Anatomy of the Human Ear

The human ear is made up of three parts, the outer, middle, and inner ear. The outer ear consists of the pinna, which serves as protection for the delicate inner parts of the ear, and the external auditory meatus, the ear canal, which collects sound and directs it towards the eardrum. The shape of the pinna also helps detect where sounds are coming from in the vertical plane. That is, from above or below the ear. The middle ear is an air-filled cavity located in the temporal area of the skull. It consists of the eardrum (the tympanic membrane), the ossicles, and the eustachian tube. The tympanic membrane is concave with an area of between sixty and ninety millimeters. And an average thickness of just seven hundredths of a millimeter. And it has a very rich blood supply. Sound waves enter the ear canal and strike the tympanic membrane, setting it in motion. The louder the sound, the more the membrane moves. The middle ear also contains three small bones known as the ossicles. These are connected in a chain across the middle ear. From the tympanic membrane to the entrance of the cochlea (called the oval window). The ossicles are called, the malleus (the hammer), the incus (the anvil), and the stapes (the stirrup). The handle of the malleus is attached to the tympanic membrane, and the base of the stapes (known as the stapes footplate) sits loosely in the oval window of the cochlea, held in place by the annular ligament. The tympanic membrane vibrates in response to sound. The eardrum moves the malleus, which in turn moves the incus, and then the stapes, the footplate of which moves in and out of the oval window. and transfers the energy to the endolymph fluid in the cochlea. The inner ear has two distinct portions, the vestibular portion, concerned with balance, and the auditory portion (the cochlea) which is concerned with hearing. The vestibular portion is made up of three semicircular canals. Each canal has a swelling at one end, the ampulla, and two membranous sacs, the utricle, and the saccule. These contain fluid and sensory cells which move in response to head movements and travel, to indicate the body’s status. The vestibular system works in conjunction with the eyes, and the receptor cells in the joints of the body to continuously maintain our balance. The cochlear appears as a coiled tube in the shape of a snail shell… with between two and a half and two and three-quarter turns. It is wrapped around the acoustic portion of the auditory nerve. If we were to unravel the cochlear, it would be about thirty-five to forty millimeters long. It is split into three distinct sections… known as the scala vestibuli, scala media, and scala tympani. The scala vestibuli and the scala tympani contain perilymph fluid. The scala media contains endolymph fluid. The reissner’s membrane separates the scala vestibuli from the scala media. and the basilar membrane separates the scala media from the scala tympani. Along the basilar membrane are two types of sensory cells (or hair cells), called the outer and inner hair cells. These are arranged in their rows according to the frequency (or pitch) of sound they detect, and covered by the tectorial membrane. Each hair cell has tiny hair-like projections (called stereocilia) attached to them. When sound energy is transferred to the cochlea, the basilar membrane vibrates up and down, causing the outer hair cell stereocilia to shear on the tectorial membrane above. This shearing action causes the stereocilia to bend, which opens the ion gates, leading to chemical changes and a resulting electrical charge inside their cells. These charges make a neural impulse which travels along the auditory nerve to the brain, and which is interpreted as sound. This complex mix of physical, chemical and mechanical processes occurs almost instantaneously, and creates our perceptions of the sounds we hear in everyday life.