Chapter 15 Notes PDF

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Comparison of Somatic and Autonomic Nervous Systems  Somatic Nervous System o Includes both sensory and motor neurons o Sensory neurons  Convey input from receptors for somatic senses (tactile, thermal, pain, and proprioceptive sensations) and from receptors for the special senses (sight, hearing, taste, smell, and equilibrium)  All of these sensations are normally consciously perceived o Motor neurons  Innervate skeletal muscles - the effectors of the somatic nervous system - and produce both reflexive and voluntary movement  When a somatic motor neuron stimulates the muscle, it contracts; the effect is always excitation  Autonomic Nervous System o The main input to the ANS comes from autonomic sensory neurons  These neurons are associated with interoceptors, which are sensory receptors located in blood vessels, visceral organs, muscles, and the nervous system that monitor conditions in the internal environment  Example - chemoreceptors that monitor blood CO2 levels  Not consciously perceived most of the time although intense activation of interoceptors may produce conscious sensations  Example - pain sensations from angina pectoris (chest pain) from not enough blood to the heart  Input that influences the ANS also includes some sensations monitored by somatic sensory and special sensory neurons o Autonomic motor neurons  Regulate visceral activities by either increasing (exciting) or decreasing (inhibiting) ongoing activities in their effector tissues (cardiac muscle, smooth muscle, glands)  Tissue innervated by the ANS often function to some extent even if their nerve supply is damaged o Most autonomic responses cannot be consciously altered to any great degree  Comparison of Somatic and Autonomic Motor Neurons o Most autonomic motor pathways consist of two motor neurons in series, one following another o The first neuron (preganglionic neuron) has its cell body in the CNS; its myelinated axon extends from the CNS to an autonomic ganglion  Ganglion is a collection of neuronal cell bodies in the PNS o The cell body of the second neuron (postganglionic neuron) is also in that same autonomic ganglion; its unmyelinated axon extends directly from the ganglion to the effector (smooth muscle, cardiac muscle, or gland) o The output part of the ANS has two divisions:  Sympathetic division  Fight-or-flight  Sympathetic activities result in increased alertness and metabolic activities in order to prepare the body for an emergency situation  Responses include rapid heart rate, faster breathing rate, dilation of the pupils, dry mouth, sweaty but cool skin, and constriction of the blood vessels to organs  Parasympathetic division  Rest-and-digest

Parasympathetic activities conserve and restore body energy during times of rest or digesting a meal  The majority of its output is directed to the smooth muscle and glandular tissue of the gastrointestinal and respiratory tracts  Conserves energy and replenishes nutrient stores o Most organs have dual innervation  They receive impulses from both sympathetic and parasympathetic neurons o In some organs, nerve impulses from one division decrease the organ's activity (excitation), and impulses from another division decrease the organ's activity (inhibition) 

Anatomy of Autonomic Motor Pathways  Each division of the ANS has two motor neurons  Preganglionic neuron o Its cell body is in the brain or the spinal cord o Its axon exits the CNS as part of a cranial or spinal nerve o The axon has a small diameter and is myelinated o It extends to an autonomic ganglion where it synapses with a postganglionic neuron o The sympathetic division is also called the thoracolumbar division  In the sympathetic division - the preganglionic neurons have their cell bodies and lateral horns of the gray matter in the 12 thoracic segments and the first two (and sometimes three) lumbar segments of the spinal cord  Thoracolumbar outflow: the axons of the sympathetic preganglionic neurons o The parasympathetic division is also called the craniosacral division  Cell bodies of preganglionic neurons of the parasympathetic division are located in the nuclei of four cranial nerves in the brain stem (III, VII, IX, X) and in the lateral gray matter of the second through fourth sacral segments of the spinal cord  Craniosacral outflow: the axons of the parasympathetic neurons  Autonomic Ganglia o Sympathetic ganglia  The sites of synapses between sympathetic preganglionic and postganglionic neurons  Sympathetic trunk ganglia  Also called vertebral chain ganglia or paravertebral ganglia  Lie in a vertical row on either side of the vertebral column  They extend from the base of the skull to the coccyx  Postganglionic axons from these ganglia primarily innervate organs above the diaphragm  Most sympathetic preganglionic axon are short  Prevertebral (collateral) ganglia  Lies anteriorly to the vertebral column and close to the large abdominal arteries  Typically innervate organs below the diaphragm o Parasympathetic ganglia  Preganglionic axons of the parasympathetic division synapse with postganglionic neurons in terminal (intramural) ganglia  Most of these ganglia are located close to or actually within the wall of visceral organs  Parasympathetic preganglionic axons are long because they are in or close to visceral organs, whereas parasympathetic postganglionic axons which are short  Postganglionic Neurons

