Anatomy of the Sensorineural Mechanism The Bony Labyrinth

Anatomy of the Sensorineural Mechanism

The Bony Labyrinth (from Zemlin, 1998) Three subdivisions of the labyrinth 1. cochlea 2. semicircular canals (vestibular system) 3. vestibule (vestibular system, mainly)

Bony labyrinth: Hard, bony outer shell. (from Minifie, Hixon, & Williams, 1973) Membranous labyrinth: Fully contained inside the bony labyrinth; like a convoluted-shaped water balloon stuff inside the bony labyrinth. The membr labyrinth is all one system; i. e. , it’s all one balloon.

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

One more: membranous labyrinth https: //www. dartmouth. edu/~humananatomy/figures/chapter_44/44 -8_files/IMAGE 001. jpg

Again with the membranous labyrinth, this time all by itself; i. e. , with the entire bony labyrinth stripped away. Note once again: It’s all one fluid system serving both hearing and balance. (from 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). (adapted from Zemlin, 1998)

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

Cut through a rolled-up cochlea modiolus (core of bone that cochlea is wrapped around) spiral ganglion (8 N cell bodies) Organ of Corti basilar membrane Reissner’s membrane tectorial membrane (from Deutsch & Richards, 1979)

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

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. (from Zemlin, Fig. 6 -48) modiolus spiral ligament

Detail of the Organ of Corti (from 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 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. )

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.

Note the different shapes of the two hair cell types: § IHC: flask § OHC: test tube

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 modiolus OHCs spiral ligament

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

The cochlear and vestibular braches of the 8 th cranial nerve