Lecture Notes: The Vestibular System

Overview

• The vestibular system is crucial for maintaining the body's equilibrium and spatial orientation.
• It detects head position and movement changes and communicates this information to the brain.

Functions and Pathways

• Brainstem Projections: Trigger reflex pathways for stability and re-equilibrium.
• Cortex Projections: Provide perception of gravity and movement.

Vestibular Reflex Pathways

• Vestibulo-Ocular Reflex
  • Controls eye muscles to maintain focus on visual objects during head movements.
  • Eyes move in the opposite direction of the head.
• Vestibulo-Spinal Reflex
  • Detects potential loss of balance.
  • Activates body muscles to prevent falling.

Anatomy of the Vestibular System

• Located in the inner ear on each side of the body.
• Consists of 3 semicircular canals and 2 otolithic organs.

Semicircular Canals

• Sense rotational movements (e.g., head turns).
• Oriented at right angles to each other to correspond with:
  • Left/right turning
  • Nodding up/down
  • Tilting to a side
• Contain fluid called endolymph.
• Each canal ends in an ampulla containing hair cells within a gel-like cupula.

Mechanism

• Head rotation causes the fluid to lag, moving opposite to head movement.
• This bends the cupula, activating hair cells to send nerve impulses to the brain.
• Direction of bend determines whether signals are excitatory or inhibitory.
• Head turns create excitatory signals on one side and inhibitory on the other.

Otolithic Organs

• Two patches of hair cells: saccule (vertical) and utricle (horizontal).
• Cilia embedded in gel with calcium carbonate crystals (otoconia).

Mechanism

• Gravity affects the otoconia, weighing down the gel.
• In an upright position, even pressure on utricle cells, no signals.
• Vertical saccule feels gravity pull, bends cilia, generates impulses.
• Horizontal head positions reverse the effect.
• Other head positions combine signals from both organs.

Sensory Detection

• Detect changes in rate of motion (acceleration/deceleration), not the motion itself.
• Example: Car movement causes utricle gel to lag and bend cilia, stimulating hair cells.
• Once stable motion is reached, gel aligns, and stimulation ceases.
• Similar reactions occur with the saccule during vertical movements like in elevators.

Key Points

• The vestibular system is essential for balance and spatial orientation.
• Relies on complex interactions of sensory organs and brain pathways.
• Important for understanding how head movements affect perception and stability.

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