Why is the anatomy and physiology of vestibular receptors so important for the vHIT?

The cupula-endolymphatic system can be described through the physical model of the pendulum with critical damping, (i.e., the cupula after deflection quickly returns to the neutral position without overshooting) of which three constituent elements are represented by:

1. Mass (M) of endolymph in the canal (M = 0.2 mg) which, in the event of angular acceleration of the head, determines the inertial force that deforms the cupula;
2. Viscosity (C) of the endolymph, which prevents the system from oscillating, allowing the cupula to return to its equilibrium position in the shortest time possible;
3. Elasticity (K) of the cupula, which determines its return to the position of equilibrium.

These three parameters that are intrinsic to the semicircular canals regulate the frequency response profile of the labyrinth and are connected to each other by a second order differential equation (Steinhausen model, 1933):

INERTIAL FORCE linked to the mass of the endolymph (M) and proportional to the head angular acceleration;
 VISCOUS FORCE linked to the viscous properties of the endolymph (C) and proportional to the head angular velocity;
 ELASTIC FORCE linked to the elastic properties of the cupula (K) and proportional to the head angular displacement.

Since elastic forces prevail at low frequencies, the cupular position will be in phase with the head acceleration.
Since viscous forces prevail at medium frequencies, the cupular position will be in phase with the head velocity.
Since inertial forces prevail at high frequencies, the cupular position will be in phase with the head position.

It can therefore be concluded that:

• at low frequencies the labyrinth will send input to the vestibular nuclei in phase with the acceleration of the head, at which point the Nucleus Prepositus Hypoglossi will perform an integration and transform the input into a velocity signal (but not a position signal). As a result, the stability of the retinal image will only be under visual control;
• at medium frequencies the labyrinth will send input to the vestibular nuclei in phase with the velocity of the head. The Nucleus Prepositus Hypoglossi will perform an integration turning it into a position signal. As a result, the stability of the retinal image will be under both vestibular and visual control;
• at high frequencies the labyrinth will send input to the vestibular nuclei in phase with the position of the head. No action will be required by the Nucleus Prepositus Hypoglossi. Therefore, the stability of the retinal image will be exclusively under vestibular control.

Vestibular receptors, both at ampullary and macular level, are distinguished in phasic (type I) and tonic (type II) cells, the differences of which also concern anatomical and functional aspects.

Phasic cells are located on top of the crista ampullaris, are connected to large-caliber afferent fibres, and respond irregularly to the high-frequency movements of the head. Tonic ones occupy the most peripheral part of the crista ampullaris, are connected to smaller-caliber afferent fibres and have regular activity both when the head is still and in the presence of lower frequency movements.

Type I cells are involved in the perception of the movements of the head at high frequency, which are configured as unidirectional elements i.e., responsive only to one direction of movement. They are also under the control of only the semicircular canals, are not inhibited, do not encounter compensation phenomena, and allow for short tests to be carried out in ambient light with excellent tolerability.

In the perception of low-frequency head movements, type II cells are involved; bidirectional receptors (responding therefore in both directions of movement) which are under the control of the brainstem are frequently subject to central inhibition. They encounter compensation phenomena and are investigated by prolonged tests performed in complete darkness and in conditions that are often poorly tolerated.

Of course, the separation is not so clear: the two types of receptors work by integrating together with the additional help of transactional cells.

Moving on to the clinical level, we can affirm that bilateral affections of phasic cells are responsible for oscillopsia, and the standard examination is the video Head Impulse Test (vHIT).

Unilateral affections of tonic cells, meanwhile, are at the basis of the spontaneous nystagmus, directional preponderance, and tonic deviations, and are mainly analysed through the Bithermal Caloric Tests and Kinetic tests.