Neurobiology 104

November 7, 2003

The Ear

The ear is an extraordinary organ with a myriad of parts and elegant functions. The purpose of my lecture and notes is to suggest which structures are important to know and to explain their important relationships.

Concept 1. The ear has two distinct functions

vestibular (balance) and auditory (hearing).

These are carried out by different structures of the ear.

Concept 2. the ear has three parts; outer, middle and inner ear

These have different embryonic origins

Outer ear

The pinna = auricle is covered with skin and supported by elastic cartilage.

The auditory canal is lined with thin skin and supported by hyaline/elastic cartilage and bone.

Its unusual features are ceruminous glands.

Middle ear

The middle ear is an air filled chamber lined with simple squamous epithelium derived from endoderm. It communicates with:

the pharynx - by way of the eustachian tube.

(You clear your ears by opening this passage way.)

the external ear - across an ear drum = tympanic membrane.

The ear drum consists of a very thin layer of skin, a layer of dense regular connective tissue and a squamous epithelium continuous with the chamber of the middle ear.

the inner ear at two small “windows” covered with a delicate membrane; the oval window and the round window.

Three tiny bones (ossicles) relay sound waves from the tympanic membrane to the oval window, the malleus, incus and stapes (hammer, anvil and stirrup).

They are covered with a thin mucosa and suspended by ligaments.

Two muscles insert on the ossicles to dampen their movement.

They are composed of involuntary skeletal muscle fibers!

The stapes is fastened so as to rock on the oval window

The ear drum is about ten times as large as the oval window.

The ossicles evolved from the hinge bones of the jaw of reptiles.

Inner ear

The inner ear is a complexly shaped epithelial membrane bag called the membranous labyrinth and filled with fluid = endolymph.

It fits loosely in a cavity in the bone, the bony labyrinth = osseous labyrinth. This cavity is lined with periosteum and mesothelium and is filled with perilymph.

Fine collagenous fibers suspend the membranous labyrinth in the bony labyrinth.

The membranous labyrinth hangs free in the perilymph in some places but adheres to one side to the bony labyrinth in others. In the cochlea it is attached along two places, thus dividing the perilymph into two parts.

The saccule, utricle and semicircular canals are vestibular.

The cochlea is auditory.

Concept 3. Both the auditory and vestibular systems depend on receptor organs developed from the epithelium of the membranous labyrinth. These are the maculae, cristae, and organs of Corti. All have the same basic organization but with individual specializations for their particular functions.

At the receptors the epithelium of the membranous labyrinth becomes taller (and pseudostratified) with, hair cells (of two types) distributed among supporting cells. A layer of jelly overlies the epithelium of the receptor.

Hair cells have an array of long microvilli called stereocilia (and also may have one aberrant immotile cilium) embedded in the overlying gelatinous layer. Moving the gelatinous layer relative to the cell bodies bends the hairs. The hair cells becomes depolarized and secrete an unknown neurotransmitter. Invisibly fine endings of sensory neuron surround the basal ends of hair cells to conduct impulses back to the brain.

It is thought that bending the hairs of a hair cell mechanically lifts a lid over a pore in the membrane and allows the cell to depolarize.

Macula – the receptor for gravity and linear acceleration, located in

the saccule and utricle

A macula is specialized by having tiny crystals of calcium carbonate embedded in the gelatinous layer. This makes the layer heavier than water and allows gravity or acceleration to push the layer across the hair cells.

The saccule and in the utricle each have one macula oriented perpendicular to the other.

Crista – the receptor organ for rotation, in the semicircular ducts

The semicircular ducts are hollow donut-like tubes filled with endolymph.

Each has crista with a tall cap of gelatin (the cupula) which extends across the canal. When the head rotates the fluid in the canal moves relative to the membrane. (In the same way that inertia keeps water in a glass stationary when you rotate the glass ---- don’t tell me that you never did this as a kid). The relative movement of the fluid pushes on the cupula and yanks the hair cells’ hairs. (Think of a crista as a flap valve in a semicircular canal - - - if this means anything to you). Each ear has three semicircular ducts in perpendicular arrangement so that they can monitor rotation in any direction.

Organ of Corti – the receptor for sound, located in the cochlea

Here the gelatin layer is fixed in place and the hair cells lie on a moveable underlying membrane. When sound vibrates that membrane the immobilized hairs are distorted (see below)

The cochlea

The cochlea is a tubular extension of the membranous labyrinth inside an elongated cavity of the bony labyrinth. This elongated structure is coiled up like a snail shell with 2½ turns around a core called the modiolus. The membranous labyrinth is attached to the wall of the bony labyrinth in two places: to the outer part by the spiral ligament and the inner side by the osseous spiral lamina. These fusions divide the bony labyrinth into two chambers called scala vestibuli and scala tympani. The membranous labyrinth itself is called the cochlear duct = Scala media. The wall of the membranous labyrinth between the scala media and scala vestibuli is called the vestibular membrane and between the scala medal and scala tympani, the basalar membrane.

The membranous labyrinth of the cochlea has two main specializations:

1. the stria vascularis.

This band of epithelium secretes the endolymph and is unusual in having capillaries running right through the epithelium itself! (Endolymph is resorbed via the endolymphatic duct and sac. Perilymph is continuous with CSF in the subarachnoid space. See the last figure for the pathways if you are interested in these details.)

