Vestibular Anatomy and Neurophysiology

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Original Editor - Megyn Robertson

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Introduction

The vestibular system is a sophisticated human postural control system (Hain, 2011). It is sensitive to two types of information: the position of the head in space and sudden changes in the direction of movement of the head (Shumway-Cook and Woollacott, 2007). The vestibular system is divided into a central and peripheral system. The vestibular system has both a sensory and motor component to help us sense and perceive motion and provides information about the movement of the head and its position with respect to gravity and other inertial forces (like those generated when driving in a car). This information is used to stabilize the eyes to our maintain gaze on targets of interest, with or without head movement.  The vestibular system also employs complicated strategies to maintain blood pressure when one quickly goes from supine to an erect posture. It helps us to maintain good head and body orientation in relation to our environment, most often in an upright posture allowing us to maximise sensory integration of our senses (see, hear, smell). 

The Peripheral Vestibular System (PVS)

The PVS is situated in the inner ear, behind the tympanic membrane. Inputs from the PVS are integrated by the central vestibular processor called the ‘vestibular nuclear complex’ which generates motor commands to drive the eyes and the body. The system is normally very accurate. To maintain accuracy, the vestibular system is monitored and calibrated by the cerebellum (Hain & Helminski, 2014).

Semi-Circular Canals

The Semi-circular canals (SCC’s) are specialised mechanoreceptors to help us access information regarding angular velocity. The sensory input received from the SCC’s enables the Vestibular Ocular Reflex (VOR) to generate an eye movement that matches the velocity of the head movement.

The 3 SCC’s are positioned at right angles to each other to give us feedback in 3 different planes of movement. Remember there are 2 ears, so effectively six SCC’s.

The six individual semi-circular canals become three coplanar pairs:

1. right and left lateral
2. left anterior and right posterior
3. left posterior and right anterior

The planes of the canals are close to the planes of the extraocular muscles, so sensory neurons and motor output neurons can give quick information to individual ocular muscles. Inside the canals there are hair cells in endolymph, and with head movement, the swishing of endolymph displaces these hair cells of the coplanar pair in opposite directions with respect to their ampullae, and neural firing increases in one vestibular nerve and decreases on the opposite side. Endolymph displacement is proportional to angular head velocity, so the semi-circular canals transmit a velocity signal to the brain.

Otoliths

The Otoliths are made up of the Utricle (vertical) and Saccule (horizontal). Their job is to give us information about linear acceleration by triggering an action potential to the brain to detect head position. Because earth’s gravitational field is a linear acceleration field, the otoliths register tilt. For example, as the head is tilted laterally (which is also called roll), shear force is exerted upon the utricle, causing excitation, while shear force is lessened upon the saccule. Similar changes occur when the head is tilted forwards or backwards (called pitch).

Otoconia are little calcium carbonate crystals embedded in the otolithic membrane. Head tilt and linear head motion cause displacement of the otoconial complex, producing a shearing force that deflects the hair bundles and subsequently depolarizes the sensory hair cells. These electrical signals are then relayed to the central nervous system (CNS) by the afferent vestibular nerve, which jointly with other proprioceptive information, stimulate the CNS to initiate neuronal responses for maintaining body balance. The correct formation and anchoring of otoconia is essential for optimal vestibular function and maintaining body balance (Jones et al., 1999, 2004; Kozel et al., 1998; Simmler et al., 2000a; Zhao et al., 2008b). Otoconia abnormalities are common and can cause vertigo and imbalance in humans.

SIde note:

Benign Positional Paroxysmal Vertigo (BPPV) is thought to be caused by dislodging of calcium carbonate stones (otoconia) from the otoconial membrane within one of the semi-circular canals of the inner ear. This dislodgment physically displaces hair cells on movement and creates persistent action potentials until the response is fatigued, generally within 30 to 60 seconds. Dizziness is a common symptom post concussion and the health professional needs to be able to differentiate dizziness from vertigo. Vertigo is most often characterised with nystagmus and dizziness particularly with positional changes of the head.

In summary, the hair cells of the canals and otoliths convert the mechanical energy generated by head motion into neural discharges directed to specific areas of the brainstem and the cerebellum. With their special orientation, the SCC’s and otolith organs can respond selectively to head motion in particular directions. It is important to remember that the otoliths and semi-circular canals have different fluid mechanics: SCC’s measure velocity whereas the otoliths measure acceleration.

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