The Autonomic Nervous System Flashcards
Autonomic System (ANS)
Involuntary motor (efferent) branch that innervates smooth and cardiac muscle, viscera, and glands
Sometimes referred to as “visceral motor system” regulating unconscious processes
*maintains homeostasis
Sympathetic vs. Parasympathetic Nervous System
Most organs receive both sympathetic and parasympathetic innervation
Sympathetic and parasympathetic systems are functionally and anatomically distinct (different)
Effects generally opposing
Functions of Sympathetic and Parasympathetic NS
Sympathetic:
-stimulates metabolism; active during stress, exertion, emergency
-increases HR, force of contraction and BP
-dilates blood vessels in skeletal muscles, downregulates digestive activity
Parasympathetic:
-promotes relaxation, nutrient uptake and energy storage
-Decrease HR, force of contraction and BP
-Stimulates digestive activity
-Dilates blood vessels in GI tract
Neurons
Basic functional unit of nervous system; highly specialized cells that carry electric signals
Axons are where signal travels to post-synaptic cell
Myelin Sheath
Lipid-rich sheath that surrounds some nerve cell axons; acts as insulating layer and increases velocity of signals
Autonomic vs. Somatic
Autonomic= involuntary, 2 neruons b/t CNS and target, innervates cardiac muscle, smooth muscle, glands and viscera
Somatic= voluntary, 1 neuron b/t CNS and target, innervates skeletal muscle
Pre-ganglionic vs. Post-ganglionic
Pre: neuronal cell bodies in brainstem or spinal cord, myelinated
Post: neuronal cell bodies in autonomic ganglion, unmyelinated
Ganglia
Ganglion= collection of neuronal cell bodies found in PNS, autonomic ganglia part of ANS
Function as relay stations b/t pre-ganglionic and post-ganglionic neurons of ANS
Autonomic Outflow (Sympathetic vs. Parasympathetic)
Sympathetic: Thoraco-lumbar division
Regions= T1-L2
Parasympathetic: Cranio-sacral division
Cranial nerves:
CN 3, 7, 9, 10
Sacral regions= S2-S4
Sympathetic Ganglia (2)
- Sympathetic chain (paravertebral ganglia)
- Prevertebral ganglia
-Celiac
-Superior mesenteric
-Inferior mesenteric
Sympathetic Chain
Paired sympathetic ganglia linked together to form a chain along length of vertebral column (C1-S5)
Preganglionic sympathetic neurons limited to T1-L2 (above and below distribution)
Sympathetic chain ganglia are connected to coordinate and spread sympathetic responses to many target tissues at the same time
Prevertebral Ganglia
Situated anterior to vertebral column along aorta
3 main ganglion:
-Celiac ganglion
-Superior mesenteric ganglion
-Inferior mesenteric ganglion
Named relative to blood vessels branching from abdominal aorta
Sympathetic Outflow Pathways
Preganglionic pathway= short
Postganglionic pathway= long
White vs. Grey Rami Communicans
Rami communicans= connection b/t spinal nerve and sympathetic trunk
White rami only from T1-L2
Grey rami at all levels along sympathetic chain
Sympathetic Outflow Origin
[Preganglionic neuron] originates in the lateral horn of spinal cord from segments T1-L2
Preganglionic neuron exits spinal cord via ventral horn and root to enter spinal nerve
Preganglionic (myelinated) neruon enters the sympathetic chain via the white ramus communicans “on ramp”
Preganglionic Neuron 3 Main Routes (Outflow)
*upon entering sympathetic chain
- Synapse within the chain at the same level
- Change level within the chain and then synapse
- Formation of a splanchnic nerve
Same Level Outflow Pathway
Preganglionic Neurons:
-Located in lateral horn between T1-L2
-Myelinated axon exits spinal cord and enters sympathetic trunk via white ramus communicans to synapse on the postganglionic neuron
-Communicating ramus called ‘white’ b/c pre fibers are myelinated
Postganglionic Neurons:
-Unmyelinated post neruons pass through grey ramus communicans to enter spinal nerve
-Communicating ramus called ‘grey’ b/c post fibers are unmyelinated
-Post fiber travels within spinal nerve to innervate target
*Nerves target body wall, abdominal and thoracic viscera, limbs
