Anatomy and Physiology Exam 2

Bony labyrinth

-Hollow spaces in the petrous portion of the temporal bone

-the bony shell that contains the membranous labyrinth and all fluid compartments

Three regions of the bony labyrinth

Vestibule, semicircular canals, and bony cochlea

Vestibule

-Ovoid central chamber (~4 mm in diameter)

-leads anteroinferiorly to the cochlea and posterosuperiorly to the semicircular canals

-oval window opens into it, communicating directly with the scala vestibuli. the round window is just below and continuous with this passageway

-contains the utricle and saccule (membranous sacs)

Number of semicircular canals per ear

3: horizontal (lateral), superior (anterior), and posterior

Horizontal (lateral) canal

Lies in the horizontal plane; openings oriented anterior to posterior

Superior (anterior) canal

~Vertical plane; openings oriented medial to lateral

Posterior canal

~Vertical plane; openings oriented superior to inferior

Functionally paired semicircular canals

Horizontal–Horizontal, Superior–Posterior, Posterior–Superior (left and right sides)

Function of the 3 semicircular canals together

Define all dimensions of head rotation; planes of rotation match the extraocular eye muscles

Bony cochlea (shape and size)

-Spiral with ~2.75 turns in humans

-about the size of a flattened pea (~9 mm base width, ~5 mm high)

~35 mm long if uncoiled

Three bony components of the cochlea

Spiral cochlear canal, modiolus, and spiral lamina

Modiolus

Central bony axis (hub) around which the cochlear spiral tube is coiled; like the shaft of a wood screw but nearly hollow

Spiral lamina

-Thin bony shelf projecting from the modiolus and winding along the cochlear canal (like a screw thread)

-does not project all the way from its medial origin to the lateral edge of the bony cochlea (bridged by the basilar membrane)

-made of two plates through which nerve fibers travel

-fibers enter the spiral lamina through openings called the habenula perforata

-the basilar membrane attaches to its lateral edge. the basilar membrane is part of the membranous cochlea, which is completely contained within the bony cochlea.

Membranous labyrinth

-Membranous tubes "threaded into" the bony labyrinth of the vestibular and cochlear regions

-filled with endolymph, a fluid similar to perilymph except its ionic (chemical) composition

-two main divisions: cochlear (scala media) and vestibular (saccule, utricle, membranous semicircular canals)

Five major structures of the membranous labyrinth

Saccule, utricle, semicircular canals (ducts), scala media, organ of Corti

Saccule

Sensitive to linear acceleration in the vertical plane

Utricle

Sensitive to linear acceleration in the lateral (horizontal) plane

Membranous semicircular canals (ducts)

-Sensitive to angular acceleration (rotation)

-the enlarged area at the base of each canal is called the ampulla

Scala media

-Also called cochlear duct or cochlear partition

-endolymph-filled, pie-shaped coiled tube that is threaded around the modiolar spiral, partially dividing the bony cochlea into scala vestibuli (superior) and scala tympani (inferior)

Walls of the scala media

-Superior = Reissner's membrane

-inferior = basilar membrane

-lateral = stria vascularis

Scala vestibuli and scala tympani

-border the scala media

-contain perilymph, a fluid which “cushions” the membranous labyrinth above and below

-Continuous compartments. communicate at the helicotrema, located at the extreme apex of the bony cochlea

-as a result the scala media is a little shorter than the other scalae because it lies between them

Organ of Corti

-contains sensory epithelium of the auditory system

-also includes many other supporting structures seated on the basilar membrane (within the membranous cochlea)

4 connections between the labyrinths and middle ear/intracranial spaces

Oval window, round window, endolymphatic duct (vestibular aqueduct), perilymphatic duct (cochlear aqueduct)

Oval window

Opening from the tympanum (middle ear) to the scala vestibuli

Round window

Opening from the tympanum (middle ear) to the scala tympani

Endolymphatic duct (vestibular aqueduct)

