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Righting reflex

The righting reflex is a reflex that occurs when the body is suddenly 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 reflex uses a combination of visual system inputs, vestibular inputs, and somatosensory inputs to make postural adjustments when the body becomes unbalanced in its normal vertical position. 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.

Vestibular System
The vestibular system is composed of inner ear organs forming the "labyrinth": the semicircular canals, the otoliths, and the cochlea. The semicircular canals are arranged at angles to the horizontal plane of the head when it is in its normal vertical posture. Fluid in these canals moves across specialized hair cells that sense the movement and spatial arrangement of the body. 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 with varying ratios between radius of curvature and canal plane area. This difference in mammalian canal structure and its connection to agility may be due to the environments in which they navigate, such as a mostly two-dimensional landscape as compared to three-dimensional spaces such as in the air, the trees, or the water.
 * Type I hair cells
 * Type II hair cells
 * Large diameter afferent fibers carry info from type I, regular afferents carry info from type II
 * Semicircular canals encode head velocity
 * Otoliths encode linear acceleration and gravity
 * Spike timing is important and associated with different distributions of ion channel activity to distinguish signals
 * Regular afferents tell info about detailed timing of a significant stimulus over a particular range of frequencies
 * Irregular afferents tell info about high frequency stimulus with a high gain

Function
The righting reflex involves a series of postural adjustments in response to a stimulus. In some cases, a startle response can evoke anticipatory postural adjustments, which involves the function of the midbrain, although the mechanisms of such an origin are yet to be elucidated. Data supporting the generation of postural adjustments from circuits in the spine connected to the supplementary motor area, the basal ganglia, and the reticular formation.

Reference frames

 * Used to perceive vertical orientation
 * Create a representation of space similar to an efference copy
 * Consistently updated and quickly adapting to process changes in vestibular input

Allocentric reference frame

 * Visual cues detected by the arrangement of external objects in the environment
 * "rod-and-frame" test used to alter perception of vertical
 * causes optokinetic-induced body tilt

Egocentric reference frame

 * Spatial information detected by the position of body in space
 * Somatosensation cues important
 * Muscle vibration can be used to create an abnormal spindle afferent discharge

Geocentric reference frame

 * Visual inputs help detect the verticality of the environment through gravity
 * Sole of foot has cutaneous receptors to detect gravity
 * Abdominal viscera contain gravireceptors
 * Roll-tilt tests used to investigate geocentric reference frame function

Pathways

 * Similar to the function of vestibuloocular reflex (VOR) and vestibulocollic reflex (VCR), explain these
 * Three-neuron arc system: primary vestibular neurons, vestibular nuclei neurons, target motorneurons
 * Input from vestibular system to spinal cord for motor function in response to change
 * Vestibular, somatosensory, and visual inputs to the cerebellum and midbrain/brainstem for comparison, then an output to the lower motor neurons in the cervical spine to skeletal muscles for automatic postural adjustments.
 * Semicircular canal stimulation generates a reflex arc to correct eye movements in the direction opposite head movement in the VOR
 * Sensorimotor integration while standing

Tests for righting reflex function

 * Vestibulospinal reflexes can be tuned by leg rotation experiments
 * Vestibular disfunction can be tested by comparing the static visual acuity test and the dynamic visual acuity test
 * Static visual acuity: patient sits at a distance from an object and is tested for visual ability
 * Dynamic visual acuity: patient is shown a series of randomly placed letters on a screen and is asked to identify them, measured for head motion toward the letter
 * Difference between results from static visual acuity and dynamic visual acuity tests is the fixation ability and VOR efficiency
 * Gaze stabilization test asks patients to move head left and right to make sure eyes stay on an object in from of them, which tests for proper VOR function (can be lost in oscillopsia or nystagmus and disrupt righting reflex function)

Development

 * Righting reflex appears at birth as the tonic labyrinthine reflex
 * Basic tests for reflexes: Babinski in babies
 * Righting inputs change as babies learn to walk
 * Fewer and fewer inputs required for normal posture, focuses on vestibular, visual, and somatosensory input
 * Vertigo and balance in children using imaging studies

Plasticity

 * Blind patients rely heavily on vestibular input when visual input is not available
 * Developmentally blind patients have a larger portion of the brain dedicated to vestibular and somatosensory input
 * Recently blind patients must form new connections where visual inputs once were

Disorders

 * Inner ear disorders causing dizziness can cause disfunction of righting reflex
 * Labyrinthitis or labyrinthectomies can cause imbalances that must be overcome through therapy

Common inner ear disorders

 * Common symptoms of vertigo in patients
 * Righting reflex can be tested for using sole of foot or knee jerk test

Benign paroxysmal positional vertigo

 * Piece of otoconia breaks off and is free floating in inner ear
 * Causes vertigo, disorientation
 * Tested for by lying back on a table and being checked for spinning sensation
 * Treatment: antihistamines, anticholinergics
 * Usually goes away without removing free otoconia

Meniere's disease

 * Fluid in inner ear buildup thought to be the cause
 * Can result from head injury, ear infection, stress, smoking, allergies
 * Characterized by pressure in ear, ringing in ears, vertigo
 * Uncontrollable eye movements
 * No known treatment, but symptoms can be treated (water pills for ear fluid, low salt diet, anti-nausea medication)

Other causes of righting reflex disorders

 * High salt diet
 * Other balance disorders: epilepsy, migraine, stroke, multiple sclerosis
 * Infectious diseases: Lyme disease, meningitis
 * Other: allergies, anemia, diabetes
 * Supporting device for postural reflexes