Anatomy of the Sensorineural Mechanism

Three Main Subdivisions of the Labyrinth The Bony Labyrinth Zemlin (1998) Three Main Subdivisions of the Labyrinth cochlea semicircular canals (vestibular system) vestibule (vestibular system, mainly)

Bony labyrinth: Hard, bony outer shell. Minifie, Hixon, & Williams (1973) Membranous labyrinth: Fully contained inside the bony labyrinth; like a convoluted-shaped water balloon stuff inside the bony labyrinth.

Another view of the membranous labyrinth Perkins & Kent (1986)

Again with the membranous labyrinth, this time all by itself; i. e Again with the membranous labyrinth, this time all by itself; i.e., with the entire bony labyrinth stripped away. Deutsch & Richards (1979)

Schematic and partially unrolled view of the middle ear and labyrinth showing the scala vestibuli, the scala media (cochlear duct), and the scala tympani, along with the cochlear fluids (endolymph & perilymph). Zemlin (1998)

Cuts through an unrolled and a rolled cochlea Deutsch & Richards (1979)

Cut through an unrolled cochlea modiolus (core of bone that cochlea is wrapped around) spiral ganglion (8N cell bodies) Organ of Corti basilar membrane Reissner’s membrane tectorial membrane Deutsch & Richards (1979)

Detail of a single cut through the cochlea showing the Organ of Corti, 8th N fibers entering through a tunnel in the spiral lamina (the habenula perforata), the spiral ligament, and the stria vascularis. Deutsch & Richards (1979)

modiolus spiral ligament Notice the stria vascularis (also area vascularis) – the only good picture I have of this. The s.v. secretes endolymph. Notice also the spiral ligament, which attaches the b.m. to the bony wall of the cochlea, and the limbus (or limbus spiralis), a fibrous covering of the spiral lamina. Zemlin (1968), Fig. 6-48) modiolus spiral ligament

Detail of the Organ of Corti Stevens (1951) Any cut through the cochlea will show 1 inner hair cell (IHC) and 3 (sometimes 4) outer hair cells (OHCs). This unit – 1 IHC & 3-4 OHCs – is referred to as a hair cell channel. There are about 3,000 channels in the human cochlea. (That number will become important later when we discuss cochlear implants.)

Another view of the Organ of Corti (Note: In this figure, and many others you will see, each HC has one nerve fiber. This is way wrong.* More later.) *The author/artist is not mistaken. The anatomy has just been simplified.

Quiz: Which way to the modiolus, which way to spiral ligament? Note: Innervation pattern shown here is accurate: many-to-one for IHCs, one-to-many for OHCs. More later.

Yet Another View of the Organ of Corti Note that the OHCs are shaped like a test tube; the IHC is flask-shaped.

Note that the cilia are arranged according to height – short hairs on the modiolus side, taller hairs on the spiral ligament side. (Figure shows three cilia per HC; the actual number is much larger – ~50-100 per HC.) IHC OHC modiolus

Electron micrograph of a hair bundle as seen from above Electron micrograph of a hair bundle as seen from above. Note: (1) cilia are arranged according to height, (2) cilia are arranged in a very distinctive pattern, variously described as a ‘W’ or sometimes a ‘V’ shape. modiolus spiral ligament

IHCs OHCs spiral ligament modiolus

reticular lamina (reticular membrane) Composed of collagen, the RL supports the HCs from above.

reticular lamina (reticular membrane)

The cochlear and vestibular braches of the 8th cranial nerve Deutsch & Richards (1979)

The cochlear and vestibular braches of the 8th cranial nerve