Lecture Notes on Equilibrium and the Vestibular System

Introduction to Equilibrium

• Equilibrium refers to a state of balance and stability in the body.
• Maintained by the vestibular system, also known as the vestibular apparatus.

Components of the Vestibular System

• Vestibule and Semicircular Canals: Detect changes in head position and body movement.
• Signals from receptors coordinate movement, maintain posture, and stabilize gaze.
• Disruptions can cause vestibular disorders, motion sickness, dizziness, vertigo, or imbalance.

Structure of the Vestibule

• Located between the semicircular canals and the cochlea.
• Contains parts of the membranous labyrinth: the utricle and saccule.
• Utricle and Saccule:
  • Fluid-filled sacs with sensory hair cells.
  • Detect linear acceleration and head position changes.
  • Contain receptors for static equilibrium.

Static Equilibrium

• Maintains body position relative to gravity.
• Stimulated by tilting the head or linear acceleration and deceleration.
  • E.g., moving in an elevator or in a speeding/slowing car.
• Utricle: Detects horizontal movement.
• Saccule: Sensitive to vertical movements.

Structure of the Semicircular Canals

• Part of the bony labyrinth.
• Semicircular Ducts:
  • Part of the membranous labyrinth.
  • Contain receptors for dynamic equilibrium.

Dynamic Equilibrium

• Maintains body position in response to rotation.
  • E.g., shaking head "no", spinning, or on a merry-go-round.
• Three semicircular canals detect rotation in different planes:
  • Horizontal canal: Rotation around the vertical axis.
  • Anterior and posterior canals: Rotation around sagittal and coronal planes.

Specific Receptors for Equilibrium

Static Equilibrium: Macula

• Located in the utricle and saccule.
• Macula consists of:
  • Hair cells (sensory receptors) and supporting cells.
  • Hair cells: Stereocilia and kinocilium form a hair bundle.
  • Otolithic Membrane: Gelatinous glycoprotein layer secreted by supporting cells.
  • Otoliths: Calcium carbonate crystals on the otolithic membrane.
• Movement or tilt affects otoliths, bending hair cells, causing depolarization or hyperpolarization.

Dynamic Equilibrium: Crista

• Located in the ampulla of each semicircular duct.
• Crista consists of:
  • Hair cells and supporting cells.
  • Cupula: Gelatinous material covering the crista; hair bundles extend into it.
• Movement affects endolymph fluid, dragging the cupula and bending hair cells, generating receptor potentials.

Pathways of Signal Transmission

• Receptor potentials travel along the vestibular branch of the vestibulocochlear nerve (Cranial Nerve VIII) to the brain.

Summary

• Vestibule and Semicircular Canals: House receptors for equilibrium.
• Utricle and Saccule: Monitor static equilibrium (tilting, linear movement).
• Semicircular Canals & Ducts: Monitor dynamic equilibrium (rotational movement).
• Signal transmission to the brain helps maintain balance and orientation.