Ninja Nerd-Autonomic Nervous System - Intro and Key Concepts
Overview of the Autonomic Nervous System (ANS)
The autonomic nervous system is the visceral motor system responsible for involuntary control of smooth muscle, cardiac muscle, and glandular tissue. It operates largely below the level of consciousness and maintains homeostasis.
The ANS has three traditional branches: sympathetic, parasympathetic, and enteric. This video focuses on the sympathetic and parasympathetic divisions; the enteric nervous system is acknowledged and covered in later GI physiology videos.
The ANS is part of the peripheral nervous system (PNS) and contrasts with the somatic nervous system, which controls voluntary motor function.
Divisions of the Nervous System (context)
Central nervous system (CNS): brain + spinal cord.
Peripheral nervous system (PNS): all neural elements outside the CNS; divided into sensory (afferent) and motor (efferent) components.
Motor division splits into:
Somatic motor (voluntary control; e.g., skeletal muscles).
Visceral motor (autonomic), which is involuntary and controls viscera; this is the autonomic nervous system (ANS).
Somatic vs Autonomic: Key Differences
Somatic nervous system (SNS):
Pathway uses a single alpha motor neuron from the spinal cord to the skeletal muscle.
Neuron location: exits via the anterior (ventral) horn of the spinal cord.
Neurotransmitter at the neuromuscular junction: acetylcholine (ACh).
Receptors on muscle: nicotinic receptors (Nm at the neuromuscular junction).
Autonomic nervous system (ANS; focusing on sympathetic and parasympathetic):
Pathway uses two motor neurons to reach the effector (a two-neuron chain): a preganglionic neuron and a postganglionic neuron.
Preganglionic cell bodies are located in the CNS (spinal cord or brainstem); postganglionic cell bodies reside in autonomic ganglia in the PNS.
Neurotransmitters:
Preganglionic fibers (both sympathetic and parasympathetic) release acetylcholine (ACh) and are typically cholinergic.
Postganglionic sympathetic fibers typically release norepinephrine (NE) and are adrenergic, except for a sweat gland exception.
Postganglionic parasympathetic fibers release acetylcholine (ACh) and are cholinergic.
Exceptions to the general rule include:
Sweat glands: postganglionic sympathetic fibers release ACh (they are cholinergic for sweat glands).
Adrenal medulla: acts as a modified sympathetic ganglion (an intramural/modified ganglionic arrangement) with preganglionic fibers directly stimulating chromaffin cells to release catecholamines into the bloodstream.
Sympathetic Nervous System (SNS): Overview
Also known as the fight/flight system; designed to use energy in stressful situations.
Thoracolumbar outflow: preganglionic cell bodies are located in the intermediolateral cell column (IML) of the spinal cord from T1 to L2
Preganglionic neurons exit via the ventral root, join the ventral ramus to form spinal nerves, and typically enter a chain of ganglia (the sympathetic trunk or paravertebral chain).
Postganglionic neurons then project to effector tissues (heart, lungs, GI tract, blood vessels, glands, etc.).
Key ganglionic pathways:
Chain ganglia (paravertebral ganglia) located adjacent to the vertebral column; also called the sympathetic trunk or chain.
White ramus communicans: myelinated preganglionic fibers entering the chain ganglia from the ventral ramus (present at each level).
White rami only found on preganglionic neurons
Gray ramus communicans: unmyelinated postganglionic fibers exiting the chain ganglia back to the ventral ramus/spinal nerves.
Postganglionic targets and fiber types in the SNS include:
Erector pili muscles (pilomotor fibers) — hair erection.
Blood vessels — vasomotor fibers causing vasoconstriction.
Sweat glands — sudomotor fibers (note: postganglionic sweat fibers are an exception and release ACh instead of NE).
Special cases and exceptions:
Sweat glands: postganglionic sympathetic fibers release ACh (cholinergic) instead of NE.
Adrenal medulla: receives directly innervating preganglionic sympathetic fibers; acts as a modified ganglion (intramural) with chromaffin cells releasing epinephrine and norepinephrine into the bloodstream.
Anatomical variability and pathways:
Preganglionic fibers can synapse at the level of their entry in the chain ganglia or descend/ascend to synapse at a different level along the chain (ascending or descending: relay through chain ganglia).
If a preganglionic fiber descends to a lower level but still synapses in a chain ganglion, the postganglionic fiber may re-enter via the gray ramus and join the spinal nerves to reach target tissues.
Some preganglionic fibers bypass the chain ganglia entirely to form splanchnic nerves, which synapse on collateral (prevertebral) ganglia located anterior to the vertebral column and around the aorta.
