drbcshah
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THE AUTONOMIC NERVOUS SYSTEM
Autonomic Reflex: Autonomic efferent fibers can be initiated in response to different types of afferent signals:
Visceral Afferent Fibers: Transmission of visceral pressure, stretch, and noxious-stimuli.
Cell bodies of these sensory neurons are located in the sensory ganglia of CN VII, IX, and X.
Enteroreceptors are specialized fibers to transmit these visceral signals.
Somatic Afferent Fibers: Temperature, pain, and light.
Cognitive Input from higher learning centers: Perceived threats, anxiety, excitement, and sexual arousal can all influence autonomic motor responses.
AUTONOMIC v/s SOMATIC NERVOUS SYSTEMS:
The autonomic system generally acts slower.
The last inter-neuronal synapse of a somatic nerve is in the CNS (in the spinal column), while the last inter-neuronal synapse of an autonomic nerve is in a peripheral ganglion.
Property
PARASYMPATHETICSYMPATHETIC
Anatomical Origin --CRANIO-SACRAL: CN III, VII, IX, X, and the Pelvic Splanchnic Nerves--THORACO-LUMBAR
Preganglion Axon: Postganglionic Axon Long Preganglion Axon and short postganglionic Axon; Ratio is nearly 1:1, yielding discrete effects.
Short Preganglionic Axon and long postganglionic axon; ratio is 1:many, yielding diffuse effects
Location of Upper Cell BodiesBrainstem nuclei and sacral segmentsIntermediolateral segments of the thoracolumbar spinal cord.
Location of Interneuronal GangliaIn or very near the target organParavertebral and Prevertebral Ganglia, far away from target organs Principle
Neurotransmitter –Acetylcholine,Norepinephrine
Other Neurotransmitters foundVasoactive Intestinal Peptide (VIP), which results in synthesis of NO ------> vasodilation. Neuropeptide Y ,Somatostatin, Enkephalins
Principle Neurotransmitter Receptors at target organs:Muscarinic acetylcholine receptors, at the end organs. Also Muscarinic Autoreceptors on the postganglionic terminal, providing feedback inhibition for release of Ach.
Beta-Adrenergic Receptors (cAMP secondary pathway) alpha-Adrenergic Receptors (IP3/DAG secondary pathway)
Neurotransmitter Inactivation
Acetylcholinesterase is the primary wayReuptake is the primary method of getting rid of NorE.Ocular ReflexMiosis: constriction of pupil is a reflex to light
Accommodation: Initiated by afferent signals from optic nerve ------> Contract ciliary muscle -----> increase natural curvature of lens ----> focus for near vision Lacrimation via Facial VII.
Pupillary Dilation: Radial smooth muscle of pupil contracts
Ciliary Muscle Relaxation These neurons for these reflexes come from Superior Cervical Ganglion ------> Carotid Plexus
Digestive ReflexSalivation via Chorda Tympani (VII) and Lingual (IX)
General increase in GI smooth muscle tone Liver promotion of glycogenesis
Salivation via sympathetics from external carotid plexus
General Relaxation of GI smooth muscle tone
Liver induction of glycogenolysis and gluconeogenesis Anal Sphincter contraction
Respiratory ReflexBronchoconstriction Vagal innervation of smooth muscle in trachea and bronchi.
BronchodilationCardiac ReflexDecrease heart rate by vagal innervation SA nodeIncrease heart rate by innervation of SA Node Increase heart contractility
Sexual ReflexPenile Erection -- vasodilation involved NO and possible VIPEjaculation
SYMPATHETIC NERVOUS SYSTEM:
Pathway of sympathetic spinal nerves out of the spinal cord:
From spinal cord, the dorsal root and ventral root of each sympathetic nerve join to form the White Communicating Ramus.
The White Communicating Ramus goes to the Paravertebral Ganglia, on either side of the spinal cord.
PREGANGLIONIC NERVE:
Once in the Paravertebral Chain, it can do one of four things:It can go up to a level rostral to the current level (to provide sympathetics to cranial region)
It can go down to a level caudal to the current level (to provide sympathetics to sacral region)
It can synapse with a ganglion cell located in the chain ganglia.
It can go straight through the chain without synapsing, thereby forming a splanchnic nerve which will go onto the Prevertebral Ganglia.
Prevertebral Ganglia: The prevertebral ganglia receive preganglionics from the Thoracic Splanchnic Nerves.
