Comprehensive notes on the Autonomic Nervous System (SNS vs PNS)
Overview and Core Concepts
Regulate the organs, glands, and glands; pursue homeostasis and allostasis (maintaining stability through change).
Allostasis: adjusting physiological set-points to cope with changing conditions; essential for adaptive responses.
Two main divisions of the nervous system:
Central nervous system (CNS): brain and spinal cord.
Peripheral nervous system (PNS): all nervous tissue outside the CNS (sensory and motor neurons).
Basic functions of the nervous system:
Monitor internal and external environmental changes.
Process and integrate perceived changes.
React to changes by producing an action or response.
Maintain homeostasis through coordinated responses.
Functional Divisions of the PNS
Sensory (afferent) neurons: detect internal and external changes.
Motor division:
Somatic nervous system (SNS): voluntary control of skeletal muscle.
Autonomic nervous system (ANS): involuntary control of cardiac muscle, smooth muscle, and glands.
Quick contrast:
Voluntary actions (reading a book, walking, typing) involve somatic motor control.
Involuntary actions (heart beating, gut activity) involve ANS.
Autonomic Nervous System (ANS)
ANS uses a two-neuron chain to connect the CNS to target organs:
Preganglionic neuron: leaves the CNS, synapses in a ganglion outside the CNS.
Postganglionic neuron: located beyond the ganglion, synapses on the target organ.
The path CNS → ganglion → target organ constitutes a ganglionic synapse and a second synapse to the organ.
ANS signals can be teased apart into two major divisions that often produce opposite effects, but there are exceptions.
Sympathetic (thoracolumbar) division: thoracic and lumbar spinal cord origins; preganglionic neurons originate from T1–L2.
Parasympathetic (craniosacral) division: brainstem (medulla) and sacral spinal cord origins; preganglionic neurons originate from cranial nerves III, VII, IX, X and sacral spinal nerves S2–S4.
Primary neurotransmitters and receptors:
Acetylcholine (ACh): used by preganglionic neurons in both sympathetic and parasympathetic divisions; acts on nicotinic receptors at ganglia.
Norepinephrine (NE): primary transmitter of the sympathetic division; acts on adrenergic receptors on target organs.
Epinephrine (adrenaline): released from adrenal medulla; acts systemically on adrenergic receptors; not a direct transmitter at all autonomic ganglia.
Receptors:
Nicotinic receptors: located at the ganglionic synapse (both sympathetic and parasympathetic divisions) and at the neuromuscular junction (somatic NS).
Muscarinic receptors: located on organs innervated by postganglionic parasympathetic neurons.
Drugs and clinical relevance:
Drugs can be agonists (activate receptors) or antagonists/blockers (block receptors).
Most autonomic-acting drugs act at the postganglionic (second) neuron's synapse onto the organ.
Terms to know: cholinergic (nerve fibers that release ACh) and anticholinergic (block ACh actions).
Summary of cholinergic vs adrenergic transmission:
ACh is the primary transmitter for parasympathetic signaling; acts on muscarinic receptors on target organs and nicotinic receptors at ganglia.
NE is the primary transmitter for sympathetic signaling; acts on alpha and beta adrenergic receptors on target organs.
In addition, preganglionic neurons release ACh to activate nicotinic receptors in both divisions; postganglionic sympathetic neurons release NE to adrenergic receptors on organs; parasympathetic postganglionic neurons release ACh to muscarinic receptors.
Practical implication: understanding these pathways helps predict drug actions and side effects (e.g., a drug that blocks muscarinic receptors will oppose parasympathetic activity).
Receptors in Focus: Nicotinic vs Muscarinic
Nicotinic receptors:
Location: ganglionic synapses in both sympathetic and parasympathetic divisions; neuromuscular junction in skeletal muscle.
Activation effects: broad physiologic effects due to widespread distribution (tachycardia, hypertension, increased GI tone, etc.).
Drugs targeting nicotinic receptors can cause broad autonomic and somatic effects.
Muscarinic receptors:
Location: on target organs innervated by postganglionic parasympathetic neurons.
Activation produces classic parasympathetic (rest-and-digest) responses: increased GI secretions and motility, bradycardia, bronchoconstriction, pupillary constriction, etc.
Drugs acting on muscarinic receptors include cholinergic agonists (activate) and anticholinergics (block).
Key term corrections from the lecture notes:
Mydriasis = pupil dilation.
Miosis = pupil constriction.
Tachycardia: rapid heart rate; bradycardia: slow heart rate.
Note: some slides in the transcript used spellings like “myosis” for miosis; correct term is miosis.
Autonomic Effects by Division
Sympathetic nervous system (SNS): fight-or-flight
Origin: thoracolumbar (T1–L2).
Primary effects on organs:
Heart: ↑ heart rate and contractility.
Lungs: bronchodilation to allow more air in.
Eyes: pupil dilation (mydriasis).
GI tract: ↓ secretions and ↓ motility.
Liver/pancreas: glucose release for energy.
Blood vessels: vasoconstriction in many areas (↑ BP); some vessels may dilate in skeletal muscle via β2 receptors.
Sweat glands: stimulated by sympathetic nerves (often via cholinergic receptors).
Neurotransmitter actions: NE (and epinephrine from adrenal medulla) acting on α and β adrenergic receptors; some postganglionic sympathetic fibers release ACh to sweat glands (muscarinic receptors).
Parasympathetic nervous system (PNS): rest-and-digest
Origin: craniosacral (medulla and sacral segments).
