Nervous System - ANS - ZD PDF

Introduction to Human Physiology FHSC 203 Chap 5 – Nervous System- ANS ANS ! The ANS: Autonomic nervous system consists of motor neurons that: " Innervate smooth and cardiac muscle and glands " Make adjustments to ensure optimal support for body activities...

Introduction to Human Physiology FHSC 203 Chap 5 – Nervous System- ANS ANS ! The ANS: Autonomic nervous system consists of motor neurons that: " Innervate smooth and cardiac muscle and glands " Make adjustments to ensure optimal support for body activities " Operate via subconscious control " Have viscera as most of their effectors ANS ! Other names for ANS: " Involuntary nervous system " General visceral motor system ANS ! Most ANS actions occur subconsciously and without our awareness ! ANS enjoys a level of functional independence ! Other terminology: " Involuntary nervous system that reflects its subconscious control " General visceral motor system that indicates location of most of its effectors ANS ANS v/s SNS ! The ANS differs from the SNS in the following three areas " Effectors " Efferent pathways " Target organ responses ! The effectors of the SNS are skeletal muscles. ! The effectors of the ANS are cardiac muscle, smooth muscle, and glands. ANS v/s SNS: Efferent pathways ANS v/s SNS: Efferent pathways ANS v/s SNS: Efferent pathways ! In the Somatic Nervous System: 1. A, thick, heavily myelinated somatic motor fiber makes up each pathway from the CNS to the muscle 2. Cell bodies are in the CNS 3. Axons extend from spinal or cranial nerves 4. Lacks ganglia ANS v/s SNS: Efferent pathways ! ANS Pathway is a 2-Neuron Chain: 1. Preganglionic neuron (in CNS) has a thin, lightly myelinated preganglionic axon 2. Ganglionic neuron in autonomic ganglion outside CNS has an unmyelinated postganglionic axon that extends to the effector organ 3. Many fibers are incorporated in spinal or cranial nerves 4. Slow ANS v/s SNS: Neurotransmitters ! All somatic motor neurons release Ach (acetylcholine), which has an excitatory effect. ! In the ANS: " Preganglionic fibers release Ach " Postganglionic fibers release norepinephrine or ACh and the effect is either stimulatory or inhibitory " ANS effect on the target organ is dependent upon the neurotransmitter released and the receptor type of the effector ANS v/s SNS: Neurotransmitters ANS v/s SNS: Overlap ! Higher brain centers regulate and coordinate both motor activities ! Nearly all spinal nerves and many cranial nerves contain both fibers ! Most of body’s adaptations to changes involve both systems activity " when skeletal muscles are working hard, ANS increases heart rate and dilate airways to provide the extra oxygen and glucose needed ANS divisions ! Sympathetic “fight and flight” And ! Parasympathetic “rest and digest” ! Innervate the same organs ! Produce opposite effects ! Continuously produce fine adjustments ANS divisions ! Role of the Parasympathetic Division " Concerned with keeping body energy use low " Involves the D activities – digestion, defecation, and diuresis " Its activity is illustrated in a person who relaxes after a meal # Blood pressure, heart rate, and respiratory rates are low # Gastrointestinal tract activity is high # The skin is warm and the pupils are constricted ANS divisions ! Role of the Sympathetic Division " Involves E activities – exercise, excitement, emergency, and embarrassment " Promotes adjustments during exercise # Visceral and cutaneous blood vessels are constricted so blood is shunted to active skeletal muscles and heart # Bronchioles are dilated to increases ventilation # Liver releases more glucose in order to accommodate for increased energy needs # Temporary non-essential activities are damped ANS divisions " Its activity is illustrated by a person who is threatened # Heart rate increases, and breathing is rapid and deep # The skin is cold and sweaty, and the pupils dilate ANS divisions Division Origin of Fibers Length of Fibers Location of Ganglia Sympathetic Thoracolumbar region Short preganglionic and Close to the spinal of the spinal cord long postganglionic cord Parasympathetic Brain and sacral spinal Long preganglionic and In the visceral cord short postganglionic effector organs ANS divisions ANS anatomy: Parasympathetic cranial outflow Cranial Outflow Cranial Nerve Ganglion Effector Organ(s) Oculomotor (III) Ciliary Eye Facial (VII) Pterygopalatine Salivary, nasal, and Submandibular lacrimal glands Glossopharyngeal Otic Parotid salivary glands (IX) Vagus (X) Located within the walls Heart, lungs, and most of target organs visceral organs ANS anatomy: Parasympathetic cranial outflow Sacral Outflow Cranial Nerve Ganglion Effector Organ(s) S2-S4 Located within the walls Large intestine, urinary of