Autonomic Nervous System

Sympathetic Division: Pathway 2

(2) the preganglionic axon ascends or descends through the sympathetic trunk and synapses with a sympathetic chain ganglion above or below where it originated from. The postganglionic neuron then exits the sympathetic trunk at the level of the synapse and courses back through the ventral/dorsal ramus of the spinal nerve.

Sympathetic Division: Pathway 3

(3) the preganglionic axon enters the sympathetic trunk, does not synapse but continues through a splanchnic nerve (greater, lesser, least & lumbar) destined for the gastrointestinal tract, and then synapses in a pre-aortic (prevertebral or collateral) ganglion. The postganglionic neuron then exits from the pre-aortic ganglion and enters an autonomic nerve plexus (generally the aortic plexus) to reach the target organ. There are 4 main pre-aortic ganglion: celiac, aorticorenal, superior mesenteric & inferior mesenteric ganglions.

Sympathetic Division: Pathway 4

(4) the preganglionic axon (mostly from T5-L2) enters the sympathetic trunk, does not synapse but continues through a splanchnic nerve destined for the celiac ganglion, however does not synapses but continues through an autonomic nerve plexus to reach the adrenal medulla. At the adrenal medulla, the preganglionic axon synapses with chromaffin cells (modified postganglionic neurons with no nerve processes). These chromaffin cells secrete 2 catecholamines into the general circulation. Epinephrine (adrenaline) and norepinephrine. Epinephrine accounts for about 80% of the total secretion.

Hirschsprung's Disease (congenital megacolon)

A birth defect in which parasympathetic innervation of the distal region of the large intestine fails to develop Feces and gas accumulate proximal to the immobile bowel segment causing this region to greatly distend Can be corrected surgically

Visceral Nerve Plexus

A visceral nerve plexus is a network of nerve fibers that rest against a visceral organ or structure. All viscera of the thorax, abdomen & pelvis are innervated by their own visceral nerve plexuses. The visceral nerve plexus is comprised of visceral sensory (VA) & autonomic fibers (VE). So by definition, a visceral nerve plexus contains: Sym/post Para/pre Visceral sensory (GVA) - pain & reflex Note: Para/post cell bodies are in the wall of the target organ, so their axons/fibers are not in the visceral nerve plexuses

Neurotransmitters Released in ANS and Somatic Motor Systems

ACh (Somatic Nervous System) Sidenote: Heavily myelinated axons NE (ANS-Sympathetic) Sidenote: Lightly myelinated and unmyelinated axons ACh (ANS-Parasympathetic) Sidenote: Lightly myelinated and unmyelinated axons

CNS Control of ANS

ANS is not considered to be under direct voluntary control but many of its activities can be regulated by the CNS. Reticular Formation (mainly in medulla) and exerts the most direct influence over the ANS Cardiac center: regulates heart rate Vasomotor center: regulates blood pressure Respiratory center: regulates rhythm & rate of breathing Additional centers regulate hiccuping, swallowing, coughing & sneezing Periaqueductal Gray Matter (midbrain) Sympathetic fear response Spinal Cord Spinal visceral reflexes Hypothalamus (main integration center) Anterior nucleus: parasympathetic control Posterior nucleus: sympathetic control These centers direct preganglionic neurons through direct connections and relays in the reticular formation & periaqueductal gray matter. Through the ANS, the hypothalamus can control heart activity, BP, body temp & digestive functions. Amygdala (limbic region for emotions) By way of the hypothalamus and periaqueductal gray matter, the amygdala stimulates sympathetic activity (previously learned fear-related behavior). Cerebral Cortex A person may be able to control the ANS through control of their thoughts and emotions. For example, meditation or recall of a scary situation.

Classifications of Autonomic Nervous System

Anatomical Classification and Functional Classification

CN III (Oculomotor)

Associated with the PARASYMPATHETIC NERVOUS SYSTEM Para/pre fibers from the Edinger-Westphal nucleus Synapse at the ciliary ganglion Para/post fibers then extend from the ciliary ganglion through the short ciliary nerves to enter the eyeball and innervate the constrictor/sphincter pupillae and ciliary muscles The constrictor/sphincter pupillae as it's name implies contricts the pupil The ciliary (body) muscle releases tension on the lens producing a more rounded structure. This results in accommodation for near vision.

