the auditory and vestibular systems

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The Auditory and Vestibular Systems

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The Auditory and Vestibular Systems. I. Functional Anatomy of the 2 Systems - Overview Parallel ascending auditory pathways. Ascending vestibular system. II. Regional Anatomy Sensory organs 1. Auditory receptor cells and apparatus. 2. Vestibular sensory organs. B. Brainstem nuclei - PowerPoint PPT Presentation

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Page 1: The Auditory and Vestibular Systems

The Auditory and Vestibular Systems

Page 2: The Auditory and Vestibular Systems

I. Functional Anatomy of the 2 Systems - OverviewA. Parallel ascending auditory pathways.B. Ascending vestibular system.II. Regional AnatomyA. Sensory organs

1. Auditory receptor cells and apparatus.2. Vestibular sensory organs.

B. Brainstem nuclei1. Vestibular and its projections.2. Auditory and its projections.a. Lateral lemniscusb. Inferior colliculusc. Medial geniculate nucleusd. Primary auditory cortex.e. Wernike’s Area – a higher-order auditory cortex.

Page 3: The Auditory and Vestibular Systems

Auditory System

Page 4: The Auditory and Vestibular Systems

I. Functional Anatomy of the 2 Systems - Overview

A. Auditory system.• Tonotopic basis for frequency coding

along the length of the cochlea.• Tympanic membrane transduced info

into neural signals.(next slides)

Page 5: The Auditory and Vestibular Systems

II. Regional Anatomy: Hearing - Equilibrium

Page 6: The Auditory and Vestibular Systems

Hearing

• Sounds are waves of compressed air traveling through space

- sound intensity wave height - pitch wave frequency

Page 7: The Auditory and Vestibular Systems

Organ of hearing (and equilibrium) – inner ear

• Cochlea

• Vestibular apparatus

Page 8: The Auditory and Vestibular Systems

Sensory Organs: Hearing1- The sound waves enter the

external auditory canal and trigger vibrations of the tympanic membrane

2- The tympanic membrane induces a vibration of the ossicles

3- the last ossicle, the stapes, transmits amplified vibrations to the oval window

4- The vibrations induce waves in the perilymph of the various inner ear chambers

5- the round window absorbs excess energy and prevent wave reverberation

6- the fluid wave is transduced into an electrical signal by the auditory receptors, the organs of Corti located on the basilar membrane

Page 9: The Auditory and Vestibular Systems

Receptors for sound: the organ of Corti

• The hair cells of the organ of Corti transduce fluid wave into an electrical signal

• The energy of the wave causes the basilar and vestibular membrane to move, thus displacing the cilia from the organ of Corti

Page 10: The Auditory and Vestibular Systems
Page 11: The Auditory and Vestibular Systems

Organ of Corti

Page 12: The Auditory and Vestibular Systems

Signal transduction• Movements of the cilia open or

close potassium channels changes in the state of polarization of the hair cell

• Changes in potassium leakage due to cilia bending trigger changes in neurotransmitters exocytosis

• The neurotransmitters send an electrical signal to an afferent neuron of the cochlear nerve

• The louder the sound, the more the cilia bend, the more numerous are the APs produced

Page 13: The Auditory and Vestibular Systems

Coding for pitch• The location of the organs

of Corti on the basilar membrane codes for pitch

- Organs of Corti located near the oval window are more sensitive to high pitch sounds while the ones located toward the tip of the cochlea respond more readily to low pitch sound

Page 14: The Auditory and Vestibular Systems

Coding for sound intensity

Page 15: The Auditory and Vestibular Systems

Neural pathway for sounds

• Cochlear nerve nucleus in medulla oblongata thalamus auditory cortex in the temporal lobe

• So, how do we perceive the direction from which a sound is coming from?

Page 16: The Auditory and Vestibular Systems

B. Brainstem nuclei and their Projections

2. Auditory nuclei:3 major auditory relay nuclei of the brainstem:A. cochlear nuclei (same side from cn VIII) (medulla).B. superior olivary nuclear complex (integration from both sides) (pons).C. inferior colliculus (midbrain).

[2 major divisions we noted earlier].-superior olivary complex is important in sound

localization (major input from AV cochlear nucleus).

Page 17: The Auditory and Vestibular Systems

Auditory System Overview: Cochlear division ofNerve VIII cochlear n. (same side) in rostral medulla.1. *Anteroventral cochlear n. sup olivary n. (bothsides) lateral lemniscus inferior colliculus.*Important for horizontal location of sounds, as well as for other aspects of sound patterns, other than location.2. Dorsal posteriorventral cochlear n. lat lemniscusInferior colliculus (opp. side). Note the decussations: Important for integration.Clin: one can experience loss in only 1 ear only if lesion is peripheral.

Page 18: The Auditory and Vestibular Systems
Page 19: The Auditory and Vestibular Systems

B. Brainstem nuclei and their Projections

2. Auditory nuclei (cont’d):• Low-freq sounds are distinguished in space by interaural

time difference.• High-freq sounds are distinguished by difference in

intensity between the ears.• Different parts of the superior olivary n. (medial and

lateral) are sensitive to these 2 types of differences.• Decussation is visible in trapezoid body.• Feedback pathway: some superior olivary neurons

project back to the cochlea (both sides).