the target organs bladder, ureters, and reproductive organs ANS anatomy: Parasympathetic cranial outflow ANS anatomy: Sympathetic outflow ! Thoracolumnar outflow: " Sympathetic division innervates more organs " Innervates structures in superficial part of the somatic nervous system " Innervates smooth muscles of vessels " Preganglionic fibers originate in lateral horns of T1 to L2 " Ventral root → white rami communicans → sympathetic trunk ganglion Sympathetic trunks and pathways ! Sympathetic trunks and pathways " The paravertebral ganglia form part of the sympathetic chain " Typically there are 23 ganglia – 3 cervical, 11 thoracic, 4 lumbar, 4 sacral, and 1 coccygeal Sympathetic trunks and pathways 1. A preganglionic fiber follows one of three pathways upon entering the paravertebral ganglia: " Synapse with the ganglionic neuron within the same ganglion " Ascend or descend the sympathetic chain to synapse in another chain ganglion " Pass through the chain ganglion and emerge without synapsing Sympathetic trunks and pathways Sympathetic trunks and pathways Sympathetic trunks and pathways Sympathetic trunks and pathways 2. A Postganglionic axons enter the ventral rami via the gray rami communicantes " These fibers innervate sweat glands and arrector pili muscles " Rami communicantes are associated only with the sympathetic division Sympathetic trunks and pathways A. Pathways to the head " Preganglionic fibers emerge from T1–T4 and synapse in the superior cervical ganglion " These fibers: # Serve the skin and blood vessels of the head # Stimulate dilator muscles of the iris # Inhibit nasal and salivary glands Sympathetic trunks and pathways Sympathetic trunks and pathways B. Pathways to the thorax " Preganglionic fibers emerge from T1–T6 and synapse in the cervical chain ganglia " Postganglionic fibers emerge from the middle and inferior cervical ganglia and enter nerves C4–C8 " These fibers innervate the heart via the cardiac plexus, as well as innervating the thyroid and the skin Sympathetic trunks and pathways Sympathetic trunks and pathways C. Pathways with Synapses in a Collateral Ganglion " These fibers (T5–L2) leave the sympathetic chain without synapsing " They form thoracic, lumbar, and sacral splanchnic nerves " Their ganglia include the celiac, the superior and inferior mesenterics, and the hypogastric Sympathetic trunks and pathways D. Pathways to the abdomen " Sympathetic nerves innervating the abdomen have preganglionic fibers from T5–L2 " They travel through the thoracic splanchnic nerves and synapse at the celiac and superior mesenteric ganglia " Postganglionic fibers serve the stomach, intestines, liver, spleen, and kidneys Sympathetic trunks and pathways Sympathetic trunks and pathways E. Pathways to the pelvis " Preganglionic fibers originate from T10–L2 " Most travel via the lumbar and sacral splanchnic nerves to the inferior mesenteric and hypogastric ganglia " Postganglionic fibers serve the distal half of the large intestine, the urinary bladder, and the reproductive organs Sympathetic trunks and pathways Sympathetic trunks and pathways F. Pathways with Synapses in the Adrenal Medulla " Fibers of the thoracic splanchnic nerve pass directly to the adrenal medulla " Upon stimulation, medullary cells secrete norepinephrine and epinephrine into the blood Visceral reflexes ! Visceral reflexes have the same elements as somatic reflexes. The main difference is that the visceral reflex arc has two neurons in the motor pathway ! They are always polysynaptic pathways. ! Afferent fibers are found in spinal and autonomic nerves. Referred pain ! Pain arising from the viscera but is perceived as somatic in origin. ! This may be due to the fact that visceral pain afferents travel along the same pathways as somatic pain fibers. Referred pain ! Pain stimuli arising in the viscera are perceived as somatic in origin " E.g. Heart attack may produce radiating pain sensation that radiates to superior thoracic wall and to left arm ! Because same spinal segments serve the heart and the regions to which pain signals from heart tissue are referred Neurotransmitters ! The sympathetic and parasympathetic nerve fibers secrete mainly one or the other of two synaptic transmitter substances: " Those fibers that secrete acetylcholine are said to be cholinergic " Those that secrete norepinephrine are said to be adrenergic Neurotransmitters ! Are cholinergic: " All preganglionic neurons in both the sympathetic and the parasympathetic nervous systems " Almost all of the postganglionic neurons of the parasympathetic system " The postganglionic sympathetic nerve fibers to the sweat glands, to the piloerector muscles of the hairs, and to a very few blood vessels Neurotransmitters Neurotransmitters