Inner Ear Balance Structures

Overview

This lecture explains the function and structure of the vestibule in the inner ear, specifically focusing on the utricle and saccule and their role in balance and equilibrium.

Structure of the Inner Ear

• The inner ear contains the cochlea (hearing), the vestibule (utricle and saccule), and the semicircular canals.
• The vestibule consists of the utricle and saccule, responsible for balance, while the cochlea is solely for hearing.
• The vestibular apparatus includes the utricle, saccule, and all three semicircular canals.

Utricle and Saccule: Function and Differences

• Both utricle and saccule are otolithic organs, containing an otolithic membrane with hair cells and otoliths (tiny pebbles).
• The utricle and saccule detect linear acceleration and changes in head position (static equilibrium).
• The utricle is oriented horizontally, best for detecting horizontal acceleration (e.g., car movement).
• The saccule is oriented vertically, best for detecting vertical acceleration (e.g., elevator movement).
• The main difference between the utricle and saccule is their orientation.

Mechanism of Detection

• Hair cells with cilia (hair tufts) are embedded in the otolithic membrane, which contains otoliths on top.
• Movement or tilt of the head shifts otoliths, bending the hair cells and altering the firing rate of connected axons.
• Baseline firing occurs when the head is upright; bending the head changes firing frequency, signaling position or acceleration changes to the brain via the vestibular nerve (cranial nerve VIII).
• Linear acceleration (e.g., starting or stopping in a car or elevator) moves the otolithic membrane, bending hair cells and modulating nerve firing.

Key Terms & Definitions

• Vestibule — The part of the inner ear containing the utricle and saccule, involved in balance.
• Vestibular apparatus — Includes the utricle, saccule, and semicircular canals, responsible for equilibrium.
• Otolithic Organ — An organ (utricle or saccule) with an otolithic membrane and otoliths, detecting linear acceleration and head position.
• Otoliths — Tiny calcium carbonate crystals atop the otolithic membrane, helping sense gravity and movement.
• Otolithic Membrane — Gel-like layer where hair cell cilia are embedded and otoliths rest.
• Linear Acceleration — Straight-line movement increase or decrease in speed (e.g., vehicle moving or stopping).
• Static Equilibrium — Detection of head position relative to gravity (upright, flexed, or extended).

Action Items / Next Steps

• Review how the utricle and saccule detect changes in head position and linear acceleration.
• Prepare for the next lecture on the semicircular canals and rotational equilibrium.