2. the organ of Corti, the receptor organ.

Organ of Corti

The elaborate organ of Corti shows up very well on your slides. you must realize that the slides and the diagrams show cross sections of the organ of Corti. The organ itself actually is a strip of tissue extending the length of the cochlear duct.

Got that? Good.

The crucial components are several rows of hair cells lying on the basal membrane. Their hairs embedded in the tectorial membrane, which is anchored to the bony osseous spiral lamina. Up to 20 sensory neurons send their end branches around a hair cell. Their cell bodies lie in a sensory ganglion in the modiolus and their axons travel to the brain in the vestibulocochlear (VIII cranial) nerve. Figure out why the ganglion is called the spiral ganglion.

Perception of sounds

Fact 1. Different positions along the organ of Corti perceive different tones of sound. High pitched sounds are perceived at the base of the cochlear duct and low pitch sounds at the distal tip.

Fact 2. A hair cell stimulates its associated neurons when the basalar membrane directly under it vibrates.

Fact 3. Each region of the organ responds to the particular frequency at which its region of the basalar membrane vibrates.

Think of the basalar membrane as like a drumhead that vibrates at a particular frequency. Increasing the tension on the drumhead or making the drumhead smaller can raise that frequency. The basalar membrane is under tension and its width varies along the length of the organ of Corti, being widest at the distal end and narrower towards the base. Therefore the basalar membrane vibrates at progressively lower resonate frequencies towards its tip

Fact 4. Sound waves in air generate motion waves in fluid of the

the middle ear.

The bony labyrinth is a closed chamber encased in rigid bone except for two tiny “windows” covered only with thin membranes. The stapes vibrates the oval window. Since water is incompressible, whenever the membrane over the oval window is pushed in, the membrane over the round window must simultaneously push out and vice versa. Therefore, air waves create motion waves in the fluid of the inner ear. Although the geometry is not shown well in the last figure, the two windows are situated so that pushing on the oval window moves fluid into the scala vestibuli and out of the scala tympani towards the round window. Somewhere the basalar membrane must be displaced

as the volume of one scala increases and the other decreases. Vibrations back and forth at the oval window will vibrates the particular part of the basalar membrane that can oscillate harmonically at that frequency. This is important to understand. (Think resonances and discuss it with your lab mates or instructor if the point is not perfectly clear.)

If a sound is a composite of two frequencies, two regions of the basalar membrane will vibrate.

Do not let two red herrings confuse you.

1. The vestibular membrane has no role here. It is very flexible and serves only to keep the perilymph from mixing with endolymph. It offers no impediment to propagation of waves. (What I said about movement of fluid volume into the scala vestibuli really should have been said about the combined scala vestibuli + scala media.

2. The arrows within the cochlea in the last figure are misleading. They seem to imply that fluid or pressure waves travel all of the way to the tip of the scala vestibuli, the helicotrema, where the scala vestibuli is continuous with the scala tympani.

THIS IS NOT SO, THE HELICOTREMA IS IRRELEVANT.

The wave for only the very lowest perceivable pitch would travel to the tip of the cochlea.

Fact 5. The ability of the basalar membrane to separate a sound into vibrations at different portions is just simple physics. Indeed, Helmholtz, a noted physicist figured out the mechanism of the cochlea in the 1800’s. However, the cochlea has far greater resolving power between close tones than simple physical principles can explain.

The organ of Corti has other tricks to sharpen discrimination.

A. Individual hair cells may be specialized to respond to particular frequencies.

B. Hair cells have both afferent and efferent innervation. Motor impulses may cause the hair cells (or their hairs) to shorten. How this activity improves discrimination is unknown. The point is that hearing is active process in a very complex way.

C. There are two distinctly different types of hair cells with different patterns of innervation: significance uncertain.

Fact 6. The cochlea is unbelievably sensitive.

Ordinary levels of sounds move the hairs of hair cells by about the width of the hair. At the limit of hearing the basalar membrane moves only 1/10 of a nanometer – that is one Angstrom. This bends the hairs by .003 degrees, equivalent to bending the tip of the Eiffel Tower by the width of your thumb! This is an order of magnitude less than the expected movement from Brownian motion.

I have been told that the variation in air pressure of the softest sounds we can hear is less than the difference in atmospheric pressure between the top and the bottom of the eardrum.

Also, the temporal discrimination is extraordinary. We detect the spatial location of sounds by the differences in arrival time of sound waves to the right and left ear. These differences can be less than 20 microseconds. This is much faster than a receptor could respond using a traditional second messenger mechanism. Therefore, hair cells use a direct mechanical method with one stereocilium pulling open a pore on its neighbor with a microfiber.

Concept 4. The ear is an extremely delicate organ and easily damaged by excessive noise. Everyone’s hearing decreases with age. Even you, at your youthful physical peak, are growing deaf and a major cause is exposure to overly loud noises.

Dr Hall advised you against abusing your skin with UV light. Good avuncular advice. However, you can simply powder over wrinkles and remove skin cancers if you catch them in time. But, once you kill a hair cell with too loud a noise you can never, never, never, you can never, NEVER, ever get that function back.

Ah, sad but true, the younger generation

Is always deaf to the wisdom of its elders.