Anatomy and physiology (A&P) of the vestibular system - the organ of equilibrium or balance. The 3 semicircular canals that sense rotational (angular) accelerations, and 2 otolith organs - saccule and utricle - that sense head positions and linear motions.
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The vestibular system is responsible for the body’s equilibrium, it maintains balance and provides awareness of the body’s spatial orientation. Vestibular sensory organs detect changes in the head’s positions and movements, and transmit this information to various regions of the brain. Projections to the brainstem trigger reflex pathways that lead to compensatory actions to maintain stability or re-establish equilibrium, while projections to the cortex provide perception of gravity and movement.
Examples of vestibular reflex pathways include:
- the vestibulo-ocular reflex that controls eye muscles to keep visual objects in focus while the head is moving. It does so by moving the eyes in the opposite direction as the head.
- and the vestibulo-spinal reflex that senses a potential loss of balance and activates body muscles to keep the body from falling.
The sensory part of the vestibular system is located in the inner ear on each side of the body. It consists of 3 semicircular canals that sense rotational movements, such as when the head is turning, and 2 otolithic organs that sense head positions, as well as straight line motions, such as when riding in a car or an elevator.
The 3 semicircular canals, or ducts, are oriented approximately at a right angle to each other, each corresponding roughly to one of the 3 planes of motions: turning left and right, nodding up and down, and tilting to a side. They contain a fluid called endolymph. Each canal has an enlargement at one end called an ampulla. Within the ampulla, there are hair cells embedded in a gel-like structure named cupula that extends the entire height of the ampulla.
When the head turns, the ducts that are located on the same plane of motion rotate, but the fluid lags behind because of inertia. This causes the fluid to briefly move in the opposite direction as the head, and either push or pull on the cupula, bending the cilia on the hair cells, and thus activating them to send nerve impulses to the brain. The direction of the bend determines if the signals generated are excitatory or inhibitory. Because the 2 sides of the head are mirror images, a head turn generates excitatory signals on one side, and inhibitory signals on the other.
The 2 otolithic organs are 2 patches of hair cells oriented nearly perpendicular to each other: the saccule being vertical, and the utricle being horizontal. The cilia of these cells are embedded in a gel-like layer sprinkled with calcium carbonate crystals called otoconia, commonly known as “ear rocks”. The crystals add weight to the layer, pulling it down with gravity.
When the head is in upright position, the gelatinous layer bears down evenly on the cells of the utricle, the cilia remain straight and no signals are generated. On the vertical saccule, however, the heavy gel is pulled down by gravity at one end, bending the cilia, generating nerve impulses. The reverse is true when the head is horizontal. Other head positions are determined by a combination of signals coming from both organs.
Vestibular sensory organs detect not the motion itself, but changes in the rate of motion, specifically acceleration or deceleration. For example, when a person is sitting in a car that starts to move, the heavy gel-like layer of the utricle lags behind at first, bending the cilia back, activating the hair cells. The more sudden the car starts, the greater the stimulation. Once the car is in stable motion, the gel catches up with the rest of the tissue, and no activation results. Similar events occur in the saccule during an elevator ride up or down.