Auditory System

Central Auditory System

Once the sound waves are turned into neural signals, they travel through cranial nerve VIII, reaching different anatomical structures where the neural information is further processed.

The cochlear nucleus is the first site of neural processing, followed by the superior olivary complex located in the pons, and then processed in the inferior colliculus at the midbrain.

The neural information ends up at the relay center of the brain, called the thalamus. The info is then passed to the primary auditory cortex of the brain, situated in the temporal lobe.

Primary Auditory Cortex

The primary auditory cortex receives auditory information from the thalamus. The left posterior superior temporal gyrus is responsible for the perception of sound, and in itthe primary auditory cortex is the region where the attributes of sound (pitch, rhythm, frequency, etc.) are processed.

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The primary auditory cortex is located deep within the lateral sulcus, on the superior surface of the superior temporal gyrus of the temporal lobe (see Figure 13.10). The two transverse temporal gyri, or the Heschl gyri, comprise the area for primary representation of auditory information from the cochlea (organ of Corti). Information from the cochlea projects to the medial geniculate nucleus of the thalamus, which, in turn, projects to the primary auditory cortex. Ascending information from the cochlea travels both ipsilaterally and contralaterally, such that each ear is represented bilaterally on the auditory cortex. The neurons in the primary auditory cortex are organized in a tonotopic arrangement, similar to the tonotopy of the cochlea.

Primary auditory area lesion

While the representation of sound in the cortex is bilateral (see further discussion in Chapter 11, “Hearing and Balance”), information from the contralateral cochlea predominates. Thus, a lesion in the primary auditory area will result in the decreased perception of sound, primarily in the contralateral ear, rather than in a loss of hearing limited to one side or the other as would occur with a lesion of the hair cells or auditory nerve on one side.

Auditory association area

Adjacent to the primary auditory area on the lateral surface of the superior temporal gyrus is the auditory association area that enables us to interpret and give meaning to the sounds we hear. A lesion in the auditory association area can result in word deafness, or acoustic verbal agnosia, in which the ability to interpret what is heard is compromised, despite intact hearing. Extending more posteriorly on the superior temporal gyrus and looping around the lateral sulcus to include the supramarginal and angular gyri is the Wernicke area, which is critical for the understanding of language (see further discussion below).

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The cochlea, within the inner ear, is the specialized organ that registers and transduces sound waves.

It lies within the cochlear duct, a portion of the membranous labyrinth within the temporal bone of the skull base.

The human ear. The cochlea has been turned slightly, and the middle ear muscles have been omitted to make the relationship clear.

Sound waves converge through the pinna and outer ear canal to strike the tympanic membrane.

Schematic view of the ear. As sound waves hit the tympanic membrane, the position of the ossicles (which move as shown in blue and black) changes.

The vibrations of this membrane are transmitted by way of three ossicles (malleus, incus, and stapes) in the middle ear to the oval window, where the sound waves are transmitted to the cochlear duct.

Two small muscles can affect the strength of the auditory signal: the tensor tympani, which attaches to the eardrum, and the stapedius muscle, which attaches to the stapes. These muscles may dampen the signal; they also help prevent damage to the ear from very loud noises.

The inner ear contains the organ of Corti within the cochlear duct. As a result of movement of the stapes and tympanic membrane, a traveling wave is set up in the perilymph within the scala vestibuli of the cochlea. The traveling waves propagate along the cochlea; high-frequency sound stimuli elicit waves that reach their maximum near the base of the cochlea (ie, near the oval window). Low-frequency sounds elicit waves that reach their peak, in contrast, near the apex of the cochlea (ie, close to the round window). Thus, sounds of different frequencies tend to excite hair cells in different parts of the cochlea, which is tonotopically organized.

Cross section through one turn of the cochlea.

The human cochlea contains more than 15,000 hair cells. These specialized receptor cells transduce mechanical (auditory) stimuli into electrical signals.

The traveling waves within the perilymph stimulate the organ of Corti through the vibrations of the tectorial membrane against the kinocilia of the hair cells. The mechanical distortions of the kinocilium of each hair cell are transformed into depolarizations, which open calcium channels within the hair cells. These channels are clustered close to synaptic zones. Influx of calcium, after opening of these channels, evokes release of neurotransmitter, which elicits a depolarization in peripheral branches of neurons of the cochlear ganglion. As a result, action potentials are produced that are transmitted to the brain along axons that run within the cochlear nerve.

Structure of hair cell. (Reproduced with permission from Hudspeth AJ: The hair cells of the inner ear. They are exquisitely sensitive transducers that in human beings mediate the senses of hearing and balance. A tiny force applied to the top of the cell produces an electrical signal at the bottom, Sci Am Jan;248(1):54–64, 1983.)

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Certainly! Let’s explore the anatomy of the vestibulocochlear nerve, also known as the eighth cranial nerve (CN VIII). This nerve plays a crucial role in transmitting sound and equilibrium information from the inner ear to the brain 1.

Origin and Components:
- The vestibulocochlear nerve consists of two divisions:
  - Vestibular root: Gives rise to the vestibular nerve.
  - Cochlear root: Gives rise to the cochlear nerve.
- These roots emerge from the vestibular and cochlear nuclei located in the brainstem.
- When the vestibulocochlear nerve reaches the inner ear, it splits into the vestibular and cochlear parts, supplying target tissues within the inner ear.
Functional Components:
- Vestibular Component: Responsible for balance and spatial orientation.
  - Superior vestibular nucleus (Bechterew): Part of the vestibular component, involved in balance and muscle tone modification.
  - Lateral vestibular nucleus (Deiters): Another nucleus within the vestibular component 2.
- Cochlear Component: Associated with hearing.
  - Dorsal and ventral cochlear nuclei: Located in the dorsolateral upper medulla, deep to the lateral angle of the rhomboid fossa 3.

In summary, the vestibulocochlear nerve is a fascinating structure that connects our inner ear to the brain, allowing us to perceive sound and maintain balance. If you’d like more detailed information or have any other questions, feel free to ask! 😊👂🧠