The righting reflex, also known as the

labyrinthine In Greek mythology, the Labyrinth (, ) was an elaborate, confusing structure designed and built by the legendary artificer Daedalus for King Minos of Crete at Knossos. Its function was to hold the Minotaur, the monster eventually killed by ...

righting reflex, is a

reflex In biology, a reflex, or reflex action, is an involuntary, unplanned sequence or action and nearly instantaneous response to a stimulus. Reflexes are found with varying levels of complexity in organisms with a nervous system. A reflex occurs ...

that corrects the orientation of the body when it is taken out of its normal upright position. It is initiated by the

vestibular system The vestibular system, in vertebrates, is a sensory system that creates the sense of balance and spatial orientation for the purpose of coordinating movement with balance. Together with the cochlea, a part of the auditory system, it constitut ...

, 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

otolith An otolith ( grc-gre, ὠτο-, ' ear + , ', a stone), also called statoconium or otoconium or statolith, is a calcium carbonate structure in the saccule or utricle of the inner ear, specifically in the vestibular system of vertebrates. The sa ...

s, and angular acceleration through the

semicircular canal In mathematics (and more specifically geometry), a semicircle is a one-dimensional locus of points that forms half of a circle. The full arc of a semicircle always measures 180° (equivalently, radians, or a half-turn). It has only one line o ...

s. The reflex uses a combination of

visual system The visual system comprises the sensory organ (the eye) and parts of the central nervous system (the retina containing photoreceptor cells, the optic nerve, the optic tract and the visual cortex) which gives organisms the sense of sight (th ...

inputs, vestibular inputs, and

somatosensory In physiology, the somatosensory system is the network of neural structures in the brain and body that produce the perception of touch (haptic perception), as well as temperature (thermoception), body position ( proprioception), and pain. It ...

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 In physiology, an efference copy or efferent copy is an internal copy of an outflowing ('' efferent''), movement-producing signal generated by an organism's motor system.Jeannerod, Marc (2003): "Action Monitoring and Forward Control of Movements". ...

. This means that the brain makes comparisons in the

cerebellum The cerebellum (Latin for "little brain") is a major feature of the hindbrain of all vertebrates. Although usually smaller than the cerebrum, in some animals such as the mormyrid fishes it may be as large as or even larger. In humans, the cerebe ...

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. ''See:

cat righting reflex The cat righting reflex is a cat's innate ability to orient itself as it falls in order to land on its feet. The righting reflex begins to appear at 3–4 weeks of age, and is perfected at 6–9 weeks. Cats are able to do this because they have a ...

Overview

Vestibular system

The vestibular system is composed of inner ear organs forming the "labyrinth": the

s, the

s, and the

cochlea The cochlea is the part of the inner ear involved in hearing. It is a spiral-shaped cavity in the bony labyrinth, in humans making 2.75 turns around its axis, the modiolus. A core component of the cochlea is the Organ of Corti, the sensory o ...

. 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 An ampulla (; ) was, in Ancient Rome, a small round vessel, usually made of glass and with two handles, used for sacred purposes. The word is used of these in archaeology, and of later flasks, often handle-less and much flatter, for holy water or ...

, that houses specialized sensory

hair cell Hair cells are the sensory receptors of both the auditory system and the vestibular system in the ears of all vertebrates, and in the lateral line organ of fishes. Through mechanotransduction, hair cells detect movement in their environment. ...

s. 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 Stereocilia (or stereovilli or villi) are non-motile apical cell modifications. They are distinct from cilia and microvilli, but are closely related to microvilli. They form single "finger-like" projections that may be branched, with normal cell ...

, 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 Afferent nerve fibers are the axons (nerve fibers) carried by a sensory nerve that relay sensory information from sensory receptors to regions of the brain. Afferent projections ''arrive'' at a particular brain region. Efferent nerve 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 The saccule is a bed of sensory cells in the inner ear. It translates head movements into neural impulses for the brain to interpret. The saccule detects linear accelerations and head tilts in the vertical plane. When the head moves verticall ...