Once axons of sympathetic preganglionic neurons pass to sympathetic trunk ganglia, they may connect with postganglionic neurons in one of the following ways:  An axon may synapse with postganglionic neurons in the ganglion it first reaches  An axon may ascend or descend to a ganglion before synapsing with a postganglionic neurons  An axon may continue, without synapsing, through the sympathetic trunk ganglion to end at prevertebral ganglion and synapse with postganglionic neurons there  An axons may also pass, without synapsing, through the sympathetic trunk ganglion and a prevertebral ganglion and then extend to chromaffin cells of the adrenal medullae that are functionally similar to sympathetic postganglionic neurons o Axons of preganglionic neurons of the parasympathetic division pass to terminal ganglia near or within a visceral effector o In the ganglion, the presynaptic neuron usually synapses with only four or five postsynaptic neurons, all of which supply a single visceral effector, allowing parasympathetic responses to be localized to a single effector Autonomic Plexuses o Autonomic plexuses: axons of both the sympathetic and parasympathetic neurons form tangles networks in the thorax, abdomen, and pelvis, many of which lie along major arteries  May contain sympathetic ganglia and axons of autonomic sensory neurons o Major plexuses in the thorax  Cardiac plexus - supplies the heart  Pulmonary plexus - supplies the bronchial tree o Major plexuses in the abdomen and pelvis  Celiac (solar) plexus  Largest autonomic plexus that surrounds the celiac trunk  Contains two large celiac ganglia, two aorticorenal ganglia, and a dense network of autonomic axons  Is distributed to the stomach, spleen, pancreas, liver, gallbladder, kidneys, adrenal medullae, testes, and ovaries  Superior mesenteric plexus  Contains the superior mesenteric ganglion  Supplies the small and large intestines  Inferior mesenteric plexus  Contains the inferior mesenteric ganglion which innervates the large intestine  Hypogastric plexus  Some sympathetic postganglionic neurons from the inferior mesenteric ganglion extend through this plexus  Anterior to the fifth lumbar vertebra  Supply the pelvic viscera  Renal plexus  Contains the renal ganglion  Supplies the renal arteries within the kidneys and ureters Structure of the Sympathetic Division o Pathway from spinal cord to sympathetic trunk ganglia  Cell bodies of sympathetic preganglionic neurons are part of the lateral gray horns of all thoracic and the first two lumbar segments of the spinal cord  The preganglionic axons leave the spinal cord along with the somatic motor neurons at the same segmental level o

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After exiting through the intervertebral foramina, the myelinated preganglionic sympathetic axons pass into the anterior root of a spinal nerve and enter a short pathway called a white ramus, before passing to the nearest sympathetic trunk ganglion on the same side  Collectively the white ramus are called the white ramus communicantes (singular is communicans)  Structures containing sympathetic preganglionic axons that connect the anterior ramus of the spinal nerve with the ganglia of the sympathetic trunk  "White" in their name indicates they have myelinated axons  Only the thoracic and first two lumbar nerves have these o Pathway from sympathetic trunk ganglia to visceral effectors  Axons leave the sympathetic trunk by entering spinal nerves (one of four ways)  The axons of some postganglionic neurons (those that synapsed with postganglionic neurons in the sympathetic trunk where they enter or farther up or down) leave the sympathetic trunk by entering short a pathway called a gray ramus and then merge with the anterior ramus of the spinal nerve  Gray rami communicantes  Structures containing sympathetic postganglionic axons that connect the ganglia of the sympathetic trunk to the spinal nerves  "Gray" in their name indicates the axons are unmyelinated  There is gray ramus leading to each of the 31 pairs of spinal nerves  The axons of the postganglionic neurons that leave the sympathetic trunk to enter spinal nerves provide sympathetic innervation to the visceral effectors in the skin of the neck, trunk, and limbs Horner's Syndrome o Sympathetic innervation to of side of the face is lost dur to an inherited mutation, injury, or disease that affects sympathetic outflow through the superior cervical ganglion o Symptoms include:  Ptosis (drooping of the upper eyelid)  Miosis (constricted pupil)  Anhidrosis (lack of sweating) o The symptoms occur on the affected side of the face Structure of the Parasympathetic Division o Cell bodies of parasympathetic preganglionic neurons are found in nuclei in the brain stem and in the lateral gray matter of the second through fourth sacral segments of the spinal cord o Their axons emerge as part of a cranial nerve or as part of the anterior root of a spinal nerve o Cranial parasympathetic outflow  Consists of preganglionic axons that extend from the brain stem in four cranial nerves o Sacral parasympathetic outflow  Consists of preganglionic axons in anterior roots of the second through fourth sacral spinal nerves o The preganglionic axons of both the cranial and sacral outflows end in terminal ganglia, where they synapse with postganglionic neurons o As preganglionic axons course through the sacral spinal nerves, they branch off these nerves to form Pelvic Splanchnic Nerves  They synapse with parasympathetic postganglionic neurons located in terminal ganglia in the walls of the innervated viscera 