Neurotransmitters ! Are adrenergic: " most of the postganglionic sympathetic neurons are adrenergic ! Neurotransmitter effects can be excitatory or inhibitory depending upon the receptor type. Neurotransmitters Neurotransmitters receptors ! Acetylcholine activates mainly two types of cholinergic receptors: " Nicotinic receptors " Muscarinic receptors Neurotransmitters receptors 1. Nicotinic receptors " Nicotinic receptors are found on: # Motor end plates (somatic targets) # All ganglionic 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 stimulatory Neurotransmitters receptors 2. Muscarinic receptors " Muscarinic receptors occur on all effector cells stimulated by postganglionic cholinergic fibers " The effect of ACh binding: # Can be either inhibitory or excitatory # Depends on the receptor type of the target organ Neurotransmitters receptors ! Norepinephrine also has 2 types of receptors: " Alpha receptors " Beta receptors Neurotransmitters receptors ! Each type has two or three subclasses (α1, α2, β1, β2, β3) ! Effects of NE binding to: " α receptors is generally stimulatory " β receptors is generally inhibitory ! A notable exception – NE binding to β receptors of the heart is stimulatory Neurotransmitters receptors ! Binding of Epi to β2 on bronchiole smooth muscle induces relaxation and dilation of the bronchiole Neurotransmitters receptors Effect of drugs ! Location of receptor subtypes determines what drug needs to be prescribed to obtain the desired inhibitory or stimulatory effects 1. Atropine—anticholinergic- blocks muscarinic receptors " Blocks salivation and lung secretions during surgery " Dilate the pupils Effect of drugs 2. Neostigmine inhibits acetylcholinesterase " Myasthenia gravis (skeletal muscle impairment due to lack of ACh stimulation) 3. Sympathomimetics (Phenylephrine) " Colds, allergies and nasal congestion 4. β2 activators and blockers " Asthma Interactions of the Autonomic Divisions ! Most visceral organs are innervated by both sympathetic and parasympathetic fibers. ! This results in dynamic antagonisms that precisely control visceral activity. " Sympathetic fibers increase heart and respiratory rates, and inhibit digestion and elimination " Parasympathetic fibers decrease heart and respiratory rates, and allow for digestion and the discarding of wastes Interactions of the Autonomic Divisions Sympathetic Tone ! The sympathetic division controls blood pressure and keeps the blood vessels in a continual state of partial constriction. ! This sympathetic tone (vasomotor tone): " Constricts blood vessels and causes blood pressure to rise as needed " Prompts vessels to dilate if blood pressure is to be decreased ! Alpha-blocker drugs interfere with vasomotor fibers and are used to treat hypertension Parasympathetic Tone ! Parasympathetic tone: " Slows the heart " Dictates normal activity levels of the digestive and urinary systems ! The sympathetic division can override these effects during times of stress ! Most glands are activated by parasympathetic fibers with Exceptions: adrenal and sweat gland ! Drugs that block parasympathetic responses increase heart rate and block fecal and urinary retention. Cooperative effect ! ANS cooperation is best seen in control of the external genitalia. " Parasympathetic fibers cause vasodilation and are responsible for erection of the penis and clitoris " Sympathetic fibers cause ejaculation of semen in males and reflex peristalsis in females " Anxiousness or upset mood → sympathetic division is in charge which impairs sexual performance Unique Roles of the Sympathetic Division ! Regulates many functions not subject to parasympathetic influence. ! These include the activity of the adrenal medulla, sweat glands, arrector pili muscles, kidneys, and most blood vessels. ! The sympathetic division controls: " Thermoregulatory responses to heat " Release of renin from the kidneys " Metabolic effects Unique Roles of the Sympathetic Division ! Thermoregulatory Responses to Heat " Sympathetic division mediates reflexes that regulate body temperature " Applying heat to skin causes reflexive dilation of blood vessels in that area # Systemic body temperature elevated → sympathetic nerves cause skin’s blood vessels to dilate, and activate sweat glands to cool the body # When body temperature falls → skin blood vessels are constricted → blood is restricted to deeper and more vital organs Unique Roles of the Sympathetic Division ! Release of Renin from the Kidneys " Sympathetic impulses stimulate the kidneys to release renin (hormone) that promotes increase in blood pressure Unique Roles of the Sympathetic Division ! Metabolic Effects " Sympathetic division exerts its metabolic effects by: (a) direct neural stimulation (b) release of adrenal medullary hormones (c ) Effects not reversed by parasympathetic activity " Metabolic effects (1) increases metabolic rate of body cells (2) raises blood glucose levels (3) mobilizes fats for use as fuels (4) medullary hormones also stimulate strong and quick skeletal muscle contraction Local and diffuse effects ! The parasympathetic division exerts short-lived, highly localized control. " One preganglionic neuron synapses with one (or few) ganglionic neurons " All fibers release ACh (quickly hydrolyzed by acetylcholinesterase) ! The sympathetic division exerts long-lasting, diffuse effects. " Preganglionic axons branch profusely and synapses with ganglionic neurons at several levels " NE is inactivated more slowly because it must be taken back up into presynaptic endings for storage or hydrolysis Effects of Sympathetic Activation ! Sympathetic activation is long-lasting because NE: " Is inactivated more slowly than Ach " Is an indirectly acting neurotransmitter, using a second- messenger system " NE and epinephrine are released into the blood and remain there until destroyed by the liver # epinephrine more potent in increasing heart rate, blood glucose levels and metabolic rate Levels of ANS Control ! The hypothalamus is the main integration center of ANS activity. ! Subconscious cerebral input via limbic lobe connections influences hypothalamic function. Levels of ANS Control Brain Stem and Spinal Cord Controls !Brain stem reticular formation exerts the most direct influence over autonomic functions " Cardiac and vasomotor centers → reflexively regulate heart rate and blood vessel diameter !Midbrain centers (oculomotor nuclei) control muscles concerned with pupil diameter and lens focus Levels of ANS Control ! Defecation and micturition reflexes are integrated at spinal cord level but are subject to conscious inhibition ! Medulla and pons are not considered part of ANS but contain respiratory centers and receive input from hypothalamus ! Most sensory impulses reach brain stem via vagus nerve afferents Levels of ANS Control ! Anterior hypothalamic regions direct parasympathetic functions; Posterior areas direct sympathetic functions ! Exert effects both directly and via relays through the reticular formation which in turn influences preganglionic motor neurons in the brain stem and spinal cord Levels of ANS Control ! Centers of Hypothalamic Control: " Heart activity and blood pressure " Body temperature, water balance, and endocrine activity " Emotional stages (rage, pleasure) and biological drives (hunger, thirst, sex) " Reactions to fear and the “fight-or-flight” response Levels of ANS Control ! Cortical Controls: Occasionally ANS is subject to voluntary controls " Sympathetic response: remembering a scary event makes our heart race " Parasympathetic response: thinking of our favorite food makes mouth water ! These inputs converge on the hypothalamus through its connections to the limbic lobe " Biofeedback: awareness of what is happening in the body provided by monitoring devices " Biofeedback training: trying to alter or control some involuntary functions Homeostatic Imbalances 1. Autonomic neuropathy " Due to damage to autonomic nerves " Common complication of diabetes mellitus " Early symptom: sexual dysfunction (erectile dysfunction or reduced vaginal lubrication) " Other symptoms: Poor blood pressure control (dizziness after standing suddenly), urinary incontinence, sluggish eye pupil reactions, and impaired sweating Homeostatic Imbalances 2. Hypertension " May result from an overactive sympathetic vasoconstrictor response promoted by continuous high level of stress " Stress-induced hypertension can be treated with adrenergic receptor-blocking drugs Homeostatic Imbalances 3. Raynaud’s disease " An exaggerated vasoconstriction response characterized by intermittent attacks causing the skin of the fingers and toes to become pale, then cyanotic and painful " Provoked by exposure to cold or emotional stress " Could result in ischemia and gangrene " Vasodilators (adrenergic blockers) usually help " Sympathectomy: severance of preganglionic sympathetic fibers in extreme cases " Blood vessels dilate and adequate blood flow to affected area is restored Homeostatic Imbalances 4. Autonomic dysreflexia " Life-threatening condition involving uncontrolled activation of autonomic neurons " Occurs in quadriplegics and individuals with spinal injuries above the T6 level " Triggered by painful stimulus to the skin or overfilling of a visceral organ " Arterial blood pressure skyrockets to life threatening levels which may cause a stroke " May be accompanied by a headache, flushed face, sweating above the level of injury, and cold clammy skin below Homeostatic Imbalances