Page 20: The Auditory and Vestibular Systems
Page 21: The Auditory and Vestibular Systems

B. Brainstem nuclei and their Projections

2. Auditory nuclei (cont’d):• Olivocochlear bundle – regulates flow of auditory info

to the brain (much like inhibitory dorsal horn n. inhibit somatic sensory info.).

Lateral LemniscusMost auditory path neurons course in lateral lemniscus

inferior colliculus.Some synapse on nucleus of lateral lemniscus

contralateral inferior colliculus.Another important site of decussation (Probst’s

Commisure)

Page 22: The Auditory and Vestibular Systems

Probst’s commisure

Page 23: The Auditory and Vestibular Systems

B. Brainstem nuclei and their Projections

2. Auditory nuclei (cont’d):Inferior Colliculus• Within the midbrain tectum.• The central nucleus within the inf colliculus receives the

auditory info, which will proceed to the medial geniculate nucleus of the thalamus and the 1° auditory cortex.

• Laminated – neurons in a single lamina are maximally sensitive to similar tonal frequencies.

• Receives input from superior olivary n., n. of lateral lemniscus (both sides), and dorsal n. + pv cochlear (direct).

• Projects to thalamus through the brachium of the inferior colliculus.

Page 24: The Auditory and Vestibular Systems

B. Brainstem nuclei and their Projections

2. Auditory nuclei (cont’d):Medial geniculate nucleus• Thalamic auditory relay nucleus.• The major part (ventral division) is tonotopically

organized (receiving its input from the central n. of the inf coll, which is also tonotopically organized.

• Therefore, the MGN is also laminated – layers maximally sensitive to similar frequencies.

• Thalamocortical auditory projections are called auditory radiations 1° auditory cortex with 2 gyri within sulcus of temporal lobe: Heschle’s Gyri

Page 25: The Auditory and Vestibular Systems

B. Brainstem nuclei and their Projections

2. Auditory nuclei (cont’d):Medial geniculate nucleus• Columnar organization of neurons sensitive to

tones of similar frequencies (isofrequency columns).

• Also binaural columns (similar interaural intensity differences – for localization of high-frequency sounds).

• Like other 1° sensory cortices, this has a prominent layer 4.

Page 26: The Auditory and Vestibular Systems

Thalamic nuclei

Page 27: The Auditory and Vestibular Systems

B. Brainstem nuclei and their Projections

2. Auditory nuclei (cont’d):Wernicke’s Area• A higher-order auditory cortex for the

interpretation of language. (language on L side of brain; interpreting emotional content of language on R side of brain).

• One projection of Wernicke’s area is to Broca’s motor speech area in the frontal lobe.

Page 28: The Auditory and Vestibular Systems

Primary Auditory cortex

Page 29: The Auditory and Vestibular Systems

Higher order auditory cortices

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Auditory System Overview: Cochlear division ofNerve VIII cochlear n. (same side) in rostral medulla.1. *Anteroventral cochlear n. sup olivary n. (bothsides) lateral lemniscus inferior colliculus.*Important for horizontal location of sounds, as well as for other aspects of sound patterns, other than location.2. Dorsal posteriorventral cochlear n. lat lemniscusInferior colliculus (opp. side). Note the decussations: Important for integration.Clin: one can experience loss in only 1 ear only if lesion is peripheral.

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Vestibular System

Page 32: The Auditory and Vestibular Systems

Sensory Organs: Vestibular:Ampulae of semicircular canalsMaculae of utricle (linear acceleration) + sacculeEndolymph – gel-like fluid flows over the hair cells with movement

and deflects them – Ca carbonate crystals (otoliths)

Page 33: The Auditory and Vestibular Systems

Equilibrium

• Ability to detect head position and movement (or acceleration)

- Change of speed = linear acceleration (utricle and saccule)

- Turning = rotational acceleration (semi-circular canals)

Page 34: The Auditory and Vestibular Systems

Utricle and saccule

• Sensory cells have cilia extending into a gelatinous material topped by otoliths

• Saccule detects backward-frontward movement

• Utricle detects changes relative to gravity

Page 35: The Auditory and Vestibular Systems
Page 36: The Auditory and Vestibular Systems
Page 37: The Auditory and Vestibular Systems

Semi-circular canals

• The receptors in the ampulla are hair cells with cilia extruding into a gelatinous mass (cupula)

• When the head rotates, the cupula moves cilia pulled APs (vestibular nerve cerebellum …)

Page 38: The Auditory and Vestibular Systems
Page 39: The Auditory and Vestibular Systems

• So why does a person become dizzy after he/she stops spinning?

Page 40: The Auditory and Vestibular Systems

B. Brainstem nuclei and their Projections

1. Vestibular nuclei – on floor of 4th ventricle.4 nuclei: inferior, medial, lateral, superior ascending projections to VPN of thalamus 1° vestibular cortex in parietal lobes (just behind the 1° somatic sensory cortex).Can project to nearby parietal areas for integration of info regarding head motion with info from somatic sensory receptors in trunk and limbs.

Page 41: The Auditory and Vestibular Systems

Vestibular Nuclei:

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Page 43: The Auditory and Vestibular Systems

Vestibular SystemOverview:Head motion vestibular hair cell receptors 4 vestibular n. in rostral medulla and caudal pons:

2 Descending projections to sc and extraocular muscles (control movements) cerebellum.

2 Ascending projections VPN of thalamus 1° vestibular ctx in parietal lobe (for conscious awareness of orientation and motion).