The ANS - Sympathetic and Parasympathetic Divisions

Lesson 19 - Chapter 16: The ANS: Sympathetic and Parasympathetic Divisions An Intro to the ANS and Higher-Order Functions  Somatic Nervous System (SNS)  Operates under conscious control.  Seldom affects long-term survival.  SNS controls skeletal muscles.  Autonomic Nervous System (ANS)  Operates without conscious instruction.  ANS controls visceral effectors.  Cardiac/smooth muscles, glands, adipocytes.  Coordinates system functions.  Cardiovascular, respiratory, digestive, urinary, reproductive. Autonomic Nervous System  Organization of the ANS  Integrative centers  For autonomic activity located in the hypothalamus.  Neurons comparable to upper motor neurons in SNS.  However, unlike SNS where the motor neurons synapses directly onto skeletal muscle targets, in the ANS, CNS motor neurons first synapse onto neurons in the autonomic ganglia. The cells in the ganglia then send out axons to their targets.  Visceral motor neurons  In the brainstem and spinal cord, they are known as preganglionic neurons.  Preganglionic Fibers: Axons of preganglionic neurons leave CNS and synapse on ganglionic neurons - located in a cluster (ganglion) outside of the CNS.  Autonomic Ganglia: Contain many ganglionic neurons.  Ganglionic neurons innervate visceral effectors.  Such as cardiac muscle, smooth muscle, glands, and adipose tissue.  Postganglionic fibers: Axons of ganglionic neurons. Divisions of the ANS  The ANS operates largely outside of our awareness.  It has two divisions: 1. Sympathetic Division: Increases alertness, metabolic rate, and muscular abilities.  Kicks in only during exertion, stress, or emergency.  “Fight or Flight” 2. Parasympathetic Division: Reduces metabolic rate and promotes digestion.  Controls during resting conditions.  “Rest and Digest”  Sympathetic and Parasympathetic Division 1. Most often, these two divisions have opposing effects.  If the sympathetic division causes excitation, the parasympathetic causes inhibition. 2. The two divisions may also work independently.  Only one division innervates some structures. 3. The two divisions may work together, with each controlling one stage of a complex process.  Sympathetic Division Preganglionic fibers (thoracic and superior lumbar spinal segments; thoracolumbar) synapse in ganglia near spinal cord.  Preganglionic fibers are short.  Postganglionic fibers are long (except at adrenal medullae - preganglionic fibers long, postganglionic fibers are short).  Prepares body for crisis, producing a “fight or flight” response.  Stimulates tissue metabolism.  Increases alertness.  Seven Responses to Increased Sympathetic Activity 1. Heightened mental alertness. 2. Increased metabolic rate. 3. Reduced digestive and urinary functions. 4. Energy reserves activated. 5. Increased respiratory rate and respiratory passageway dilate. 6. Increased heart rate and blood pressure. 7. Sweat glands activated.  Parasympathetic Division  Preganglionic fibers originate in the brainstem and sacral segments of the spinal cord; craniosacral.  Synapse in ganglia close to (or within) target organs.  Preganglionic fibers are long.  Postganglionic fibers are short.  Parasympathetic division stimulates visceral activity.  Conserves energy and promotes sedentary activities.  Five Responses to Increased Parasympathetic Activity 1. Decreased metabolic rate. 2. Decreased heart rate and blood pressure. 3. Increased secretion by salivary and digestive glands. 4. Increased motility (movement of material in digestive tract) and blood flow in digestive tract. 5. Urination and defecation stimulation.  Enteric Nervous System (ENS)  Third division of ANS.  Extensive network in digestive tract walls.  Complex visceral reflexes coordinated locally.  Can be influenced by sympathetic or parasympathetic divisions.  Roughly 100 million neurons.  Remember sensory neurons ~ 10 million, motor neurons ~ ½ million  All neurotransmitters found in this division are also found in the brain. The Sympathetic Division  The Sympathetic Division  Preganglionic neurons located between segments T1 and L2 of spinal cord.  Ganglionic neurons in ganglion near vertebral column.  Cell bodies of preganglionic neurons in lateral gray horns of spinal cord.  Axons leave the spinal cord and enter ventral roots of segments before terminating on ganglionic neurons.  