-Bony channel linking the vestibule with the epidural space

-contains the endolymphatic sac, which is filled with endolymph

• endolymph exits the duct into the epidural space

-communicates with the utricle and saccule via the utricular and saccular ducts, and indirectly with the cochlea via the saccule

Perilymphatic duct (cochlear aqueduct / cochlear canaliculus)

-Bony channel linking the perilymphatic space of the basal turn of the cochlea with the subarachnoid space

-continuous with CSF

-in all normal temporal bones studied, the cochlear aqueduct was patent (open)

Basilar membrane (structure)

-Underlies the organ of Corti

-medial attachment = bony spiral lamina,

-lateral attachment = spiral ligament

-Narrower (0.1 mm) and thicker/stiffer at the base, wider (0.5 mm) and thinner/more flexible at the apex

-acellular plate of radially oriented fibers (type II collagen) in a connective matrix, not a true membrane

Reissner's membrane

Separates scala vestibuli from scala media; maintains the ionic difference between perilymph and endolymph

Stria vascularis

-Lateral wall of the scala media

-3 cell layers: basal, intermediate, marginal

Basal level of stria vascularis

border between other 2 layers and the spiral ligament

intermediate level of stria vascularis

contains capillary network, which supplies blood to much of the lateral portion of the scala media

Marginal cells of the stria vascularis

-Innermost layer bordering scala media

-single cell layer

• covered with microvilli (associated with secretion/absorption)

• secrete endolymph

-involved in maintaining the ionic composition of the endolymph by pumping potassium ions into the fluid and sodium ions out

• this function is critical to the normal physiology of the cochlea

Spiral ligament

-Connective tissue forming the lateral attachment of the basilar membrane to the outer bony wall of cochlea

-extends into perilymphatic spaces (scala tympani and vestibuli)

• pump K+ from perilymph spaces into endolymph

• adds support to the stria vascularis

-complimentary to spiral lamina (bony shelf) medially

Spiral limbus

-Periosteal projection on the superior surface of the bony spiral lamina

• similar tissue composition as spiral ligament

-medial attachment of the tectorial membrane

Tectorial membrane

-attaches medially to the spiral limbus

-attached laterally to the supporting cells of Hensen comprising the marginal net

-becomes broader toward the apex, almost filling the scala media at the apex

• similar to changes in width of the basilar membrane

-primary function is thought to be passive: provides resistance necessary to cause shearing of the stereocilia attached to or making contact with its inferior surface. only the tallest stereocilia are attached.

-the stereocilia of the inner hair cells are apparently not firmly attached, but fit loosely into Hensen’s stripe

composition of tectorial membrane

-gelatinous sac containing collagen II fibers and other protein filaments acellular… not a true membrane

-About 97% water. approximately the same density as the surrounding endolymph

Hensen's stripe

Raised groove on the inferior surface of the tectorial membrane where inner hair cell stereocilia fit loosely

Functional definition of the organ of corti

the structure that transduces hydromechanical (fluids, stereocilia) motion of neuroelectric pulses (action potentials) that can be conducted by central auditory structures

anatomical definition of the organ of corti

the spiral strip of sensory epithelia, supporting cells, and related structures seated on the basilar membrane, extending from the basal to apical end of the cochlear duct

Inner sulcus cells

-Innermost border (lining) of the inner spiral sulcus

-may support the inner hair cells medially

Inner phalangeal cells

Surround each inner hair cell, cupping the rounded base of each cell

Inner hair cells (IHCs)

~3,500 cells in a single row

- gourd or flask/wine-bottle shape, 4–8 µm long

-entirely encompassed by inner phalangeal cells (could be inner sulcus cells), except for the synaptic zones at the base

-superior surface embedded in the reticular lamina

Inner pillar cells (rods of Corti)

-Support IHCs laterally

-form the medial border of the tunnel of Corti

-superior process forms part of the reticular lamina

-body of the cells rests on the basilar membrane

Outer pillar cells (rods of Corti)

-Body rests on the basilar membrane

-club-shaped superior process forms part of the reticular lamina

-form the lateral wall of the tunnel of Corti

-the superior processes of the inner and outer pillars meet forming the section of the reticular lamina that bridges the tunnel of corti