Primary effects on organs:
Heart: ↓ heart rate and ↓ contractility (primarily via muscarinic receptors).
Lungs: bronchoconstriction (slower breathing when at rest).
Eyes: pupil constriction (miosis).
GI tract: ↑ secretions and ↑ motility (digestion).
Urinary and reproductive systems: promotion of urination and various reproductive functions.
Neurotransmitter actions: ACh acting on muscarinic receptors on target organs; nicotinic receptors at ganglia.
Special Topics and Practical Insights
Exceptions to opposite effects:
Some tissues are primarily under sympathetic control (e.g., arterioles and sweat glands), with limited parasympathetic counterbalance.
Arteriolar constriction is controlled predominantly by the sympathetic branch.
Clinical relevance of dual innervation:
Most organs receive input from both divisions, allowing fine-tuned regulation (dominant tone may vary by tissue and situation).
In disease or pharmacology, shifting the balance can predict symptoms and drug effects.
The role of the adrenal medulla:
Epinephrine is released into the bloodstream, acting on adrenergic receptors throughout the body; it is not a direct postganglionic neurotransmitter at a specific organ but amplifies SNS effects systemically.
Receptor distribution and drug implications:
Targeting nicotinic receptors can cause broad autonomic and somatic effects due to their presence in ganglia and NMJ.
Targeting muscarinic receptors yields parasympathetic-specific effects on organs, useful for cholinergic agonists and anticholinergics.
Practical pharmacology points:
Drugs acting on the ANS often aim at the final effector organ (postganglionic junction).
Pharmacologic effects depend on receptor type and tissue distribution (α vs β adrenergic, nicotinic vs muscarinic).
Key terms to recognize:
Tachycardia: rapid heart rate.
Bradycardia: slow heart rate.
Mydriasis: pupil dilation.
Miosis: pupil constriction.
Cholinergic agonist: drug that mimics ACh.
Anticholinergic (muscarinic blocker): drug that blocks muscarinic receptors.
Adrenergic receptor subtypes: alpha (α) and beta (β) receptors on organs.
Spinal and Cranial Anatomy References
Vertebral and spinal counts mentioned in the lecture:
Cervical vertebrae: 7
Thoracic vertebrae: 12
Lumbar vertebrae: 5
Coccyx (tailbone): part of the sacral/pelvic region.
Primary CNS connection points:
Sympathetic: thoracic and lumbar nerves (thoracolumbar).
Parasympathetic: medulla (cranial nerves) and sacral sacral nerves (craniosacral).
Critical Thinking and Example Scenarios
Case-based reasoning (from the transcript): ER patient after motor vehicle accident with rapid heart rate, dilated pupils, and decreased digestive activity.
Most likely dominant division: Sympathetic nervous system (fight-or-flight).
Expected: tachycardia, mydriasis, reduced GI secretions and motility, etc.
Exam-style practice prompts:
Compare parasympathetic and sympathetic effects on heart rate, pupil size, airway tone, and GI activity.
Identify which receptor types mediate specific organ responses (muscarinic vs nicotinic vs adrenergic).
Predict drug effects if a muscarinic receptor blocker is used.
Summary of Key Takeaways
The ANS uses a two-neuron chain (preganglionic → ganglion → postganglionic → organ) to regulate cardiac muscle, smooth muscle, and glands.
Sympathetic (thoracolumbar) promotes fight-or-flight responses; parasympathetic (craniosacral) promotes rest-and-digest functions.
ACh is the main neurotransmitter for parasympathetic and somatic NMJ; NE is the main transmitter for sympathetic pathways; epinephrine acts systemically from the adrenal medulla.
Receptors:
Nicotinic: ganglia and NMJ; broadly excitatory when stimulated.
Muscarinic: target organs of the parasympathetic system; produce rest-and-digest effects.
Drugs target the ANS at the postganglionic junction and can be either agonists or antagonists, with effects on heart rate, glandular secretions, GI motility, pupil size, airway caliber, and more.
Important numbers to remember from the lecture:
Cervical vertebrae: 7
Thoracic vertebrae: 12
Lumbar vertebrae: 5
Tachycardia and bradycardia thresholds in the lecture: tachycardia is described as > 400 in one slide (clinically this is typically > 100 bpm), bradycardia as < 60 bpm.
Terms to know: mydriasis, miosis, tachycardia, bradycardia, nicotinic receptors, muscarinic receptors, cholinergic, anticholinergic, adrenergic (α and β) receptors.
Quick Reference Glossary (narrative form)
Allostasis: adaptive stability through change to meet environmental demands.
Thoracolumbar: relating to the sympathetic outflow from thoracic and lumbar regions of the spinal cord.
Craniosacral: relating to parasympathetic outflow from brainstem and sacral spinal cord.
Mydriasis: pupil dilation.
Miosis: pupil constriction.
Tachycardia: fast heart rate; Br
adycardia: slow heart rate.Cholinergic: nerve fibers releasing acetylcholine; cholinergic agonists mimic ACh, anticholinergics block ACh.
Adrenergic: receptors responsive to adrenaline/epinephrine and norepinephrine.
Nicotinic vs Muscarinic receptors: different locations and roles in autonomic signaling.
Note: Some numerical values and spellings in the source transcript contain inconsistencies (e.g., reported tachycardia threshold of >400 bpm). In clinical practice, tachycardia is generally defined as a heart rate > 100 bpm and bradycardia as < 60 bpm. This notes compilation preserves the transcript content while marking such discrepancies for your awareness during study.