muscarinic receptors therefore blocks parasympathetic effects, increase heart rate and fecal and urinary retention parasympatholytic Tricyclic antidepressants prolong the activity of NE on postsynaptic membranes sympathomimetic Over-the-counter drugs for colds, allergies, and nasal congestion stimulate α-adrenergic receptors sympathomimetic Effects of Drugs Beta-blockers blocks cardiac B1 receptors, decreases HR and BP sympatholytic Alpha-blockers - interfere with vasomotor fibers and are used to treat hypertension sympatholytic Salbutamol activates B2 receptors, dilates bronchioles (asthma treatment) sympathomimetic 12
Age and the ANS In old age, ANS efficiency decreases, resulting in constipation, dry eyes, and orthostatic hypotension Orthostatic hypotension is a form of low blood pressure that occurs when sympathetic vasoconstriction centers respond slowly to positional changes 13