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Autonomic Nervous System Comprehensive Notes

Autonomic Nervous System: Comprehensive Notes

  • Autonomic nervous system (ANS) comprises motor neurons that innervate smooth muscle, cardiac muscle, and glands, and make subconscious adjustments to support body activities (e.g., shunting blood, regulating heart rate, blood pressure, digestion, etc.). Also called the involuntary nervous system or general visceral motor system.

  • ANS functions alongside somatic nervous system (SNS); higher brain centers coordinate both systems.

  • Major comparison points between ANS and SNS:

    • Effectors:
    • SNS: skeletal muscles
    • ANS: cardiac muscle, smooth muscle, glands
    • Efferent pathways and ganglia:
    • SNS: a single, thick, heavily myelinated axon from CNS to effector; no ganglia along the way
    • ANS: two-neuron chain from CNS to effector with a ganglion in between (preganglionic and postganglionic neurons)
    • Neurotransmitters at effector:
    • SNS: all somatic motor neurons release Acetylcholine (ACh); effect is always stimulatory
    • ANS: preganglionic fibers release ACh; postganglionic fibers release norepinephrine (NE) or ACh depending on the target; effects can be stimulatory or inhibitory depending on receptors
    • Pathways:
    • SNS: one-neuron arc from CNS to effector
    • ANS: two-neuron arc with an autonomic ganglion in between
    • Ganglia and axon characteristics:
    • SNS: thick, heavily myelinated preganglionic axon extends directly to skeletal muscle
    • ANS: preganglionic fiber is lightly myelinated; postganglionic fiber is nonmyelinated
    • Neurotransmitters at target:
    • ACh and NE; adrenal medullary hormones epinephrine and norepinephrine augment activity

  • Two divisions of the ANS and their roles:

    • Parasympathetic division (rest-and-digest): promotes maintenance, conserves energy, stimulates digestion and urination; drives activities that occur during relaxation
    • Sympathetic division (fight-or-flight): mobilizes the body during activity, exercise, emergency, or stress; increases heart rate and respiration, redirects blood to skeletal muscle and heart, dilates bronchi, increases glucose release by the liver
    • Dual innervation: most visceral organs receive input from both divisions but with opposite effects; maintains homeostasis
    • Basal tone: both divisions are partially active at rest (tone); one may predominate, but in some cases they cooperate

Anatomical foundations: sites of origin

  • Parasympathetic division (craniosacral):
    • Fibers originate in brain stem (cranial nerves III, VII, IX, X) and in spinal cord segments S2–S4
    • Preganglionic fibers are long; postganglionic fibers are short
    • Ganglia are within or near visceral effector organs (intramural or terminal ganglia)
  • Sympathetic division (thoracolumbar):
    • Fibers originate in thoracic and lumbar spinal cord segments T1–L2
    • Preganglionic fibers are short; postganglionic fibers are long
    • Ganglia lie near CNS in sympathetic trunk ganglia or collateral (prevertebral) ganglia; contains gray and white rami communicantes

Parasympathetic Division: overview and cranial part

  • Also called craniosacral division

  • Preganglionic fibers originate in brain stem or sacral spinal cord; long preganglionic fibers extend nearly to target organs

  • Postganglionic fibers are short

  • Ganglia are intramural (within walls of target organs) or near organs

  • Cranial part of parasympathetic division:

    • Cell bodies of preganglionic fibers located in brain stem
    • Cranial nerves involved: ext{III (oculomotor)}, ext{VII (facial)}, ext{IX (glossopharyngeal)}, ext{X (vagus)}
    • Actions include pupil constriction and lens focusing, lacrimal and nasal gland activation, and salivary gland activation

  • Vagus nerve (X) pathway:

    • ~90 ext{%} of preganglionic parasympathetic fibers in the body
    • Arises from brain and synapses in terminal ganglia within walls of target organs
    • Cardiac plexus: slows heart rate
    • Pulmonary plexus: serves lungs and bronchi
    • Esophageal plexus: innervates most of the digestive system

  • Sacral part of the parasympathetic division:

    • Originates from sacral region; serves pelvic organs and part of large intestine
    • Gives rise to pelvic splanchnic nerves
    • Controls distal half of large intestine, urinary bladder, ureters, and reproductive organs

Sympathetic Division: anatomy and organization

  • More complex and widespread than parasympathetic

  • Short preganglionic neurons and longer postganglionic neurons

  • Thoracolumbar division: fibers originate in T1-L2 spinal cord segments

  • Some structures receive sympathetic innervation only (e.g., sweat glands, arrector pili muscles, smooth muscles of all blood vessels)

  • Sympathetic trunk (paravertebral) ganglia:

    • Preganglionic fibers enter trunk ganglia via white rami communicantes (myelinated)
    • Short preganglionic fibers may:
      1) synapse in the same trunk ganglion, 2) ascend/descend the trunk to synapse in another trunk ganglion, or 3) synapse in collateral (prevertebral) ganglia outside trunk
    • Postganglionic fibers exit via gray rami communicantes (unmyelinated)
    • Some preganglionic fibers pass through trunk ganglia to synapse in collateral ganglia near the vertebral column

