If a medium is set into vibration within certain frequency limits (average between 25 cycles per second and 18,000 cycles per second), we have what is called a sound stimulus. The sensation of sound, of course, occurs only when these vibrations are interpreted by the cerebral cortex of the brain at the conscious level.
The human ear is the special sensory receptor for the sound stimulus. As the stimulus passes from the external medium (air, water, or a solid conductor of sound) to the actual receptor cells in the head, the vibrations are in the form of airborne waves, mechanical oscillations, and fluid-borne pulses.
The ear is organized in three major parts: external ear, middle ear, and internal (inner) ear. Each part aids in the transmission of the stimulus to the receptor cells.
External Ear Process:
The external ear begins with a funnel-like auricle. This auricle serves as a collector of the airborne waves and directs them into the external auditory meatus. At the inner end of this passage, the waves act upon the tympanic membrane (eardrum).
The external auditory meatus is protected by a special substance called earwax (cerumen).
Middle Ear Process:
The tympanic membrane separates the middle and external ears. It is set into mechanical oscillation by the airborne waves from the outside.
Middle Ear Cavity.
Within the petrous bone of the skull is the air-filled middle ear cavity.
Function of the auditory tube. Due to the auditory tube, the air of the middle ear cavity is continuous with the air of the surrounding environment. The auditory tube opens into the lateral wall of the nasopharynx.
Thus, the auditory tube serves to equalize the air pressures on the two sides of the tympanic membrane. If these two pressures become moderately unequal, there is greater tension upon the tympanic membrane; this reduces (dampens) mechanical oscillations of the membrane.
Extreme pressure differences cause severe pain. The passage of the auditory tube into the nasopharynx opens when one swallows; therefore, the pressure differences are controlled somewhat by the swallowing reflex.
The middle ear cavity extends into the mastoid bone as the mastoid air cells. The relatively thin roof of the middle ear cavity separates the middle ear cavity from the middle cranial fossa.
There is a series of three small bones, the auditory ossicles, which traverse the space of the middle ear cavity from the external ear to the internal ear. The auditory ossicles function as a unit.
The first ossicle, the malleus, has a long arm embedded in the tympanic membrane. Therefore, when the tympanic membrane is set into mechanical oscillation, the malleus is also set into mechanical oscillation.
The second ossicle is the incus. Its relationship to the malleus produces a leverage system which amplifies the mechanical oscillations received through the malleus.
The third ossicle, the stapes, articulates with the end of the arm of the incus. The foot plate of the stapes fills the oval (vestibular) window.
The auditory muscles are a pair of muscles associated with the auditory ossicles. They are named the tensor tympani muscle and the stapedius muscle. The auditory muscles help to control the intensity of the mechanical oscillations within the ossicles.
Internal Ear Process:
Transmission of the Sound Stimulus.
The foot plate of the stapes fills the oval (vestibular) window, which opens to the vestibule of the internal ear. As the ossicles oscillate mechanically, the stapes acts like a plunger against the oval window. The vestibule is filled with a fluid, the perilymph. These mechanical, plunger-like actions of the stapes impart pressure pulses to the perilymph.
Organization of the Internal Ear.
The internal ear is essentially a membranous labyrinth suspended within the cavity of the bony (osseous) labyrinth of the petrous bone. The membranous labyrinth is filled with a fluid, the endolymph. Between the membranous labyrinth and the bony labyrinth is the perilymph.
The cochlea is a spiral structure associated with hearing. Its outer boundaries are formed by the snail-shaped portion of the bony labyrinth. The extensions of the bony labyrinth into the cochlea are called the scala vestibuli and the scala tympani These extensions are filled with perilymph.
The basilar membrane forms the floor of the cochlear duct, the spiral portion of the membranous labyrinth. The basilar membrane is made up of transverse fibers. Each fiber is of a different length, and the lengths increase from one end to the other. Thus, the basilar membrane is constructed similarly to a harp or piano.
Acting like the strings of the instrument, the individual fibers mechanically vibrate in response to specific frequencies of pulses in the perilymph. Thus, each vibration frequency of the sound stimulus affects a specific location of the basilar membrane.
Organ of Corti. Located upon the basilar membrane is the organ of
Corti. The organ of Corti is made up of hair cells. When the basilar membrane vibrates, the hair cells are mechanically deformed so that the associated neuron is stimulated.
Nervous Pathways for Hearing Process
The neuron (associated with the hair cells of the organ of Corti then carries the sound stimulus to the hindbrainstem. Via a special series of connections, the signal ultimately reaches Brodmann's area number 41, on the upper surface of the temporal lobe.
Here, the stimulus is perceived as the special sense of sound. It is interesting to note that speech in humans is primarily localized in the left cerebral hemisphere, while musical (rhythmic) sounds tend to be located in the right cerebral hemisphere.