ANS Lecture Notes: Sympathetic and Parasympathetic Pathways with Neurotransmitters, Receptors, and Backup Systems

• Overview: Autonomic nervous system (ANS) has two major divisions with largely opposite effects: sympathetic (fight/flight) and parasympathetic (rest/digest). The lecture emphasizes lab vs lecture content, pathways, key ganglia, backup systems, neurotransmitters, receptors, and practical exam tips.

Sympathetic Division: Key Architecture and Backup Systems

• Main idea: Sympathetic division orchestrates widespread body responses during stress, via a chain of ganglia and collateral pathways.
• Spinal origin and distribution:
  • Exits CNS from the thoracolumbar region: $T<em>1 ext{ through } L</em>2$ (thoracolumbar division).
  • Preganglionic fibers are typically short; postganglionic fibers are long for most targets.
• Sympathetic trunk (chain) and collateral ganglia:
  • Preganglionic fibers can synapse in the sympathetic chain or bypass to collateral ganglia via splanchnic nerves.
  • Collateral ganglia (found in the abdominal-pelvic region):
  • Celiac ganglia (go to the stomach)
  • Superior mesenteric ganglia
  • Inferior mesenteric ganglia
  • For lab exams, you must know these three collateral ganglia names specifically.
• Pathways illustrated:
  • Some fibers go to the head/neck/chest via the sympathetic pathway.
  • Other fibers go to the abdominal-pelvic region via collateral ganglia.
• Backup (adrenal medulla) as a special pathway:
  • If nerves to a tissue are severed (e.g., after heart transplant where nerves are cut), the body has a backup system.
  • The adrenal medulla acts as a modified ganglion in the sympathetic pathway:
  • A single preganglionic sympathetic fiber travels via the adrenal medulla nerve and bypasses collateral ganglia to reach the adrenal medulla.
  • In the adrenal medulla, preganglionic fibers activate chromaffin cells (modified neurons) that secrete catecholamines into the bloodstream.
• Adrenal medulla: modified ganglia concept and chromaffin cells
  • Adrenal medulla is a two-gland organ (cortex and medulla); the medulla is the termination point for this backup pathway.
  • Chromaffin cells originated as neurons in development; now neuroendocrine cells that release hormones (catecholamines) when stimulated by acetylcholine (ACh).
  • Primary hormones released: catecholamines, especially epinephrine (adrenaline) with a smaller amount of norepinephrine and a trace of dopamine.
  • Hormonal release vs neurotransmitter action:
  • The adrenal medulla releases epinephrine and norepinephrine into the bloodstream, producing widespread, slower-onset effects compared to fast synaptic neurotransmission.
  • Epinephrine is the major hormone released (~$80 ext{ extpercent}$ of adrenal output), with norepinephrine making up ~$20 ext{ extpercent}$, and dopamine in small amounts.
  • In Europe, epinephrine is called adrenaline; in the US, epinephrine is used, but the hormone is the same chemical.
• Why the adrenal medulla is a backup system:
  • If sympathetic nerves to an organ are damaged, epinephrine in the bloodstream can still trigger fight/flight-like effects via circulating receptors.
  • Immediate effect: neurotransmitters produce millisecond responses; the adrenal hormone pathway is slower but still effective.
  • Timeline comparison: neurotransmitter signaling occurs in the order of $ext{~milliseconds}$, while hormonal signaling via adrenal medulla can take longer (on the order of seconds) but still provides necessary systemic activation.
• Practical takeaway: Even though the adrenal medulla pathway is a backup system, it is active concurrently with the main sympathetic responses; it is not a replacement but a supplement.
• The adrenal medulla as a “modified ganglia”:
  • When preganglionic sympathetic fibers stimulate chromaffin cells, those cells release catecholamines into the bloodstream.
  • This mechanism explains rapid, systemic effects such as increased heart rate and bronchodilation during stress, even if direct neural innervation is compromised.
• Hormone details:
  • Major hormone: Epinephrine (adrenaline) binds to adrenergic receptors (both α and β receptors).
  • Minor hormone: Norepinephrine (noradrenaline) also binds to adrenergic receptors.
  • Dopamine is present in small amounts.
• Mechanism summary: Sympathetic preganglionic fibers → nicotinic receptors on postganglionic neurons → (a) postganglionic neurotransmitter norepinephrine (to most targets) binding to adrenergic receptors, or (b) adrenal medulla pathway via chromaffin cells releasing epinephrine/norepinephrine into blood (hormonal route).
• Neurotransmitter and receptor overview (sympathetic):
  • Pre-to-post ganglionic synapse: acetylcholine (ACh) binds to nicotinic receptors (ionotropic, excitatory).
  • Post-to-target synapse (majority): norepinephrine (NE) binds to adrenergic receptors (metabotropic, G-protein coupled) – alpha or beta subtypes.
  • Special case (skin): sympathetic to skin can be cholinergic to sweat glands via acetylcholine binding to muscarinic receptors (MACHR) on sweat glands.
• Speed and redundancy concept:
  • The body maintains redundancy with the adrenal medulla to ensure fight/flight responses even if direct neural connections are damaged or severed.

