Activity 6A: How We Hear

The ears are on duty 24 hours a day. Unlike the eyes, the ears have no "lids" to shut out stimuli. Simply put, the ears collect vibrations from the air and change them into nerve impulses. The brain "hears" these impulses, making useful information of the pitch, volume, and timbre of the sound waves.

Use the handout "Anatomy of the Human Ear." This handout has an outline of the auditory system. A labeled master copy of the handout ("Anatomy of the Human Ear: Teacher Handout") is provided for the teacher to use in the lesson. Give each student a copy of the handout. Using the master copy as a guide, trace the path of sound from the object to the auditory cortex of the cerebrum (temporal lobes).

Steps:

1. Sound waves (vibrations) are collected by the outer ear (pinna) and channeled into the external auditory canal.
2. The sound waves travel down the external auditory canal (lined with hair and wax) to strike the circular tympanic membrane (ear drum) causing it to vibrate.
3. The tympanic membrane's vibrations are transmitted to the middle ear ossicles (stapes [stirrup], incus [anvil], and malleus [hammer]). The middle ear increases the intensity of the vibrations through these three bones which are the smallest bones in the human body.
4. The ossicles in turn transmit the vibrations to the oval window, the interface between the middle ear and cochlea (inner ear).
5. In the inner ear, the sound vibrations that were first transmitted through air and then through bone, are now transmitted in fluid called endolymph.
6. The endolymph is divided into three compartments, the scala vestibuli (upper), the scala tympani (lower), and cochlear duct (middle). These three compartments are wound like a snail's shell to form the cochlea. Inside the cochlear duct are the hair cells that will convert the vibrations into neurochemical signals.
7. Each hair cell is coated with cilia - hairlike projections that bend in response to particular frequencies and intensities of vibrations. As the cilia are bent by the vibrations in the endolymph, the hair cells generate a neurochemical signal.
8. The neurochemical signal is sent by the auditory nerve to the auditory cortex of the brain. The auditory cortex is located in the temporal lobes of the cerebrum. Both ears have neural connections with both temporal lobes. It is in the temporal lobes that sound is perceived and interpreted.