Auditory System

Ascending Auditory Pathways

Overview: spiral ganglion (organ of corti) Þ cochlear nuclei Þ superior olivary nucleus Þ (via lateral lemniscus) Þ nucleus of the lateral lemniscus Þ inferior colliculus Þ medial geniculate nucleus Þ auditory cortex in the temporal lobe

Note: there are extensive decussations from the level of the superior olivary nucleus to the inferior colliculus
there are no decussations at the level of the medial geniculate nucleus
ends at Superior Temporal Gyrus (there is a great deal of redundancy so each cochlea is bilaterally represented in brain)
There are several processing stations, or nuclei, along the pathway from the cochlear nuclei to auditory cortex

at each processing level (nucleus), fibers entering may do one of four things:

(1) Synapse on cells in that nucleus
(2) Decussate to the opposite nucleus and synapse in the contralateral nucleus
(3) Pass through the nucleus without synapsing
(4) Decussate to the opposite nucleus, pass through that contralateral nucleus without synapsing and continue to ascend on the contralateral side

The important nuclear groups that comprise the brainstem auditory pathways are: Superior olivary nuclei; Nuclei of the lateral lemniscus; Inferior colliculi; Medial geniculate nuclei of the thalamus
memory key: SLIM = Superior olivary, Lateral lemniscus, Inferior colliculus, Medial geniculate nucleus

(1) Spiral Ganglion (organ of corti)

spiral ganglion within cochlea Þ bipolar cells (~30,000 in spiral ganglion): dendrites in hair cells of organ of Corti in the cochlea; axons into CNS

hair cells have stereocilia (< 1 micron in diameter) – cells are extremely sensitive (can detect very minute movements)

inner hair cells (1 row): (1 to 1 relationship) 1 hair cell innervated by a single fiber from spiral ganglion

1 hair synapses on several bipolar cell dendrites and 1 bipolar cell is innervated by only 1 inner hair cell

outer hair cells (3 rows): (both spread and collapse) many hair cells innervated by a single fiber

involved in sensing presence and intensity of sound Þ precise freq. distribution of sound you’re hearing

each hair cell synapses on several dendrites from several bipolar cells and an individual bipolar cell can synapse on several outer hair cells

afferent fiber system – fine tunes the sensitivity of the hair cells

Cochlea – adjacent to semicircular canals Þ movement of tectorial membrane

controlled by acoustic energy from scalar media due to oval window
round window is a pressure release system – enables sound to be tuned, and reverberation of the stalks

(2) Cochlear nuclei (Contain the Secondary Neurons in the Auditory Pathway)

cochlear nuclear complex

located on lateral surface of inferior cerebellar peduncle (restiform body) in rostral medulla and caudal pons

composed of dorsal and ventral cochlear nuclei

ventral nucleus is larger and subdivided into anteroventral and posteroventral cochlear nuclei

ventral and dorsal cochlear nuclei can ascend ipsilateral or contralateral Þ depends if they travel to med.lemniscus

auditory nerve fibers: enter ventral cochlear nucleus and bifurcate into one branch traveling rostral into anteroventral cochlear nucleus and the other branch traveling caudal into posteroventral division then into dorsal cochlear nucleus

Within the cochlear nuclear complex: cells respond maximally to different aspects of sound (ex: some fire maximally at the onset, others when the sound stops, and others only if the sound is of constant intensity)

the functional organization of cells responding to different aspects of sounds is not completely known

tonotopically organized: high frequency fibers synapse in dorsal aspect of ventral nucleus, fibers that respond maximally to low frequency sounds synapse in ventral portion of ventral nucleus

tumors of 4^th ventricle early on produce problems in hearing and balance since CN 6,7,8 arise near the 4^th ventricle

(3) Superior Olivary Nucleus: Each superior olivary nucleus receives input from both cochlear nuclei and appear to be involved in localizing sound.

