Sounds are detected by the ears in the form of vibrations and transformed into nerve impulses by specialized neurons which then transmit the sound (auditory) information to the auditory cortex for processing.
The auditory cortex is located in the temporal lobe of the brain.
Sounds are that are received by the ear are transmitted by the vestibulocochlear nerve (8th cranial nerve) and processed by the auditory sensory system.
The auditory sensory system includes the cochlear nuclei, the superior olivary complex, the inferior colliculus, the medial geniculate nucleus, and the auditory cortex
1. What Are the Steps of Hearing?
Sound processing is ensured by the auditory system that includes the ears and the sensory system and follows the following steps:
- Step 1: The sound enters the ear, travels through the ear canal, and reaches the eardrum.
- Step 2: The sound waves are amplified by the eardrum and the ossicles.
- Step 3: The amplified sound waves is received by the cochlea where it is transformed into nerve impulses by the hair cells (stereocilia) found in the organ of Corti.
- Step 4: The nerves impulses are transmitted to the sensory cortex for processing by the auditory cortex.
1.1. What Is Ear and Its Function?
The ear is the first organ of hearing and is composed of the outer ear, middle ear, and inner ear .
- What Is Outer Ear?
The outer ear is composed of the visible part of the ear (Pinna or auricle), and the ear canal.
- What Is Middle Ear?
The middle of the ear contains the eardrum and 3 small bones known as ossicles and which are also involved in amplifying the sound waves before reaching the cochlea in the inner ear.
The eardrum is the membrane involved in the amplification of the sound waves that travel through the ear canal. It separates between the outer ear and the inner ear.
- What Is Inner Ear?
The inner ear contains the utricle, saccule, and the bony labyrinth which includes the semicircular canals, and the cochlea.
The utricle and saccule are involved in displacement and linear accelerations such as tilting the head and orientation.
The semicircular canals are filled with fluids and are involved in maintaining balance and coordination.
The cochlea is a cavity that is shaped like a spiral filled with fluids. It contains the organ of Corti where the hair cells (stereocilia) are found. The hair cells are responsible for transforming sound waves into impulses in coordination with sensory neurons in proximity.
1.2. What Is the Auditory Sensory System Function?
The auditory sensory system includes the vestibulocochlear nerve (8th cranial nerve), the cochlear nuclei, the superior olivary complex, the inferior colliculus, the medial geniculate nucleus, and the auditory cortex.
- What Does the 8th Cranial Nerve Do?
The 8th cranial nerve is known as the vestibulocochlear nerve (cranial nerve VIII) or auditory nerve. It has two nerve branches, the cochlear nerve, and the vestibular nerve .
The cochlear nerve is involved in transmitting auditory information to the cochlear nucleus in the medulla oblongata found in the brainstem.
The vestibular nerve transmits information to the brain for the processing of body balance.
- What Is the Cochlear Nucleus?
The cochlear nucleus or cochlear nuclei are located in the brainstem and receive auditory information from the cochlear nerve. They work like a distribution center through the processing of different acoustic waves.
- What Does the Superior Olivary Nuclei do?
The superior olivary nuclei are also known as superior olive or superior olivary complex. It is located in the pons of the brain where it is involved in measuring the difference in sound intensity and azimuth.
The superior olivary complex receives auditory information from the cochlear nuclei.
- What Does the Inferior Colliculus in the Brain?
The inferior colliculus is located in the midbrain and plays a role as a relay for auditory information coming from the two ears and its integration. It is also involved in distinguishing between a pitch and a rhythm .
The inferior colliculus receives auditory information from the superior olivary complex and the auditory cortex.
- What is the medial geniculate nucleus?
The medial geniculate nucleus is located in the part of the brain known as the thalamus and is involved in the detection of sound intensity and duration.
The median geniculate nucleus receives auditory information from the medial geniculate nucleus.
- What is the Role of the Auditory Cortex?
The auditory cortex is part of the temporal lobe involved in transforming acoustics into perceptual representation such as recognizing the sound and its identification. It is also involved in language switching .
2. What Are Hearing Disorders?
Hearing disorders can affect both the ear and the auditory sensory system.
2.1. What Are the Most Common Ear Disorders?
What Are disorders of the Outer Ear?
There are several disorders of the outer ear that are due to the following causes:
- Absence of the outer ear (Anotia)
- Malformation of the outer ear (Microtia)
- Infection (Otitis externa)
- Wax build-up
- Bony tumor (Osteoma)
- Absence of ear canal (Atresia)
- Narrowing of the ear canal (stenosis)
- Fungal infection (Otomycosis)
- Disorders of the eardrum (perforation, thickening (tympanosclerosis), and inflammation (Myringitis).
What Are disorders of the Middle Ear?
- Otitis media (infection of the middle ear)
- Otosclerosis (bony growth in the middle ear)
- Eustachian tube dysfunction
- Ossicular chain discontinuity (loss of connectivity between the ossicles)
- Cholesteatoma (Tumor)
What Are disorders of the Inner Ear?
- Ménière’s disease (increased build-up of fluids in the inner ear) characterized by symptoms such as vertigo, ringing in the ear, and loss of hearing.
- Nose-induced hearing loss (NIHL)
- Presbycusis (age related degeneration of the cochlea)
- Perilymph fistula (fistula causing a leak of the inner ear fluid into the middle ear)
2.2. What Are the Most Common Disorders of the Auditory Sensory System?
What Are the Disorders of the Auditory Nerve?
- Acoustic Neuroma (tumor growing on the auditory nerve) causing hearing loss, dizziness, and ringing in the ear (tinnitus)
- Auditory Neuropathy Spectrum Disorder (auditory neuropathy) is due to alterations in the transmission of auditory information between the cochlea and the cochlear nuclei in the brainstem.
What Are the Disorders of the Cochlear Nuclei?
- These disorders are due to alterations in the transmission of the auditory information within the cochlear nuclei that can be caused by diseases (e.g., syphilis, multiple sclerosis, congenital malformation), stroke, or aging.
What Are the Disorders of the Higher Auditory Pathways?
- Minimal Auditory Deficiency Syndrome is due to language deprivation in children which may be caused by undiagnosed otitis media
- Central Deafness is rare and can be caused by a vascular lesion in the brain hemispheres.
- Auditory Processing Disorder is cause by a deficit of transmission of auditory information between the ear and the brain.
The auditory system is very complex and fascinating and involves both the ears and the brain processing sounds that are extremely diverse. Meanwhile, I am certainly going to enjoy listening to a nice piece of music while I still can.