Autonomic nervous system by Khaled Shoghy

The Autonomic Nervous System

Autonomic nervous system  The autonomic nervous system is the subdivision of the peripheral nervous system that regulates body activities that are generally not under conscious control  Visceral motor innervates non-skeletal (non-somatic) muscles  Composed of a special group of neurons serving:  Cardiac muscle (the heart)  Smooth muscle (walls of viscera and blood vessels, Internal organs and Skin) 2

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Basic anatomical difference between the motor pathways of the voluntary somatic nervous system (to skeletal muscles) and those of the autonomic nervous system

 Somatic division:  Cell bodies of motor neurons reside in CNS (brain or spinal cord)  Their axons (sheathed in spinal nerves) extend all the way to their skeletal muscles  Autonomic system: chains of two motor neurons  1st = preganglionic neuron (in brain or cord)  2nd = gangionic neuron (cell body in ganglion outside CNS)  Slower because lightly or unmyelinated

Diagram contrasts somatic (lower) and autonomic (upper): autonomic

this dorsal root ganglion is sensory

somatic

Note: the autonomic ganglion is motor

Visceral Efferent Two-Neuron Pathway Visceral efferent (VE) pathways that innervate smooth muscle, cardiac muscle, and glands involve two neurons and a synapse within an autonomic ganglion. The cell bodies of the preganglionic neurons are in the brainstem or spinal cord of the central nervous system (CNS). The cell bodies of the postganglionic neurons are in autonomic ganglia located peripherally. Axon terminal of preganglionic neurons synapse on dendrites and cell bodies of postganglionic neurons. (In contrast, somatic efferent (SE) innervation of skeletal muscle involves only a single neuron; its cell body in the CNS.)

The advantage of two neurons is conservation of space in the CNS, by shifting neurons into the spacious periphery. The small number of CNS neurons utilize divergent circuits to drive the numerous peripheral neurons. (This strategy has succeeded, in just gut wall alone there are more neurons than in the entire spinal cord.) The disadvantage of moving neurons out of the CNS is reduced brain control, but decreased brain control is not a problem in the case of visceral organs which have a limited repertoire of actions. 6

ď&#x201A;§ Preganglionic axons are myelinated, which means that they conduct more rapidly than do the numerous nonmyelinated postganglionic axons. However, non-myelinated axons are metabolically more efficient and reduced conduction speed is not a problem in cases of smooth muscle contraction and gland secretion.

Divisions of the autonomic nervous system  Parasympathetic division  Sympathetic division Serve most of the same organs but cause opposing or antagonistic effects Parasysmpathetic: routine maintenance “rest &digest”

Sympathetic: mobilization & increased metabolism “fight, flight or fright” or “fight, flight or freeze” 8

Comparison of the sympathetic and parasympathetic nervous systems ( simple funny ) Dr Aidy - YouTube.flv

“rest & digest” 11

Parasympathetic nervous system “rest & digest”

 Also called the craniosacral system because all its preganglionic neurons are in the brain stem or sacral levels of the spinal cord  Cranial nerves III,VII, IX and X  In lateral horn of gray matter from S2-S4

 Only innervate internal organs (not skin)  Acetylcholine is neurotransmitter at end organ as well as at preganglionic synapse: “cholinergic” 12

The parasympathetic division of the ANS originates (bilaterally) from the brainstem and from sacral segments of the spinal cord. Preganglionic axons synapse in discrete autonomic ganglia except in the case of the vagus nerve. Preganglionic axons in the vagus nerve synapse on terminal ganglia located within plexuses inside the visceral organ being innervated. A microscope is required to view terminal ganglia. Sacral parasympathetic preganglionic neurons are located in intermediate gray matter of spinal cord segments (S1,2,3 in the carnivore). Bilaterally, preganglionic axons run through the lumbosacral plexus and pelvic nerve to synapse on pelvic ganglia within the pelvic plexus. Post ganglionic axons innervate pelvic viscera including the descending colon. Note: it is reported that 50% of the axons in the pelvic nerve are afferent (General Visceral Afferent).

