Autonomic Nervous system
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- The Autonomic Nervous System (ANS)
- is a general visceral efferent (GVE) motor system
- controls and regulates smooth muscle, cardiac muscle, and glands.
- has three division
- The Autonomic Nervous System (ANS)
- the sympathetic
- the parasympathetic
- enteric.
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- consists of two types of projection neurons
- preganglionic neurons
- postganglionic neurons
- sympathetic ganglia have interneurons
- output that is influenced by the hypothalamus.
- consists of two types of projection neurons
- Divisions of the Autonomic Nervous System
- Sympathetic division
- is also called the thoracolumbar, or adrenergic, system.
- Sympathetic division
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- Preganglionic neurons
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- are located in the intermediolateral cell column (T1–L3).
- project via ventral roots and white communicating rami to the sympathetic trunk or via splanchnic nerves to prevertebral (collateral) ganglia
- They synapse at both locations with postganglionic neurons.
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- Postganglionic neurons
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- are located in the
- sympathetic trunk (paravertebral ganglia)
- prevertebral (collateral) ganglia.
- in the sympathetic trunk, project via gray communicating rami to spinal nerves
- innervate blood vessels, arrector pili muscles, and sweat glands.
- in prevertebral ganglia, project to abdominal and pelvic viscera.
- are located in the
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- Interneurons
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- are called small intensely fluorescent (SIF) cells.
- are located in sympathetic ganglia.
- are dopaminergic and inhibitory.
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- Neurotransmitters
- Acetylcholine (ACh)
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- is the neurotransmitter of preganglionic neurons
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- Norepinephrine
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- is the neurotransmitter of postganglionic sympathetic neurons, with the exception of sweat glands and some blood vessels that receive cholinergic sympathetic innervation.
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- Epinephrine
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- is produced by the chromaffin cells of the adrenal medulla
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- exists in a 4:1 ratio to norepinephrine.
- Dopamine
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- is the neurotransmitter of the SIF cells.
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- Vasoactive intestinal polypeptide (VIP)
- is co-localized with ACh in some postganglionic parasympathetic fibers.
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- is a vasodilator.
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- Parasympathetic division
- is called the craniosacral, or cholinergic, system.
- uses ACh as the neurotransmitter for both preganglionic and postganglionic synapses
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- Cranial division is associated with four cranial nerves.
- Oculomotor nerve (CN III)
- Facial nerve (CN VII)
- Superior salivatory nucleus
- projects preganglionic fibers to the pterygopalatine and submandibular ganglia.
- Pterygopalatine ganglion
- projects postganglionic fibers to the lacrimal gland and to the mucosa of the nose and palate.
- Submandibular ganglion
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- projects postganglionic fibers to the submandibular and sublingual glands.
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- Glossopharyngeal nerve (CN IX)
- Inferior salivatory nucleus
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- projects preganglionic fibers to the otic ganglion.
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- Otic ganglion
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- projects postganglionic fibers to the parotid gland.
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- Vagal nerve (CN X)
- Dorsal motor nucleus
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- projects preganglionic fibers to intramural (terminal) ganglia within or adjacent to visceral organs.
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- Intramural (terminal) ganglia
- innervate, via short postganglionic fibers, viscera of the thorax and abdomen as far as the left colic flexure.
- Nucleus ambiguus
- projects preganglionic fibers to the intramural ganglia of the heart (sinoatrial and atrioventricular nodes).
- Sacral division
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- originates from the sacral parasympathetic nucleus of sacral segments S2 to S4.
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- postganglionic neurons lie on, near, or in the wall of the innervated viscus (intramural ganglia).
- innervates via pelvic nerves the lower abdomen and pelvic viscera, including the colon distal to the left colic flexure, urinary bladder (detrusor muscle), and genital viscera.
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- is involved with micturition, defecation, and sexual function.
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- Neurotransmitters
- ACh
- VIP
- is co-localized with ACh.
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- stimulates secretomotor neurons and vasodilator neurons.
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- Nitric oxide
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- responsible for relaxation of smooth muscle.
- is responsible for penile erection
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- Enteric division
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- consists of
- intramural (enteric) ganglia
- plexuses of the gastrointestinal tract, including the
- submucosal (Meissner) plexus
- myenteric (Auerbach) plexus.
- is influenced by postganglionic adrenergic sympathetic input.
- is influenced by preganglionic cholinergic parasympathetic input.
- functions independently when deprived of central nervous system (CNS) innervation.
- plays a major role in the control of gastrointestinal motility.
- consists of
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- Visceral Afferent Fibers and Pain
- All sympathetic and parasympathetic nerves contain both general visceral afferent (GVA) and GVE fibers.
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- GVA fibers and innervated structures
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- Most visceral reflexes and organic sensations are mediated by parasympathetic afferent fibers.
- GVA cell bodies
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- are found in
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- dorsal root ganglia
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- inferior ganglia of the glossopharyngeal nerve (CN IX)
- the vagal nerve (CN X),
- geniculate ganglion of the facial nerve (CN VII).
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- GVA pain fibers
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- are found in the white communicating rami.
- accompany sympathetic nerves exclusively.
- have their cell bodies in the dorsal root ganglia of the thoracolumbar region (T1–L3).
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- GVA reflex fibers
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- accompany both sympathetic and parasympathetic nerves.
