AP2 3.2 The Auditory and the Vestibular Systems Flashcards
The ear
The ear is the sensory organ for hearing. The ear can be divided into three areas: the external, middle, and inner ear.
external ear
The external ear consists of the auricle and external acoustic canal (meatus).
auricle
external ear
The auricle is shaped to funnel sound waves into the external acoustic canal so that sounds can be detected. The auricle is the part of the ear that can be seen externally. It is composed of elastic cartilage covered with thin skin making up the rim (helix) and the lobule that lacks cartilage
external acoustic meatus
external ear
The external acoustic meatus is the tunnel between the auricle and tympanic membrane. The external acoustic meatus is composed of elastic cartilage near the auricle and becomes a cylinder through the temporal bone. The canal is lined with skin containing hairs and glands that secrete cerumen (earwax), which serves to trap foreign materials.
tympanic membrane
The inner end of the canal terminates at the tympanic membrane (eardrum), a thin membrane of connective tissue whose vibration transmits sound energy to the middle ear.
middle ear
The middle ear is an air-filled chamber containing the three smallest bones in the body, which are known as the ossicles: the malleus (hammer), incus (anvil), and stapes (stirrup). The malleus receives the vibrations from the eardrum and transfers them along through the incus, then finally the stapes, which conveys them to the inner ear
inner ear
The inner ear is composed of the bony and membranous labyrinths filled with fluid. The bony labyrinth system is divided into three sections: the vestibule, semicircular canals, and cochlea.
vestibule
The vestibule connects to three semicircular canals, which house receptors to provide the body’s vestibular (equilibrium and balance) system.
vestibular sense
The semicircular canals contribute in this way to the body’s vestibular sense (balance and upright orientation). Note that CNVII (facial nerve) is also located in the inner ear region. The facial nerve innervates a muscle called the stapedius, which is attached to the stapes. The stapedius controls the amount of tension on the bone, allowing for increased or decreased vibration to help control the intensity of sounds.
semicircular canals
The three semicircular canals are also filled with fluid. Each semicircular canal contains equilibrium receptors, which cause action potentials in response to different types of motion (head movements or rotation). The action potentials are sent through the vestibular portion of CN VIII.
cochlea
The cochlea is a spiral, bony chamber containing the membranous endolymph-filled cochlear duct. The cochlea houses the organ of Corti and terminates at the cochlear nerve.
The process of hearing
The process of hearing happens in a series of events. First, sound waves are funneled into the external auditory canal causing the eardrum to vibrate. The vibrations transmit to the ossicles, which push against fluid inside the cochlear duct. The movement of the fluid causes movement of the stereocilia in the organ of Corti. The movement of the stereocilia then stimulates the neurons to send impulses through the cochlear nerve to the primary auditory cortex of the temporal lobe. The auditory pathway is a unique sensory pathway because the ascending pathways cross in addition to sending signals to the same side of the brain. The result is that sounds from each ear are equally interpreted on the left and right sides of the brain.
Damage to the auditory pathway
Damage to the auditory pathway results in hearing loss called deafness. Deafness can be partial (if hearing is impaired but not completely lost) or total.
organ of Corti
The organ of Corti is the sense organ for the auditory system. The organ of Corti contains the tectorial membrane and tiny hairs called stereocilia . As the fluid within the cochlear duct receives vibrations, the tectorial membrane moves and bends the attached stereocilia. The mechanical movement of the stereocilia initiates an action potential in neurons that is transmitted through the cochlear nerve portion of CN VIII
Conduction deafness
Conduction deafness occurs from damage to the outer or middle ear structures. For example, a build up of cerumen can partially block sound waves entering the external acoustic meatus. Another cause of conduction deafness is if the ossicle bones fuse, decreasing the amount of vibrations transmitted to the inner ear. Conduction deafness is not complete because the sound waves can be conducted through the cranial bones to move the organ of Corti.