Ganglionic Neurons  Occurs in three locations: 1. Sympathetic Chain Ganglia 2. Collateral Ganglia 3. Adrenal Medullae       Sympathetic Chain Ganglia  On both sides of the vertebral column.  Control effectors:  In the body wall.  Inside the thoracic cavity.  In the head.  In the limbs. Collateral Ganglia  Are anterior to vertebral bodies.  Contain ganglionic neurons that innervate tissues and organs in abdominopelvic cavity. Adrenal Medullae (Suprarenal Medullae)  Very short axons (the ganglionic neurons are endocrine cells).  When stimulated, release neurotransmitters into the bloodstream (not at synapse).  Function as hormones to affect target cells throughout the body.  Releases the hormones norepinephrine and epinephrine into the bloodstream. Organization and Anatomy of the Sympathetic Division  Ventral roots of spinal segments T1-L2 contain sympathetic preganglionic fibers.  Give rise to myelinated white ramus.  Carry myelinated preganglionic fibers into sympathetic chain ganglion.  May pass through sympathetic chain ganglion and synapse at collateral ganglia or in adrenal medullae. Sympathetic Chain Ganglia  Preganglionic Fibers  One preganglionic fiber synapses on many ganglionic neurons.  Fibers interconnect sympathetic chain ganglia.  Each ganglion innervates a particular body segment(s).  Postganglionic Fibers  Paths of unmyelinated postganglionic fibers depend on targets.  Postganglionic fibers control visceral effectors.  In body wall, head, neck, or limbs.  Enter gray ramus.  Return to spinal nerve for distribution.  Postganglionic fibers innervate effectors.  Sweat glands of skin.  Smooth muscles in superficial blood vessels.  Postganglionic fibers innervating structures in thoracic cavity form bundles called sympathetic nerves.  Innervates heart, lungs, other structures in thoracic cavity.  Preganglionic neurons  Only to spinal cord segments T1-L2 have white rami (myelinated preganglionic fibers).  Innervate neurons in:  Cervical, inferior lumbar, and sacral sympathetic chain ganglia (preganglionic axons run along the length of the chain to innervate the ganglionic neurons in these areas).  Chain ganglia provide postganglionic fibers  Every spinal nerve contains a gray (unmyelinated) ramus that carries sympathetic postganglionic fibers distributed for body wall (distributes signal for whole body response).  Only spinal nerves T1-L2 have white rami  Remember, white ramus carries visceral motor fibers to sympathetic ganglion. Collateral Ganglia  Receive sympathetic innervation via sympathetic preganglionic fibers (does not form synapse in sympathetic chain).  Postganglionic fibers leave collateral ganglia.  Extend throughout the abdominopelvic cavity.  Innervates variety of visceral tissues and organs.  Reductions of blood flow and energy by organs not vital to short-term survival (Ex: Digestive tract).  Causes release of stored energy reserves.  Adrenal Medullae  Preganglionic fibers entering adrenal gland proceed to center (adrenal medulla).  Adrenal Cortex: The outer region of adrenal gland.  Modified sympathetic ganglion.  Preganglionic fibers synapse on neuroendocrine cells.  Specialized ganglionic neurons secrete the hormones epinephrine and norepinephrine into the bloodstream.  Neuroendocrine cells  Secrete neurotransmitters epinephrine (E) and norepinephrine (NE).  Epinephrine  Also called adrenaline.  Is 75-80% of secretary output.  Remaining 20-25% of secretary output is norepinephrine.  Noradrenaline  Bloodstream carries neurotransmitters through the body (since these chemical messengers are carried through blood, they are called hormones).  Causing changes in metabolic activities of different cells including cells not innervated by sympathetic postganglionic fibers.  Effects last longer.  Hormones continue to diffuse out of bloodstream after they are no longer released by the adrenal medullae.  Sympathetic Activation: Activates the entire sympathetic division during crisis.  Change activities of tissues and organs by:  Releasing NE at peripheral synapses.  Target specific effectors, smooth muscle fibers in blood vessels of the skin.  Are activated in reflexes.  Does not involve other visceral effectors.  Distributing E and NE throughout the body in the bloodstream.  Entire division responds (sympathetic division).  