Autonomic Nervous System (ANS) Overview

Overview of Autonomic vs. Somatic Nervous Systems

• The body’s motor output can be divided into two broad pathways:

  • Somatic Nerves (Voluntary / Skeletal Muscle)

  • Direct conscious control from the cerebral cortex OR via reflex arcs.

  • Single‐neuron pathway: \text{Spinal Cord} \rightarrow \text{Skeletal Muscle}.

  • Neurotransmitter: \text{Acetylcholine (ACh)} – always excitatory at the neuromuscular junction.

  • Autonomic Nerves (Involuntary / Organs)

  • Regulate cardiac muscle, smooth muscle, and glands automatically, independent of conscious intent.

  • Examples of involuntary responses:

    • Heart rate rises when climbing stairs if blood \text{O}_2 drops.

    • Saliva & gastric activity begin when smelling food.

    • Bladder contracts when stretched to capacity (though cortical control can delay urination temporarily).

  • Two‐neuron chain: \text{Preganglionic Neuron} \rightarrow \text{Autonomic Ganglion} \rightarrow \text{Postganglionic Neuron} \rightarrow \text{Target Organ}.

  • Allows the system to use different neurotransmitters at each synapse (excitatory at one, inhibitory at another).

  • Subdivisions:

    • Sympathetic ("Fight, Flight, Fright, or Freeze")

    • Parasympathetic ("Rest, Digest & Restore")

Key Functional Differences

• Skeletal Muscle

  • Can only be turned ON (contract) via somatic nerves; relaxation is passive (absence of stimulation).

• Smooth & Cardiac Muscle / Glands

  • Can be turned ON or OFF because two different autonomic branches often exert opposing actions.

  • Sympathetic vs. parasympathetic balance = organ’s final response.

Sympathetic Nervous System (SNS)

• Motto: Exercise, Excitement, Embarrassment, Emergency.

• Preganglionic cell bodies: Thoracic & Lumbar spinal cord (T1–L2).

• Neurotransmitters:

  • Preganglionic: \text{ACh} (nicotinic receptors in ganglia).

  • Postganglionic: Mostly \text{Norepinephrine (NE)}; adrenal medulla releases \text{Epinephrine (EPI)} & \text{NE} into blood.

• Representative organ effects ("Fight or Flight"):

  • Dilate pupils ⇒ more light for vision.

  • Inhibit salivation ⇒ redirect fluid to blood volume.

  • Increase heart rate & contractility ⇒ raise cardiac output.

  • Relax airways (bronchodilation) ⇒ improve \text{O}_2 intake.

  • Inhibit stomach & intestinal activity ⇒ conserve energy.

  • Stimulate liver to release glucose ⇒ quick energy.

  • Relax bladder & inhibit defecation ⇒ prevent elimination during danger.

  • Stimulate sweat glands & peripheral vasoconstriction ⇒ thermoregulation & minimize bleeding.

  • Activate adrenal medulla ⇒ systemic adrenaline surge amplifies all above effects.

Parasympathetic Nervous System (PNS)

• Motto: Digestion, Diuresis, Defecation, Decreased HR.

• Preganglionic neurons emerge from:

  • Cranial Nerves: \text{CN III (Oculomotor)},\; \text{VII (Facial)},\; IX (Glossopharyngeal),\; X (Vagus).

  • Sacral Spinal Cord: segments S2–S4 (pelvic splanchnic nerves).

• Neurotransmitters:

  • Preganglionic: \text{ACh} (nicotinic).

  • Postganglionic: \text{ACh} (muscarinic) – can excite or inhibit depending on receptor subtype.

• Representative organ effects ("Rest & Digest"):

  • Constrict pupils ⇒ protect retina.

  • Stimulate salivation ⇒ start digestion.

  • Slow heart rate ⇒ conserve energy.

  • Constrict airways slightly ⇒ match reduced metabolic demand.

  • Stimulate stomach & intestinal motility, enzyme release & acid secretion.

  • Stimulate gallbladder (bile release) & pancreatic secretions.

  • Contract bladder detrusor & relax internal sphincter ⇒ promote urination.

  • Promote erection of genitals; in females, vaginal contraction aiding arousal.

Why Each Division Acts the Way It Does

• Evolutionary Logic:

  • During acute stress, blood & energy must be diverted to skeletal muscle & heart ⇒ digestion, urination, & reproduction are temporarily unimportant.

  • During quiet times, the body prioritizes nutrient assimilation, waste removal, and reproductive readiness.

• Example – Bladder Control:

  • SNS relaxes detrusor muscle & contracts internal sphincter ⇒ urinary retention while fleeing.

  • PNS does the opposite ⇒ controlled voiding during safety.

Neurotransmitter Paradigm

• Dual neurotransmitter strategy gives the ANS flexibility:

  • Example on the heart:

  • Sympathetic: \text{ACh (preganglionic)} \rightarrow \text{NE (postganglionic)} ⇒ \uparrow heart rate (β₁ receptors).