Chain Ganglia and Ramus Communicans (SNS details)
Chain ganglia (paravertebral ganglia): a series of ganglia forming the sympathetic trunk along the length of the spine.
White ramus communicans: myelinated preganglionic fibers from the ventral ramus entering the chain ganglia.
Gray ramus communicans: unmyelinated postganglionic fibers exiting the chain ganglia back to the ventral ramus/spinal nerves.
Within the chain/ganglia, preganglionic fibers can:
Synapse on postganglionic neurons at their level.
Descend to lower levels to synapse (and possibly ascend thereafter).
Ascend to higher levels to synapse.
Bypass synapsing in the chain and form splanchnic nerves to collateral ganglia (prevertebral/preaortic) for abdominal/pelvic targets (these are the prevertebral or collateral ganglia such as celiac, superior mesenteric, inferior mesenteric, etc.).
Some fibers pass through to reach the adrenal medulla (as intramural evidence of direct innervation).
Splanchnic nerves (SNS): preganglionic sympathetic fibers that bypass the chain and form nerves that travel to collateral/prevertebral ganglia located around the aorta to innervate abdominal/pelvic viscera. They are named based on location and target:
Thoracic splanchnic nerves to thoracic organs (heart, lungs, esophagus).
Abdominal splanchnic nerves to abdominal organs via prevertebral ganglia (celiac, superior mesenteric, inferior mesenteric regions).
Subdiaphragmatic and other collateral ganglia names reflect their anatomical positions:
Pre-aortic, prevertebral, collateral, subdiaphragmatic.
Functional outputs include piloerection, vasomotor control, and sudomotor (sweat) control, with the noted exception for sweat glands (ACh release from postganglionic sympathetic fibers).
Parasympathetic Nervous System (PNS): Overview
Craniosacral outflow: parasympathetic outflow arises from the brainstem (cranial nerves) and from sacral spinal levels.
Cranial outflow: four cranial nerves provide parasympathetic input and have distinct nuclei:
Oculomotor nerve (CN III) — Edinger–Westphal nucleus; constricts pupil and slows accommodation; fibers travel to pirripable targets in the eye via parasympathetic ganglia.
Nucleus: Edinger–Westphal nucleus (parasympathetic).
Facial nerve (CN VII) — Superior Salivatory Nucleus; parasympathetic outflow to lacrimal and salivary glands; targets numerous structures via various ganglia.
Glossopharyngeal nerve (CN IX) — Inferior Salivatory Nucleus; parasympathetic outflow to the parotid gland and other targets.
Vagus nerve (CN X) — Major parasympathetic outflow to thoracic and most abdominal organs; extensive distribution and a large portion of viscera innervation.
Sacral outflow: pelvic organs receive parasympathetic input via sacral nerves from S2-S4; these are the pelvic splanchnic nerves.
Outflow type: craniosacral outflow.
Preganglionic vs postganglionic organization:
Parasympathetic preganglionic neurons are long; postganglionic neurons are short.
Terminal (or intramural) ganglia: the parasympathetic preganglionic fibers are long enough to reach/near the organ, with postganglionic neurons located in ganglia at or within the organ.
Cranial nerve specifics (brief):
CN III (oculomotor): parasympathetic fibers originate in the Edinger–Westphal nucleus and target eye structures.
CN VII (facial): superior salivatory nucleus → lacrimal and salivary glands among others.
CN IX (glossopharyngeal): inferior salivatory nucleus → parotid gland.
CN X (vagus): widespread parasympathetic distribution to thoracic and abdominal viscera.
Sacral/pelvic parasympathetic: S2–S4 provide parasympathetic innervation to pelvic organs via pelvic splanchnic nerves.
Autonomic Ganglia: Definitions and Locations
Ganglion: a cluster of neuronal cell bodies in the peripheral nervous system.
Parasympathetic ganglia: terminal or intramural, located near or within target organs due to long preganglionic fibers.
Sympathetic ganglia:
Chain/paravertebral ganglia: sympathetic trunk along the vertebral column, containing postganglionic neuron cell bodies.
Collateral (prevertebral) ganglia: anterior to the aorta, receiving preganglionic fibers via splanchnic nerves for abdominal/pelvic targets (pre-aortic, prevertebral, collateral, subdiaphragmatic names describe their locations).
Splanchnic nerves: preganglionic sympathetic fibers that bypass the chain and synapse in collateral ganglia; then postganglionic fibers travel to abdominal viscera.
Terminal/intramural ganglia (parasympathetic): located very close to or inside target organs due to long preganglionic fibers.