Are controlled by sympathetic centers in the hypothalamus.  Effects are not limited to peripheral tissues.  Alters CNS activity.  Changes Caused by Sympathetic Activation  Increased alertness - stimulates reticular activating system.  Feelings of energy and euphoria.  Change in breathing - increase activity in respiratory and cardiovascular centers of pons and medulla.  Elevation in muscle tone.  Mobilization of energy reserves. Various Sympathetic Neurotransmitters  Stimulation of Sympathetic Preganglionic Neurons  Releases ACh at synapses with ganglionic neurons.  ACh is always released at cholinergic synapses.  Always excitatory on ganglion neurons. Ganglionic Neurons  Release neurotransmitters at specific target organs - mostly release NE, but some ganglionic neurons release acetylcholine (ACh).  Axon Terminals  Form branching networks of telodendria instead of synaptic terminals.  Telodendria form sympathetic varicosities.  Resembles a string of pearls.  Swollen segment packed with neurotransmitter vesicles.  Pass along or near the surface of effector cells.  No specialized postsynaptic membranes.  Membrane receptors on surfaces of target cells.  Release NE at most varicosities.  Called adrenergic neuron.  Some ganglionic neurons release ACh instead.  Called cholinergic neurons because they release ACh.  These cholinergic neurons are located in the body wall, skin, brain, and skeletal muscles. Sympathetic Stimulation and the Release of NE and E  Primarily from interactions of NE and E with two types of adrenergic membrane receptors. 1. Alpha Receptors (NE more potent) 2. Beta Receptors  Activates enzymes on the inside of cell membrane via G protein (remember indirect effect).  Alpha-1 (ɑ₁)  More common type of alpha receptor.  Releases intracellular calcium ions from reserves in endoplasmic reticulum.  Has excitatory effect on target cell (causes constriction of smooth muscle blood vessel at digestive and urinary tract).  Alpha-2 (ɑ₂)  Lowers cAMP levels in cytoplasm.  Has inhibitory effect on the cell.  Helps coordinate sympathetic and parasympathetic activities.  Beta (β) Receptors  Affect membranes in many organs (stimulates skeletal muscles, lungs, heart, and liver).  Trigger metabolic changes in target cell.  Stimulation increases intracellular cAMP levels.  Binding of NE/E causes opposite effect on cAMP compared to ɑ₂ receptor.  Beta-1 (β₁): Increases metabolic activity - increase activity of skeletal and cardiac muscle.  Beta-2 (β₂): Triggers relaxation of smooth muscles along respiratory tract - increases diameter of air passages.  Beta-3 (β₃): Leads to lipolysis, the breakdown of triglycerides in adipocytes - releases nutrients into the bloodstream. Sympathetic Stimulation and the Release of ACh and NO  Cholinergic (ACh) sympathetic terminals.     Innervate sweat glands of skin and blood vessels of skeletal muscles and brain. Stimulate sweat gland secretion and dilate blood vessels to skeletal muscles and brain.  Nitroxidergic Synapses  Release nitric oxide (NO) as neurotransmitter.  Neurons innervate smooth muscles in walls of blood vessels in skeletal muscles and the brain.  Produce vasodilation and increased blood flow.  Summary:  Dilation of blood vessel  To skeletal muscles and the brain by ACh, NO on cholinergic and nitroxidergic receptors, respectively.  Constriction of blood vessels  To digestive and urinary system by NE, N and Alpha-1 receptors. The Parasympathetic Division  Autonomic Nuclei  Are contained in the mesencephalon, pons, and medulla oblongata (brainstem).  Associated with cranial nerves III, VII, IX, X.  In lateral gray horns of spinal segments S ₂-S₄.  Ganglionic Neurons in Peripheral Ganglia  Types:  Terminal Ganglion: Near target organ; usually paired.  Intramural Ganglion: Embedded in tissues of target organ; interconnected masses; clusters of ganglion cells.  Organization and Anatomy of the Parasympathetic Division  Parasympathetic preganglionic fibers leave the brain as components of cranial nerves.  III (Oculomotor)  VII (Facial)  IX (Glossopharyngeal)  X (Vagus)  III, VII, and IX control visceral structures in the head.  Parasympathetic preganglionic fibers leave the spinal cord at sacral level - leaves as pelvic nerves.  Note: Vagus nerve innervates structures in neck, thoracic, and abdominopelvic cavities.  