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From the terminal ganglia, parasympathetic postganglionic axons innervate smooth muscle and glands in the walls of the colon, ureters, urinary bladder, and reproductive organs Structure of the Enteric Division o Also called the enteric nervous system (ENS) o The gastrointestinal tract must respond to surrounding stimuli to generate proper homeostatic control o The gastrointestinal tract has its own nervous system with intrinsic input, processing, and output o Functions independently of the central nervous system activity, but an receive controlling input from the CNS o Specialized collection of nerves and ganglia forming a complex, integrated neuronal network within the wall of the gastrointestinal tract, pancreas, and gallbladder o The enteric network of nerves and ganglia contains sensory neurons capable of monitoring tension in the intestinal wall and accessing the composition of the intestinal contents o These sensory neurons relay their input signals to interneurons within the enteric ganglia which process the incoming signals and generates a regulatory output signal to motor neurons through plexuses within the wall of the digestive organs o The motor neurons carry the output signals to the smooth muscle and glands of the gastrointestinal tract to exert control over its motility and secretory activities o Myenteric plexus  Positioned between the outer longitudinal and circular muscle layers from the upper esophagus to the anus  Most nerve fibers that innervate the digestive organs arise from this plexus and the submucosal plexus  Communicates extensively with the smaller plexus - the submucosal plexus  Occupies the gut wall between the circular muscle layer and the muscularis mucosae and runs from the stomach to the anus  Neurons emerge from the ganglia of these two plexuses to form smaller plexuses around blood vessels and within the muscle layers and mucosa of the gut wall 

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ANS Neurotransmitters and Receptors  Cholinergic neurons and receptors o Release the neurotransmitter acetylcholine (ACh) o In the ANS, cholinergic neurons include:  All sympathetic and parasympathetic preganglionic neurons  Sympathetic postganglionic neurons that innervate most sweat glands  All parasympathetic postganglionic neurons o Acetylcholine (ACh)  Stored in synaptic vesicles and released by exocytosis  Diffuses across the synaptic cleft and brings with specific cholinergic receptors: integral membrane proteins in the post synaptic membrane  There are two types of cholinergic receptors, both of which bind ACh:  Nicotinic receptors  Present in the plasma membrane of dendrites and cell bodies of both sympathetic and parasympathetic postganglionic neurons  Present in the plasma membranes of chromaffin cells of the adrenal medullar  Present in the motor end plate at the neuromuscular junction  Nicotine mimics the action of ACh by binding to these receptors