Parasympathetic continued  Cranial outflow    

III - pupils constrict VII - tears, nasal mucus, saliva IX – parotid salivary gland X (Vagus n) – visceral organs of thorax & abdomen:  Stimulates digestive glands  Increases motility of smooth muscle of digestive tract  Decreases heart rate  Causes bronchial constriction

 Sacral outflow (S2-4): form pelvic splanchnic nerves  Supply 2nd half of large intestine  Supply all the pelvic (urogenital) organs

Parasympathetic (only look at this if it helps you)

Parasympathetic Pathway to the Eye

Origin: parasympathetic nucleus of the oculmotor nerve Preganglionic Pathway: oculomotor nerve Ganglion: ciliary ganglion (located within the orbit) Postganglionic Pathway: short ciliary nerves Target Organs: pupillary constrictor smooth muscle & ciliary smooth muscle 16

Parasympathetic Pathway to Nasal & Lacrimal Glands Origin: parasympathetic nucleus of the facial nerve Preganglionic Pathway: facial nerve (pars intermedia), major petrosal nerve, nerve of ptyerygoid canal Ganglion: pterygopalatine ganglion Postganglionic Pathway: branches to lacrimal & nasal glands Target Organs: lacrimal gland & nasal cavity glands 18

Parasympathetic Pathway to Mandibular & Sublingual Glands Origin: parasympathetic nucleus of the facial nerve Preganglionic Pathway: facial nerve (pars intermedia), chorda tympani nerve (travels through the middle ear), lingual nerve, lingual nerve branch to salivary glands Ganglion: mandibular ganglion Postganglionic Pathway: branches to mandibular & sublingual salivary glands Target Organs: mandibular salivary gland & sublingual salivary glands

Parasympathetic Pathway to Parotid & Zygomatic Salivary Glands Origin: parasympathetic nucleus of the glossopharyngeal nerve Preganglionic Pathway: glossopharyngeal nerve, tympanic n. (travels through the middle ear), minor petrosal nerve Ganglion: otic ganglion Postganglionic Pathway: branches to parotid & zygomatic salivary glands Target Organs: parotid salivary gland & zygomatic salivary gland

Parasympathetic Pathway to Thoracic Viscera Origin: parasympathetic nucleus of the vagus nerve (nucleus ambiguus for cardiac preganglionic neurons) Preganglionic Pathway: left/right vagus nerve, vagosympathetic trunk & vagus nerve (in thoracic cavity) vagal branches to heart & lungs, etc. Ganglion: terminal ganglia (within organs innervated) Postganglionic Pathway: microscopic nerve plexuses within organs Target Organs: heart (nucleus ambiguus preganglionic origin); lungs: bronchial smooth muscle smooth muscle of the esophagus & trachea.

Parasympathetic Pathway to Thoracic Viscera

Parasympathetic Pathway to Abdominal Viscera Origin: parasympathetic nucleus of the vagus nerve Preganglionic Pathway: vagus nerve, vagosympathetic trunk, vagus nerve (in thorax), dorsal & ventral vagal branches (bilateral) dorsal & ventral vagal nerve trunks, dorsal vagal nerve trunk joins celiacomesenteric plexus, ventral vagal nerve trunk goes directly to stomach & liver Ganglion: terminal ganglia (within organs innervated) Postganglionic Pathway: microscopic nerve plexuses within organs, e.g., myenteric & submucosal nerve plexuses within the gut wall Target Organs: smooth muscle & glands of alimentary tract (to descending colon); 26 liver; gallbladder; pancreas; & cranial ureter

Parasympathetic Pathway to Abdominal Viscera

Parasympathetic Pathway to Pelvic Viscera Origin: sacral segments (S1-S3) of the canine spinal cord Preganglionic Pathway: ventral root, spinal nerve, ventral primary branch, lumbosacral nerve plexus, pelvic n. & pelvic plexus Ganglion: pelvic ganglia (also some in terminal ganglia) Postganglionic Pathway: direct branches from pelvic plexus to pelvic viscera Target Organs: smooth muscle & glands associated with pelvic viscera, i.e., urinary tract; genitalia; descending colon, rectum & anal canal; 28 blood vessels to penis/clitoris

Parasympathetic Pathway to Pelvic Viscera

“fight, flight or fright” 30

Sympathetic nervous system “fight, flight or fright”  Also called thoracolumbar system: all its neurons are in lateral horn of gray matter from T1-L2  Lead to every part of the body (unlike parasymp.)  Easy to remember that when nervous, you sweat; when afraid, hair stands on end; when excited blood pressure rises (vasoconstriction): these sympathetic only  Also causes: dry mouth, pupils to dilate, increased heart & respiratory rates to increase O2 to skeletal muscles, and liver to release glucose

 Norepinephrine (aka noradrenaline) is neurotransmitter released by most postganglionic fibers (acetylcholine in preganglionic): “adrenergic” 31

For all regions of the body, the initial pathway for sympathetic preganglionic axons is identical: ventral root spinal nerve ramus communicans sympathetic trunk. Only rami communicantes T-1 to L-4 contain myelinated preganglionic axons. All rami communicantes contain non-myelinated postganglionic axons. Axons from sympathetic preganglionic neurons synapse in autonomic ganglia, shown in yellow to the left. 32