  • Adrenal medulla pathway:

    • Some preganglionic fibers pass directly to adrenal medulla without synapsing
    • Medullary cells secrete norepinephrine (NE) and epinephrine (adrenaline) into the blood
    • Sympathetic ganglia and adrenal medulla share a common embryologic origin

  • Summary of sympathetic pathways (pathways and termination):

    • Preganglionic fibers enter trunk ganglia (via white rami) and may synapse in trunk ganglia, ascend/descend to other ganglia, terminate in collateral ganglia, or pass through to collateral ganglia without synapsing in trunk
    • Postganglionic fibers travel to target organs (via gray rami or other routes)
    • Adrenal medulla acts as a modified sympathetic ganglion with hormone release into blood

Autonomic ganglia: types and locations

  • Terminal (intramural) ganglia: within wall of the organ served (parasympathetic)
  • Sympathetic trunk ganglia: along the vertebral column; paired, beside spinal cord
  • Collateral (prevertebral) ganglia: anterior to the vertebral column; unpaired

Neurotransmitters and receptors

  • Major ANS neurotransmitters: Acetylcholine (ACh) and norepinephrine (NE)
  • ACh roles:
    • Released by all preganglionic neurons (both parasympathetic and sympathetic) and by parasympathetic postganglionic neurons
    • Effect at cholinergic receptors is always excitatory for nicotinic receptors
  • NE roles:
    • Released by almost all sympathetic postganglionic neurons; sweat gland postganglionic fibers release ACh instead
    • Effects depend on receptor subtype and organ; can be stimulatory or inhibitory
  • Receptor families:
    • Cholinergic receptors: Nicotinic and Muscarinic
    • Adrenergic receptors: Alpha (α) and Beta (β) subtypes
  • Overview of receptor locations and actions:
    • Nicotinic receptors: on all postganglionic neurons and adrenal medullary cells; skeletal muscle at neuromuscular junction; ACh binding is always excitatory
    • Muscarinic receptors: on all effector cells stimulated by postganglionic cholinergic fibers; can be inhibitory or excitatory depending on the target
    • Adrenergic receptors: located on most sympathetic target organs; responses depend on receptor subtype

Cholinergic receptors

  • Nicotinic ACh receptors (N):
    • Location: all postganglionic neurons; adrenal medullary cells; skeletal muscle neuromuscular junction
    • Effect: excitatory (opens cation channels, depolarizing postsynaptic cell)
  • Muscarinic ACh receptors (M):
    • Location: all parasympathetic target organs; some sympathetic sweat glands
    • Mechanism: G protein-coupled; effects can be excitatory or inhibitory depending on organ
    • Examples: ACh binding to cardiac muscle slows heart rate; ACh binding to intestinal smooth muscle increases motility

Adrenergic receptors

  • Major classes:

    • Alpha (α): α1, α2
    • Beta (β): β1, β2, β3

  • Functional outcomes depend on which subtype predominates at the target organ

  • Classic examples (simplified):

    • α1: constricts blood vessels (vasoconstriction)
    • α2: inhibits NE release at adrenergic terminals; modulates insulin release
    • β1: increases heart rate and force of contraction; stimulates renin release from kidneys
    • β2: relaxes smooth muscle (bronchial dilation); dilates certain vessels; reduces uterine tone
    • β3: mobilizes fat (lipolysis) in adipose tissue

  • Related receptor table (summary for context):

    • Adrenergic receptors are the targets of NE and epinephrine; their activation yields varied responses across tissues

Effects of drugs on autonomic receptors

  • Some drugs target cholinergic receptors:

    • Nicotinic agents (e.g., nicotine): activate nicotinic receptors
    • Parasympathomimetic agents (muscarinic agents, e.g., bethanechol): activate muscarinic receptors to enhance parasympathetic activity (e.g., bladder contraction)

  • Some drugs target cholinesterase (AChE inhibitors):

    • Neostigmine: increases ACh availability at receptors; used in myasthenia gravis; prolongs ACh action; also associated with sarin (nerve agent) effects by inhibiting AChE

  • Some drugs target adrenergic receptors or NE release:

    • Albuterol: β2 agonist; dilates bronchioles in asthma
    • Propranolol: β-adrenergic receptor antagonist; beta-blocker; reduces sympathetic effects and lowers blood pressure

  • Additional examples (from drug table):

    • Nicotinic agents: nicotine (nicotinic receptor agonist)
    • Parasympathomimetic agents: pilocarpine (muscarinic agonist)
    • Acetylcholinesterase inhibitors: neostigmine (increases ACh at all ACh receptors)
    • Sarin: AChE inhibitor with widespread ACh receptor effects
    • Sympathomimetics (adrenergic agonists): phenylephrine (alpha-adrenergic agonist)
    • Sympatholytics (adrenergic antagonists): propranolol (beta-blocker)

  • Practical note: drug effects depend on receptor distribution and subtypes across tissues; some drugs have selective effects (e.g., β2-selective bronchodilators) while others have broad actions (e.g., AChE inhibitors increase ACh broadly)

