Innear Ear

The inner ear detects sound, acceleration and position. It consists of fluid filled sacs (membranous labyrinth) that lie in cavities of the temporal bone (bony labyrinth).

The membranous labyrinth is divided into two inter-connecting regions:

• Cochlea - detects sound vibration (organ of Corti).
• Vestibular Apparatus - detects acceleration (ampulla of semi-circular canals) and gravity direction and static position (macula of utricle and saccule).

The next pages explain these in more detail.

Cochlea

The cochlea is a coiled tube that spiral 2 and 2/3rds times around a central pillar of bone (modiolus).

• Cochlear Divisions
• Two ribbons of tissue a lower basilar membrane and upper vestibular membrane, divide the cochlea throughout its length into three compartments.
• Upper compartment (scala vestibuli)
• Middle compartment (scala media)
• Lower compartment (scala tympani)
• Helicotrema - at the apex of the spiral the scala tympani communicates with the scala vestibuli.
• Organ of Corti - the sensory compartment of the cochlea.
• Basilar Membrane - extends from a medial projection of bone to a lateral projection of connective tissue.
• The cells of the organ of Corti project into the wedge-shaped scala media. The cells and spaces from medial to lateral are:
  • Spiral limbus
  • Internal spiral tunnel over which lies the tectorial membrane
  • Inner hair cells
  • Inner tunnel and outer hair cells
  • Outer tunnel
  • Cells of Hensen, Boettcher and Claudius
• The axons of hair cells exit the organ Corti through short, cochlear nerves and enter spiral ganglia located within the modiolus.

Vestibular Apparatus

The vestibular apparatus detects acceleration (ampulla of semi-circular canals) and gravity direction and static position (macula of utricle and saccule).

(Only parts of it can be seen in this section.)

• Membranous Labyrinth - associated with the vestibular system which consists of two sacs (utricle and saccule) and three semi-circular canals.
• Crista Ampullaris - sensory regions within the ampulla of the semi-circular canals and similar structures in the utricle and saccule.
  • Two types of simple columnar epithelium:
    • Supporting - cells seen in the basal area of the epithelium.
    • Sensory - cells that project to the surface of the epithelium.
      • Apical surface is covered with kinetocilium and stereocilia that project into the overlying gelatinous material.