Twenty Seven Flashcards
What are the 3 parts of the ear including subparts?
The external ear includes the auricle or pinna that gathers sound waves, the external
acoustic meatus that amplifies sound waves, and the tympanic membrane or ear drum
that vibrates when struck by sound waves. The middle ear or tympanic cavity contains
the auditory ossicles and muscles (malleus (tensor tympani muscle), incus, stapes (stapidus muscle)) that transmit vibrations from the tympanic membrane to the
internal ear. The internal ear contains the cochlea, within which is the cochlear duct.
What are the parts located within the cochlear duct that sense hearing? What cells are located there? What are the two types of neuroepithelial cells? What overlies them? How are they excited? What are they innervated by? Which are more innervated and thus become the primary auditory receptors? What do they others do?
Within the cochlear duct is the spiral organ (of Corti) which contains the receptor
cells for hearing. The spiral organ rests on the basilar membrane. Histologically the
spiral organ is very complex, but fundamentally there are only two types of cells:
neuroepithelial hair cells and supporting cells.
The neuroepithelial cells are columnar in shape and are of two types: type I, the inner
hair cells arranged in a single row, and type II, the outer hair cells which are arranged in
three rows at the base of the cochlea and four or five rows at the apex. Overlying the hair
cells is the gelatinous tectorial membrane in which the stereocilia (hairs) are embedded.
Vibration of the basilar membrane bends these hairs causing excitation of their cells.
The hair cells are innervated by the primary auditory neurons in the spiral ganglion.
The vast majority of these primary neurons supply the inner hair cells on a 1:1 synaptic
relationship. The outer hair cells are supplied by very few spiral ganglion cells, and on a
1: 10 synaptic relationship. Hence, the inner hair cells are the chief auditory receptors,
while the outer hair cells appear to play a role in modulation of the spiral organ.
What is the bony cochlea filled with? What is the membranous cochlea filled with? What are the two chambers inside the bony cochlea? Which membranes separate them from the membranous cochlea? Where do they meet?
Perilymph
Endolymph
Scala tympani on the outside
Scala vestibuli on the inside
They meet in the center at the helicotroma
The scala vestibuli is separated by the vestibular membrane, and the scala tympani by the basilar membrane on which the hair cells sit.
What are hair cells innervated by? What kinds of cells are they? What do their axons form? Where do they enter the cranial cavity? At which height do they reach the brainstem?
The peripheral processes of the bipolar cells of the spiral ganglion make synaptic
contact with the hair cells in the spiral organ. Their central processes form the cochlear
division of CN VIII which enters the cranial cavity through the internal acoustic meatus
and reaches the brainstem in the cerebellopontine angle.
Where does the cochlear nerve innervate? Where are they? What are the two separations? Where do the fibers go from there? Where do they synapse?
Upon entering the brainstem the cochlear nerve fibers bifurcate and enter the cochlear nuclei. These nuclei hang upon the inferior cerebellar peduncle like saddle bags, with the dorsal cochlear nucleus posterior and the ventral cochlear nucleus anterior. The efferent fibers of second order neurons in the cochlear nuclei form ventral, intermediate, and dorsal acoustic striae. The ventral acoustic stria courses rostro-medially and becomes incorporated in the trapezoid body which crosses to the
contralateral side. Upon reaching the ventrolateral part of the pontine tegmentum, fibers of the trapezoid body turn rostrally and become the lateral lemniscus. The hardly
noticeable dorsal and intermediate acoustic striae also pass rostro-medially, decussate, and join the contralateral lateral lemniscus.
The lateral lemniscus ascends to the inferior colliculus where its fibers synapse.
Where do the auditory fibers pass from the inferior colliculus?
Axons from the auditory relay neurons in the inferior colliculus form the brachium of the
inferior colliculus which passes along the lateral surface of the midbrain to the medial geniculate body or nucleus.
Where do the axons pass from the medial geniculate nucleus?
The medial geniculate nucleus is the highest subcortical auditory center. After receiving the auditory impulses from the brachium of the inferior colliculus it gives rise to thalamocortical projections, the geniculotemporal tract or auditory radiation. This tract travels through the sublenticular part of the posterior limb of the internal capsule to reach the primary auditory cortex.