Activation of nicotinic receptors by ACh causes depolarization and thus excitation of the postsynaptic cell, which can be a postganglionic neuron, an autonomic effector, or a skeletal muscle fiber  Muscarinic receptors  Present in the plasma membranes of all effectors innervated by parasympathetic postganglionic axons  A mushroom poison called muscarine mimics the actions of ACh by binding to them  Activation of muscarinic receptors by ACh sometimes causes depolarization (excitation) or hyperpolarization (inhibition), depending on which particular cell bears the muscarinic receptors  ACh activates both types of cholinergic receptors  Acetylcholinesterase (AChE)  The enzyme that inactivates ACh  Causes the effects triggered by cholinergic neurons to be brief Adrenergic Neurons and Receptors o Adrenergic neurons  Release norepinephrine (NE) - also known as noradrenalin  Most sympathetic postganglionic neurons are adrenergic  Stored in synaptic vesicles and released by exocytosis  Molecules of NE diffuse across the synaptic cleft and bind to specific adrenergic receptors on the post synaptic membrane, causing either excitation or inhibition of the effector cell o Adrenergic receptors  Norepinephrine can either be released as a neurotransmitter by sympathetic postganglionic neurons or released as a hormone into the blood by chromaffin cells of the adrenal medullae  Two main types of adrenergic receptors:  Alpha receptors and Beta receptors  Found on visceral effectors innervated by most sympathetic postganglionic axons  Further classified into subtypes based on the specific responses they elicit and by their selective binding of drugs that activate or block them  Typically activation of A1 and B1 receptors causes excitation  Typically activation of A2 and B2 receptors causes inhibition of effector tissues  Typically activation of B3 receptors causes activation of thermogenesis and are only found in cells of brown adipose tissue  Cells of most effectors contain either alpha or beta receptors  Some visceral effector cells contain both  Norepinephrine stimulates alpha receptors more than beta receptors  The activity of norepinephrine at a synapse is terminated either when NE is taken up by the axon that released it or when the NE is enzymatically inactivated by either catechol-0-methyltranferase (COMT) or monoamine oxidase (MAO)  Norepinephrine lingers in the synaptic cleft for longer therefore the effects triggered by adrenergic neurons typically last longer than those triggered by cholinergic neurons Receptor Agonists and Antagonists o A large variety of drugs and natural products can selectively activate or block specific cholinergic or adrenergic receptors o Agonist 

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A substance that binds to and activates a receptor, in the process mimicking the effect of a natural neurotransmitter or hormone Antagonist  A substance that binds to and blocks a receptor, thereby preventing a natural neurotransmitter or hormone from exerting its effect

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Physiology of the ANS  Autonomic Tone o Autonomic tone: the balance between the sympathetic and parasympathetic activity that is regulated by the hypothalamus o Typically, the hypothalamus turns up sympathetic tone at the same time it turns down the parasympathetic tone, and vice versa o The two divisions can affect body organs differently because their postganglionic neurons release different neurotransmitters and because the effector organs posses different adrenergic and cholinergic receptors o An increase in sympathetic tone has one effect, and a decrease produces the opposite effect  Sympathetic response o During physical or emotional stress, the sympathetic division dominates the parasympathetic division o High sympathetic tone favors body functions that support vigorous physical activity and rapid production of ATP o At the same time the sympathetic division reduces body functions that favor the storage of energy o Visualize body changes that occur during "E situations"  Exercise  Emergency  Excitement  Embarrassment o Activation of the sympathetic division and release of hormones by the adrenal medullae set in motion a series of physiological responses collectively called "fight-or-flight", which includes the following effects:  Pupils dilate  Heart race and blood pressure increase  The airways dilate  Blood vessels that’s supply kidneys and gastrointestinal tract constrict to decrease blood flow  Slowing of urine formation and digestive activities  Blood vessels that supply organs involved in exercise or fighting off danger dilate to allow greater blood flow  Skeletal muscles, cardiac muscle, liver, and adipose tissue  Liver cells perform glycogenolysis  Breakdown of glycogen to glucose  Adipose tissue cells perform lipolysis  Breakdown of triglycerides to fatty acids and glycerol  Release of glucose by liver increases blood glucose level  Processes that are not essential for meeting the stressful situation are inhibited o The effects of sympathetic stimulation are longer lasting and more widespread than the effects of parasympathetic stimulation for three reasons:  Sympathetic postganglionic axons diverge more extensively

Acetylcholinesterase quickly inactivates acetylcholine while norepinephrine lingers in the synaptic cleft for longer  Epinephrine and norepinephrine secreted into the blood from the adrenal medullae intensify and prolong the responses cause by NE liberated from sympathetic postganglionic axons Parasympathetic Responses o The parasympathetic division enhances "rest-and-digest" activities o Parasympathetic responses support body functions that conserve and restore body energy during times of rest and recovery o Parasympathetic impulses to the digestive glands and the smooth muscle of the gastrointestinal tract predominate over sympathetic impulses o This allows energy-supplying food to be digested and absorbed o Parasympathetic responses reduce body functions that support physical activity o SLUDD - the five parasympathetic responses  Salivation (S)  Lacrimation (L)  Urination (U)  Digestion (D)  Defecation (D) o Other important parasympathetic responses are the "three decreases":  Decreased heart rate  Decreased diameter of airway...