Reticular lamina

-Layered structure formed by superior surfaces of hair cells (cuticular plates), their adjoining support cells, and connective tissue (fibers)

-forms a barrier to endolymph so only the superior surfaces of hair cells (including stereocilia) are in contact with the endolymph

• this separation is critical in maintaining the proper ionic concentration within the hair cell cytoplasm

• can be damaged by high levels of noise exposure

-adds rigidity to the organ of corti

Tunnel of Corti

-Triangular space formed by the pillar cells

-filled with cortilymph, which also surrounds the hair cell bodies

-nerve fibers that innervate the OHCpass between the pillar cells and across the tunnel of corti

Outer hair cells (OHCs)

-Only in mammals

~15,000 cells in 3 rows

-tubed shaped sensory cells, 20–70 µm long (longer than IHCs)

-supported primarily at their base by Deiters' cells

-superior surface forms part of reticular lamina

-tilted medially toward the stapes

Deiters' cells

“Outer phalangeal cells”

-support the base of a single OHC

-long thin phalangeal process extends superiorly to form part of the reticular lamina between adjacent rows of OHC

Hensen's cells

Several rows of tall supporting cells lateral to Deiters' cells; may be part of the lateral attachment of the tectorial membrane

Cells of Claudius

Not part of the organ of Corti; lie between Hensen's cells and the spiral ligament/stria; likely an ionic barrier between scala media and scala tympani

Why cochlear hair cells do not regenerate

The cochlea has no basal (undifferentiated) cells

Cuticular plate

Thickened cap on the top of the hair cell made of contractile proteins (actin, fimbrin, myosin, calcium-binding protein); stereocilia are rooted in it; helps position stereocilia

Kinocilium

Large stereocilium present next to the stereocilia in the vestibular system; NOT present in the mature cochlea

Stereocilia (structure)

Rigid, finger-like extensions (0.2–0.8 µm) with actin filament cores; pivot at the base (rootlets anchor to cuticular plate); ~20–50 per IHC (broad U), ~150 per OHC (W or V); open end faces medially

Stereocilia cross-links

Within-row links, between-row links, and tip links; make the bundle move en masse

Tip links

Link the tip of a shorter stereocilium to the side of a taller neighbor in the next row; involved in transduction (open/close K+ channels)

Excitatory direction of stereocilia deflection

Toward the tallest stereocilia (stereocilia are ~2x stiffer in this direction)

Most vulnerable part of the inner ear to mechanical trauma

Stereocilia

Cochlear gradients from base to apex

Tectorial and basilar membranes wider and thinner; OHC and IHC bodies longer; stereocilia taller and fewer; Deiters' and pillar cells taller; fluid spaces narrower; sensitivity to noise/toxins decreases

Afferent vs. efferent fibers

Afferents carry information from hair cells to the brainstem; efferents carry information from the brainstem to the hair cells

Efferent synapse location (IHC vs OHC)

IHC: indirect, onto Type I afferent dendrites (inhibitory/modulatory); OHC: direct synapses onto the hair cell base

IHC neurotransmitter and afferent type

IHCs release glutamate onto Type I afferents

OHC neurotransmitter and afferent type

OHCs release glutamate onto Type II afferents

Efferent neurotransmitters to IHC synapses

Acetylcholine (most abundant), dopamine, GABA, CGRP, others

Efferent neurotransmitters to OHCs

Acetylcholine (most abundant), GABA, CGRP, others

Ribbon synapse

Specialized synapse in vertebrate sensory cells (hair cells, retinal photoreceptors and bipolar cells, pineal gland) that allows rapid, reliable, sustained glutamate release; ~20 proteins required

Synaptopathy

Loss of ribbon synapses even though the hair cell remains (from aging or noise exposure)

Hidden hearing loss

Normal hearing thresholds but reduced acuity and dynamic range (e.g., poor hearing in noise) due to cochlear synaptopathy

