Chapter 4 - Vestibular Physiology

Question 1

Who first described the resting discharge?

Answer 1

Otto Lowenstein and Alexander Sand (1936). They found in dogfish and Ray that a resting discharge is modulated in each ampulla by angular rotation.

Question 2

What are the 3 main advantages of a resting discharge?

Answer 2

1. Bidirectional response of each afferent fiber.
2. Reduction or elimination of a sensory threshold.
3. Resting activity provides a powerful excitatory input to the brain. (See changes after labyrinthectomy)

Question 3

How would you determine the resting discharge of a an otolith organ?

Answer 3

You could do this for an individual afferent if you discovered the polarization vector by rotating the animal 360 degrees. The resting rate can be obtained when the polarization vector is orthogonal to the gravity vector. This would occur at 2 points around the arc. (Angelaki and Dickman, 2000, Fernandez 1972)

Question 4

Is the resting rate of afferents higher or lower in otolith organs compared to canals? Regular compared to irregular?

Answer 4

Resting rates are lower in otolith afferents, lower in irregularly discharging afferents (moreso for canals).

Question 5

Give 3 reasons why discharge regularity is useful for classifying units?

Answer 5

1. Discharge regularity is characteristic of each unit.
2. It’s easy to calculate by using CV.
3. There are many other characteristics that are unique among them.

Question 6

How is the coefficient of variation calculated?

Answer 6

⁃ Relates standard deviation to the mean interval (sd/t). The coefficient of variation varies with the mean interval, so cv* is a normalized statistic, or standard mean interval. For mammals this is 15 ms

Question 7

What are characteristics of irregular afferents?

Answer 7

• Much larger galvanic responses
• Shallower and faster afterhyperpolarization
• Thick and medium-sized axons ending as calyx and dimorphic terminals
• Central (striolar) zone
• Sensitive to the velocity of cupular displacement
• High sensitivity to angular or linear forces

Question 8

What are characteristics of regular afferents?

Answer 8

• Weak galvanic responses.
• Deeper, slower afterhyperpolarization
• Medium size, thin axons ending as dimorphic and bouton terminals in the peripheral zone
• Low sensitivity to angular or linear forces

Question 9

What is the only difference discussed above that is not causally related to discharge regularity?

Answer 9

Response dynamics. This is from a comparison of response dynamics obtained with sinusoidal galvanic currents and sinusoidal head rotations. (Azure et al 1983, Goldberg et al 1982). They are highly correlated, but not causally related.

Question 10

How efficiently do the different afferents encode information?

Answer 10

• Regular units much better estimate the stimulus, so basically the regular unit has fidelity to the stimulus, including small changes in head motion.
• Of note, there are high gain irregular afferents which should enhance information transmission, and in fact, these units have signal-to-noise ratios similar to regular units.’ Maybe irregular units have a distinct function, maybe for vestibular sensation.

Question 11

What is Ewalds first law (1892)?

Answer 11

Responses are in the plane of the semicircular canals (tested horizontal).

Ewald’s first law, that the eyes move in the plane of the stimulated canal, was not novel, as Marie-Jean-Pierre Flourens (1794-1867) had established that cutting the nerve to a semicircular canal of pigeons caused eye movements in its own plane.

Question 12

What is Ewalds second and third laws (1892)?

Answer 12

One stimulus direction leads to distinctly larger responses in each canal for the horizontal canals (2nd law) and the vertical canals (3rd law). Ewald’s second and third laws is the basis for the head-impulse test currently used as a bedside method of determining whether or not vestibular function is lost in one horizontal semicircular canal, such as is often the case after a bout of vestibular neuritis (Cremer, Halmagyi et al. 1998).

Question 13

Describe a Bode plot:

Answer 13

⁃ A graphical plot in which gain and phase of cupular displacement is plotted as a function of sinusoidal frequency.

Question 14

Over what frequency range is the VOR best suited, due to the torsion-pendulum model Bode plot?

Answer 14

⁃ Mid frequencies from 0.02 to 20 Hz (The corner frequencies are 0.025 and 25 Hz). Most daily activities fall between 0.5 and 20 Hz, with a peak at 2-5 Hz. From both the macro mechanics and the way the brain interprets the signal, the SCC are velocity sensors at these frequencies.

Question 15

At low frequencies (below 0.02 Hz, is acceleration, velocity or position transducer? Mid-frequency? High-frequency?

Answer 15

⁃ very low frequencies, canals serve as angular acceleration - transducers, mid frequency velocity and high frequency position.

