Lesson 20

ANS Reflexes: Overview

  • Bainbridge Reflex: Increases heart rate due to increased venous return to prevent blood sludging.
  • Bezold-Jarisch Reflex: Triad of bradycardia, hypotension, and coronary artery dilation due to harmful stimuli.

Bainbridge Reflex

  • Stimulus: Increase in blood volume.
  • Sensor: Low-pressure stretch receptors in atria and lung vasculature.
  • Afferent Pathway: Vagus nerve to nucleus tractus solitarius (NTS).
  • Efferent Pathway: Parasympathetic and sympathetic to SA node, leading to increased heart rate.
  • Response: Counterbalances the baroreceptor reflex; negligible effect on contractility and stroke volume.

Bezold-Jarisch Reflex

  • Stimulus: Noxious stimuli (e.g., myocardial infarction, low venous return).
  • Sensor: Chemo- and mechanoreceptors in the left ventricle wall.
  • Afferent Pathway: Nonmyelinated C fibers through vagus to NTS.
  • Control Center: NTS and medullary cardiovascular nuclei.
  • Efferent Pathway: Vagus nerve effects on heart rate and conduction.
  • Clinical Relevance: May be less pronounced in patients with cardiac hypertrophy.

Comparison of Reflexes

  • Bainbridge: Increases HR during venous congestion; aims to minimize blood congestion.
  • Bezold-Jarisch: Decreases HR during severe hypovolemia; allows for heart filling.

Chemoreceptor Reflex

  • Stimulus: Hypoxia.
  • Effect Responses: Increased respiratory rate and tidal volume; cardiovascular response includes decreased heart rate initially, but persistent hypoxemia leads to increased heart rate and cardiac output due to sympathetic activation.

Vasovagal Reflex

  • Triggers: Psychological stress, pain, or peritoneal distension.
  • Physiological Changes: Massive vasodilation and decreased cardiac output leading to syncope.
  • Symptoms: Pallor, sweating, nausea, potential cerebral ischemia and loss of consciousness.

Oculocardiac Reflex (Five & Dime Reflex)

  • Afferent Pathway: CN 5 (trigeminal).
  • Efferent Pathway: CN 10 (vagus).
  • Stimuli: Traction on extraocular muscles, ocular trauma etc.
  • Clinical Presentation: Bradycardia, hypotension, AV block.
  • Management: Remove stimulus, ensure ventilation, administer anticholinergics if necessary.