There are four major prevertebral ganglia:
Celiac Ganglion
Superior Mesenteric Ganglion
Inferior Mesenteric Ganglion
Aorticorenal Ganglion
There are four major thoracic splanchnic nerves, which go straight through the paravertebral chain to synapse in the prevertebral ganglia.
Greater Thoracic Splanchnic (T10)
Lesser Thoracic Splanchnic (T11)
Least Thoracic Splanchnic (T12)
POSTGANGLIONIC NERVE: For those nerves that synapse in the paravertebral chain, they can do one of two things afterwards:
It can form a grey communicating ramus which then goes on to form a spinal nerve and provide sympathetic innervation to the appropriate dermatomal level.
Many blood vessels, sweat glands, hair follicle, and piloreceptors are innervated in this way.
It can leave the paravertebral ganglion and go straight to its target organ. This happens mainly in the cranial and sacral regions.
ADRENAL MEDULLA: Releases 80% Epi and 20% NorEpi into bloodstream.
It is innervated by sympathetic preganglionics, which have come by way of the celiac ganglion, but they didn't synapse there.
It is sympathetic cholinergic, with nicotinic ganglionic acetylcholine receptors.
ADRENERGIC RECEPTORS:
Beta Receptors: Activated by Isoproterenol
beta1-Receptor: Sympathetics in heart and kidney (renin release) -- they respond equally to both epinephrine and NorE
beta2-Receptor: All other tissues except adipocytes -- responds primarily to NorE
beta3-Receptor: Adipocytes
Alpha Receptors: Activated by Phenylephrine
GANGLIONIC NICOTINIC CHOLINERGIC RECEPTORS: Both sympathetic and parasympathetic neurons use acetylcholine. Their receptors are of the nicotinic type, but they are different structurally in that they respond differently to drugs.
GROUND PLEXUS: Autonomic postganglionics are unmyelinated. Near the target organ they divide to form a meshwork-like web called a ground-plexus.
Axonal Varicosity: The autonomic axon becomes wider near the target.
NEUROEFFECTOR JUNCTION: The name of an autonomic synapse. It is not proper to call it a synapse.
It is not called a synapse because there are no ultrastructural membrane specializations between the neuron and the target cell. Thus not a synapse.
Prejunctional Element: Autonomic name for the presynapse.
Postjunctional Element: Autonomic name for the postsynapse.
Autonomic Tone: The continual visceral innervation of target organs. It is the job of autonomic nerves to modulate, either up or down, the tone of the target organ, rather than to discretely stimulate it.
Denervation Supersensitivity: As a compensatory mechanism, a target loses autonomic innervation, it becomes hypersensitive to said neurotransmitter.
Hypersensitivity due to increased synthesis of neuroreceptors.
SYMPATHETIC LESIONS:
Preganglionic Lesion: Not so much hypersensitivity at target.
Postganglion Lesion: Pronounced Hypersensitivity because there is no longer a way for neurotransmitter reuptake to occur!!
Five Ways to Autonomically regulate End-Organ Activity:Antagonistic Effects on the same organ: Sympathetic and Parasympathetic have same effect.
Heart, Respiratory Passages
Urinary Bladder
Gut
Antagonistic Effects through opposing organs: Sympathetic and Parasympathetic innervate different targets to achieve antagonistic effects.
Pupil (Pupil dilator and pupil constrictor)
Anal Sphincter
Agonistic Effects on complementary organs: Salivary glands
Agonistic Effects on the same target organ:
Innervation by only one component: Lots of end organs
PARASYMPATHETIC ONLY: Lacrimal glands (CN VII), Nasopharyngeal Glands (CN VII), Tracheal / Bronchial Glands
SYMPATHETIC ONLY: Tarsal Muscles of eyelid. Loss of this function (drooping eyelids) is called Ptosis, a component of lost sympathetic innervation, or Horner's Syndrome.
Pilomotor smooth muscle
Sweat Glands: Excitatory sudomotor innervation
Eccrine Sweat Glands are cholinergic sympathetic. They are present all over and are responsible for thermoregulation.
Apocrine Glands = Sebaceous Sweat Glands. Mainly in axilla. They receive adrenergic innervation.
Spleen Capsule
Kidney stimulation for renin release
PENILE ERECTION:
Parasympathetic vasodilation of penile arteries to yield erection.
Sympathetic constriction of vas deferens and seminal vesicles for ejaculation.
SYMPATHETIC CIRCULATORY REFLEXES: Conform to the needs of fight or flight.
Vasodilation of pathways leading to the heart and lungs.
Vasoconstriction of pathways in the portal / GI system.
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