CH 14; Autonomic Nervous System PDF

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Mareib and Cohen textbook ed. 11...

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CHAPTER 14 The Autonomic Nervous System  

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The system of motor neurons that innervates smooth and cardiac muscles and glands The ANS controls the functions of the body that are involuntary (you don’t think to do it; breathing, HR, digestion, etc.) o Subconscious control or general visceral motor system, which indicates the location of most of its effectors 2 branches: a. Sympathetic (Fight or Flight) b. Parasympathetic (Rest and Digest)

NOTE: THERE WILL ALWAYS BE SOME SORT OF OVERLAP IN FUNCTION BETWEEN THE SYMPATHETIC AND PARASYMPATHETIC NS 14.1 THE ANS DIFFERS FROM THE SOMATIC NERVOUS SYSTEM IN THAT IT CAN STIMULATE OR INHIBIT ITS EFFECTORS 

The Somatic NS and ANS have some key difference despite the overlap in function. Both have motor fibers but differ in a. Effectors, b. Efferent Pathways + Ganglia, and c. Target Organ Response to their NT’s SOMATIC NS Skeletal Muscles

Effectors Efferent Pathways + Ganglia

- Motor neuron cell body is located in CNS - Axons are in spinal or cranial nerves - Myelinated fibers - Group A fibers (conducts quickly)

Neurotransmitters (NT’s)

Acetylcholine (Ach); normally stimulates muscle fiber contraction

OVERLAP OF SOMATIC AND AUTONOMIC FUNCTION

AUTONOMIC NS Cardiac Muscles Smooth Muscles Uses a 2-neuron chain to reach its effectors: First (Preganglionic) Neuron: - Cell body resides in brain stem or spinal cord First (Preganglionic) Axon: - Synapses with the second motor neuron Second (Postganglionic) Neuron: - Cell body is in an autonomic ganglion outside of CNS Second (Postganglionic) Axon: - Extends to effector organ 2 NT’s are released: Norepinephrine (NE) - Secreted by most sympathetic fibers Acetylcholine (Ach) - Secreted by sympathetic fibers - Depending on the type of receptors on the target organ you can get either a inhibitory or excitatory response

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Higher brain centers regulate and coordinate both somatic and autonomic motor activities, and most spinal nerves, and many cranial nerves. This is because for the most part they carry both somatic and autonomic fibers. Most of the body’s adaptations require both skeletal muscles and visceral organs.  EXAMPLE: When skeletal muscles are used (i.e. workouts) they need an increase of oxygen and glucose. In order to achieve this the ANS will then increase HR and dilate the airways

14.2 THE ANS CONSISTS OF PARASYMPATHETIC + SYMPATHETIC DIVISIONS  Potato Salad

2 divisions: a. Parasympathetic Division (Rest + Digest): Promotes maintenance functions and conserves body energy b. Sympathetic Division (Fight or Flight): Mobilizes the body during activity

Sh*t Po-po

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Both divisions generally serve the same visceral organs BUT cause opposite effects = Dual Innervation o Dual Innervation: Two divisions counterbalance each other to keep body systems running smoothly

Role of the Parasympathetic Division     

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Keeps body energy use as low as possible Does housekeeping duties: digesting food, eliminating feces + urine BP + HR are regulated GI tract is actively digesting food Pupils are constricted and the lenses are accommodating for close vision to improve clarity (close up image) “D” Division: Deification, Digestion, Diuresis

Role of the Sympathetic Division  

Is evident when one is excited and in a threatening situation or emergency S/S of Sympathetic activation: - Rapidly pounding HR - Dry mouth

- Deep breathing - Sweaty Skin

- Dilated Pupils -Cold

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During any type of vigorous physical activity, the sympathetic promotes several adjustments: o Constricts visceral (sometimes cutaneous) blood vessels to shunt blood to active skeletal muscles and the heart o Dilates bronchioles to ↑ airflow = ↑ oxygen delivery o Stimulates liver to release more glucose into the blood to accommodate the increased energy needs to body cells “E” Division: Exercise, Excitement, Emergency, Embarrassment

NOTE: At the same time the sympathetic NS temporality reduces nonessential activities (ex: GI mobility) Key Anatomical Differences

14.3 LONG PREGANGLIONIC PARASYMPATHETIC FIBERS ORIGINATE IN THE CRANIOSACRAL CNS 

Parasympathetic division = craniosacral division o This is because the preganglionic fibers spring

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from opposite ends of the CNS – the brain stem and the sacral region of the spinal cord Preganglionic axons extend from the CNS nearly all the way to the structures they innervate o There axons synapse with postganglionic neurons

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located in terminal ganglia that lie close to or within the target organs Very short postganglionic axons issue from the terminal ganglia and synapse with effectors cells in the immediate area

Cranial Part of Parasympathetic Division 

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Preganglionic fibers run in the oculomotor, facial, glossopharyngeal, and vagus cranial nerves o Their cell bodies lie in the brain stem in the motor nuclei of associated cranial nerves CN III, VII, and IX supply the entire sympathetic innervation of the head, whereas the vagus nerves have a much more widespread distribution

NOTE: CN III, VII, IX preganglionic fibers only lie within the 3 pairs of CN’s. The postganglionic fibers DO NOT.

