How We Hear
Below is a picture of the auditory, or hearing, system, which we generally break into three parts: The outer, middle, and inner ears. The outer ear describes everything from the ear outside the head to the eardrum, the middle ear describes the space and structures from the eardrum to the cochlea (the snail shell structure inside the skull), and the inner ear describes the cochlea with all the structures inside it and the hearing nerve that carries sound to the brain.
Each of these three parts of the auditory system are described below, along with how they contribute to hearing and some issues that can arise to cause hearing loss.
THE OUTER EAR
Sound first enters the outer ear at the pinna, the portion of our ear visible on the outside of our head. The pinna collects sound waves and funnels them down the ear canal to the eardrum. Together the pinna and ear canal are referred to as the outer ear.
THE MIDDLE EAR
The middle ear begins at the tympanic membrane or eardrum. When sound waves from the outer ear strike the tympanic membrane, it vibrates like a drum (hence the term eardrum). Behind the eardrum is an air-filled space containing three middle ear bones, the smallest bones in the body. The eardrum vibrations cause the middle ear bones to vibrate. A picture of a healthy eardrum appears below, where you can see the first of the middle ear bones, the Incus, attached in the upper right portion.
THE INNER EAR
The cochlea, our hearing and balance organ, together with the auditory (hearing) nerve, are referred to as the inner ear. Sound passes to the inner ear via the vibrations of the middle ear bones, which are connected to the cochlea at one end. Tiny hair cells within the cochlea convert the sound vibrations into an electrical signal which is carried up the hearing nerve to the brain, where sound happens.
Several rows of hair cells are pictured below. A healthy cochlea contains about 20,000 hair cells which fire about 1,000 times per second.