Question 16

In what ways does the vestibular system behave linearly? (define 3)

Answer 16

1. No evidence of a threshold or discontinuity as the curves pass through zero response.
2. Responses are close to sinusoidal with <10% nonlinear distortions, with this amount reflecting asymmetries between excitation/inhibition.
3. Gains and phases at a given frequency are almost constant as peak velocity varies over a wide range.

Question 17

Where on the hair cell/afferent are Galvanic currents thought to work? (afferent terminals, hair cells, or axons?)

Answer 17

Galvanic currents work on afferent terminals, rather than on hair cells or parents axons. (Goldberg, 1984)

Question 18

Provide a short explanation of the torsion-pendulum model

Answer 18

Oman’s piecewise model (1987), the torsion-pendulum model is a differential equation that relates the angular head acceleration to the displacement of endolymph

Question 19

What are the three factors that determine the differential that converts angular acceleration to cupula displacement as part of the torsion pendulum model?

Answer 19

1) canal geometry, specifically radius of curvature, as well as its cross-sectional area.
2) physical properties of endolymph (density, viscosity)
3) elasticity of the cupula

Question 20

Type 1 hair cells and calyx endings are limited to which types of animals?

Answer 20

These are a new evolutionary feature, being present in reptils, birds, and mammals. They are not present in fish or amphibians.

Question 21

What might be a benefit of having a type 1 hair cells and calyx endings?

Answer 21

They are present in turtles, mammals, birds, and reptiles, all of which have necks, freeing head rotation from the body. Calyx afferents are low gain, allowing them to permit high acceleration head movements without reaching afferent spike saturation. It is unclear why low gain and phasic responses require calyx endings, however.

Question 22

How are directional properties of otolith afferents determined?

Answer 22

The properties are summarized by a unit polarization vector (v = (x,y,z), expressed in head-fixed coordinates (see photo). Peripheral afferents are one-dimensional (i.e. they have one vector). Central otolith afferents can have multiple vectors.

Question 23

Are otolith afferents sensitive to compressional forces, shearing forces, both or neither?

Answer 23

Compressional forces would be as if the macula were being pressed downward into its surface, shearing are orthogonal to these, or along the macula plane. There is no effect of compression on afferents, and it does not modify the response with a combined shearing and compressional force. The structure is like a hydrated gel, a substance that has a turgor pressure and can therefore resist compressional forces. Hair bundles are also embedded in tunnels in the columnar layer, which may help protect from compressional forces.

Question 24

Is the utricle preferentially excited by ipsilateral or contralateral head roll?

Answer 24

In the squirrel monkey, utricular units excited by ipsilateral roll tilts outnumber those excited by contralateral roll tilts by a 3:1 ratio (Fernández et al. 1972 ; Fernández and Goldberg 1976a ). A similar preponderance of ipsilaterally excited utricular units was observed in the cat (Loe et al. 1973 ). In contrast, in the chinchilla, ipsilaterally excited utricular units were only in a slight majority (Goldberg et al. 1990a ). This slightly exceeds the area for ipsi and contra preferences.

Question 25

What is the mean frequency of human movement?

Answer 25

2 hertz, measured for daily activities over a 10-hour period. ( Macdougall and Moore, 2005)

Question 26

What are the three layers to the otolith organs?

Answer 26

The otolithic membrane is 60 micrometers high, and composed of three layers. Outer otoconial layer, middle gelatinous layer, and the lowest columnal layer.

Question 27

Describe the lower 2 layers of the otolithic membrane.

Answer 27

The gelatinous layer and the columnar layer (also called subcupular meshwork). Lying underneath the OL is the gelatinous layer (GL), a relatively rigid structure consisting of a dense, randomly oriented, cross-linked fi lamentous network (Fig. 4.21D ) that couples motion of the OL to underlying structures. The lowest or columnar layer (CL) (Fig.4.21C ) is a looser meshwork of vertically arranged filaments,anchored to the apical surfaces of supporting cells and sometimes referred to as the subcupular meshwork (Dohlman 1971 ; Lim 1984). Given its anisotropic structure, the CL is likely to be the most compliant of the three layers in shear.

Question 28

What is the saturation limit for otolithic hair cell transduction and for afferent discharge stimulus?

Answer 28

Studies of solitary hair cells suggest that maximal transduction occurs when hair bundles of 10μ m height are deflected in the excitatory direction by 1 μ m — that is, X(Z = 10 μ m) = 1 μ m (Holt et al. 1997 ). From afferent recordings (Fernández and Goldberg 1976b ), discharge in the excitatory direction typically saturates for linear forces near 5 g (see Fig. 4.25 ). This is well above the 1 g required for static tilts.