Neurotransmitters and Receptors (Summary of Synapses)
Sympathetic pathways:
Preganglionic: releases ACh at the autonomic ganglion (cholinergic).
Postganglionic target tissue: releases norepinephrine (NE) onto effector tissue (adrenergic) for most targets; exceptions exist (see sweat glands).
Major exception: sweat glands — postganglionic sympathetic fibers release ACh (cholinergic) to sweat glands.
Adrenal medulla: preganglionic sympathetic fibers directly stimulate chromaffin cells to release catecholamines (epinephrine and norepinephrine) into the bloodstream (functional analog of an intramural ganglion).
Parasympathetic pathways:
Preganglionic: releases ACh at the ganglion (cholinergic).
Postganglionic: releases ACh onto target tissues (cholinergic).
General receptor context (to be expanded in later lessons):
Nicotinic receptors: located on autonomic ganglia (Nn) and at the neuromuscular junction (Nm). Early synapse (ganglia) involves nicotinic receptors.
Muscarinic receptors: located on visceral effector tissues for parasympathetic signaling; activated by ACh released from postganglionic parasympathetic fibers.
Pathway Details: Visualizing the Sympathetic Pathways
Preganglionic cell bodies location:
In the thoracic and upper lumbar spinal cord: T1-L2 (intermediolateral cell column). This region is the IML.
Entry into the sympathetic chain:
Preganglionic fibers exit via the ventral root, join the ventral ramus, enter the white ramus communicans (myelinated), and reach the chain ganglia.
Within the chain ganglia:
They may synapse at the level of entry or travel up/down the chain to synapse at a different level.
They may descend to lower levels before synapsing.
Postganglionic paths after synapse:
Exit via the gray ramus communicans (unmyelinated) to rejoin the ventral ramus/spinal nerves and then reach target tissues such as:
Erector pili (pilo motor)
Blood vessels (vasomotor)
Sweat glands (sudomotor)
Alternative routes from preganglionic fibers:
Some preganglionic fibers bypass the chain ganglia and form splanchnic nerves to collateral/prevertebral ganglia (pre-aortic, collateral, subdiaphragmatic) to reach abdominal/pelvic targets.
Those that synapse in collateral ganglia may send postganglionic fibers via their own nerves to abdominal/pelvic organs.
Thoracic vs abdominal thoracoabdominal innervation:
Thoracic splanchnic nerves mainly target thoracic organs (heart, lungs, esophagus) via collateral/prevertebral ganglia.
Abdominal splanchnic nerves target abdominal organs via collateral/prevertebral ganglia around the aorta (celiac, superior mesenteric, inferior mesenteric, etc.).
Summary: Key Concepts to Remember
Outflows and general organization:
Sympathetic: thoracolumbar outflow T1-L2; chain (paravertebral) ganglia; white ramus (myelinated) and gray ramus (unmyelinated) communicants; splanchnic nerves to prevertebral/collateral ganglia; postganglionic NE to targets; exceptions include sweat glands (ACh) and adrenal medulla (direct preganglionic innervation).
Parasympathetic: craniosacral outflow (cranial nerves III, VII, IX, X and S2-S4); long preganglionic fibers, short postganglionic fibers; terminal or intramural ganglia near or within target organs; ACh at both pre- and postganglionic synapses.
Major anatomical terms and structures:
Intermediolateral (IML) cell column: location of sympathetic preganglionic cell bodies in the spinal cord (levels T1-L2).
Chain/paravertebral ganglia: sympathetic trunk along the vertebral column.
White ramus communicans: myelinated sympathetic preganglionic fibers entering the chain.
Gray ramus communicans: unmyelinated postganglionic fibers exiting the chain.
Splanchnic nerves: preganglionic sympathetic fibers that bypass the chain to reach collateral/prevertebral ganglia (pre-aortic, collateral, subdiaphragmatic).
Terminal/intramural ganglia: parasympathetic ganglia near or within target organs.
Neurotransmitters and receptors (core pattern):
Preganglionic (both SNS and PNS): acetylcholine (ACh) — cholinergic.
Postganglionic SNS: norepinephrine (NE) — adrenergic (with sweat gland exception: ACh).
Postganglionic PNS: acetylcholine (ACh) — cholinergic.
Nerve targets and physiological effects (high-yield examples):
Erector pili (hair erection): pilomotor response (SNS).
Blood vessels: vasomotion/vasoconstriction (SNS).
Sweat glands: sudomotion (SNS; exception uses ACh).