Parasympathetic Division: Architecture and Function

• Overall effects: Parasympathetic division produces opposite effects to the sympathetic division (e.g., slows heart rate, constricts pupils, promotes digestion).
• Exit points (where fibers leave CNS):
  • Cranial nerves: III (Oculomotor), VII (Facial), IX (Glossopharyngeal), X (Vagus) – exit via cranial nerves above the sacral region.
  • Sacral nerves: $S<em>2, S</em>3, S_4$ – contribute to parasympathetic outflow in the pelvic region.
• Nerves and distribution:
  • Cranial outflow (primarily via the vagus nerve, X): broad distribution, including most thoracic and abdominal viscera up to the end of the abdominal-pelvic region (except the final urinary and reproductive portions).
  • The vagus nerve (X) is the major parasympathetic highway, providing about $75 ext{ extpercent}$ of parasympathetic outflow.
  • If one vagus nerve is damaged, the other may still maintain some function; both injured is highly detrimental to digestive and respiratory control.
• Cranial nerves and their targets:
  • III (Oculomotor): pupil constriction and lens focus; pupil constriction is a parasympathetic effect.
  • VII (Facial): stimulates lacrimation and salivation; provides tears and other secretions.
  • IX (Glossopharyngeal): stimulates salivation.
  • X (Vagus): innervates most thoracic and abdominal viscera; extensive distribution.
• Sacral parasympathetic output:
  • Pelvic nerves (Sacral outflow): $S<em>2, S</em>3, S_4$ supply remaining pelvic organs for parasympathetic control (e.g., distal digestive tract, urinary, reproductive organs).
• General anatomy terminology:
  • Parasympathetic ganglia are typically terminal or intramural (within or near the walls of target organs):
  • Terminal ganglia: located close to the organ or within its wall (intramural).
  • Intramural or mitral ganglia (often abbreviated as intramural) – within the organ wall.
• Neurotransmitter and receptor in parasympathetic signaling:
  • Neurotransmitter: Acetylcholine (ACh) is the sole neurotransmitter used in the parasympathetic division.
  • Receptors at pre-to-post synapse: nicotinic acetylcholine receptor (NACHR) on the postganglionic neuron.
  • Receptors at post-to-target synapse: muscarinic acetylcholine receptor (MACHR) on target tissues.
  • Nicotinic receptor characteristics (parasympathetic):
  • Location: preganglionic to postganglionic synapse (ganglionic neurons).
  • Type: ionotropic (ligand-gated ion channel) – excitatory (depolarizes the postganglionic neuron).
  • Muscarinic receptor characteristics (parasympathetic post-to-target):
  • Location: postganglionic to target tissue.
  • Type: metabotropic (G-protein coupled) – effects depend on tissue; can be excitatory or inhibitory.
• Effects on organs (parasympathetic):
  • Heart: decreased heart rate (negative chronotropic effect) via muscarinic receptors on cardiac muscle.
  • Bronchial smooth muscle: bronchoconstriction (via muscarinic receptors).
  • Pupils: pupil constriction (via muscarinic receptors).
  • Digestive tract: increased motility and secretions (excitation via muscarinic receptors).
  • Salivation and lacrimation: increased secretions (via muscarinic receptors).
  • Defecation and urination: increased activity (parasympathetic dominance).
• Note on pharmacology and toxins:
  • Nerve gas and certain poisons can massively activate the parasympathetic division, causing extreme salivation, tearing, bronchoconstriction, and defense reactions (foam at the mouth, etc.).

