CoverModule 1.0. Homeostasis, Membranes, Electrophysiology and ANS1.1. Homeostasis1.1.1. Homeostasis Defined1.1.2. Homeostatic Control Systems1.1.3. Feedback Response Loop1.2. Cell Transport; Water & Solutes1.2.1. Fluid Compartments1.2.2. Osmosis1.2.3. Diffusion of Solutes1.2.4. Active Transport1.2.5. Bulk Transport1.3. Electrophysiology1.3.1. Ions and Cell Membranes1.3.2. Membrane Potentials1.3.3. Graded Potential1.3.4. Action Potentials1.3.5. Refractory Periods1.3.6. Propagation of an Action Potential1.4. The Synapse1.5. The Autonomic Nervous System1.5.1. Organization of the Nervous System1.5.2. Structural Organization of the ANS1.5.3. The SNS and the PNS1.5.4. The Enteric Nervous System1.5.5. Physiology of the ANS1.5.6. Neurotransmitters of the ANS1.5.7. Receptors of the ANS1.5.8. Actions of the Autonomic Nervous System1.5.9. Table of Actions for the SNS and PNS and Some Common DrugsModule 2.0. Skeletal Muscle and Special Senses2.1. Structural Organization of Skeletal Muscle2.2.1. Neuromuscular Junction, Excitation-Contraction Coupling2.2.2. Muscle Contractures and Cramps2.3. Whole Muscle Contraction, Fiber Type, Fatigue and Muscle Pharmacology2.3.1. Motor Units2.3.2. Factors that Influence the Force of Contraction2.3.2. Factors that Influence the Force of Muscle Contraction2.3.3. Energy Source for Muscle Contraction2.3.4. Skeletal Muscle Fiber Types2.3.5. Fatigue2.3.6. Muscle Pharmacology2.4. Smooth Muscle2.4.1. Smooth Muscle Contraction2.5. Control of Body Movement2.5.1. Voluntary Control of Muscle2.5.2. Reflexes2.6. Taste and Smell2.6.1. Taste2.6.2. The Sense of Smell2.7. Vision2.7.1. Structure of the Eye2.7.2. Focusing Light on the Retina2.7.3. Converting Light to Action Potentials2.7.4. The Retina2.7.5. Phototransduction2.7.6. Receptive Fields2.8. Hearing and Equilibrium2.8.1. The Nature of Sound2.8.2. The Hearing Apparatus2.8.3. Sound Vibrations to Action Potentials2.8.4. The Sense of Balance and EquilibriumModule 3.0. Cardiovascular System3.1. Structure of the Heart3.1.1. Chambers and Circulation3.2. Cardiac Cell Action Potentials3.2.1. Action Potentials in Cardiac Muscle Cells3.2.2. Action Potentials in Cardiac Autorhythmic cells3.2.3. Cellular Mechanisms of Inotropy and Chronotropy3.3. Electrophysiology of Heart Muscle3.3.1. Heart Conduction System3.3.2. Electrocardiogram (ECG)3.3.3. Abnormal ECG - Current of Injury3.4. The Cardiac Cycle3.4.1. Cardiac cycle3.4.2. Cardiac Measurements and Pressure Volume Loops3.5. Blood vessels and Blood Pressure3.5.1. Arteries and Veins3.5.2. Capillaries3.5.3. Blood Pressure Regulation and Shock3.5.4. Capillary Exchange3.5.5. Myogenic and Paracrine Regulation of Vasoconstriction and Vasodilation3.6. Blood3.6.1. Composition of Blood3.6.2. Hematopoeisis3.6.3. Breaking Down Red Blood Cells3.6.4. HemostasisModule 4.0. Urinary and Respiratory Systems4.1. Function and Structure of the Kidney4.1.1. Urinary System Function4.1.2. Functional Anatomy of the Urinary System4.1.3. The Nephron: Functional Unit of the Kidney4.1.4. The Renal Corpuscle: Bowman's Capsule4.2. Physiology of Urine Production4.2.1. Filtration4.2.2. Renal Clearance4.2.3. Tubular Reabsorption4.2.4. Urine Concentration and Dilution4.2.5. Hormonal Regulation of Urine Production4.3. Acid/Base Balance4.3.1. Buffers4.3.2. Acid/Base Disturbances4.4. The Respiratory System4.4.1. Respiratory System Structure and Function4.4.2. Respiratory Membrane4.4.3. Respiratory pressures and Inspriation/Expiration4.4.4. Alveoli and Surfactant4.4.5. Pneumothorax4.4.6. Pressure-Volume Loops and the Work of Breathing4.5. Gas Exchange and Transport4.5.1. Gas Laws4.5.2. Partial Pressure Gradients in the Lung4.5.3. Alveolar Gas Equation4.5.4. Oxygen and Carbon Dioxide Transport in the Blood4.5.5. Alveolar Ventilation4.5.6. Ventilation/Perfusion Ratio4.6. Chronic Bronchitis and Emphysema4.6.1. Respiratory Control by the Medulla Oblongata4.6.2. Chemicals that Regulate VentilationModule 5.0. Digestive, Endocrine and Reproductive Systems5.1. Functional Anatomy of the Digestive System5.1.1. Layers of the Digestive Tract5.1.2. Enteric Nervous System5.1.3. Organs of the Digestive System5.2. Digestion5.2.1. Carbohydrates5.2.2. Proteins5.2.3. Lipids5.2.4. Lipoproteins5.3. Regulation of Digestive Secretions5.4. Endocrine System5.4.1. Overview of the Endocrine System5.4.2. Hormone Receptors5.4.3. Hormones of the Body5.4.4. Other Hormones: Melatonin and Pheromones5.5. The Hypothalamus and Pituitary Gland5.5.1. Structure and Function of the Hypothalamus and Pituitary Gland5.5.2. The Posterior Pituitary5.5.3. The Anterior Pituitary5.5.4. Growth Hormone5.5.5. Prolactin5.5.6. Thyroid Hormones5.5.7. Adrenal Hormones5.6. Pancreas5.6.1. Insulin and Glucagon5.6.2. Diabetes Mellitus5.7. Reproductive System Anatomy5.7.1. Female Reproductive Anatomy5.7.2. Male Reproductive Anatomy5.7.3. Sexual Development at Puberty5.7.4. Male Reproductive Endocrine Axis5.7.5. Spermatogenesis5.7.6. Female Reproductive System: Oogenesis5.7.7. Ovulation and Fertilization5.7.8. The Ovarian Cycle5.7.9. The Uterine Cycle5.7.10. PregnancyAppendix A. GenderAppendix B. The Placebo EffectB.2.1. The Placebo EffectB.2.2. Examples of the Placebo EffectB.2.3. How do Placebos Work?B.2.4. Are Placebos Ethical?B.2.5. How do we validate actual effectiveness of placebosB.2.6. Tips for evaluating scientific evidenceB.2.7. What about Faith Healingstest chapter

