Vestibular System

Introduction

Role of Vestibular System: detect motion in space and to offset the effects of that motion upon sensory organs and motor actions via compensatory reflexes.

Three Functions of Vestibular System

(1) Stabilizes the eyes in space so that motion of the head does not degrade vision

(2) Compensates for forces acting upon the animal so that these perturbations do not cause the animal to fall over

(3) Adjusts an animal’s motor activities (particularly those acting upon the outside world) so that these activities remain accurately targeted, despite self motion

Anatomy of Vestibular System – consists of two structures (located in inner ear):

(1) Semicircular Canals – Detect and respond to angular acceleration and deceleration of the head

made up of three semicircular canals (anterior, posterior, and lateral) that lie in a mutually perpendicular plane
contain hair cells (cells with cilia on them that act as receptors) which are activated by movement of endolymph
these canals are like hula-hoops filled with some water; as the hula-hoop spins, the water moves with the same speed in the opposite direction and thus appears to the outside observer as if it is not moving

(2) Otolith System – Detects and responds to position of head with respect to linear acceleration and the pull of gravity

made up of utricle and saccule, which are endolymp-containing dilatations of membranous labyrinth of inner ear
the macula of the utricle and saccule have two patches of sensory epithelium consisting of hair cells
the utricular macula is disposed in horizontal plane and responds maximally to tilting head forward or backward
the saccular macula is disposed in vertical plane and responding maximally to tilting the head to the side
if these structures are damaged the person will lose sense of being pulled down by gravity

Seven Vestibular Reflexes – most important are aVOR, IVOR, and VSR

(1) Angular Vestibulo-Ocular Reflex (aVOR)

challenge – prevent object of regard from slipping across retina during rotation of head in space (rotational degrees of freedom: yaw, roll, pitch)

price of failure – degraded acuity

strategy – rotate eyes opposite direction of head rotation in order to maintain focus on moving object.

sensor – semicircular canals in vestibular labyrinth

effector – extraocular muscles

(2) Linear Vestibulo-Ocular Reflex (IVOR)

challenge – prevent the object of regard from slipping across retina during translation of head (translational degrees of freedom: up/down, fore/aft, left/right) in space

price of failure – degraded acuity

strategy – rotate eyes to offset effect of translation of head – amount depends on direction and distance of target

sensor – otolith organs (aka Macular organs located in the Utricle and Saccule of the vestibular labyrinth)

effector – extraocular muscles

(3) Optokinetic reflex (OKR) – ex: when sit in train and another moves, you feel like you’re moving backwards

challenge – aVOR doesn’t compensate well for slow rotations or prolonged rotations – OKR makes up for this

price of failure – degraded acuity

strategy – (1) deduce rotation of body from optic flow (patterns that the entire visual scene describes as it slips across the retina during self motion), (2) rotate eyes opposite direction of deduced head rotation

sensor – retina

effector – extraocular muscles

(4) Vestibulocollic reflex (VCR) – minor reflex in humans, cats use it to land on their feet when dropped upside down

challenge – maintain head upright in space during applied accelerations

price of failure – the smelly cat will break its head

strategy – adjust neck tone to counter translational/rotational accelerations of head

sensor – otolith organs and semicircular canals

effector – neck muscles

(5) Vestibulospinal reflex (VSR) – a.k.a. Tonic Labyrinthine Reflexes

challenge – maintain body upright in space during applied accelerations

price of failure – fall-over

strategy – adjust limb tone to restore body to upright position in space

sensor – Otolith organs and semicircular canals

effector – antigravity limb muscles

(6) Cervico-ocular reflex (COR) – identical to aVOR

challenge – prevent image slipping across retina during rotation of head in space

price of failure – potential degraded acuity

strategy – deduce head rotation from cervical proprioceptors, rotate eyes opposite deduced head rotation

sensor – upper cervical proprioceptors

effector – extraocular muscles

(7) Tonic neck reflexes (Cervicocollic and Cervicospinal Reflexes) – seen in infants and comatose patients

challenge – maintain body level with earth

price of failure – poor self- righting

strategy – deduce misalignment from neck proprioceptors, adjust neck tone (CCR) or limb posture (CSR) to restore alignment

sensor – cervical proprioceptors

effector – neck, spine, limb muscles

How do the semicircular canals control eye movements?

semicircular canals function can be described using the right hand rule: wrap fingers in direction of rotation and thumb shows direction of vector. Left horizontal canal stimulated by body rotation to the left (vector up), right horizontal canal stimulated by body rotation to the right (vector down)

eye muscles are arranged in a way to oppose each one of these six vectors! So if the head moves left the eyes will receive signal to move right

brain connects the correct canal in pairs (right horizontal canal and left horizontal canal, left posterior with right posterior) with correct eye muscles. Brain can also suppress reflex when disadvantageous, calibrates self so that eyes rotate the correct amount to cancel head rotation

Horizontal Reflex

vestibular nuclei have connections to the lateral and medial rectus muscles (CN VI and CN III)
as the canals sense spin in one direction they signal (via CN VIII) to motor neurons of lateral rectus of one eye. They also have motor fibers which relay to nucleus of medial rectus muscles in the other eye. This results in both eyes moving in the same direction!
the above is allowed to happen because the same system also sends signals via inhibitory interneurons to stop the movement of the opposing muscles in each eye. Understand that the circuitry controls both eyes with movement.

Clinical: Vertigo

Acute Vertigo

Peripheral – infectious (viral) labyrinthitis, acoustic artery ischemia, alcohol, head/neck injury

Central – brainstem/cerebellar stroke hemorrhage, multiple sclerosis, intoxicants

Recurrent Vertigo

Peripheral – benign paroxysmal positional vertigo (dislocation of utricular macular otoliths, most common cause of recurrent vertigo), Ménière’s disease (Ý endolymphatic fluid pressure), perilymph fistula

Central – brainstem ischemia, chiari malformation, migraine/epilepsy

Young person with vertigo – probably viral (peripheral acute)

Old person with vertigo – check for stroke! (central acute)