Neurotransmitters and Receptors: Core Rules for the ANS

• Two main neurotransmitters used by the ANS:
  • Acetylcholine (ACh)
  • Norepinephrine (NE) (also called noradrenaline; epinephrine from adrenal medulla can act as a circulating hormone)
• Receptor rules (summary):
  • Pre-to-post (both sympathetic and parasympathetic): always ACh acting on nicotinic acetylcholine receptors (NACHR) – ionotropic and excitatory.
  • Post-to-target sympathetic: NE (or epinephrine from adrenal medulla) acting on adrenergic receptors (α and β subtypes; metabotropic).
  • Post-to-target parasympathetic: ACh acting on muscarinic receptors (MACHR) – metabotropic with variable excitatory/inhibitory effects depending on tissue.
• Special skin case:
  • Sympathetic innervation of sweat glands uses ACh on muscarinic receptors (MACHR) rather than norepinephrine on adrenergic receptors.
• Receptor details:
  • Nicotinic acetylcholine receptor (NACHR): ionotropic; always excitatory at pre-to-post ganglionic synapses.
  • Muscarinic acetylcholine receptor (MACHR): metabotropic; effects depend on tissue and G-protein signaling.
  • Adrenergic receptors (α and β): metabotropic; effects depend on receptor subtype and tissue (e.g., α causing vasoconstriction; β2 causing bronchodilation and vasodilation in certain beds; β1 increasing heart rate).
  • For cardiac tissue, β1 receptors can be excitatory (increasing heart rate) in the sympathetic pathway; in other tissues, β receptors can be inhibitory.
• Practical exam pointers:
  • When a question asks about a neurotransmitter at a given synapse, answer with the correct transmitter (ACH or NE) and the correct receptor type (NACHR, MACHR, α, β).
  • For parasympathetic pre-to-post: ACH + NACHR.
  • For parasympathetic post-to-target: ACH + MACHR (muscarinic).
  • For sympathetic post-to-target (most tissues): NE + α or β adrenergic receptors; speed and direction depend on receptor type and tissue.
  • For skin sympathetic post-to-target: ACh + MACHR (sweat glands).
• Exam prep tip on wording:
  • If asked for a single primary hormone released by the adrenal medulla, answer the one main hormone: epinephrine (adrenaline). If asked for the set, you can mention epinephrine with norepinephrine and a little dopamine, but when asked for the primary hormone, respond with epinephrine only.
• Pathway terminology to know for diagrams and labs:
  • Pre-to-post ganglionic synapse: label as ACH + NACHR.
  • Post-to-target synapse: label with the specific transmitter (NE for sympathetic targets) and receptor (α or β) or MACHR if parasympathetic to organ.
• Lab exercise tasks (to prepare for exams):
  • Illustrate the sympathetic pathway to skin and to other targets with correct pre-to-post (short pre, long post) and post-to-target (neurotransmitter and receptor).
  • For sympathetics to skin: show ACh release and MACHR on target tissue.
  • For parasympathetic: show ACh release at both pre-to-post (NACHR) and post-to-target (MACHR).
• Quick clinical connection:
  • Beta blockers: pharmacological agents that block β-adrenergic receptors, commonly used to manage high blood pressure by reducing cardiac output and sympathetic stimulation; blocking β receptors can have widespread effects and side effects due to receptor distribution.
  • Nicotine and muscarine interactions illustrate agonist effects: nicotine binds to nicotinic receptors (NACHR) and is an agonist; muscarine binds to muscarinic receptors (MACHR) and can also be an agonist for those receptors.
  • Nicotinic receptors are present at the neuronal ganglia (pre-to-post) and at the neuromuscular junction (somatic motor). In the ANS, the ganglionic NACHR is the key site for pre-to-post synapses.