Neurotransmitters of the ANS

Neurotransmitters are chemicals that travel across the synapse connecting two neurons, or between a neuron and an effector. For example, acetylcholine (ACH) is an important neurotransmitter that we will discuss in this section and again when we discover that it is the neurotransmitter between neurons and skeletal muscle. Sometimes neurons can be classified by the type of neurotransmitter they release. Cholinergic neurons produce ACH and store ACH in their synaptic terminals. The preganglionic neuron for both the parasympathetic and sympathetic nervous systems is cholinergic. The postganglionic neuron of the parasympathetic division is also cholinergic. The postganglionic neuron for the sympathetic division is usually an adrenergic neuron which means that it produces neurotransmitter molecules that are related to "adrenaline". As a group, the molecules that can stimulate adernergic receptors are called catecholamines.  Catecholamines are an organic chemistry group that includes norepinephrine (NE), epinephrine (EPI) and dopamine. In the Sympathetic nervous system, NE is the neurotransmitter found at the synapse between postganglionic neurons and the organ. There is one exception to this rule that we should know and remember.  Sympathetic postganglionic neurons innervating general sweat glands and some reproductive system blood vessels are cholinergic and release ACH.


Image by JS S22

The image above shows how catecholamines are produced. The synthesis starts with the amino acid called Tyrosine. Adding a hydroxyl group to the aromatic ring creates DOPA. A carboxyl group is then removed from DOPA to yield dopamine. Once dopamine is created we have the first of the "catecholamines" used by the body to bind adrenergic receptors (shown in yellow in the picture). Dopamine is converted to Norepinephrine by the addition of a hydroxyl group to the side chain and finally Epinephrine is generated when a methyl group is added. The catecholamines are similar to each other and can all bind the several subtypes of alpha and beta adrenergic receptors. However, there are enough small molecular difference among the catecholamines to cause subtle differences in half-life and binding affinity. Dopamine is generally found as a neurotransmitter between neurons in the central nervous system. Norepinephrine (NE) is a neurotransmitter that is produced between neurons of the central nervous system as well.  However,  NE is also important as a neurotransmitter released from sympathetic post ganglionic neurons and the adrenergic receptors of an effector (smooth muscle, cardiac muscle and gland). Epinephrine is generally produced in the adrenal medulla and secreted into the blood where it can travel through the body and affect a large breadth of adrenergic receptors. 

Clinical Pearl  (L-DOPA)

Patients with Parkinson's Disease have a deficiency of dopamine secreting neurons that arise from an area of the brain called the substantia nigra. As the symptoms of this disease begin to manifest, patients can often find relief from medication that increases dopamine levels in the brain. A molecule called L-DOPA (the L references an isomer of dopa that is preferred by the human body) can be given orally to patients. L-DOPA can cross the blood brain barrier where it can be converted to dopamine. 

While this drug can certainly increase dopamine in the brain and relieve symptoms of early Parkinson's disease, there is an unfortuante side effect. As L-DOPA travels through the blood on its way to the brain, it can meet up with an enzyme called aromatic L-amino acid decarboxylase that is expressed in many tissues of the body. This enzyme will remove a carboxyl group from the L-DOPA molecule and produces dopamine. The dopamine then becomes free to float through the circulation and bind to adrenergic receptors (similar to what epinephrine does). Therefore, you might imagine that the side effects of taking L-DOPA would involve similar effects to having an overdose of adrenaline. Among other things, blood pressure rises substantially and can be dangerous. 

Clever scientists, in an effort to mitigate the side effects of taking L-DOPA for Parkinson's disease, discovered a drug called Carbidopa that functions as an L-DOPA decarboxylase inhibitor. However, this inhibitor cannot cross the blood brain barrier. Thus, if a patient takes L-DOPA simultaneously with Carbidopa, then the L-DOPA will not be converted as readily to dopamine, and instead finds its way to the brain.  

Modern drug regiments involve a simultaneous consumption of both drugs. 

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