Hearing and balance

Hearing and balance

The vestibulocochlear (VIII) cranial nerve has two components:

• The cochlear nerve, which carries information for hearing
• The vestibular nerve, which carries information used to maintain balance and posture

Both the auditory and vestibular sensory organs are located in the inner ear. They share some structural features, but they also have important functional differences.

Cochlear system: The ear is divided into the external, middle, and inner ear.

• The middle ear is air-filled and is separated from the external ear by the tympanic membrane (eardrum).
• The middle ear contains three bony ossicles: malleus, incus, and stapes.
• The footplate of the stapes fits into the oval window, which forms the boundary between the middle and inner ear.

The inner ear is fluid-filled and contains both a bony labyrinth and a membranous labyrinth.

• The bony labyrinth includes three semicircular canals: lateral, posterior, and superior.
• The membranous labyrinth includes ducts called the scala vestibuli, scala media, and scala tympani.

The fluids in these compartments differ:

• Perilymph (in the scala vestibuli and scala tympani) is similar to extracellular fluid.
• Endolymph (in the scala media) is high in potassium.

A spiral structure called the cochlea contains the organ of Corti, which is responsible for sound perception. The organ of Corti sits on the basilar membrane and contains two types of ciliated hair cells:

• Inner hair cells
• Outer hair cells

Their cilia are embedded in the tectorial membrane. Hair cells are tuned by location:

• Cells near the base of the basilar membrane respond best to high frequencies.
• Cells near the apex respond best to low frequencies.

When sound waves strike the eardrum, the tympanic membrane vibrates. This vibration is transmitted through the ossicles, pushing the stapes footplate into the oval window. That movement displaces inner ear fluid and produces vibrations that reach the organ of Corti, bending the hair cell cilia.

• Bending in one direction causes increased potassium efflux and hyperpolarization.
• Bending in the opposite direction causes increased potassium influx and depolarization.

Depolarization increases intracellular calcium, which triggers release of excitatory neurotransmitters and stimulates cochlear nerve endings.

Neurons in the spiral ganglion form the first-order neurons of the auditory pathway. These are bipolar neurons:

• Their shorter processes innervate the hair cells.
• Their longer axons enter the CNS as cochlear nerve fibres.

The cochlear nerve synapses in the ventral and dorsal cochlear nuclei in the medulla (second-order neurons). From this level onward, auditory fibres project both ipsilaterally and bilaterally.

• Some fibres from the ventral cochlear nucleus cross the midline and synapse in the superior olivary nucleus. These crossing fibres form the trapezoid body.
• Other fibres synapse in the ipsilateral superior olivary nucleus.
• Fibres from the superior olivary nucleus ascend to the inferior colliculus.
• Fibres from the dorsal cochlear nucleus ascend in the ipsilateral lateral lemniscus.

The lateral lemniscus carries ipsilateral and bilateral fibres and synapses with the superior colliculus and the medial geniculate body of the thalamus. Thalamic fibres then project to the primary auditory cortex in the superior transverse temporal gyrus, near the lateral (Sylvian) fissure.

Because the central auditory pathways have characteristic bilateral innervation, unilateral CNS lesions do not cause deafness, whereas local cochlear lesions can cause ipsilateral deafness.

Vestibular system: Vestibular receptors are located in:

• The semicircular canals, which detect angular (rotational) acceleration
• The otolith organs (utricle and saccule), which detect linear acceleration and gravitational forces

There are three semicircular canals - horizontal, superior, and posterior - arranged perpendicular to each other. The semicircular canals and otolith organs are filled with endolymph and surrounded by perilymph.

Vestibular hair cells have:

• A kinocilium (a single, thick cilium)
• Multiple stereocilia (smaller cilia)

In the semicircular canals, hair cells are located in the dilated ampulla at one end of each canal. They are embedded in a gelatinous cupula. The hair cells plus cupula form the crista.

In the utricle and saccule, hair cells are embedded in a gelatinous otolith mass.

Canal function relates to head movement:

• The horizontal canals respond to lateral head movement (for example, turning the head sideways).
• The superior and posterior canals respond to vertical head movements (moving the head up and down).

