Righting reflex

The righting reflex, also known as the labyrinthine righting reflex, or the Cervico-collic reflex; is a reflex that corrects the orientation of the body when it is taken out of its normal upright position. It is initiated by the vestibular system, which detects that the body is not erect and causes the head to move back into position as the rest of the body follows. The perception of head movement involves the body sensing linear acceleration or the force of gravity through the otoliths, and angular acceleration through the semicircular canals. The reflex uses a combination of visual system inputs, vestibular inputs, and somatosensory inputs to make postural adjustments when the body becomes displaced from its normal vertical position. These inputs are used to create what is called an efference copy. This means that the brain makes comparisons in the cerebellum between expected posture and perceived posture, and corrects for the difference. The reflex takes 6 or 7 weeks to perfect, but can be affected by various types of balance disorders.

The righting reflex has also been studied in cats and other non-human mammals.

The vestibular system is composed of inner ear organs forming the "labyrinth": the semicircular canals, the otoliths, and the cochlea. The section below is an overview of the vestibular system, as it is crucial to the understanding of the righting reflex. Sensory information from the vestibular system allows the head to move back into position when disturbed as the rest of the body follows. The semicircular canals (brown, see figure) are arranged at angles to the horizontal plane of the head when it is in its normal vertical posture. Each canal has a widened base, called an ampulla, that houses specialized sensory hair cells. Fluid in these canals surrounds the hair cells, and moves across them as the head moves to gather information about the movement and position of the body. The hair cells are covered in tiny sensory hairs called stereocilia, which are sensitive to displacement forces as the body is moved in different positions. When the head is moved, the force moves the hair cells forward, which sends signals to afferent fibers and on to the brain. The brain can then decide which muscles in the body need to become active in order to right itself.

The semicircular canals have a superior, posterior, and horizontal component. Studies have shown that the horizontal canal is most correlated with agility, as shown with several mammals. Curvature and size of these canals seems to affect agility, and may be due to the environments in which animals navigate, such as a mostly two-dimensional landscape as compared to three-dimensional spaces (i.e. in the air, the trees, or the water).

The otoliths have two components: the utricle and the saccule. Both are made of the same sensory tissue containing hair cells, which is covered by a gelatinous layer and the otolithic membrane on top. Embedded in this membrane are calcium carbonate crystals, called otoconia, or "ear rocks." As the head is tilted forward or backward, the otoconia move the hair cells in a similar fashion to the semicircular canal fluid movement and cause depolarization of the hair cells. Signals from these cells are also transmitted along afferent fibers and on to the brain.

Vestibular afferent signals are carried by type I or type II hair cells, which are distinguished by a larger amount of stereocilia per cell in type I cells than in type II cells. Nerve fibers attached to these hair cells carry signals to the vestibular nuclei in the brain, which are then used to gain information about the body's position. Larger diameter afferent fibers carry information from both type I and type II hair cells, and regular afferent fibers carry signals from type II hair cells. The semicircular canals encode head velocity signals, or angular acceleration, while the otoconia encode linear acceleration signals and gravitational signals. Regular afferent signals and irregular afferent signals travel to the vestibular nuclei in the brain, although irregular signals are at least two times more sensitive. Because of this, it has been questioned why humans have regular afferent signals. Studies have shown that regular afferent signals give information about how long the motion of the head or body lasts, and irregular afferent signals occur when the head is moved more violently, such as in falling.

The righting reflex involves complex muscular movements in response to a stimulus. When startled, the brain can evoke anticipatory postural adjustments, or a series of muscle movements, which involves the function of the midbrain. However, the mechanisms of such an origin are yet to be elucidated. Data support the generation of these movements from circuits in the spine connected to the supplementary motor area, the basal ganglia, and the reticular formation.

Visual input for proper righting reflex function is perceived in the form of reference frames, which create a representation of space for comparison to expected orientation. Three types of reference frames are used to perceive vertical orientation; they are consistently updated and quickly adapting to process changes in vestibular input.