SIW - Hearing and Balance Lesson

AandP_Lesson_TopBanner.png 

Hearing and Balance

Anatomy of the Ear

The anatomy of the ear is detailed.

 

The ear is the major sense organ for hearing.  Download the "Anatomy of the Ear" note outline Links to an external site. here and complete the interactive learning object here Links to an external site. to learn about the structure of the ear.

How Do We Hear?

Sound waves are longitudinal waves that move through a medium such as water or air. In order for us to hear a sound, a wave must enter into the outer ear, be transformed into fluid waves by our middle ear, transduced into an action potential in the inner ear, and then processed by our brains.

Reception of Sound

The human ear can be divided into three parts due to differences in anatomical position and function:   the outer ear, middle ear, and inner ear.   Learn about the functions of each in the learning object below.

Sound waves are collected by the external, cartilaginous outer part of the ear called the auricle (pinna). The auricle helps to funnel sound waves through the auditory canal. These waves then cause the thin diaphragm called the tympanum (eardrum) at the innermost part of the outer ear to vibrate.  

On the interior of the tympanum is the middle ear, which holds three small bones called ossicles ("little bones"), that transfer energy from the moving tympanum to the inner ear. The three ossicles are the smallest bones in the body and are called the malleus (the hammer), the incus (the anvil), and stapes (the stirrup). The malleus is attached to the interior surface of the tympanum. The incus then attaches the malleus to the stapes, which is attached to the oval window of the cochlea. When the tympanic membrane begins to vibrate due to sound waves, its movement is transferred through the ossicles to the oval window.

Transduction of SoundA diagram of the anatomy of the ear. The outer ear, middle ear, and inner ear are detailed. The ossicles (small bones) of the ear are detailed.

The inner ear can be divided into three parts: the semicircular canals, the vestibule, and the cochlea. The semicircular canals and the vestibule affect the sense of balance and are not concerned with hearing. The cochlea is where the transduction of sound occurs.

In the fluid-filled cochlea, energy from a sound wave is transferred from the stapes through the flexible oval window, creating pressure waves. The cochlea consists of three fluid-filled chambers that are separated by membranes. Its middle chamber contains the organ of Corti, which is the organ of hearing. The cochlea got its name from the Greek word for "snail" because its structure is whorled like the shell of a snail. It contains receptors for the transduction of the mechanical wave into an electrical signal.   Inside the cochlea, the basilar membrane is a mechanical analyzer that runs the length of the cochlea, curling toward the cochlea's center.

As the fluid moves, thousands of sensory hair cells detect its motion and convert that motion to action potentials, sending their signals through the auditory nerve.

As with sight, there are many different types of signals that need to be transmitted in order for a human to hear. Signals must indicate qualities such as sound source, pitch, loudness, and location. The physical characteristics of a sound wave are responsible for some of these qualities.  For example, loudness is determined by the amplitude of a sound wave which is measured in decibels (dB). The softest sound that a human can hear is 0 dB.   Humans speak normally at 60 dB. Pitch is determined by the frequency of a sound wave which is measured in Hertz (Hz). The average human is able to perceive sounds between 30-20,000 Hz. Women are typically better at hearing high frequencies, but everyone's ability to hear high frequencies decreases with age. In the diagram below, you can see that the basilar membrane registers frequencies at different points as pressure waves spiral through the cochlea. 

A detailed view of the anatomy of the cochlea. The process of transduction of sound is illustrated.

 

Audio Processing

The sound information from the cochlea travels via an action potential through the auditory nerve where it is processed in locations such as the brain stem, thalamus, and the primary auditory cortex of the temporal lobe. As with sight, what we hear is always a result of processing by the brain. This means that injury or illness can affect what we perceive to hear.

Balance

Balance is typically a sense that is not thought of unless it is affected. The vestibular system in the ear helps to maintain balance and equilibrium in order to maintain homeostasis by detecting forces such as gravity, acceleration, and deceleration. The vestibular system utilizes sensory hair cells like the auditory system, but it excites them in different ways. There are five vestibular receptor organs in the inner ear, all of which help to maintain balance: the utricle, the saccule, and three semicircular canals. Together, they make up what is known as the vestibular labyrinth.

The vestibular system utilizes sensory hair cells to maintain balance and equilibrium in order to maintain homeostasis by detecting forces such as gravity, acceleration, and deceleration.

Head position is sensed by the utricle and saccule. The sensory hairs in these receptor organs lie below a gelatinous layer which is also embedded with calcium carbonate crystals. When the head is tilted, the crystals continue to be pulled straight down by gravity, but the new angle of the head causes the gelatin to shift, thereby bending the sensory hair cells. This bending stimulates specific neurons that signal to the brain that the head is tilted, allowing the maintenance of balance.

Head movement is sensed by the semicircular canals. In these fluid-filled structures, one canal lies horizontally, while the other two lie at about 45-degree angles to the horizontal axis. When the brain processes input from all three canals together, it can detect acceleration or deceleration in three dimensions. When the head turns, the fluid in the canals shifts, thereby bending sensory hair cells and sending signals to the brain. When acceleration or deceleration stops, the movement of the fluid within the canals slows or stops.

As the head rotates, the cupula bends in the opposite direction of the rotation.

Review

 

AandP_BottomBanner.png 

IMAGES CREATED BY GAVS OR OPENSOURCE