Parasympathetic Activation  Centers on relaxation, food processing, and energy absorption.  Localized effects, last a few seconds at most.  Major Effects of Parasympathetic Division  Constriction of the pupils.  To restrict the amount of light that enters the eyes.  And focusing of the lenses of the eyes on nearby objects.  Secretion by digestive glands.  Including salivary glands, gastric glands, duodenal glands, intestinal glands, the pancreas (exocrine and endocrine), and the liver.  Secretion of hormones.  That promotes the absorption and utilization of nutrients by peripheral cells (Ex: Increase insulin secretion).  Changes in blood flow and glandular activity.  Associated with sexual arousal.  Increase in smooth muscle activity.  Along the digestive tract.   Stimulation and coordination of defecation. Contraction of the urinary bladder during urination. Constriction of the respiratory passageways. Reduction in heart rate and in the force of contraction. Parasympathetic Neurons Release ACh  Neuromuscular and Neuroglandular Junctions  All release ACh as a neurotransmitter.  Both the preganglionic and ganglionic neurons release acetylcholine (ACh).  Small, with narrow synaptic cleft.  Effects of stimulation are short lived (few seconds).  Inactivated by acetylcholinesterase (AChE) at synapse.  ACh is also inactivated by tissue cholinesterase in surrounding tissues.  Membrane Receptors and Responses  Nicotinic Receptors  On surfaces of ganglionic cells (sympathetic and parasympathetic).  Exposure to ACh causes excitation of ganglionic neuron or muscle fiber.  Opens chemically-gated Na⁺ channels, allowing Na⁺ influx into cell and depolarization of ganglion cells.  Muscarinic Receptors  At cholinergic neuromuscular or neuroglandular junctions (parasympathetic).  At few cholinergic junctions (sympathetic).  G Proteins  Effects are longer lasting than nicotinic receptors.  Response reflects activation or inactivation of specific enzymes.  Can be excitatory or inhibitory.  Dangerous Environmental Toxins  Produce exaggerated, uncontrolled responses.  Nicotine  Binds to nicotinic receptors in sympathetic and parasympathetic ganglion cells.  Targets autonomic ganglia and skeletal neuromuscular junctions.  50 mg ingested or absorbed through skin causes poisoning.  Signs and symptoms:  Vomiting, diarrhea, high blood pressure, rapid heart rate (tachycardia), sweating, profuse salivation, convulsions.  May result in coma or death.  Muscarine: Toxin produced from some poisonous mushrooms.  Binds to muscarinic receptors.  Targets parasympathetic neuromuscular or neuroglandular junctions.  Signs and symptoms:  Salivation, nausea, vomiting, diarrhea, constriction of respiratory passages, low blood pressure, slow heart rate (bradycardia). Dual Innervation  Sympathetic Division  Widespread impact.  Reaches organs and tissues through the body.  Parasympathetic Division  Innervates only specific visceral structures - served by cranial nerves or lie within the abdominopelvic cavity.  Sympathetic and Parasympathetic Division  Most vital organs receive instructions from both sympathetic and parasympathetic divisions.          Two divisions commonly have opposing effects. Anatomy of Dual Innervation  Parasympathetic postganglionic fibers accompany cranial nerves to peripheral destinations.  Sympathetic innervation reaches the same structures.  By traveling directly from the superior cervical ganglia of the sympathetic chain.  In thoracic and abdominopelvic cavities, sympathetic postganglionic fibers mingle with parasympathetic preganglionic fibers to form nerve network (autonomic plexuses) before innervating target organs.  Note: Blood vessels and sweat glands are not dual innervated. Only innervated by the sympathetic division. Autonomic Tone: Caused by resting level of spontaneous activity.  Is an important aspect of ANS function.  If the nerve is inactive under normal conditions, it can only increase activity.  If the nerve maintains a background level of activity, it can increase or decrease activity.  Autonomic Motor Neurons  Maintain a resting level of spontaneous activity.  Background level of activation determines autonomic tone.  Significant where dual innervation occurs.  Two divisions have opposing effects.  However, even more important when dual innervation does not occur.  