Sympathetic nervous system continued  Preganglionic axons exit spinal cord through ventral root and enter spinal nerve  Exit spinal nerve via communicating ramus  Enter sympathetic trunk/chain where postganglionic neurons are has three options… 33

Options of preganglionic axons in sympathetic trunk 1. Synapse on postganglionic neuron in chain ganglion then return to spinal nerve and follow its branch to the skin 2. Ascend or descend within sympathetic trunk, synapse with a posganglionic neuron within a chain ganglion, and return to spinal nerve at that level and follow branches to skin 3. Enter sympathetic chain, pass through without synapsing, form a splanchnic nerve that passes toward thoracic or abdominal organs ď&#x201A;§ These synapse in prevertebral ganglion in front of aorta ď&#x201A;§ Postganglionic axons follow arteries to organs 34

Synapse in chain ganglia at same level or different level

Pass through ganglia and synapse in prevertebral ganglion

Adrenal gland is exception  Synapse in gland  Can cause body-wide release of epinephrine aka adrenaline and norepinephrine in an extreme emergency (adrenaline “rush” or surge) 37

Sympathetic ganglia

Sympathetic Pathway to the Head Origin: most cranial thoracic segments (T1-T3) Preganglionic Pathway: ventral root, spinal nerve, ramus communicans, sympathetic trunk, ansa subclavia, cervical sympathetic trunk Ganglion: cranial cervical ganglion Postganglionic Pathway: internal and external carotid nerve plexuses, plexuses named for the particular vessels on which they run. Target Organs: pupillary dilator muscle; nasal glands; salivary glands; sweat glands; blood vessel smooth muscle

Sympathetic Pathway to the Head

Sympathetic Pathway to the Neck Origin: cranial thoracic segments (T3-T5) Preganglionic Pathway: ventral root, spinal nerve, ramus communicans, sympathetic trunk Ganglion: cervicothoracic ganglion Postganglionic Pathway: vertebral nerve, rami communicantes to cervical spinal nerves, dorsal or ventral primary branches of cervical spinal nerves Target Organs: arrector pili smooth muscle (piloerection); sweat glands; blood vessel smooth muscle

Sympathetic Pathway to the Neck

Sympathetic Pathway to Body Wall & Limbs Origin: thoraco-lumbar segments (T1-L4) Preganglionic Pathway: ventral root, spinal nerve, ramus communicans, sympathetic trunk Ganglion: sympathetic trunk ganglia (paravertebral ganglia) Postganglionic Pathway: rami communicantes to spinal nerves, dorsal or ventral primary branches of spinal nerves. Target Organs: arrector pili smooth muscle (piloerection); sweat glands; blood vessel smooth muscle

Sympathetic Pathway to Body Wall & Limbs

Sympathetic Pathway to Thoracic Viscera Origin: mid-thoracic segments (T5-T7,8) Preganglionic Pathway: ventral root, spinal nerve, ramus communicans, sympathetic trunk, [ansa subclavia] Ganglion: cervicothoracic ganglion [& middle cervical ganglion] Postganglionic Pathway: branches from the caudal limb of the ansa subclavia, [branches that run with the vagus nerve] Target Organs: heart; bronchiole smooth muscle; blood vessel smooth muscle

Sympathetic Pathway to Thoracic Viscera

Sympathetic Pathway to Abdominal Viscera Origin: caudal thoracic & cranial lumbar segments (T7-L2) Preganglionic Pathway: ventral root, spinal nerve, ramus communicans, sympathetic trunk, major, minor, & lumbar splanchnic nerves Ganglion: prevertebral ganglia within plexuses (i.e., celiac,cranial & caudal mesenteric, gonadal, and renal ganglia); adrenal medulla Postganglionic Pathway: nerve plexuses on abdominal arteries, e.g., celiac, hepatic, cranial mesenteric, middle colic, renal, etc. nerve plexuses Target Organs: alimentary tract smooth muscle and glands; liver; pancreas; blood vessel smooth muscle

Sympathetic Pathway to Abdominal Viscera

Sympathetic Pathway to Pelvic Viscera Origin: lumbar segments (L1-L3) Preganglionic Pathway: ventral root, spinal nerve, ramus communicans,sympathetic trunk, lumbar splanchnic nn. & caudal mesenteric plexus Ganglion: caudal mesenteric ganglion (also some pelvic ganglia) Postganglionic Pathway: left or right hypogastic nerve to the pelvic plexus, branches to pelvic viscera, from the pelvic plexus Target Organs: smooth muscle & glands associated with pelvic viscera, i.e., urinary tract, genitalia, rectum & anal canal blood vessel smooth muscle

Sympathetic Pathway to Pelvic Viscera

Summary