When the head rotates to the left:

• The left semicircular canals are excited (depolarized).
• The right semicircular canals are inhibited (hyperpolarized).

Head movement shifts the cupula, bending the hair cell cilia:

• If stereocilia bend towards the kinocilium, the hair cell depolarizes.
• If stereocilia bend away from the kinocilium, the hair cell hyperpolarizes.

In the utricle and saccule, movement of the otolith mass bends the cilia in the same way, producing the same depolarization/hyperpolarization pattern.

Vestibular nerve neurons are bipolar and located in the vestibular (Scarpa’s) ganglion. Their fibres synapse in the vestibular nuclei in the medulla.

• The medial and superior vestibular nuclei receive input from the semicircular canals and relay to cranial nerve nuclei III, IV, and VI via the medial longitudinal fasciculus.
• The lateral vestibular nucleus receives input from the utricles and relays to spinal motor neurons via the vestibulospinal tract.
• The inferior vestibular nucleus receives input from the semicircular canals, utricle, and saccule and relays via the medial longitudinal fasciculus to the cerebellum and brainstem.

The lateral and superior vestibular nuclei project to the ventral posterior nuclei of the thalamus. Thalamic projections end in the vestibular cortex (the parieto-insular cortex), which is under MCA supply.

Olfaction

Odorant chemicals activate olfactory receptors in the nasal epithelium. These olfactory receptors are primary afferent neurons, and their receptor proteins are located on their cilia.

• There are more than 1000 types of receptor proteins.
• These receptors are GPCRs that activate adenylyl cyclase, increasing cAMP.
• Increased cAMP opens sodium channels, causing sodium influx and depolarization of the receptor cell membrane.

This depolarization generates action potentials that travel along the olfactory nerve. Olfactory nerve axons are small and unmyelinated. They pass through the cribriform plate at the base of the skull to reach the olfactory bulb, which lies on the inferior surface of the frontal lobes.

In the olfactory bulb, mitral cells receive input from approximately 1000 olfactory receptor axons, forming a glomerulus-like structure. The mitral cell is the second-order neuron of the olfactory pathway.

Mitral cell axons form the olfactory tract, which projects to the cerebral cortex. The tract divides into:

• A medial tract that synapses in the anterior commissure and the contralateral olfactory bulb
• A lateral tract that ends in the primary olfactory cortex (the piriform cortex)

The piriform cortex is located inferomedially at the junction of the frontal and temporal lobes, near the entorhinal cortex. It is a well-known site of origin of focal epilepsy (temporal lobe epilepsy) characterized by an olfactory aura.

Taste

Taste receptors are located in taste buds (papillae). Different papillae are associated with different taste qualities:

• Fungiform papillae (tip of the tongue): salty, sweet, and umami
• Foliate papillae (sides of the tongue): mainly sour
• Circumvallate papillae (posterior tongue): mainly bitter

Taste transduction depends on the type of stimulus:

• Bitter substances activate GPCRs, increasing IP3 and DAG. This opens transient receptor potential (TRP) channels, causing depolarisation.
• Sweet and umami use a similar GPCR-based mechanism but with different GPCRs.
• Sour substances contain H+, which closes K channels on the receptor, causing depolarization.
• Salty substances contain Na, which enters directly through Na channels, causing depolarization.

Depolarization leads to action potentials in the nerve fibres innervating the taste receptors.

Taste afferent innervation is divided by region:

• Anterior ⅔ of the tongue: facial nerve
• Posterior ⅓ of the tongue: glossopharyngeal nerve
• Posteriormost tongue and epiglottis: vagus nerve

So, bitter taste is sensed by the vagus nerve, and so on.

Afferent fibres from these three nerves ascend in the ipsilateral brainstem as the tractus solitarius and synapse in the nucleus of the tractus solitarius in the medulla (second-order neurons of the gustatory pathway). Fibres from there project to the VPM nucleus of the thalamus, which then connects to the gustatory cortex in the insula and inferior frontal gyrus.