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Most preganglionic fibers “hitch a ride” with branches of Trigeminal nerve (CN V) o It does this to take advantage of the wide distribution

Oculomotor Nerves (CN III) 

Innervates smooth muscles in the eyes that cause the pupils to constrict and the lenses to bulge

Facial Nerves (CN VII)   

Stimulate large glands in the head Fibers that activate the nasal glands and the lacrimal glands of the eye The preganglionic neurons that stimulate the submandibular and sublingual salivary glands

Glossopharyngeal Nerves (IX) 

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The parasympathetic fibers in the glossopharyngeal nerves originate in the inferior salivatory nuclei in the of the medulla and synapses to the optic ganglia; located inferior to the foramen ovale of the skull The postganglionic fibers course to and activate the parotid salivary glands anterior to the ears

Vagus Nerve (CN X) 

Accounts for about 90% of all preganglionic parasympathetic fibers in the body o Provides fibers to the neck and to nerve plexuses that serve virtually every organ in the thoracic and abdominal cavity

NOTE: These nerve plexuses contain fibers from both the parasympathetic and sympathetic divisions 

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Preganglionic axons arise mostly from the dorsal motor nuclei of the medulla and synapse in terminal ganglia usually located in the walls of the target organ o Intramural Ganglia: Ganglia within the walls As the Vagus nerve passes into the thorax, they send branches through: 1. Cardiac Plexus → Supplies fibers to the heart 2. Pulmonary Plexus → Serves the airways of the lungs 3. Esophageal Plexus → Serves the esophagus and part of the stomach o When the main trunks of the Vagus nerve reaches the esophagus the fibers run down to the abdominal. o This then innervates the liver, gallbladder, stomach, small intestine, pancreas, and proximal ½ of large intestine

Sacral Part of Parasympathetic Division 

The sacral part of the parasympathetic divisions serves the pelvic organs and the distal ½ of large intestine o Arises from S₂ - S₄

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The axons of these neurons branch off into the Pelvic Splanchnic Nerves which passes through the Inferior Hypogastric Plexus in the pelvic floor o This then controls the distal ½ of large intestine, urinary bladder, ureters, and reproductive organs

14.4 SHORT PREGANGLIONIC SYMPATHETIC FIBERS ORIGINATE IN THE THORACOLUMBAR CNS

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The sympathetic division is anatomically more complex than the PSNS o Partially because it innervates more organs All preganglionic fibers of the SNS arise from cell bodies of preganglionic neurons in T₁ - L₂ o Therefore SNS is also known as Thoracolumbar Division Also supplies smooth muscles, cardiac muscles, and glands in the internal body cavities Also supplies all smooth muscles and glands in the superficial (somatic) part of the body o These superficial structures are innervated by only the SNS. These include: 1. Sweat Glands (Both eccrine and apocrine) 2. Arrector pili muscles (skin) 3. Smooth muscle in the walls of all arteries and veins (both deep + artificial) The cell bodies of preganglionic sympathetic neurons are from the grey matter in the spinal. They then leave the spinal cord via the ventral root and enter the adjoining sympathetic trunk ganglion which forms the sympathetic trunk

The sympathetic trunk runs down both sides of the spinal cord; ganglia and fibers run from one ganglion to another Once a preganglionic axon reaches a trunk ganglion, 1 of 3 things can happen. The preganglionic and postganglionic neurons can: o

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1. Synapse at the same level: As the trunk ganglion 2. Synapse at a higher or lower level: the preganglionic axon ascends and descends the sympathetic trunk to another trunk ganglion

3. Synapse in a distant collateral ganglion: The preganglionic axon passes through the trunk ganglion and emerges from the sympathetic trunk without synapsing. 