the cells in the medial superior olivary nuclei appear to respond to differences in the time of arrival of auditory stimuli from the cochlear nuclei and so may be sensitive to the slight difference in the time that sound takes to reach the ear nearer to the sound source compared to the distal ear
cells in the lateral superior olivary nuclei are tonotopically organized so that a given cell will respond to sound of a given frequency; firing frequency of the superior olivary nucleus cell will depend on where the sound is in space

(4) Nucleus of the Lateral Lemniscus

the major fiber pathways that connects the auditory processing nuclei in the brainstem are the:

(a) trapezoid bodies: contain fibers from cochlear nuclei that decussate and either synapse in the superior olivary nucleus or travel rostrally in the lateral lemniscus

(b) lateral lemniscii: fiber pathways that ascend from the level of the superior olivary nuclei to the inferior colliculi

the fibers in the lateral lemniscus rostral to nuclei of lateral lemniscus all synapse in inferior colliculus

the cells in the inferior colliculus are tonotopically arranged

if cut lateral lemniscus Þ no deafness, but lose ability to discern localization of sound and/or freq distribution

(5) Inferior Colliculus

Outputs from the inferior colliculi mostly synapse in the ipsilateral medial geniculate nucleus, some cross to the contralateral medial geniculate nucleus.

(6) Medial Geniculate Nucleus

Posterior aspect of thalamus Þ med. geniculate (auditory processing; input from inf. collicular nucleus) and lateral geniculate (visual)

cells in the medial geniculate nucleus are tonotopically arranged
medial geniculate nucleus is the thalamic projection nucleus for the auditory system – similar to the lateral geniculate for vision and the ventral posterior nucleus (VPL, VPM) for somatic sensation

The medial geniculate nuclei project in the posterior limb of the internal capsule to the ipsilateral superior temporal gyrus – primary auditory cortex (Brodmann’s areas 41 and 42)

(7) Auditory Cortex in the Temporal Lobe

The cells in auditory cortex are organized in columns that maximally respond to bilateral auditory input or to auditory stimulation of the ipsilateral or contralateral ear

Auditory cortex (superior posterior temporal gyrus) Þ involved with speech and language Þ overlaps with Wernicke’s area

Descending auditory pathways (Pathways from Cortex to Periphery)

modulate the transmission of auditory info (allow you to focus on sound of a particular type or from a particular region)

cortex Þ medial geniculate nuclei Þ inferior colliculus

inferior colliculus Þ superior olivary nucleus

inferior colliculus Þ cochlear nuclei

nucleus of the lateral lemniscus Þ superior olivary nucleus (primrily excitatory)

nucleus of the lateral lemniscus Þ cochlear nuclei

superior olivary nucleus Þ cochlear (inhibits hair cells)

Efferent Connections from Auditory Cortex may play a Role in the Ability to Focus on Specific Sounds (ex: one instrument in an orchestra, one person speaking amongst a group of people)

auditory cortex projects to the medial geniculate nucleus and the inferior colliculus

inferior colliculi cells project back to the cochlear nuclei

cells near the sup. olivary nucleus give rise to the olivocochlear bundle which project to and inhibit the cochlea hair cells

primary auditory afferent fibers arising from bipolar cells of origin of the olivocochlear bundle have not been delineated

Superior olivary nuclei Þ regulate info passing through it so an important gate in auditory pathway system Þ turn up or turn down the gain to the hair cells (via efferent of 8^th cranial nerve) – turn on one area of the cochlea or turn off others so you can focus on one particular sound of an orchestra

Effects of Lesions in the Central Nervous System on Sound Perception

deafness is rare and requires a bilateral lesion in the brainstem or a lesion of both auditory cortices

a lesion that involves the posterior portion of the temporal lobe of the dominant hemisphere can produce a condition of "word deafness" or "auditory agnosia for speech" in which the patient can hear sounds and even differentiate tones of different frequencies, but cannot understand spoken language

syndrome of pure word deafness may result from disruption of the connections from the auditory cortices to Wernicke’s area so that auditory information does not get to a language processing area

lesions of one auditory cortex can also interfere with localization of sound in space on the contralateral side of the body