. 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 An otolith ( grc-gre, ὠτο-, ' ear + , ', a stone), also called statoconium or otoconium or statolith, is a calcium carbonate structure in the saccule or utricle of the inner ear, specifically in the vestibular system of vertebrates. The sa ...

, 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.

Signal transduction

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 The vestibular nuclei (VN) are the cranial nuclei for the vestibular nerve located in the brainstem. In Terminologia Anatomica they are grouped in both the pons and the medulla in the brainstem. Structure Path The fibers of the vestibular n ...

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 In physics, angular acceleration refers to the time rate of change of angular velocity. As there are two types of angular velocity, namely spin angular velocity and orbital angular velocity, there are naturally also two types of angular accelera ...

, while the otoconia encode

linear acceleration In mechanics, acceleration is the rate of change of the velocity of an object with respect to time. Accelerations are vector quantities (in that they have magnitude and direction). The orientation of an object's acceleration is given by th ...

signals and

gravitational In physics, gravity () is a fundamental interaction which causes mutual attraction between all things with mass or energy. Gravity is, by far, the weakest of the four fundamental interactions, approximately 1038 times weaker than the str ...

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.

Function

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 The midbrain or mesencephalon is the forward-most portion of the brainstem and is associated with vision, hearing, motor control, sleep and wakefulness, arousal ( alertness), and temperature regulation. The name comes from the Greek ''mesos'', ...

. 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 The basal ganglia (BG), or basal nuclei, are a group of subcortical nuclei, of varied origin, in the brains of vertebrates. In humans, and some primates, there are some differences, mainly in the division of the globus pallidus into an ext ...

, and the

reticular formation The reticular formation is a set of interconnected nuclei that are located throughout the brainstem. It is not anatomically well defined, because it includes neurons located in different parts of the brain. The neurons of the reticular formatio ...

Reference frames

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.

Allocentric reference frame

The allocentric reference frame describes a visual reference frame based on the arrangement of objects in an organism's environment. To test for the use of an allocentric reference frame, a "rod-and-frame" test, in which a subject's perception of virtual objects in an environment are altered, can be used to cause a body tilt as the subject believes to be correcting for the shift.

Egocentric reference frame

The egocentric reference frame refers to a

proprioceptive Proprioception ( ), also referred to as kinaesthesia (or kinesthesia), is the sense of self-movement, force, and body position. It is sometimes described as the "sixth sense". Proprioception is mediated by proprioceptors, mechanosensory neurons ...

reference frame using the position of an organism's body in a space. This reference frame relies heavily on

information, or feedback from the body's sensory system. Muscle vibrations can be used to alter a subject's perception of the location of their bodies by creating an abnormal somatosensory signal.

Geocentric reference frame

The geocentric reference frame involves visual inputs to help detect the verticality of an environment through gravitational pull. The sole of the foot contains receptors in the skin to detect the force of gravity, and plays a large role in standing or walking balance. The abdominal organs also contain receptors that provide geocentric information. "Roll-tilt" tests in which a subject's body is mechanically moved can be used to test for geocentric reference frame function.

Pathways

The righting reflex can be described as a three-neuron arc system composed of primary vestibular neurons, vestibular nuclei neurons, and target motorneurons. Input from the vestibular system is received by sensory receptors in the

s of the

s and the

s, which are processed in the vestibular nuclei. The

is also active at this time for processing of what is called an

, which compares expectations of the body's posture with how it is oriented at the time. The difference between expected posture and actual posture is corrected for via motorneurons in the spinal cord, which control muscle movements for righting the body. These automatic postural adjustments can be explained in terms of two reflexes similar to the righting reflex: the vestibulo-ocular reflex (VOR) and the vestibulocollic reflex (VCR). The VOR involves movement of the eyes while the head turns to remain fixated on a stationary image, and the VCR involves control of neck muscles for correction of the head's orientation. During the VOR, the semicircular canals send information to the brain and correct eye movements in the direction opposite head movement by sending excitatory signals to motor neurons on the side opposite to the head rotation. Neurons in the otoliths control not only these signals for control of eye movements, but also signals for head movement correction through the neck muscles. The righting reflex utilizes the VOR and VCR as it brings the body back into position. Visual information under the control of these reflexes creates greater stability for more accurate postural correction.