  • Parasympathetic: \text{ACh (preganglionic)} \rightarrow \text{ACh (postganglionic)} ⇒ \downarrow heart rate (M₂ receptors).

• Diagrammatic summary:
  \begin{aligned}
  \text{Somatic: } & \text{ACh} \; (+) \; \text{skeletal muscle contraction}\
  \text{Sympathetic: } & \text{ACh} \; (+) \text{ganglion} \rightarrow \text{NE} \; (+) \text{heart}\
  \text{Parasympathetic: } & \text{ACh} \; (+) \text{ganglion} \rightarrow \text{ACh} \; (-) \text{heart}\
  \end{aligned}

Anatomical Routes

• Parasympathetic Pathways:

  • Cranial outflow ("craniosacral") travels with CN III, VII, IX, X.

  • CN III ⇒ pupillary constrictor & ciliary muscles.

  • CN VII ⇒ lacrimal, nasal, submandibular, sublingual glands.

  • CN IX ⇒ parotid gland.

  • CN X ⇒ heart, lungs, most abdominal viscera up to proximal colon.

  • Sacral outflow (S2–S4) ⇒ distal colon, rectum, bladder, reproductive organs.

• Sympathetic Pathways ("thoracolumbar"):

  • Preganglionic fibers exit T1–L2 ventral roots, enter sympathetic chain (paravertebral ganglia).

  • Three possibilities:

  • Synapse at entry level; rejoin spinal nerve (to sweat glands, vessels).

  • Ascend/descend chain before synapsing (to head/neck or pelvis).

  • Pass through chain as splanchnic nerves to prevertebral ganglia (celiac, superior & inferior mesenteric) then to abdominal organs.

  • Adrenal medulla functions as a specialized sympathetic ganglion (preganglionic ACh directly stimulates chromaffin cells that secrete \text{EPI/NE} into blood).

Integrated Functional Scenarios

• "Relaxing at a sad movie eating popcorn":

  • Pupils dilated (sympathetic emotion) + active salivation & digestion (parasympathetic from food cues) + lacrimal gland tear production (parasympathetic) ⇒ Illustrates concurrent activity; not pure on/off.

• Exercise sprint:

  • SNS elevates HR, bronchodilates, releases glucose, diverts blood to muscles; PNS withdrawal simultaneously removes brakes on heart.

Clinical Case Analysis

• Case: Gunshot severing spinal cord at \text{T6}.

  • Loss of voluntary somatic motor control & sympathetic outflow below lesion.

  • Parasympathetic innervation to most digestive organs is via CN X (Vagus), which emerges above the lesion (brainstem) ⇒ digestion remains intact.

  • Enteric nervous system adds semi‐autonomous peristalsis.

  • Bladder & bowel may lose coordinated sympathetic modulation but sacral parasympathetic reflexes can still trigger voiding/defecation (though voluntary control is impaired).

Key Numbers & Facts to Memorize

• Craniosacral vs. Thoracolumbar origins.

• Two‐neuron ANS pathway vs. single‐neuron somatic pathway.

• Major neurotransmitters: \text{ACh}, \text{NE}, \text{EPI}.

• Receptor types: Nicotinic (ganglia), Muscarinic (parasympathetic targets), Adrenergic (α, β on sympathetic targets).

• "E‐Division" activities = Sympathetic; "D‐Division" activities = Parasympathetic.

Ethical & Practical Considerations

• Autonomic drugs exploit receptor selectivity:

  • β₁ blockers slow heart; β₂ agonists dilate bronchi (asthma inhalers).

  • Anticholinergics reduce GI motility but risk urinary retention.

• Stress management (mindfulness, biofeedback) can consciously modulate ANS balance (e.g., vagal breathing slows HR).

• Spinal cord injuries require autonomic training (bowel/bladder schedules) to compensate for lost sympathetic circuits.

High‐Yield Mnemonics & Connections

• "Sympathetic = Stress." "Parasympathetic = Peace." (simple recall)

• Cranial nerve list for PNS: "3, 7, 9, 10 — keep the body zen."

• ANS receptor summary: "Nicotinic for Nerve synapse; Muscarinic for end‐Muscle/gland; Adrenergic for Adrenal‐like effects."

Quick Comparison Table (Mental Map)

• Pupils: SNS \uparrow size / PNS \downarrow size.

• Saliva: SNS \downarrow / PNS \uparrow.

• Heart: SNS \uparrow rate, force / PNS \downarrow.

• Airways: SNS dilate / PNS constrict slightly.

• GI Motility: SNS \downarrow / PNS \uparrow.

• Bladder: SNS relax detrusor, tighten sphincter / PNS contract detrusor, relax sphincter.

• Genitals: SNS ejaculation, vaginal contraction / PNS erection, engorgement.

Use this structured overview to replace or supplement the original slides while studying for autonomic physiology.