Part 3: Basic Anatomy and Regulation of the Ear

• Sense of hearing is based on the physics of sound, the physiology of the ear, nerves that travel to the brain, and brain regions that are involved in sensing and perceiving acoustic information.
• Sound is a form of energy that comes and travels in waves.
• The number of wave cycles per second = frequency = pitch. The higher the frequency of the waves (waves occur faster), the higher the pitch is of a sound.
• Amplitude is how tall the sound waves are. The more amplitude (the bigger the wave), the louder the sound.
• The anatomy of the ear, from outside to inside, go from the pinna, the external auditory canal, the tympanic membrane (ear drum), the malleus, the incus, the semicircular canal, the stapes (oval window), the middle ear cavity, the vestibulocochlear nerve (vestibular branch and cochlear branch), the cochlea, and the auditory (eustachian) tube (where there is equalized pressure).
• The helicotrema is the apex (top) of the cochlea.
• The cochlear duct, scala vestibuli, and scala tympani are all filled with fluid that help sound travel.
• The round window of the middle ear is a membrane-covered opening in the cochlea that responds to fluid movement in the scala tympani.
• The scala vestibuli and the scala tympani are two outer layers of the cochlea structure. In the middle of them is the cochlear duct. The basilar membrane lies between the scala and the cochlear duct (imagine it like a filling of a sandwich).
• As sound moves, the tympanic membrane deflects the waves and vibrates. This causes the middle ear bones (malleus and incus) to move. The membrane in the oval window moves, causing waves to move down the scala vestibuli and scala tympani, which causes the basilar membrane to move. The fluid in the cochlear duct moves as it hears sound. Then, through all this fluid movement, the membrane in the round window moves.
• The basilar membrane has hairs that can move around.
• The lower the frequency of the sound, the more it can get to the helicotrema (top, apex). In other words, higher frequency sounds can't travel as far as lower frequency sounds.
• High-frequency sounds vibrates the basilar membrane near the oval window. Basically, instead of going all the way around the helicotrema, it cuts through the cochlear duct and basilar membrane towards the scala tympani, and therefore get through the round window.
• The Organ of Corti is a structure in the inner ear that is capable of transducing sound wave energy into action potentials (directly from textbook e15).
• Since the Organ of Corti produces action potentials, we would not be able to hear without it.
• When there is fluid and hair movement of the cochlea, the tectorial membranes (of Organ of Corti) move. This causes outer hair cells (hearing cells) to move back and forth. The outer hair cells are attached to the tectorial membrane by stereocilia.
• Stereocilia contain K+ channels (on the tip links) that open as the tectorial membrane moves around (this causes tip links to stretch). This is how action potentials are generated. They also act as receptor cells to afferent neurons.
• When the stereocilia's tips slack back (when there is no sound movement), there are no K+ channels open, and therefore no action potentials generated.
• There are two auditory pathways: primary and secondary.
• The primary sensory neurons project to the medulla oblongata in the brainstem.
• The secondary sensory neurons project to both sides of the brain so both sides of get signals from both ears (basically information stays on one side, but some information also crosses over to the other side).
• The localization of a sound source requires simultaneous input from both ears.
• The secondary sensory neurons also synapse in the nuclei in the midbrain and thalamus before they project into the auditory cortex.
• There are three types of hearing loss: conductive, central, and sensorineural.
• Conductive hearing loss is when there is no transmission through the external or middle ear (e.g., blocked because of ear wax, physical obstruction).
• Central hearing loss is when there is damage to the neural pathway between the ear and the cerebral cortex. There could also be damage to the cortex itself (e.g., because of a stroke, but this is uncommon).
• Sensorineural hearing loss is where there is damage to the structure of the inner ear (e.g., damage of the hair cells, and this is IRREVERSIBLE). This is the most common form of hearing loss, and can be caused by old age or loud noises (e.g., listening to music at max volume all the time).
• There are two methods to restoring the sense of hearing: through hearing aids and/or cochlear implants.
• Hearing aids are sound amplifiers that are places in the auditory canal which activates existing auditory "machinery".
• Cochlear implants are externally located audio sensors that receive input and activates electrodes which physically stimulate the cochlear nerve. Basically, the implant directly activates neurons and skips all the middle ear structures to go straight into the scala.
• When the Organ of Corti sends electrical signals about sound to the brain, the perceived pitch of a sound is determined by the location of the activated hair cells on the basilar membrane. Basically, the pitch is determined by how far into the basilar membrane the frequency got to.