A&P Inner Ear - Cochlea

3 Parts of Osseous Labyrinth

• 3 Semicircular canals

• Cochlea

• Vestibule

3 Semicircular Canals

Superior/anterior, lateral/horizontal, posterior/inferior

• sensory receptor for balance/equilibrium

Cochlea

• sensory receptor for hearing

• cochlea spirals about 2 &3/4 - turns around a central core of bone called the modiolus

Vestibule

• central portion of the labyrinth

• semicircular canals and cochlea open at either end into the vestibule

Vestibule

• central portion of the labyrinth

• semicircular canals & cochlea open at either end into the vestibule

Cochlea structure

3 scalae (fluid filled canal)

• Scala vestibuli

• Scala media

• Scala tympani

Osseous spiral lamina

body shelf extending from the modiolus, spiraling up the cochlea

Reissner’s membrane

• seperates S.Vestibuli from S. Media

  • runs the entire length/spiral of the cochlea

  • reaches diagonally from near the osseous spiral lamina to the outer cochlear wall

Basilar membrane

• separates S. Media from S. Tympani

  • stretches from the osseous spiral lamina to the outer wall of the cochlea

  • held in place by the spiral ligament

Scala vestibuli

• filled with perilymph

• base is at oval window articulating with the middle ear

• runs up and down the cochlear spiral until it reaches the helicotrema (located at apex of the cochlea)

• at the helicotrema, the scala vestibuli meets the scala tympani

Scala Media

• contains the organ of corti

• filled with endolymph

  • continuous with the vestibular portion of the inner ear via the ductus reuniens

• between and essentially seperates the S. vestibuli and the S. tympani

  • aka cochlear duct/partition

Organ of corti

• the end organ of hearing (sits on the basilar membrane)

• 3 rows of outer hair cells, 1 row of inner hair cells

Tunnel of corti

• also known as corti’s arch

• supportive structure on one end of the organ of corti

• formed by 2 pillars which lean toward eachother to form a triangle (seperates the inner and outer hair cells)

Stria vascularis

• vascular strip the lies along the outer wall of the scala media

• resposible for secretion and absorption of endolymph

• supplies oxygen/nutrients to organ of corti

Spiral ligament

• thickened lateral portion of the cochlear duct

• forms a spiral band along the outside edge of cochlear spiral

• connects to the basilar membrane

Tectorial membrane

• sits above organ of corti

• tips of the steriocilia of the outer hair cells (only) are embedded in the underside of this membrane

Outer hair cells of cochlea

• 3 rows

• have stereocilia protruding from top

• ohc ARE imbedded in the underside of the tectorial membrane

• lateral to tunnel of corti

• 20,000 in each ear

• sensitive to noise and ototoxic medications

• test tube shaped

• “cochlear amplifier” - source for otoacoustic emissions (OAEs)

Inner hair cells

• 1 row

• have steriocilia protruding from the top

• IHC steriocilia NOT embedded in tectorial membrane

• medial to tunnel of corti

• 3,500 in each ear

• flask shaped

• actual transducers of hydraulic energy to electrochemical (nerve) energy

Scala tympani

• runs along “other” side of cochlea

• ends at round window

• filled w/perilymph

• meets S. vestibuli at helicotrema

• the base is at the round window forming a connection point with the middle ear

Tonotopic organization of cochlea

• for every frequency ear is able to percieve, there is a specific place in the cochlea that is sensitive to that frequency (what we percieve as pitch)

• basilar membrane widens as it goes from base to apex of cochlea

  • causes elasticity variance (stiffer at base, floppier at apex)

  • each stiffness level allows maximum displacement by a certain frequency

Traveling wave theory

• mechanical energy from the stapes transduces to hydraulic energy as it pushes against oval window

• a wave is created in the perilymph of the S. vestibuli, and travels up then crosses the basilar membrane (or Reissner membrane) at the point of maximum displacement

• only that point of maximum displacement is stimulated and a neural message is sent to the central nervous system

• energy then travels back down the s. tympani and exits out the round window (pressure relief valve) and goes back to middle ear (we dont perceive this)

Shearing action

• the mechanical bending of the cilia of the hair cells on the organ of cori by the tectorial membrane and fluids in the cochlea move (traveling wave)

  • stimulates hair cells and causes them to fire

    • discharge creates an action potential

    • firing of the hair cells= transduction from hydraulic to electrochemical energy through neurological impulses

Current theory of frequency (pitch) perception

• uses place theory (explained by traveling wave theory) for high frequencies

• uses frequency or the rate of firing (explained by temporal theory) for low frequencies

• uses both theories for mid frequencies

current theory of loudness perception

• different neurons with different thresholds of activation

  • low threshold neurons process lower intensity sounds

  • high threshold neurons process higher intensity sounds

current theory of temporal perception

• time-intensity trade to maintain audibility

• lower intensity sounds need to be played longer (more time) to be audible

• higher intensity sounds can be played for a shorter amount of time and still be audible

CN VII: Vestibulocochlear Nerve

• transmits information to the brain

• bundle of nerve fibers consisting of

  • vestibular nerves (maintain balance and related eye movements)

  • cochlear nerves (hearing)

Auditory Pathway: Ear to Brain

• VIII Nerve: recieves signal from cochlea (sensory only-afferent pathway)

• Cochlear nuclei- refines coding done by the cochlea (frequency and intensity)

• Superior olivary complex - receive information from both ear (important for localization, acoustic reflex, more frequency and intensity coding)

• Inferior colliculi - frequency and duration coding

• Medial geniculate nucleus- frequency and intensity coding; supports localization

• Primary auditory cortex- process complex signals