- terminate centrally in the solitary nucleus
- mediate the gag reflex.
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- Carotid sinus
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- is a slight dilation of the common carotid artery at the bifurcation.
- It contains baroreceptors
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- when baroreceptors are stimulated cause bradycardia and a decrease in blood pressure.
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- is innervated by GVA fibers from CN IX.
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- Carotid body (glomus caroticum)
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- is a small structure just above the bifurcation of the common carotid artery
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- it contains chemoreceptors that respond to carbon dioxide, oxygen, and pH levels.
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- is innervated by GVA fibers from CN IX and CN X.
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- Autonomic Innervation of Selected Organs
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- Eye
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- Sympathetic input
- Hypothalamic neurons project directly to the intermediolateral cell column at T1 and T2, the ciliospinal center of Budge.
- The intermediolateral cell column (T1–T2) projects preganglionic fibers via the sympathetic trunk to the superior cervical ganglion.
- The superior cervical ganglion projects postganglionic fibers via the internal carotid artery to the cavernous sinus.
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- Pupillodilator fibers reach the dilator pupillae muscle of the iris via the superior orbital fissure and via the nasociliary and long ciliary nerves (CN V)
- How does Horner syndrome occur with otitis media.
- Some pupillodilator fibers accompany the caroticotympanic nerves prior to entering the orbit;
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- Fibers to the tarsal muscles of Müller reach the eyelids via the ophthalmic artery (in the optic canal).
- Interruption of sympathetic input to the eye at any level results in Horner syndrome.
- Preganglionic versus postganglionic Horner syndrome
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- Instill 1% hydroxyamphetamine into the conjunctional sac.
- A dilated iris indicates preganglionic (central) Horner syndrome
- An undilated iris indicates postganglionic Horner syndrome.
- This test is used to rule out other causes of miosis (e.g., anisocoria).
- Instill 1% hydroxyamphetamine into the conjunctional sac.
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- Parasympathetic input
- The Edinger-Westphal nucleus projects preganglionic fibers via the oculomotor nerve (CN III) to the ciliary ganglion.
- The ciliary ganglion projects postganglionic fibers via the short ciliary nerves to the
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- sphincter pupillae (which acts to contract the pupil)
- ciliary muscle (which affects lens shape in accommodation).
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- Postganglionic fibers mediate the efferent limb of the pupillary light reflex.
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- Interruption of the parasympathetic input results in
- internal ophthalmoplegia (a fixed unresponsive] and dilated pupil)
- cycloplegia (paralysis of accommodation).
- Interruption of the parasympathetic input results in
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- Blood vessels
- receive their innervation from the sympathetic division of the ANS.
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- Arteries and arterioles
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- Constriction of cutaneous and splanchnic blood vessels results from sympathetic stimulation of alpha-receptors.
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- Dilation of skeletal muscle arteries results from sympathetic stimulation of Beta-receptors.
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- Blood vessels are not affected by parasympathetic stimulation.
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- Large veins and venules are moderately innervated.
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- Capillaries seem to have no innervation.
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- Cerebral blood vessels respond to circulating metabolites (carbon dioxide and oxygen).
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- Heart
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- Sympathetic input
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- The intermediolateral cell column (T1–T5)
- projects preganglionic fibers to the upper thoracic ganglia and to the three cervical ganglia of the sympathetic trunk.
- The rostral sympathetic trunk
- projects postganglionic fibers via cardiac nerves to the ventricular and atrial walls and the pacemaker tissue.
- The intermediolateral cell column (T1–T5)
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- Stimulation of cardiac nerves results in an increase in heart rate and in the force of cardiac contractility (via Beta 1-adrenergic receptors).
- Parasympathetic input
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- The nucleus ambiguus of CN X projects preganglionic fibers via the vagal nerve to the intramural ganglia of the atria and the sinoatrial node.
- Postganglionic fibers from the intramural ganglia innervate the heart.
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- Vagal stimulation lowers the strength and rate of cardiac contraction.
- Bladder (control of micturition)
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- Control is predominantly parasympathetic.
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- Sympathetic input
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- is from T12 to L3 via the inferior mesenteric plexus and via the inferior hypogastric plexus to the detrusor muscle and the internal sphincter.
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- Damage to sympathetic fibers has no effect on bladder function.
- Parasympathetic input
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- is from S2–S3 via the pelvic splanchnic nerves to the detrusor muscle and the internal sphincter.
- Stimulation results in emptying the bladder.
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- Paralysis produces an atonic bladder, with no reflex or voluntary control.
- Somatomotor input
- is from S2 to S4 via the pudendal nerves to the external sphincter muscle.
- Sensory input to spinal cord
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- is via hypogastric, pelvic, and pudendal nerves.
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- Damage results in an atonic bladder with overflow incontinence (dribbling).
- Ascending pathway for bladder sensation
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- controls the urge to void.
- is found with sacral fibers of the lateral spinothalamic tract.
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- Transection results in loss of the urge to void and overflow incontinence.
- Upper motor neuron (UMN) input
- controls volitional micturition.
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- is from the paracentral lobule via the corticosacral tract
- Bilateral transection results in an uninhibited neurogenic bladder
- Sensation is normal
- patient has no control over voiding
- bladder fills and suddenly empties without cortical control.
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