CN X (Vagus)

Associated with the PARASYMPATHETIC NERVOUS SYSTEM Para/pre fibers from the dorsal motor nucleus of vagus Synapse at the smaller "intramural" ganglion found in the submucosa of its target organs

CN IX (Glossopharyngeal)

Associated with the PARASYMPATHETIC NERVOUS SYSTEM Para/pre fibers originate from the inferior salivatory nucleus and travel through the tympanic nerve, tympanic plexus and finally the lesser petrosal nerve Synapse at the otic ganglion Para/post fibers then extend from the otic ganglion through the auriculotemporal nerve (V3) to innervate the parotid gland

CN VII (Facial)

Associated with the PARASYMPATHETIC NERVOUS SYSTEM Para/pre fibers originate from the superior salivatory nucleus and course through the greater petrosal nerve (synapse at pterygopalatine) Para/post fibers then extend from the pterygopalatine ganglion onto V2 branches that course to and innervate most of the mucosal glands of the nasal cavity and lacrimal gland in the orbit Para/pre fibers also course through the chorda tympani nerve to reach the lingual nerve (V3) (synapse at submandibular) Para/post fibers from the submandibular ganglion are responsible for innervating the sublingual and submandibular glands.

Anatomical Classification

Composed of the CNS and PNS

Achalasia of the Cardia (achalasia = "failure to relax")

Defect in the parasympathetic autonomic innervation of the esophagus Results in a loss of the esophagus' ability to propel a food bolus inferiorly Smooth muscle of the cardiac sphincter, located at the junction of the esophagus and stomach, remains contracted preventing passage of the food bolus Cause not really understood but tends to happen in young adults Longitudinal incision through the muscle near the esophagus/stomach junction is made to alleviate the sustained contraction

Raynaud's Disease

Exaggerated, sympathetic response resulting in vasocontriction of vessels Commonly provoked by exposure to cold or by emotional response Intermittent attacks of the fingers and toes making them pale, blue and painful due to the vasoconstriction of blood vessels in that region Treatment involves administration of drugs that inhibit vasoconstriction Rather common in the elderly affecting 9% of all elderly women and 3-5% elderly men

Mass Reflex Reaction

Massive uncontrolled activation of autonomic and somatic motor neurons Affects quadriplegics and paraplegics with spinal cord injuries above T6. Temporary loss of all reflexes inferior to the injury When reflex activity returns it is exaggerated because of the lack of inhibitory input from higher brain centers Brought on by overfilling of a visceral organ; muscle spasms occur, with the colon and bladder emptying

Parasympathetic Division Pathway

Preganglionic Cell Body-is located in brain stem nuclei associated with 4 different cranial nerves (CN III, VII, IX, X) or oculomotor, facial, glossopharyngeal & vagus respectively) or the lateral horns between S2-S4 spinal cord segments Preganglionic Axon-generally very long Postganglionic Cell Body-Located in the wall (submucosa) of the target organ below the head or within 4 special ganglia (ciliary, pterygopalatine, submandibular & otic) in the head Postganglionic-very short

Sympathetic Division Cell Body/Axons

Preganglionic Cell Body-located in the lateral horns (intermediolateral columns) between T1-L2 spinal cord segments Preganglionic Axon-can be relatively short depending on where the axon originates from because the spinal cord is close to the sympathetic chain Postganglionic Cell Body-is located in the sympathetic chain (paravertebral) ganglia or in a pre-aortic (prevertebral) ganglia (i.e. celiac, superior mesenteric, aorticorenal & inferior mesenteric ganglion) in the abdomen Postganglionic Axon-can be relatively long

Functional Classification

Somatic (afferent and efferent) Nervous System and Visceral (afferent and efferent-ANS) Nervous System