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Preganglionic fibers help form several splanchnic nerves and synapse in collateral (or prevertebral) ganglia located anterior to the vertebral column o They occur only in abdomen and pelvis Collateral (Prevertebral) Ganglia: Sympathetic ganglia which lie between the paravertebral ganglia and the target organ o ENGLISH VERSION: A cluster of cell bodies that are between the sides of the spinal cord and the target organ

Sympathetic Pathways with Synapses in Trunk Ganglia 

When synapses are made in sympathetic trunk ganglia, postganglionic axons enter the ventral (or dorsal) ramus of adjoining spinal nerves by way of communicating branches called the gray rami communicantes o From there they travel via branches to their effectors, including sweat glands and arrector pili muscles

(a) Pathways to the Head 

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Sympathetic preganglionic fibers serving the head emerge from T₁-T₄ and ascend the sympathetic trunk to synapse in the superior cervical ganglion o Contributes sympathetic fibers that run in several cranial nerves and in the upper 3 or 4 cervical spinal nerves Superior cervical ganglion serves: (i) Skin and BV of head (ii) Dilator muscles of irises (iii) Inhibit the nasal and salivary glands (iv) Innervates the smooth muscle that lifts upper eyelid (v) Direct branches to the heart

(b) Pathways to the Thorax 

Preganglionic fibers innervate the thoracic organs at T₁- T₆ o Some of these axons synapse in the nearest sympathetic trunk ganglion

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Postganglionic axons run directly to organs supplied Postganglionic axons pass through the cardiac, pulmonary, and esophageal plexus o Fibers to esophagus and lungs are a direct route o

Fibers to the heart mainly take less direct routes; however, there are few direct routes

Sympathetic Pathways with Synapses in Collateral Ganglia  

Most preganglionic fibers from T₅ and down synapse in a collateral ganglia o Most of these fibers enter and leave the sympathetic trunks without synapsing They form several nerves called the Splanchnic Nerves o Splanchnic nerve contributes to a number of nerve plexuses collectively known as the abdominal aortic plexus (complex that serves abdominopelvic viscera; including celiac, superior mesenteric, and inferior mesenteric)

(a) Pathways to the Abdomen

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Sympathetic preganglionic fibers serving the head emerge from spinal segments T₅-L₂ innervate the abdomen o They will travel in the thoracic splanchnic nerves to synapse to the celiac and superior mesenteric ganglia Postganglionic fibers issuing from the celiac and superior mesenteric ganglia then serve the: 1. Stomach 2. Intestines (except the distal ½ of large intestine) 3. Liver 4. Spleen 5. Kidneys

(b) Pathways to the Pelvis 

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Preganglionic fibers innervating the pelvis originate from T₁₀- L₂ and then descend into the sympathetic trunk to the lumbar and sacral trunk ganglia o Some fibers synapse at the ganglia in which the postganglionic fibers run into the lumbar and sacral splanchnic nerves to plexuses on the lower aorta and in the pelvis o Other preganglionic fibers pass directly to the autonomic plexuses and synapse in collateral ganglia, such as the inferior mesenteric ganglion Postganglionic fibers proceed from these plexuses to the pelvic organs 1. Urinary bladder 2. Reproductive organs 3. Distal ½ of large intestine For the most part sympathetic fibers inhibit the activity of muscles and glands in the abdominopelvic visceral organs

Sympathetic Pathways with Synapses in the Adrenal Medulla 

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Fibers branch from the thoracic splanchnic nerves, through the celiac ganglion, without synapsing and terminate by synapsing with hormone-producing medullary cells of the adrenal glands When stimulated preganglionic fibers medullary cells secrete epinephrine and norepinephrine (AKA adrenaline and noradrenaline) into the blood o Produces excitatory effects

14.5 VISCERAL REFLEX ARCS HAVE THE SAME FIVE COMPONENTS AS SOMATIC REFLEX ARCS Visceral Sensory Neurons    

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Visceral sensory neurons send information about chemical changes, stretch, temperature, and irritation of the viscera The brain interrupts this information as hunger, fullness, pain, or nausea o Almost all receptors for these visceral senses are free-nerve endings (non-encapsulated) Cell bodies of visceral sensory neurons are located in the dorsal root ganglia and in sensory ganglia of cranial nerves Axons travel along the same nerves as the autonomic motor fibers o EXAMPLE: Many visceral sensory neurons accompany the parasympathetic fibers of the Vagus nerve As fibers pass through sympathetic ganglia (on the way to spinal cord) branches of the visceral sensory fibers may synapse with postganglionic located in ganglia Visceral sensory neurons are also involved in referred pain

Visceral Reflexes  

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Visceral reflex arcs are essentially the same as somatic reflex arcs 3 key differences: 1. Visceral reflex arcs have 2 consecutive neurons on its motor component 2. Afferent fibers are visceral sensory (as oppose to somatic) 3. Effector organs are different; Smooth muscle, cardiac muscle, and glands EXAMPLES OF VISCERAL REFLEX ARC: Emptying of rectum and bladder

14.6 ACETYLCHOLINE AND NOREPINEPHRINE ARE MAJOR NEUROTRANSMITTERS OF THE ANS 

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Cholinergic Fibers: Ones that release Ach o Includes all ANS preganglionic axons and all parasympathetic postganglionic axons at synapses with effectors Adrenergic Fibers: Ones that release NE o Includes the majority of sympathetic postganglionic axons o EXCEPTION: Sympathetic postganglionic fibers that secrete Ach onto sweat glands

NOTE: EFFECTS OF ACh AND NE ARE NOT CONSISTENTLY EXCITATORY OR INHIBITON BECAUSE IT DEPENDS ON THE TYPE OF RECEPTOR IT BINDS TOO!! 