Where is the primary auditory cortex located? Where are the high tones localized to? Low tones? Where do they receive their input?
The primary auditory cortex is located in the transverse temporal gyri (of Heschl). These gyri are on the dorsal surface of the superior temporal gyrus and are buried in the floor of the lateral fissure. Tonotopic localization exists in the auditory cortex: high tones posteromedially and low tones anterolaterally.
What are 3 accessory auditory nuclei? Where are they located? Where do they send fibers? How is bilaterality achieved?
Three groups of nuclei are found along the auditory pathway between the cochlear nuclei
and the inferior colliculus. These are the superior olivary nucleus, the nucleus of the
trapezoid body, and the nuclei of the lateral lemniscus.
The superior olivary nucleus is located lateral to the trapezoid body. It receives
collaterals of the ipsilateral and the contralateral secondary cochlear fibers and it gives
rise to fibers that join the ipsilateral and contralateral lateral lemnisci.
The nucleus of the trapezoid body is scattered among the trapezoid bundles and its
afferent and efferent connections are similar to those of the superior olive.
The nuclei of the lateral lemniscus are located in and adjacent to the lateral lemniscus.
They receive lemniscal fibers or their collaterals and give axons to both the ipsilateral
and the contralateral lateral lemnisci.
Thus, the bilateral representation of auditory impulses has as its anatomical basis
decussations that occur at four levels. In the caudal part of the pons, the superior
olivary nuclei and to a lesser extent the trapezoid nuclei, relay auditory impulses from
both ears to both lateral lemnisci. Thus, each lateral lemniscus transmits impulses from
both ears. Additional bilaterality occurs through decussations emanating from the nuclei
of the lateral lemnisci in the rostral pons and isthmus and from the inferior colliculi in
the midbrain. The latter occur through the commissure of the inferior colliculus.
How is sound reception modulated? How do the reflexes involving the tensor tympani muscle and stapidus muscle occur? What are these muscles innervated by?
Reciprocal connections between the various central auditory nuclei ensure modulation of
the ascending auditory impulses. Thus, the auditory cortex sends axons back to the
medial geniculate nucleus and inferior colliculus. The latter, as well as the lateral
lemniscus and superior olivary nuclei, send fibers to the cochlear nuclei. Moreover, the
efferent olivocochlear bundle which arises from neurons in the superior olivary and
trapezoid nuclei, as well as the adjacent reticular formation, terminates on the outer hair
cells of the spiral organ and on the afferent terminals innervating them. This auditory
feedback system provides a mechanism for the auditory sharpening associated with
selective attention to certain sounds.
Reflex control of the tensor tympani and stapedius muscles occurs through the motor
trigeminal and facial nuclei, respectively. Both receive input from the superior olivary
and trapezoid nuclei via the reticular formation.
Flinching of the body, turning the head, and closure of the eyes in response to a startling
sound occurs via auditory connections with the superior colliculus and reticular formation
which then connect with the appropriate motor nuclei.
What is conduction deafness? How does it occur? Why is it never complete or total? What is sensorineural deafness? How can it occur? What do unilateral lesions above the cochlear nuclei result in?
Conduction deafness results from any interference with the passage of sound waves through the external or middle ear (air-ossicular route). Bone conduction (transmission of sound through the cranial bones) can occur and, therefore, conduction deafness is never complete or total.
Nerve deafness (perception deafness) results from damage to the receptor cells of the spiral organ or to the cochlear nerve. The defect or damage is in the portion of the auditory mechanism common to both air and bone conduction and, therefore, there is a hearing failure or loss in both routes. The degree of hearing loss is, of course, related to the amount of damage to the spiral organ or nerve.
Unilateral lesions in the spiral organ, spiral ganglion, cochlear nerve, or cochlear nuclei result in deafness in the ipsilateral ear. Due to the bilaterality of auditory
impulses as they ascend in the brainstem, unilateral lesions in the pathway beyond the cochlear nuclei cause few, if any, symptoms. Large unilateral lesions of the auditory cortex mainly impair the ability to localize the direction from which sounds reach the contralateral ear.
