The Ear


The Human Ear provides several functions in addition to providing us with the ability to hear including detecting the heads position and motion and providing a sense of balance. Within the ear there are distinct areas that are involved in hearing and balance. The ear is divided into 3 different sections each having a particular purpose in the process of converting sound into electrical signals and its interpretation. Figure 1.1 and Table 1.1 shows the anatomy and function of the human ear.

Fig. 3.1
earanatomy.jpg

Table 3.1 - Anatomy of the human ear
Structure
Description of Structure
Region of Ear
Function
Pinna
Large, fleshy, external region of ear. Contains cartilage to shape the ear.
Outer
Collects sound and directs into the auditory canal. The shape of the ear is vital as it helps determine the direction of sound and enables the maximum collection of sound. It also serves as protection for the inner parts of the ear .
Tympanic membrane
(eardrum)
Thin but tight membrane situated between the external and middle ear.
Outer/
middle
Vibrates at the same frequency as the sound waves, transferring this mechanical energy to the hammer in the middle ear. It also provides an airtight protection between the external ear and the middle ear.
Ear ossicles
Three small, hinged bones connected together (hammer, anvil and stirrup)
Middle
The three bones transfer sound's vibration, by movement of these bones, from the eardrum to the oval window of the cochlea. The sound is amplified in the process as a result of the levering actions of these bones as they move
Oval window
Flexible membrane covering the opening of the upper canal of the cochlea. Connected to the ear ossicles by the stirrup
Middle/
Inner
The fluid (the perilymph) in the cochlea (held by a membrane) receives and transfers the sound vibration from the stirrup
Round window
Membrane found at the bottom end of the lower canal of cochlea
Inner
Receives the sound vibrations from the oval window causing it to bulge outwards due to the constant changes in pressure of the sound vibrations in the cochlea.
Cochlea
A spiral-shaped structure containing three canals filled with fluid.
Inner
Detects the different pressure changes caused by the sounds as different frequencies in different parts: High pitched sounds are detected at the base of the cochlea, lower pitched sounds towards the end of the cochlea's spiral (see organ of Corti)
Organ of Corti
Possesses the hair cells on the basilar membrane. Occurs in the cochlea
Inner
The hair cells here convert the mechanical energy of sound into electrochemical signals (nerve impulses). These impulses transfer sound's frequencies, the sound's intensity and duration of the sound
Auditory nerve
The axons of the hair cells connecting cochlea to the hearing centres of the brain

Transfers the electrochemical signals from the cochlea to the auditory centres of the brain
Fig. 3.2 - The anatomy of the ear

The Eustachian Tube

The Eustachian tube is a tube that originates in the back of the nose, runs a slightly uphill course, and ends in the middle ear space as indicated in Figure 3.1. The primary role of the Eustachian tube is to equalise the pressure between the outer and inner ear, so that the tympanic membrane is not forced into a position it would not usually occupy. It manages to equalise the air pressure as it connects the inner ear with the outside air in the pharynx. A rapid change in
in external pressure will be noticed and a ‘popping’ may be experienced as pressures are adjusted for example when yawning. Normally, the Eustachian tube is closed, which helps prevent the inadvertent contamination of the middle ear space by the normal secretions found in the back of the nose.

Path of a sound wave


When sound is emitted, it causes the air to vibrate. The vibrating air enters the outer ear and strikes the tympanic membrane, or the eardrum.The eardrum vibrates and transfers the sound energy to the ossicles, which then start vibrating. These vibrations are transferred to the fluids in the cochlea. When the fluids press against the membranes in the inner ear, they cause forces to be exerted on the small hairs in the organ of Corti. As the cochlear hair cells bend, the hair cells release neurotransmitters into the synapses between them and the neurons that lead to the auditory nerves. The neurotransmitter molecules set up the action potential in the neurons and travel to the cerebral cortex for interpretation. Figure 3.3 is a short animation of how sound travels into the ear.

Fig. 3.3 - Sound entering the ear.

The Organ of Corti

The organ of Corti is the sensitive element in the inner ear and can be thought of as the body's microphone. It is situated on the basilar membrane in one of the three compartments of the Cochlea.. There are thin fibres of various lengths in the basilaar membrane of the organ of Corti. These fibres are of different lengths and each length vibrates at a different frequency. Thus different frequencies of sound will activate the basiliar membrane at different locations, activating different sets of hair cells. The hair cells are arranged along the basilar membrane from those stimulated by low frequencies at the apex and those stimulated by high frequencies at the base.

Fig. 3.4 - The Organ of Corti
14300-004-5FF07709.gif

Sound Shadow


When sound is coming from one side, the receptors in the ear closest to the sound will be stimulated slightly earlier and also more intensely. The brain then locates the sound as coming from one side of the body. The head is said to cast a sonic shadow or sound shadow on the sound coming into the ear from the opposite side of the body.
Most animals use the difference in time between arriving sound and the difference in intensity to find the source of the sound. The head casts a sound shadow that causes one ear to receive less intense sound than the other.