Mechanoreception – Audition and Equilibrium

Hearing and Equilibrium Figure 10.37

The Middle Ear

Inner ear Membranous labyrinth contains endolymph Bony labyrinth surrounds and protects membranous labyrinth Vestibule Semicircular canals Cochlea

The Inner Ear

Components of the inner ear Vestibule contains the utricle and saccule Semicircular canals contain the semicircular ducts Cochlea contains the cochlear duct

Windows Round window separates the perilymph from the air spaces of the middle ear Oval window connected to the base of the stapes Basic receptors of inner ear are hair cells Provide information about the direction and strength of stimuli

Hearing Sounds are waves of compressed air traveling through space - sound intensity wave height - pitch  wave frequency

Organ of hearing (and equilibrium) – inner ear Cochlea Vestibular apparatus

Hearing 1- 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

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

The Organ of Corti

Hearing Cochlear duct lies between the vestibular duct and the tympanic duct Hair cells of the cochlear duct lie within the Organ of Corti Intensity is the energy content of a sound Measured in decibels

The Cochlea

Pathway of Sound Sound waves travel toward tympanic membrane, which vibrates Auditory ossicles conduct the vibration into the inner ear Tensor tympani and stapedius muscles contract to reduce the amount of movement when loud sounds arrive Movement at the oval window applies pressure to the perilymph of the cochlear duct Pressure waves distort basilar membrane Hair cells of the Organ of Corti are pushed against the tectoral membrane

Sound and Hearing

Sound and Hearing

Neural pathway Sensory neurons of hearing are located in the spiral ganglion of the cochlea Afferent fibers form the cochlear branch of cranial nerve VIII Synapse at the cochlear nucleus

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

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

Coding for sound intensity

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?

Equilibrium Anterior, posterior and lateral semicircular ducts are continuous with the utricle Each duct contains an ampulla with a gelatinous cupula and associated sensory receptor Saccule and utricle connected by a passageway continuous with the endolymphatic duct Terminates in the endolymphatic sac Saccule and utricle have hair cells clustered in maculae Cilia contact the otolith (statoconia)

Equilibrium Ability to detect head position and movement (or acceleration) Change of speed = linear acceleration (utricle and saccule) Turning = rotational acceleration (semi-circular canals)

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

The Vestibular Complex

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 …)

The Vestibular Complex

The Vestibular Complex

Vestibular Neural Pathway Vestibular receptors activate sensory neurons of the vestibular ganglia Axons form the vestibular branch of cranial nerve VII Synapses within the vestibular nuclei

Pathways for Equilibrium Sensation

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