Study Notes on the Mechanisms and Pathways of Vomiting

Mechanisms and Pathways of Vomiting

Overview of Vomiting Control

  • Vomiting is a physical reflex controlled by brainstem nuclei known as the emetic center.
  • The emetic center is the convergence point for all vomiting stimuli.

Pathways of Vomiting Stimuli

  • Vomiting stimuli can travel through two pathways: central and peripheral.
Central Pathway
  • Description: Stimuli travel directly to the emetic center from higher brain centers.
Peripheral Pathway
  • Description: Stimuli can reach the emetic center in two ways:
    • Directly via neural afferents.
    • Indirectly through the chemoresceptor trigger zone (CRTC), which detects circulating toxins in the blood.

Examples of Conditions Leading to Vomiting

  • Dietary Indiscretion: Causes vomiting stimuli to travel directly to the emetic center.
  • Liver Failure: Causes vomiting stimuli to travel indirectly via the CRTC to the emetic center.
  • Renal Failure: Can trigger vomiting signals reaching the emetic center by either direct route or indirect route via the CRTC.
  • Motion Sickness: Involves signals from the vestibular apparatus traveling through the CRTC to reach the emetic center.

Detailed Structure of the Chemoresceptor Trigger Zone (CRTC)

  • Location: The CRTC is positioned outside the blood-brain barrier.
  • Function: Detects abnormal concentrations of circulating toxins and signals the emetic center to induce vomiting.
Receptors in the CRTC
  • Total Known Receptor Types: Seven types mediating the vomiting signal:
    • Neurokinin 1 (NK1)
    • 5-HT3 (serotonin receptor)
    • Adrenergic Receptors
    • Dopaminergic Receptors
    • Muscarinic Receptors
    • Histamine Receptors

Receptors in the Emetic Center

  • Total Known Receptor Types: Three types identified within the emetic center:
    • Adrenergic Receptors
    • 5-HT2 (serotonin receptor)
    • NK1 Receptors
Role of NK1 Receptor
  • Recognition: NK1 receptor has been recognized as a key receptor involved in the vomiting reflex.
  • Concentration: High concentrations exist in both the emetic center and the CRTC.
Interaction with Substance P
  • Substance P: A neurotransmitter with a crucial role in the vomiting response.
    • Function: It demonstrates a high affinity for NK1 receptors.
    • When Substance P binds to NK1 receptors, a lock and key effect occurs, resulting in a cascade of events that triggers vomiting.

Summary of Physiology of Vomiting

  • Vomiting is the result of impulses from various underlying conditions.
  • All signals to vomit ultimately converge on the emetic center.
  • Neurotransmitters like Substance P bind with receptors (like NK1) in the emetic center, triggering the physical reflex of vomiting.