Normally, background release of NE from adrenergic fibers keeps blood vessels partially constricted. When more blood flow is needed, NE release drops and different sympathetic cholinergic neurons are stimulated to release ACh which dilates the blood vessels. The Heart Rate Receives Dual Innervation  Two divisions have opposing effects on heart function: 1. Parasympathetic Division: Acetylcholine released by postganglionic fibers slows heart rate. 2. Sympathetic Division: NE released by varicosities accelerates heart rate.  Balance between two divisions.  Autonomic tone is present.  Releases small amounts of both neurotransmitters continuously.  Parasympathetic innervation dominates under resting conditions.  Crisis accelerates heart rate by:  Stimulation of sympathetic innervation.  Inhibition of parasympathetic innervation. Autonomic Tone  Blood vessel dilates and blood flow increases.  Blood vessel constricts and blood flow is reduced.  Sympathetic postganglionic fibers release NE.  Innervates smooth muscle cells in walls of peripheral vessels.  Note: Blood vessel diameter is regulated only by the sympathetic division and not innervated by the parasympathetic division.  Background sympathetic tone keeps muscles partially contracted.  To increase blood flow:  Rate of NE release decreases.  Sympathetic cholinergic fibers are stimulated (releasing ACh).  Smooth muscle cells (lining the blood vessel) relax.  Vessels dilate and blood flow increases. Effects of Aging on the Nervous System  Effects of Aging  Anatomical and physiological changes begin after maturity (age 30).  Accumulate over time.  85% of people over age 65 have changes in mental performance and CNs function.  Common Age-Related Anatomical Changes in the Nervous System 1. Reduction in brain size and weight.  Decrease in volume of cerebral cortex.  Narrower gyri and wider sulci.  Larger subarachnoid space (since brain matter is shrinking). 2. Reduction in number of neurons.  Brain shrinkage is linked to loss of cortical neurons.  No neuronal loss in brainstem nuclei. 3. Decrease in blood flow to the brain.  Arteriosclerosis  Fatty deposits in walls of blood vessels.  Reduces blood flow through arteries.  Increases chances of rupture.  Cerebrovascular Accident (CVA) (Stroke)  May damage surrounding neural tissue. 4. Changes in synaptic organization of the brain.  Number of dendritic branches, spines, and interconnections decreases.  Synaptic connections lost.  Rate of neurotransmitter production declines. 5. Intracellular and extracellular changes in CNS neurons.  Neurons in the brain accumulate abnormal intracellular deposits.  Lipofuscin: Granular pigment with no known function.  Neurofibrillary Tangles: Masses of neurofibrils form dense mats inside the cell body and axon.  Plaques  Extracellular accumulations of fibrillar proteins.  Surrounded by abnormal dendrites and axons.  Plaques and Tangles  Contain deposits of several peptides.  Primarily two forms of amyloid ß (Aß) protein.  Appear in brain regions, such as the hippocampus, that are specifically associated with memory processing.  Alzheimer’s Disease: Correlated with abnormal plaques and neurofibrillary tangles in brain areas involved with memory, emotions, intelligence. Genetic factors play a major role.  Anatomical Changes  Linked to functional changes.  Neural processing becomes less efficient with age.  Memory consolidation is more difficult.  Secondary memories (recent past) harder to access.  Sensory Systems  Hearing, balance, vision, smell, and taste become less acute.  Reaction time slowed.  Reflexes weaken or disappear.  Motor Controls  Precision decreases.  Takes longer to perform. Incapacitation  85% of elderly population develops changes that do not interfere with abilities.  Some individuals become incapacitated by progressive CNS changes.  Senility  Also called senile dementia.  Degenerative changes:  Memory loss.  Anterograde amnesia (lose ability to store new memories).  Emotional disturbances.  Alzheimer’s disease is most common. The Nervous System Integration  The Nervous System  Monitors all other systems.  Issues commands that adjust their activities.  Like a conductor of an orchestra.  Neural Tissue  Extremely delicate.  Extracellular environment must maintain homeostatic limits.  If regulatory mechanisms break down, neurological disorders appear. 