Chapter 2 - Structure of the Labyrinth

Question 1

Describe the composition of the membranous labyrinth:

Answer 1

Simple layer of squamous epithelium, encased in fibrous tunic and tethered to the bony labyrinth by connective tissue trabeculae.

Question 2

Describe the three types of nerve fibers that innervate the sensory ends of the vestibular neuroepithelium.

Answer 2

1) Afferents, bipolar cells with cell body in the vestibular ganglion (continue into the neuroepithelium)
2) efferents originating in the brainstem (continue into the neuroepithelium)
3) autonomic fibers from the ipsilateral superior cervical ganglion (The autonomics remain in the connective tissue stroma)

Question 3

Scarpa’s ganglion holds the cell bodies for the bipolar neurons innervating what structures?

Answer 3

Superior, lateral and utricle, as well as the hook region of the saccule, via Volt’s anastamosis. Inferior vestibular ganglion holds cell bodies for those of the saccule majority and the posterior canal. It is unclear if these are seperate structures or just different portions of the same structure.

Question 4

There is an anastamosis between the facial nerve and vestibular nerves (as well as cochlear). What neural fibers are carried here?

Answer 4

Sympathetic nerve fibers

Question 5

How are the endolymphatic spaces connected?

Answer 5

The cochlea and saccule are connected via the ductus reuniens. The utricle and saccule are connected by the utricular duct. The saccule is connected to the endolymphatic sac with the saccular duct.

Question 6

What is Oort’s anastamosis?

Answer 6

Von Oort initially described the vestibulocochlear anastomosis in 1918. It is believed to carry all or part of the cochlear efferents, and links the saccular nerve and the cochlear nerve along the floor of the IAC.

Labrousse, M., et al. “An anatomical study of the vestibulocochlear anastomosis (anastomosis of Oort) in humans: preliminary results.” Surgical and Radiologic Anatomy 27.3 (2005): 238-242.

Question 7

What creates the gelatinous supporting structure (cupula and otoconial membrane)?

Answer 7

The neuroepithelium supporting cells secrete it. They probably also removal neurotransmitters from the extracellular space and regulate the ionic environment but this is not known.

Question 8

What is the synaptic body?

Answer 8

An electron dense area at the basal pole of the hair cell that is surrounded by a halo of vesicles. These are connected by filaments to the synaptic body.

Question 9

What makes up the afferent synapse, also known as ribbon synapses?

Answer 9

Synaptic body, associated vesicles, and pre-and post-synaptic membrane specializations. Similar structures are in the inner ear, lateral lines, electroreceptors, photoreceptors and bipolar cells in the retina and in pinealocytes. There is variability in ribbons, with some attaching 500 vesciles and others 25, depends on species and location.

Question 10

Where are type 1 hair cells found?

Answer 10

Only in the vestibular system of amniotes (mammals, reptiles and birds), type 2 hair cells are found in just about any other hair cell organ.

Question 11

Where are the neuroepithelium supporting cells found?

Answer 11

Completely surrounding hair cells (so much so that they are seldom in direct contact), extends from the apical surface to the basement membrane. Their apical area has microvilli and large secretory-like granules.

Question 12

What are transitional cells?

Answer 12

They look like neuroepithelial supporting cells, but have nuclei that are more central, microvilli only on the edges, less secretory granules. The otolith organs are completely surrounded by these, whereas in the crista, they are found in the flanks.

Question 13

What are dark cells?

Answer 13

Resemble secretory cell in other organs, have irregular apical surfaces, rich in mitochondria, with melanocytes in the connective-tissue stroma beneath them. Possibly involved in the productive of endolymph.

Question 14

What are the patterns of hair cell distribution in the crista?

Answer 14

Simple calices innervating type 1 cells are found throughout, but complex afferents innervating multiple calicies are only central. Bouton fibers branch extensively and provide bud-shaped endings to several type 2 hair cells located in irregularly shaped zones. In short, calyces are central and boutons are peripheral. Dimorphic are scattered.

Question 15

How big is the utricular macula and what is its location?

Answer 15

It’s less than 1 mm and is in the anterior end of the utricular sac. It’s anterior portion is slightly curved upwards, with the utricular nerves entering directly into this portion. The posterior portion has fibers entering a fibrous layer and sharply turn upwards to innervate the utricle.

Question 16

What is the striola?

Answer 16

It’s a ribbon-shaped zone that runs throughout much of the macula length, divides in medial and lateral extrastriola. The line of reversal is just lateral to the striola. There are not many otoconia at the striola!

Question 17

How are the hair cells oriented in the utricle and saccule?

Answer 17

In the utricle, it’s as if you’re on a mountain when you’re at the striola, looking down, i.e. the kinocillium are immediately adjacent to each other at the line of reversal. The saccule is like a valley, if standing at the line of reversal looking up the stereocillia toward the kinocillium at the top on either side. The sack in the valley.

Question 18

What are the distribution of type 1 and type 2 hair cells in the utricular maccula? How about afferents?

Answer 18

About 2/3 of the striolar cells are type 1, whereas roughly equal in extrastriola areas. Calyx fibers are confined to the striola, there are a few dimorphic fibers, but these are more extrastriola. There are very few boutons peripherally. The afferents respect the striola, juxtastriola and extrastriola boundaries, and don’t send fibers to both sides.

Question 19

What is the location of the saccular macula?

Answer 19

It’s in the ventromedial wall of the sacculus.

Question 20

What is the saccule’s innervation?

Answer 20

Posterior portion is the inferior vestibular nerve. Anterior is Voit’s anastomosis to the superior nerve.

Question 21

ow are efferents recognized histologically?

Answer 21

On electron microscopy, they have many synaptic vesicles.

Question 22

Roughly how many afferents and efferents innervate the vestibular end organs?

Answer 22

Probably 10-20,000 afferents, but only 500 to 1000 efferents. Surprisingly, despite this, afferent boutons only outnumber efferent boutons by about 3:1, meaning efferents broadly! branch to touch hair cells.

Question 23

Where do efferents originate?

Answer 23

1 mm column between abducens and superior vestibular nuclei (group e). In monkeys roughly half project ipsilateral and half contralateral. They send collaterals to the flocculus and ventral paraflocculus

Question 24

Does the vestibular system behave linearly as would be expected from the linear differential equation of the torsion-pendulum model?

Answer 24

Yes, for one, there is no discontinuity as the curves of sinusoidal rotation pass through 0, responses are mostly sinusoidal with some nonlinear distortions <10%, reflecting excitatory/inhibitory asymmetry. Gains and phases are almost constant for a wide range of velocities at the same frequency.