IHC vs OHC: number and rows

IHC: single row, ~3,500; OHC: 3 rows, ~12,000–15,000

IHC vs OHC: shape and height

IHC: wine bottle/flask, 4–8 µm; OHC: test tube/cylinder, 20–70 µm

IHC vs OHC: stereocilia and tectorial membrane

IHC: not embedded; OHC: tallest stereocilia embedded

IHC vs OHC: afferent innervation

IHC: 90–95%; OHC: 5–10%

IHC vs OHC: motility

IHC: not motile; OHC: motile

IHC vs OHC: location

IHC: on the spiral lamina; OHC: on the basilar membrane

Vascular pathway to the cochlea

Vertebral arteries → basilar artery → AICA → labyrinthine (internal auditory) artery → common cochlear artery and anterior vestibular artery

Labyrinthine artery

Branches off the basilar artery or AICA; terminal blood supply to the cochlea and vestibule; passes through the internal auditory canal with CN VII and VIII

Spiral modiolar artery

Main cochlear artery; supplies all of the cochlea except the extreme basal end; enters the modiolus and coils to the apex giving off radiating arterioles

Type I radiating arterioles

Supply the stria vascularis and spiral ligament

Type II radiating arterioles

Supply the spiral lamina near the habenula perforata and the basilar membrane under the tunnel of Corti (organ of Corti and spiral ganglion)

Vestibulocochlear artery

Supplies the extreme basal end ("hook region") of the cochlea

Cochlear venous drainage

Capillaries → spiral modiolar veins and vein of the cochlear aqueduct → jugular vein

Endolymph

In the scala media; high K+, low Na+ (like intracellular fluid); secreted by the marginal cells of the stria vascularis

Perilymph

In scala vestibuli and scala tympani; high Na+, low K+ (like CSF/extracellular fluid); may be a capillary filtrate rather than CSF-derived

Effect of interrupted blood flow to the stria vascularis

Endocochlear potential drops rapidly (even within one minute)

Proposed endolymph outflow route

Ductus reuniens → endolymphatic duct → endolymphatic sac → epidural space (evidence is conflicting)

Frequency range of human hearing

About 20 Hz to 20 kHz (some texts say 16 Hz–16 kHz)

Traveling wave

Wave that propagates from base to apex along the basilar membrane as a traveling bulge, peaking at a frequency-dependent location

Place theory of frequency encoding

Frequency is encoded by the place of maximum basilar membrane displacement: high frequencies at the base, low frequencies at the apex; von Békésy won a Nobel Prize for it

Helmholtz resonance theory

Earlier (1869) idea that transverse fibers in the basilar membrane act as tuned resonators; contradicted by von Békésy

Why the displacement peak varies with frequency

The basilar membrane is narrow and stiff at the base and wide and flexible at the apex

Tonotopic (cochleotopic) organization

Orderly high-to-low frequency map maintained from the cochlea through the auditory nerve and central pathways to cortex

Characteristic (best) frequency (CF)

The frequency at which a neuron has its lowest threshold; basal neurons have high CFs, apical neurons low CFs

Bone vs. air conduction displacement maxima

The basilar membrane displacement maxima are the same for both

Basilar membrane upward deflection

Structures on it move upward (toward the tectorial membrane) and inward (toward the modiolus); stereocilia shear laterally (outward), corresponding to stapes outward

Basilar membrane downward deflection

Structures move away from the tectorial membrane; stereocilia shear medially (inward), corresponding to stapes inward

Resting state of mechanotransduction channels

~15% of K+ channels at the stereocilia tips are open at rest, giving a small K+ influx balanced by passive K+ efflux

Excitatory stereocilia deflection (transduction)

Toward tallest stereocilia → K+ channels open → K+ enters from endolymph → depolarization → voltage-gated Ca2+ channels open → glutamate release

Inhibitory stereocilia deflection (transduction)

Toward shortest stereocilia → all K+ channels close → net K+ outflow → hyperpolarization

Resting potential of IHCs and OHCs

IHC about -40 mV (notes also say -45 mV); OHC about -60 to -70 mV