From an anatomical point of view, the ear is divided into three parts: the outer ear, the middle ear, and the inner ear, as shown in the figure below. Physiologically, it can be further divided into the conducting part (including the outer and middle ear) and the sensory part (including the inner ear, the auditory nerve and the brain center that ultimately senses sound). Strictly speaking, the fluid in the labyrinth plays a conduction role and usually the entire cochlea is considered to be the sensory part.
The outer ear
The outer ear is composed of the visible part of the ear (the auricle) and the external auditory canal that leads to the eardrum. In humans, the auricle is composed mainly of cartilage and skin. This makes it very soft. The deep central part of the auricle is called the auricular cavity, which leads to the opening of the external ear canal.
The outer ear is the channel through which most of the sound is transmitted, but does not have the function of sensing sound. The human outer ear only directs sound and gives it a degree of enhancement, so we need to be in a good position to align with the direction in which the sound is coming from.
Most animals have muscles in the outer ear, so it makes more sense that they can usually hold their ears up. The human outer ear also has these muscles but cannot actually make the ear stand up.
The outer ear causes an enhancement or amplification of sound by 10-15 dB in the frequency range of approximately 1. 5 kHz to 7 kHz, due to resonance. The outer ear canal frequency is approximately 2. 5kHz, and the resonance frequency of the auricle is closer to 5kHz.
Middle Ear
Anatomically, the middle ear is more complex and it begins at the tympanic membrane at the end of the ear canal. The tympanic membrane is cone-shaped and translucent toward the inner ear, and it separates the middle ear cavity or tympanic chamber from the ear canal. The tympanic membrane consists of two layers of fibers; one layer radiates outward from the center of the tympanic membrane and its outer layer consists of circular fibers. The main part of the middle ear cavity lies between the tympanic membrane and a bony wall (the capsule), which is filled with air and communicates with the pharynx through the eustachian tube.
Hearing Bones
The middle ear has three small auditory bones: the hammer bone, the anvil bone, and the stapes bone, also collectively known as the auditory chain, which connect the eardrum to the inner ear and transmit the vibrations produced by sound to the eardrum to the inner ear.
Middle ear muscles
The middle ear has two small muscles: the tympanic tensor muscle and the stapedius muscle. The tympanic tensor muscle is connected to the hamate stalk (the tympanic membrane is attached to the hamate stalk). The stapedius muscle is attached to the wall of the tympanic cavity. These small muscles have the following two functions.
1. The middle ear muscle is an important part of the suspension system of the auditory chain.
2. The middle ear muscles also play a protective role. Contraction of the middle ear muscles reduces the sound pressure transmission through the auditory chain allowing people to be less exposed to loud sounds
Eustachian tube
It connects the tympanic cavity to the pharyngeal cavity and is open when coughing or swallowing. It has the function of regulating the pressure inside the tympanic cavity and maintaining a balance between the pressure in the tympanic cavity and the external atmospheric pressure.
Role of the middle ear
The leverage of the auditory chain and the difference in the size of the tympanic membrane (60 mm2) and the oval window (3 mm2) area help to enhance the air pressure in the external ear canal, so that this pressure can drive the denser inner ear fluid. Approximately 30 dB gain enhancement.
Inner Ear
The inner ear (labyrinth) is the most complex part of the ear. It is located just behind the middle ear and is a cavity made of rock-like bone that is filled with fluid. The rock-like bones act as a protection. The inner ear is very important because it converts vibrating sound waves into nerve impulses.
From an anatomical point of view, the inner ear is composed of three main parts: the vestibule, the semicircular canal and the cochlea. In its outer wall there are two windows, one called the oval window and connected to the pedicle of the stapes and the other called the round window. The vestibule is connected to the semicircular canal and the cochlea. The semicircular canal is involved in the understanding of sound and it is also an important part of the balance organ.
Cochlea
The cochlea is a progressively smaller diameter tube that spirals and rotates to a point similar to the shell of a snail. In humans, the cochlea has 2 and 5/8 turns.
The cochlea is divided into three parts by the basilar membrane and Reissner’s membrane: the vestibular, middle and tympanic steps. The vestibular and tympanic steps contain ectolymphatic fluid, while the middle step contains endolymphatic fluid.
The middle stage contains endolymphatic fluid.
Corti’s apparatus
The organ of Cortical is located on the basilar surface of the middle stage and is capped by a membrane that runs parallel to the base.
The inner side of the cortical apparatus is lined with hair cells, and the other side has three layers of outer hair cells, the cilia of which are in contact with the capping membrane. The hair cells and nerve fibers are held in position by the supporting cells. When the basilar membrane is stimulated by stapedial movements, it produces traveling wave movements. The different movements of the basilar membrane and the lid membrane result in shear stress movements of the hair cell cilia and stimulation of the nerve fibers.
Auditory processes
In the following we will look in detail at how the human ear perceives sound:
Our starting point is any directional sound source in the environment around the human ear. Changes in air pressure transmitted to the ear cause the eardrum to vibrate, and the handle of the hammer bone, which is connected to the tympanic membrane, also vibrates and transmits the vibrations to the anvil and stapes, whose pedals are connected to the oval window, which transmits the vibrations to the fluid (lymphatic fluid) in the vagus. Since the fluid cannot be compressed and the circular window is flexible, when the oval window advances, the circular window moves in the opposite direction and the basement membrane is thus moved.
The movement results in shear stress movement of hair cell cilia and stimulation of nerve fibers.
Tone perception
The top of the basilar membrane senses low frequencies and the bottom senses high frequencies. Different hair cells are stimulated to perceive different frequencies.
Because the structure of the basement membrane is quite complex, not all vibrations reach the entirety of the basement membrane. In fact, the traveling waves travel along the membrane from the window to the top of the cochlea like waves traveling across the surface of a pond. The amplitude gradually increases to a maximum and then decreases sharply. The place where the maximum is generated depends on the frequency of the sound.
Perception of loudness
The perception of loudness seems to be related to the number of stimulated hair cells.
We have found that sound responds as soon as it reaches the basilar membrane of the ear and is eventually transmitted to the brain. The result is that the human ear is able to accurately distinguish sounds that are sustained for a rapid and very brief period of time, such as the sound of people talking.