Static and dynamic equilibrium: vestibular and cochlear structures and sensory pathways

Vestibular and Olfactory Sensory Systems

Static and Dynamic Equilibrium

Mechanisms of Vestibular-Mediated Equilibrium

There are two types of equilibrium that the vestibular system helps to maintain: static and dynamic (Virtual Medical Centre, 2010, para. 31). Static equilibrium provides feedback concerning head position or head movement when the body is stationary. In contrast, dynamic equilibrium involves sensing motion or acceleration/deceleration of the head. Acceleration can be further divided into sensing a change in linear velocity, either horizontally or vertically, and angular velocity associated with rotation of the head.

The vestibular system's contribution to maintaining equilibrium critically depends on inner ear structures. The saccule and utricle together provide sensory information concerning static equilibrium and linear acceleration, while the semicircular canals contribute information about angular acceleration (Virtual Medical Centre, 2010, para. 31-32). Both the saccule and utricle contain a small patch of hair cells and supporting cells, which are known as maculae. The saccule and utricle maculae are positioned vertically and horizontally, respectively. The macula hair cells are embedded in a jelly-like material called the otolithic membrane. On top of the otolithic membrane are otoliths, which act to weigh down the membrane to give it greater inertia. If a person shifts head position then the otoliths and membrane moves lags behind the movement, thereby mechanically stimulating a large number of cilia elaborated by the hair cells.

If a person tilts their head forward, or starts or stops running, then the horizontally-positioned otolithic membrane in the utricle will provide the necessary linear acceleration/deceleration information (Virtual Medical Centre, 2010, para. 33-34). If a person gets on an elevator the saccule macula will sense the vertical acceleration/deceleration.

The semicircular duct consists of three semicircular canals that lie at opposing angles to each other (Virtual Medical Centre, 2010, para. 35). Two canals are oriented in a vertical direction, but with a difference of 90 degrees. The third canal is angled about 30 degrees from the horizontal plane. Nodding, tilting the head to the side, or scanning the horizon, are the types of head motions that will cause the endolymph, or internal fluid, to shift position and stimulate hair cells located in a special structure called the ampulla. The ampulae are located at the end of each canal and also contain a jelly-like substance called the cupula, within which the hair cells are embedded. The cupula stretches to the roof of the ampulla, creating a membrane against which the endolymph will push during head movement to mechanically stimulate the hair cells. For example, turning the head left to check for oncoming traffic at an intersection would cause the endolymph in one or more semicircular canals to push against the cupula. For this reason only acceleration or deceleration are sensed, since the endolymph would return to its normal position in a head at constant velocity.

Vestibular Neural Anatomy

Equilibrium information is transmitted to the vestibular nucleus complex and the cerebellum for processing, which in turn controls motor neuron activity to maintain the desired head and eye position during angular or linear acceleration/decelerations, or head movements independent of body movement (Hain and Helminski, 2001, p. 2). The vestibular nerve is distinct from the cochlear nerve, the latter transmitting acoustic information to two cochlear nuclei located in the inferior cerebellar peduncle.

The vestibular pathway mediates three reflexive behaviors: (1) the vestibulo-ocular reflex helps to maintain visual stability during head movements, by controlling the activity of motor neurons innervating the extraocular muscles of the eyes, (2) the vestibulo-collic reflex helps to maintain head position during body movements by controlling the motor neurons innervating the neck musculature, and (3) the vestibulo-spinal reflex controls peripheral motor neuron activity to help maintain balance (Hain and Helminski, 2001, p. 2). Although these pathways can operate reflexively, they can are modified or overridden to some extent by intentional cognitive processes via the cerebellum.

The vestibular nucleus, which is located primarily in the pons and extends into the medulla, consists of four major nuclei that have overlapping duties (Hain and Helminski, 2001, p. 8-10). The vestibulo-ocular reflex depends on the superior and medial nuclei, and the vestibulo-spinal reflex depends primarily on the lateral nucleus and to a lesser extent on the medial. The fourth major vestibular nucleus, the descending nucleus, connects the other three nuclei to the cerebellum and has no distinct function. Although these nuclei can directly control motor neuron activity to reflexively maintain head and eye positions during movement, without the cerebellum to provide adaptive control the reflexive responses become ineffective.

Olfactory Sensation

The human olfactory system consists of two specialized structures within the nasal passages, the vomeronasal organ (VNO) and the main olfactory epithelium (MOE) (Lledo, Gheusi, and Vincent, 2005, 281-282). The primary function of the VNO is the detection of pheromones to regulate sexual and social interactions with others. The MOE represents what most of consider to be the primary structure mediating our sense of smell, but contrary to popular beliefs the MOE is actually located in a protected cleft in the posterior roof of the nasal passages. The MOE is capable of sensing the presence, and to some extent the concentration, of over 1000 compounds called odorants.