Localized vs diffuse effects

  • Parasympathetic responses tend to be short-lived and highly localized because ACh is rapidly degraded by acetylcholinesterase
  • Sympathetic effects are longer-lasting and often more widespread since NE is inactivated more slowly and adrenal medullary hormones (epinephrine and NE) persist in the blood after sympathetic signals stop

Tone in the autonomic nervous system

  • Sympathetic tone (vasomotor tone): a constant, partial constriction of most blood vessels maintained by sympathetic nerve activity; keeps blood pressure at a stable baseline
    • If blood pressure drops: sympathetic activity increases leading to greater vasoconstriction and higher pressure
    • If blood pressure rises: sympathetic activity decreases leading to vasodilation and lower pressure
  • Parasympathetic tone: predominates in heart and many digestive and urinary tract organs; slows heart rate and promotes digestion and excretion
  • Some organs exhibit parasympathetic tone (always slightly activated)
  • Sympathetic division can override parasympathetic effects during stress

Antagonistic and cooperative interactions

  • Dynamic antagonism (classic example of dual innervation): sympathetic division increases heart and respiratory rates and inhibits digestion/elimination; parasympathetic division slows heart and respiration and allows digestion and waste elimination
  • Cooperative effects: most notable in control of external genitalia
    • Parasympathetic: vasodilation leading to erection
    • Sympathetic: ejaculation in males and reflex vaginal contraction in females

Unique roles of the sympathetic division

  • Thermoregulatory responses to heat:
    • When body temperature rises: sympathetic nerves dilate cutaneous blood vessels and activate sweat glands to promote cooling
    • When body temperature falls: blood vessels constrict to reduce heat loss
  • Renin release from kidneys increases blood pressure (beta-adrenergic effect; covered in detail in the blood pressure section)
  • Metabolic effects: increases metabolic rate, increases blood glucose, mobilizes fats for fuel

Summary of neuronal signaling and receptor actions (quick reference)

  • Major neurotransmitters:
    • Acetylcholine (ACh): cholinergic fibers (preganglionic and parasympathetic postganglionic)
    • Norepinephrine (NE): adrenergic fibers (most sympathetic postganglionic)
    • Adrenal medullary hormones: epinephrine and NE released into blood
  • Receptors:
    • Cholinergic: Nicotinic (N) and Muscarinic (M)
    • Adrenergic: Alpha (α1, α2) and Beta (β1, β2, β3)
  • Functional outcomes depend on receptor type and tissue context (stimulatory vs inhibitory and varying tissue responses)

Functional anatomy recap: key concepts to memorize

  • Origins and divisions:
    • Parasympathetic: ext{craniosacral}; brain stem and S2–S4; long preganglionic, short postganglionic; terminal/intramural ganglia
    • Sympathetic: ext{thoracolumbar}; T1-L2; short preganglionic, long postganglionic; trunk and collateral ganglia near spinal cord; white and gray rami communicantes
  • Pathways:
    • Sympathetic trunk ganglia pathways include synapse in same ganglion, ascend/descend to another ganglion, synapse in collateral ganglia, or pass through trunk to collateral ganglia
    • Adrenal medulla acts as a modified ganglion—preganglionic fibers stimulate medullary cells to secrete NE and epinephrine into the bloodstream
  • Functional relationships:
    • Dual innervation with opposite effects for many organs; tone-based regulation; occasional cooperative effects (e.g., erection vs ejaculation) and evolutionary advantages for rapid, coordinated responses

Connections to foundational principles and real-world relevance

  • Homeostasis depends on balanced parasympathetic and sympathetic activity; imbalances can contribute to hypertension, peptic ulcers, urinary retention, and other disorders
  • Pharmacology of the ANS underpins many clinical therapies (beta-blockers for hypertension, muscarinic agonists for bladder function, bronchodilators for asthma, AChE inhibitors for myasthenia gravis, etc.)
  • Understanding receptor subtypes and tissue distributions is essential for predicting drug effects and side effects across organ systems

Notation and key references (for quick lookup)

  • Parasympathetic origin: craniosacral; brain stem and S2-S4
  • Sympathetic origin: T1-L2
  • Ganglia terminology: terminal/intramural, sympathetic trunk, collateral (prevertebral)
  • Neurotransmitters: ACh (cholinergic), NE (adrenergic); adrenal medullary hormones: epinephrine and NE
  • Receptors: Nicotinic (N), Muscarinic (M), and Adrenergic (α1, α2, β1, β2, β3)
  • Drugs to remember (examples): bethanechol (muscarinic agonist), pilocarpine (muscarinic agonist), neostigmine (AChE inhibitor), sarin (AChE inhibitor), albuterol (β2 agonist), propranolol (β blocker)

Note: Figures and tables referenced in the source (e.g., Figure 14.2, Table 14.1, Table 14.3, Table 14.4) align with the descriptions above and provide visual summaries of the pathways, receptors, and drug effects. If you have access to the material, review those visuals to reinforce these notes.