Each autonomic NT binds with two or more kinds of receptors; allows it to exert different effects (activation or inhibition) at different body targets

Cholinergic Receptors 

2 types (ACh-binding) receptors o Named after drugs that bind to them and mimic acetylcholine’s effects

(a) Nicotinic Receptors  

Responds to nicotine Found on: o All postganglionic receptors (cell bodies and dendrites) of sympathetic and parasympathetic o

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NS Sarcolemma of skeletal muscle cells at NMJ

o Hormone-producing cells of adrenal medulla When ACh binds to a nicotinic receptor effects are ALWAYS stimulatory o EXAMPLE: Skeletal muscle; directly opens ion channels and causes depolarization

(b) Muscarinic Receptors  

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Can be activated by mushroom poison muscarine Occur on all effector cells stimulated by postganglionic cholinergic fibers o IN SUMMARY: All parasympathetic target organs and a few sympathetic targets (eccrine sweat glands) When ACh binds to muscarinic receptors can be either inhibitory or stimulatory o Depends on a subclass of receptor on the target organ EXAMPLES: Cardiac; when ACh binds to cardiac muscle receptors it slows heart activity// G.I Tract; when ACh binds to smooth muscle it increases motility

Adrenergic Receptors 

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2 major classes of NE-binding receptors: 1. Alpha (α) Receptors 2. Beta (β) Receptors Receptors are then further divided into sub-divisions (α₁ ,α₂ , β₁ , β₂ , β₃) o Organs that respond to adrenergic receptors (NE) have one or more subtypes NE or EPI can be either excitatory or inhibitory depending on which subclass of receptor predominates in the target organ o EXAMPLE: NE binding to β₁ receptors of cardiac muscle prompts the heart into more vigorous activity o EXAMPLE: EPI binding to β₂ receptors on the bronchiole smooth muscle causes it to relax; dilate

14.7 THE PARASYMPATHETIC AND SYMPATHETIC DIVISIONS USUALLY PRODUCE OPPOSITE EFFECTS 

Both ANS divisions are particularly active ( dual innervation)

Antagonistic Interactions 

Antagonist effects are most clearly seen with the cardiac, respiratory, and G.I organs o Fight or Flight (Sympathetic): Increased heart rate, dilated airways, inhibition of digestion and elimination o Rest and Digest (Parasympathetic): Restores heart rate, airway diameter is resting, body continues digestion and elimination

Sympathetic and Parasympathetic Tone (a) Sympathetic Tone

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Sympathetic division is major contributor to BP (even at rest) o With few exceptions blood vessels are entirely controlled by the SNS o Sympathetic fibers keep the blood vessels in a continual state of partial constriction (keeps them from being saggy) = Sympathetic (Vasomotor) Tone When the BP is too low to maintain blood flow sympathetic vasomotor fibers fire more rapidly → blood vessels constrict→ raises BP When BP is too high→ sympathetic vasomotor fibers fire less rapidly→ vessels dilate Circulatory Shock: Inadequate blood flow to body tissues o More blood is needed for vital organs and needs of skeletal muscle; blood vessels constrict and “shunt” blood to abdominal viscera to maintain proper blood flow in said vital organs = blood shunting

Homeostatic Imbalance 14.1 Autonomic NeuropathyDamage to autonomic nerves - Common complication of diabetes mellitus- Early s/s is sexual dysfunction; Up to 75% of male diabetics experience erectile dysfunction and female diabetics experience reduced vaginal lubrication Other s/s: (i) Dizziness after standing suddenly (because of poor BP control)(ii) Urinary incontinence(iii) Sluggish pupil reactions (iv) Impaired sweatingTreatment/Prevention:Strict control of blood glucose levels

(b) Parasympathetic Tone    

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Controls heart and smooth muscle of digestive and urinary tracts Parasympathetic Tone: Slowing of HR and dictates the normal activity levels of digestive and urinary tracts However, sympathetic NS can override the parasympathetic tone during times of stress Drugs that block parasympathetic responses cause: (i) Increase HR (ii) Fecal Retention (iii) Urinary Retention Parasympathetic activates most glands o EXCEPTION: Adrenal and sweat glands

Cooperative Effects 

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Best example is exte...