Autonomic Nervous System & Eye Physiology – Key Vocabulary

Autonomic Nervous System (ANS) – Core Framework

• Two branches:
  • Sympathetic – “fight or flight”
  • Parasympathetic – “rest and digest”
• Neither branch is inherently “better.”
  • Health = proper autonomic tone → the body shifts to parasympathetic when safe and to sympathetic when threatened.
• Growing issue in modern life: autonomic dysfunction (e.g.
  • Students taking a written exam show sympathetic signs even while seated.)

Importance of Autonomic Tone & Homeostasis

• Goal of physiology: homeostasis—stable internal conditions.
• Autonomic tone = balanced, adaptive interplay of both branches.
  • Too much sympathetic tone → panic attacks, chronic stress markers.
  • Too much parasympathetic tone → lethargy, impaired responsiveness.

Key Sympathetic vs Parasympathetic Physiological Responses

• “The body doesn’t lie” – patients may deny stress, but vitals reveal sympathetic state (elevated pulse, dilated pupils, rapid breathing).

Pupil Dynamics & Eye-Related Clinical Points

• Pupil: dark central hole – always black; cannot give eye color.
  • Iris (colored ring) = true source of eye color.
• Sympathetic mode ⇒ pupil dilation to widen visual field for hazard detection.
• Parasympathetic mode ⇒ pupil constriction; wide visual field unnecessary at dinner table.

Applications & Examples

• Police sobriety tests: many recreational drugs/ETOH are sympathomimetic → unnatural, persistent pupillary dilation.
• Eye clinic dilation drops: widen pupil so clinician can view retina through a larger “window.”
• “Red-eye” in flash photography: light reflects off retinal/choroidal vasculature (red reflex).
  • Clinicians use red reflex angle when learning ophthalmoscopy.
• Diabetes screening: retina = only place to directly view blood vessels → ocular exam detects inflamed or hemorrhagic vessels early.
• Jaundice (yellow sclera): excess bilirubin from liver failure; eyes (and skin) serve as visible diagnostic clues.

Spinal Cord Segmentation & Branch Activation

• Spinal cord regions: Cervical → Thoracic → Lumbar → Sacral.
• Sympathetic drive: signals exit mainly from T{1-12} (thoracic) + L{1-2} (upper lumbar) segments.
• Parasympathetic drive:
  • Cranial nerves from brainstem, plus
  • Sacral segments S_{2-4}.
• Neurological imaging/studies can localize autonomic state by observing segmental activity.
• Total spinal nerves: 31 pairs (mixed sensory + motor).

Preganglionic vs Postganglionic Neurons

• Signal direction: always one-way, spinal cord → organ.
  • Ganglion = PNS “relay station” with concentrated neuronal cell bodies.
• Naming rule (reference = ganglion):
  • Preganglionic neuron: from CNS to ganglion.
  • Postganglionic neuron: ganglion to effector organ.
• Length differences:
  • Sympathetic: Short preganglionic, Long postganglionic.
  • Parasympathetic: Long preganglionic, Short postganglionic.
• This architecture reinforces branch separation—prevents “confusion.”

Neurotransmitters & Chemical Messengers

• Most common neurotransmitter overall: Acetylcholine (ACh).
  • Abbrev: ACH.
• ACh present at:
  • ALL preganglionic synapses (symp & parasymp).
  • ALL parasympathetic postganglionic synapses.
• Sympathetic‐specific postganglionic transmitters:
  • Epinephrine (Adrenaline).
  • Norepinephrine (Noradrenaline).
  • Basis of the “adrenaline rush.”
• Clinical note: EpiPen delivers epinephrine IM to counteract anaphylactic airway closure (bronchodilation, ↑BP, etc.).

Adrenal Gland – Central Sympathetic Amplifier

• Anatomy: triangular gland on top of each kidney (two total).
• Regions:
  • Adrenal cortex (outer) → steroid hormones (e.g.
  • Cortisol; discussed weeks 6–7).
  • Adrenal medulla (inner) → epinephrine & norepinephrine.
• Only the adrenal medulla is considered part of the sympathetic nervous system.
  • Thoracic/lumbar sympathetic preganglionic axons synapse directly on chromaffin cells → catecholamine release into bloodstream.
• Blood-borne epinephrine/norepinephrine → systemic effects:
  • ↑ HR, bronchodilation, pupillary dilation, shunt blood to skeletal muscle, mobilize glucose, etc.

Additional Clinical & Practical Observations

• Eyes & skin are highly informative organs:
  • Skin = largest organ; color, turgor, lesions reveal systemic issues.
• Crocs ≠ closed-toe shoes in lab → mandatory for dissection safety.
• Sheep brain dissection upcoming; closed-toe shoes + safety quiz (≥18) required.
• Reinforcement exercise (strength/opp. report) worth 30 points—check with IT if access issues.

Study & Exam Tips Summarized

• Memorize sympathetic vs parasympathetic triggers, spinal levels, neuron lengths, neurotransmitters, adrenal involvement.
• Be ready to label diagrams:
  • Spinal cord segments, ganglia, pre/postganglionic pathways, adrenal gland.
• Recognize clinical scenarios:
  • Panic attack physiology, drug-induced pupillary dilation, epinephrine use, diabetic retinopathy screening, jaundice.
• Understand functional reasoning behind each physiological change (e.g.
  bronchodilation → ↑ O$_2$ for muscle ATP).