Change Level Outflow Pathway
Preganglionic nerve fibers can ascend or descend within sympathetic chain, above/below/within T1-L2 distribution before synapsing
*grey ramus above and below + post ganglionic, white ramus within + pre ganglionic
Ascending vs. Descending Nerves
Ascending:
-Head/neck viscera
-Thoracic viscera
-Upper limbs
Descending:
-Abdominopelvic viscera
-Lower limbs
*thoracic viscera supplied by various plexuses, post ganglionic contribute to autonomic plexuses including cardiac plexus (heart) and pulmonary plexus (lungs)
Splanchnic Nerves Outflow Pathway
Also known as visceral nerves (associated with abdominoplenvic viscera)
-Form when autonomic fibers do not re-join the spinal nerve; instead form a splanchnic nerve
-Preganglionic fibers bypasses sympathetic chain completely and synapses in a peripheral (pre-vertebral) ganglion near the target organ (celiac, sup. mesenteric or inf. mesenteric)
Originate from thoracic and lumbar vertebral levels of spinal cord as sets of splanchnic nerves including:
- Greater splanchnic
- Lesser splanchnic
- Least/lumbar splanchnic
Thoracic and Lumbar Splanchnic Nerves
Greater (T5-9)- synapse in celiac ganglion
Lesser (T10-11)- synapse in superior mesenteric ganglion
Least/lumbar (T12-L2)- synapse in inferior mesenteric ganglion
Sympathetic NS Outflow Pathways Summary
- Preganglionic neuron synapses in sympathetic ganglion, postganglionic axon exits to re-join spinal nerve at same level to innervate target
- Preganglionic neruon ascends/descends within sympathetic chain before synapsing, postganglionic axon exits via spinal nerve above or below original spinal level
- Preganglionic neuron passes through sympathetic chain without synapsing, forming a splanchnic nerve, which synapses at corresponding prevertebral ganglion
Parasympathetic Outflow Pathways
Preganglionic= long
Postganglionic= short
PNS
“Rest and digest”
Targets:
-Viscera of head and neck
-Abdominal and thoracic viscera
-Pelvic viscera
*sweat glands and limb vasculature only receive sympathetic innervation
PNS Cranial Outflow
-Preganglionic cell bodies within brainstem cranial nuclei
-Axons travel via cranial nerves 3, 7, 9, 10
-Synapse on ganglia in or near effector organ
Cranial Outflow: Head and Neck
Pre-ganglionic neurons leave brainstem from parasympathetic nuclei, synapse on post-ganglionic neuron within parasympathetic ganglia
CN 3- ciliary ganglion (eye- ciliary muscles and sphincter pupillae control shape of lens and size of pupil)
CN 7- pterygopalatine ganglion (lacrimal gland- tear production) and submandibular ganglion (sublingual/submandibular glands- secretion of saliva)
CN 9- otic ganglion (parotid gland)
Cranial Outflow: Thoracic and Abdominal Viscera
Preganglionic neurons leave brainstem via CN 10 which contribute to autonomic plexuses, supplies parasympathetics to pharynx, larynx, thorax, and mid-gut structures (to transverse colon) e.g. synapse on post neuron via intra-mural ganglion
Sacral Outflow: Distal GI Tract and Pelvic Viscera
Preganglionic neuronal cell bodies located in spinal segments S2-S4
-Axons travel via pelvic splanchnic nerves which contribute to autonomic plexuses (e.g. inferior hypogastric plexus)
Visceral Afferent Fibers and Visceral Reflexes
Visceral afferent fibers conduct sensory info from autonomic targets to the CNS (e.g. organs)
*Visceral afferent fibers are DISTINCT from ANS and not considered sympathetic or parasympathetic
Visceral reflexes are the simplest functional units in ANS
-Visceral reflex arc consists of sensory nerve (visceral afferent) and visceral motor neurons (e.g. shining a light in eye triggers visceral reflex that constricts pupils- dilation= symp, constriction= parasymp)
Enteric Nervous System
ANS includes a 3rd division called the enteric nervous system
-Network of neurons located in the walls of the digestive tract
Enteric system receives regulatory signals via sympathetic and parasympathetic divisions (but many complex visceral reflexes are still initiated and coordinated independently)