Exam-Ready Summary Cheat Sheet

• Sympathetic division:
  • Origin: $T<em>1 ext{ through } L</em>2$; short preganglionic, long postganglionic fibers.
  • Major pathways: sympathetic trunk; collateral ganglia: $ext{celiac}, ext{superior mesenteric}, ext{inferior mesenteric}$.
  • Backup: adrenal medulla via adrenal medulla nerve; chromaffin cells; catecholamines (epinephrine, norepinephrine, dopamine).
  • Neurotransmitters: pre-to-post = ACh + NACHR; post-to-target = NE + α/β adrenergic receptors (metabotropic).
  • Skin special case: ACh + MACHR for sweat glands.
• Parasympathetic division:
  • Origin: cranial nerves $III, VII, IX, X$ and spinal nerves $S<em>2, S</em>3, S_4$ (sacral outflow).
  • Major nerve: vagus (X) supplies most thoracic and abdominal viscera; 75% of parasympathetic outflow.
  • Ganglia: terminal or intramural (within organ wall).
  • Neurotransmitters: pre-to-post = ACh + NACHR; post-to-target = ACh + MACHR.
• Receptors and effects: main pairing
  • Nicotinic acetylcholine receptor (NACHR): ionotropic, excitatory, pre-to-post (ganglionic).
  • Muscarinic acetylcholine receptor (MACHR): metabotropic, post-to-target; effects depend on tissue.
  • Adrenergic receptors: α and β (metabotropic);
  • α: often excitatory (vasoconstriction).
  • β: often inhibitory (bronchodilation, vasodilation) but β1 can increase heart rate; general rule: α excitatory, β typically inhibitory in vascular beds, with exceptions.
• Important physiological ideas:
  • Effects depend on receptor type, not just transmitter.
  • Adrenal medulla provides a hormonal backup that uses epinephrine and norepinephrine to maintain systemic sympathetic tone when direct innervation is compromised.
  • Timing differences: neural signaling is faster (milliseconds) vs hormonal adrenal signaling (seconds).
• Common exam pitfalls to avoid:
  • Mixing up pre-to-post vs post-to-target terminology.
  • Assuming a single transmitter equates to a single effect; receptor type determines effect.
  • Forgetting the skin’s cholinergic sympathetic pathway (Ach + MACHR) versus the classical NE/adrenergic pathways in other tissues.

Practical Lab Instructions (as discussed in lecture)

• Part 1: Draw the sympathetic pathways to skin vs other organs:
  • Sympathetic to skin: short preganglionic, long postganglionic; post-to-target uses ACh on MACHR.
  • Other sympathetic targets: NE on α/β receptors; label the receptor types on the target organ.
• Part 2: Draw parasympathetic pathways:
  • Show long preganglionic fibers, short postganglionic fibers; pre-to-post: ACh + NACHR; post-to-target: ACh + MACHR.
  • For both parasympathetic and sympathetic, clearly label pre-to-post vs post-to-target synapses.
• Always annotate neurotransmitter and receptor at each synapse for exam clarity: e.g.,
  • Pre-to-post (sympathetic): ACh + NACHR
  • Post-to-target (heart): NE + β1 (or α1 depending on tissue)
  • Parasympathetic target (heart): ACh + MACHR

Notes on Terminology Used in the Lecture

• Pre-to-post ganglionic synapse = the junction between the preganglionic neuron and the postganglionic neuron; transmitter = ACh; receptor = NACHR.
• Post-to-target synapse = junction between postganglionic neuron and the effector tissue; transmitter = NE (mostly sympathetic) or ACh (parasympathetic); receptor = adrenergic (α/β) or muscarinic (MACHR).
• Modified ganglia = adrenal medulla (neuroendocrine gland) acting as a backup ganglion; chromaffin cells = adrenal medulla cells that release catecholamines when stimulated by ACh.
• Epinephrine vs norepinephrine:
  • Epinephrine (E or epi): major adrenal hormone; 80 ext{ extpercent} output; agonist at both α and β receptors.
  • Norepinephrine (NE or norepi): 20 ext{ extpercent} of adrenal output; primarily acts on α receptors but also β receptors in some tissues.
• Key exam phrases to memorize:
  • “Adrenal medulla nerve” stimulates chromaffin cells via preganglionic sympathetic fibers.
  • “Modified ganglia” refers to adrenal medulla; its hormones provide backup sympathetic drive.
  • When asked for the primary hormone released by the adrenal medulla, answer: epinephrine only.
  • For parasympathetic neurotransmitter: ACH (acetylcholine) in all relevant synapses.

If you’d like, I can tailor these notes to focus more on specific slides or convert this into a printable study sheet with diagrams and labeled pathways for quick review during your prep.