Eye (pupil, accommodation): parasympathetic control via CN III (Edinger–Westphal).
Quick Reference: Notable Nomenclature and Alternatives
Chain ganglia synonyms: paravertebral ganglia, sympathetic trunk.
Ganglia types for SNS: chain (paravertebral), collateral/prevertebral (pre-aortic, prevertebral), subdiaphragmatic (abdominal/pelvic viscera targets).
Presentable nerve terms:
White ramus communicans: myelinated preganglionic fibers entering the chain.
Gray ramus communicans: unmyelinated postganglionic fibers exiting the chain.
Outflow descriptors:
Thoracolumbar outflow: T1-L2 (SNS).
Craniosacral outflow: cranial nerves III, VII, IX, X and S2-S4 (PSNS).
Connections to Foundational Principles and Real-World Relevance
The two-neuron autonomic circuit is fundamental for understanding how visceral control is organized and how systemic responses (e.g., heart rate, digestion, sweating) are coordinated during stress and rest.
The variety of ganglia and nerve routes allows the autonomic system to regulate widespread targets efficiently, and anticipates clinical correlates such as sympathetic chain lesions affecting multiple body regions or vagal dysfunction impacting abdominal organs.
The adrenal medulla’s role as a modified ganglion explains rapid systemic catecholamine release during stress, effectively amplifying the sympathetic signal.
Understanding the neurotransmitter patterns (ACh vs NE and the cholinergic vs adrenergic designations) underpins pharmacology and therapeutic targets (e.g., beta-adrenergic blockers, muscarinic agonists/antagonists).
Ethical, Philosophical, and Practical Considerations
The autonomic system operates largely outside voluntary control, raising considerations for how medical interventions (drugs, surgery, implants) manipulate homeostatic processes without conscious consent.
Understanding autonomic regulation informs treatment of autonomic dysfunctions and highlights the balance between sympathetic and parasympathetic activity in health and disease (e.g., hypertension, arousal disorders, GI dysmotility).
Notation and Formulas Used in This Note
Spinal cord levels and outflow:
Sympathetic outflow: T1-L2
Parasympathetic cranial nerves: III, \, VII, \, IX, \, X
Parasympathetic sacral levels: S2-S4
Ganglia and Pathways Terms
The Intermediolateral cell column (IML) is where sympathetic preganglionic cell bodies are located.
Sympathetic ganglia include:
Chain (paravertebral) ganglia: Along the vertebral column (sympathetic trunk).
Collateral (prevertebral) ganglia: Located anterior to the aorta (e.g., pre-aortic, subdiaphragmatic).
White ramus communicans: Myelinated preganglionic fibers entering the sympathetic chain.
Gray ramus communicans: Unmyelinated postganglionic fibers exiting the sympathetic chain.
Neurotransmitters and receptors:
Preganglionic: ACh (cholinergic)
Postganglionic sympathetic: NE (adrenergic)
Postganglionic parasympathetic: ACh (cholinergic)
Sweat gland exception: postganglionic sympathetic to sweat glands uses ACh (cholinergic)
Adrenal medulla: intramural sympathetic ganglion analog; chromaffin cells release catecholamines into bloodstream
The preganglionic cell bodies located in the Intermediolateral cell column (IML) in the spinal cord (T1-L2) can follow several pathways as part of the sympathetic nervous system:
Entry into the Sympathetic Chain: Fibers exit via the ventral root, join the ventral ramus, and enter the paravertebral chain ganglia (sympathetic trunk) via the white ramus communicans.
Within the Chain Ganglia: Once in the chain, the preganglionic fibers can:
Synapse on a postganglionic neuron at the same level of entry.
Ascend or descend within the chain to synapse at a different vertebral level.
Bypass the chain ganglia entirely to form splanchnic nerves.
Postganglionic Exit from Chain Ganglia: After synapsing in the chain, postganglionic fibers typically exit via the gray ramus communicans to rejoin spinal nerves and innervate targets like erector pili muscles, blood vessels, and sweat glands.
Splanchnic Nerve Pathway: Preganglionic fibers that bypass the chain ganglia form splanchnic nerves, which travel to collateral (prevertebral) ganglia located anterior to the aorta (e.g., celiac, superior mesenteric, inferior mesenteric ganglia). There, they synapse with postganglionic neurons that then innervate abdominal and pelvic viscera.
Adrenal Medulla Pathway: Some preganglionic sympathetic fibers directly innervate the chromaffin cells of the adrenal medulla, which acts as a modified sympathetic ganglion, leading to the release of catecholamines (epinephrine and norepinephrine) into the bloodstream.