Brainstem Reflex Pathways

probably not involved in conscious perception of sound (ie are not part of the projections to auditory cortex)

tympanic membrane energy transfered to incus/malleus/stapes Þ to oval window (affects how effectively sound is transferred to cochlea (as ossicles contract – efficency to transmit sound diminishes)

stapedius (facial nerve) and tensor tympani (innervated by trigeminal nerve) – takes while for these muscles to turn down in order to increase the efficiency of sound transmission – doesn’t turn on quickly, doesn’t turn off quickly

Stapedius reflex (actually stapedius and tensor tympani reflex)

Pathways:

(1) reticular formation Þ CN VII nucleus Þ stapedius muscle

(2) reticular formation Þ motor nucleus of V Þ tensor tymapani muscle

Action: 8^th nerve stimulus Þ contraction of stapedius muscles Þ reduces acoustic efficiency of middle ear ossicles

Purpose: Not clearly known; at first this was thought to be a reflex mechanism to protect the ear from high intensity sounds, but it is not effective for the higher frequencies

reflex probably serves to reduce level of acoustic stimuli reaching brainstem when ambient level of sound is high
also descending input to superior olivary nucleus may be able to modify this response so that this pathway may contribute to the ability to reduce auditory input and focus on specific sound patterns

can measure stapedious reflex Þ measures integrity of transmission of impulse thru the reflexes (put beep in left ear, put probe in right ear and see if tympanic membrane stiffens to decrease efficiency of sound transmission (stapedius reflex test: measures reflex of tensor tympani muscle)

Alerting sound (tectobulbar and tectospinal tracts)

Pathway: brainstem reticular formationÞ nucleus of inferior colliculus Þ midbrain tectum Þ tectobulbar and tectospinal tracts

cells of origin located on both the superior and inferior colliculi
fibers cross over to the opposite side of brainstem in the dorsal tegmental decussation and then descend in a pathway that is close to the medial longitudinal fasciculus (MLF)
fibers project to several brainstem structures including the abducens nucleus, pontine paramedian reticular formation, nucleus ambiguous, and the anterior gray matter of the cervical spinal cord including the cells of origin of CN XI

Action: believed to serve in arousal to sound (as to your alarm clock) and in head and eye turning toward sound

alerting to sound: see above path (visual stimuli also innervate tectospinal tract) – when startled: increases sensitivity of motor neuron pools so you can react to stimuli much better; can see/hear better) begin in superor colliculus for sight and inferior col for sound

Evoked Responses

EEG tenchnique Þ represent repeated stimulus (average over 1000 times); will be left with signal associated with stimulus

early wave (occur within 1^st 10 msec) – associated with different regions of brainstem (electroactivity of acoustic nerve; after that is cochlear nucleus; after this point the rest of pathway is not precise, can go back and forth, great deal of crossing and processing so not a direct pathway – but have a major route)

later waves – cortical activity after cortex is activated Þ more processing going on; processing in cortex varies depeding on the type of stimulus (boop or bleep), cortex can modulate its own activity and feed back down

Tumor – Acoustic neuroma Þ tumor of 8^th nerve

perception of sound when in fact there is not (tinnitis) Þ starts in vestibular side so perception of movement

as it grows it damages vestibular fibers Þ people usually become deaf (but occurs at such a slow rate and other ear compensates, so often not aware of it)

effect on facial nerve: get weakness of facial muscles on one side; rare for it to advance to this stage

may even enlarge to hit 5^th; sensation to face, motor innervation to jaw and tensor tympani (very rare)

can hit 9^th and 10^th Þ swallowing and larynx/pharynx sensations damaged; never seen it happen but hypothetically possible