Sympathetic Pathway Cheats

Sym/pre fibers innervating the head originate from T1-T4 and synapse primarily at the superior cervical ganglion. The sym/post fibers then course along the arterial supply before synapsing at glands and smooth muscle. Loss of sympathetic innervation to the head (generally one side) leads to Horner's Syndrome. Sym/pre fibers innervating the thoracic organs originate from T1-T4 (T5-T6) and synapse in the nearest sympathetic chain ganglion. The sym/post fibers pass through visceral nerve plexuses before synapsing on the target organ. Sym/pre fibers innervating the abdominal organs originate from T5-L2 and pass through the sympathetic trunk and thoracic/lumbar splanchnic nerves before synapsing at pre-aortic (prevertebral or collateral) ganglion of the aortic plexus. The sym/post fibers then course along the main arterial branches of the aorta to reach and then innervate their target organs. Sym/pre fibers innervating the pelvic organs originate from T10-L2 and pass through the sympathetic trunk before synapsing in the lower lumbar & sacral sympathetic chain ganglia. From here the sym/post fibers will course through the sacral splanchnic nerves and corresponding pelvic plexuses (inferior hypogastric, vesical, rectal, prostatic & uterovaginal) to reach and then innervate their target organs.

Sympathetic Division: Pathway 1

Sympathetic preganglionic (sym/pre) neurons that originated somewhere in the thoracolumbar (T1-L2) portion of the spinal cord, will send their motor axons through the adjacent ventral rootlets/root into the spinal nerve, white ramus communicans and associated sympathetic trunk ganglion. (1) the preganglionic axon synapses with the postganglionic neuron in the sympathetic trunk ganglion at the same level and exits on the spinal nerve at that level.

Sacral Outflow

The sacral outflow originates from the sacral spinal cord (S2-S4) and innervates some additional organs in the abdomen as well as the pelvic organs The para/pre fibers from the sacral region originate from lateral horns of the S2-S4 region of spinal cord. They then travel through ventral rootlets/roots of spinal nerves S2-S4 and the pelvis splanchnic nerves that arise from their ventral rami. From pelvic splanchnic nerves, para/pre fibers travel to abdomen and pelvis via autonomic plexuses (inferior mesenteric plexus, pelvic plexus) specifically hindgut, and pelvic organs

Sympathetic Division: Basic Organization

The sympathetic nervous system innervates visceral organs within the internal body cavity as well as superficial visceral structures like sweat glands, smooth muscle of vessels and arrector pili muscles. (1) sympathetic trunk ganglia (2) pre-aortic (prevertebral or collateral) ganglia Also contains splanchnic nerves leading to visceral organs. There are typically 3 cervical, 11 thoracic, 4 lumbar, 4 sacral & 1 coccygeal ganglion on each side of the vertebral column

Visceral Sensory Neurons: Referred Pain

Visceral pain is not felt if the viscera is cut or scrapped but will be felt when there may be chemical irritation or inflammation, smooth muscle spasms (cramping) or from excessive stretching of the visceral organ. This pain will be perceived as somatic but is a phenomenon known as referred pain and is closely associated with the sympathetic nervous system.

Visceral Reflexes

Visceral sensory and autonomic neurons are involved in visceral reflex arcs. Many of these reflexes involve simple spinal reflexes. Defecation reflex Micturition reflex Some involve cranial nerves Baroreceptor reflex (blood pressure) Chemoreceptor reflex (blood chemistry) Some do not involve the CNS Enteric Nervous System

Visceral Sensory Neurons

Visceral sensory fibers (GVA) monitor stretch, temperature, chemical changes as well as irritation within the visceral organs. The brain interprets this visceral information as the feeling of pain, nausea, hunger, fullness or well-being. Almost all visceral sensory fibers are free nerve endings and widely scattered throughout the visceral organ. However, visceral sensations are difficult to localize. Visceral pain fibers generally course along with sympathetic fibers and their cell bodies are found in dorsal root ganglion. Visceral reflex fibers course along with parasympathetic fibers and their cell bodies are located in either dorsal root ganglion (sacral region) or sensory ganglia of cranial nerves (CN VII, IX & X).