Tests for righting reflex function

Vestibular function can be tested through a series of visual acuity tests. The static visual acuity test investigates a patient's ability to see an object from a distance by placing a subject at a certain distance from a letter fixed on a screen. The dynamic visual acuity test involves a patient's ability to control eye movements by following letters that appear on a screen. The difference between these two test results is the patient's fixation ability and vestibuloocular reflex (VOR) efficiency.Badaracco C, Labini FS, Meli A, Tufarelli D: Oscillopsia in labyrinthine defective patients: comparison of objective and subjective measures. Am J Otolaryngol 2010, 31(6):399-403. Vestibular reflexes can also be examined using body tilt experiments. Patients with vestibular disorders may go through the Dix-Hallpike maneuver, in which the patient is seated with legs extended and rotates the head 45 degrees. The patient is then asked to lie down on the table and checked for

nystagmus Nystagmus is a condition of involuntary (or voluntary, in some cases) eye movement. Infants can be born with it but more commonly acquire it in infancy or later in life. In many cases it may result in reduced or limited vision. Due to the invol ...

, or uncontrollable eye movements. Nystagmus in patients indicates dysfunction of the vestibular system, which can lead to dizziness and inability to complete a righting reflex. Proprioceptive ability tests are important in testing for righting reflex function. A therapist may ask a patient whether they know where a certain limb or joint is located without looking at it. These tests are often conducted on uneven surfaces, including sand and grass. Recently, vestibular reflexes have been investigated using leg rotation experiments. A leg and foot rotation test can be used to investigate changes in neuron activity within the

labyrinth In Greek mythology, the Labyrinth (, ) was an elaborate, confusing structure designed and built by the legendary artificer Daedalus for King Minos of Crete at Knossos. Its function was to hold the Minotaur, the monster eventually killed by ...

, or the inner ear. When the head is rotated while the leg and foot are rotated 90 degrees, the vestibular signals cause the brain to inhibit movement in the direction of the rotation. At the same time, it activates the muscles on the opposite side in an attempt to correct for the displacement.Grasso C, Barresi M, Scattina E, Orsini P, Vignali E, Bruschini L, Manzoni D: Tuning of human vestibulospinal reflexes by leg rotation. Hum Mov Sci 2011, 30(2):296-313.

Plasticity

Because visual input is so critical in proper righting reflex function, impairment of vision can be detrimental. Blind patients can rely on vestibular input where visual input is not available, and the

visual cortex The visual cortex of the brain is the area of the cerebral cortex that processes visual information. It is located in the occipital lobe. Sensory input originating from the eyes travels through the lateral geniculate nucleus in the thalamus ...

can become rewired to accommodate other senses taking control. Developmentally blind patients have a larger portion of the brain dedicated to vestibular and

input than patients with normal visual function. Recently blind patients must form new connections where visual inputs once were, and vestibular therapy may enhance this ability. This principle, called

neuroplasticity Neuroplasticity, also known as neural plasticity, or brain plasticity, is the ability of neural networks in the brain to change through growth and reorganization. It is when the brain is rewired to function in some way that differs from how it p ...

, is of growing interest to researchers today.

Disorders

Many inner ear disorders can cause dizziness, which leads to dysfunctional righting reflex action. Common inner ear disorders can cause

vertigo Vertigo is a condition where a person has the sensation of movement or of surrounding objects moving when they are not. Often it feels like a spinning or swaying movement. This may be associated with nausea, vomiting, sweating, or difficulties w ...

in patients, which can be acute or chronic symptoms. Labyrinthitis, or inflammation of the inner ear, can cause imbalances that must be overcome through therapeutic exercises. Labyrinthectomy, or removal of inner ear organs, is an operation conducted for patients with severe inner ear disorders whose vertigo is debilitating. Imbalances result from the procedure, but therapy can help overcome the symptoms.Lao Z, Sha Y, Chen B, Dai C-F, Huang W-H, Cheng Y-S: Labyrinthine sequestrum: four case studies. Otolaryngology–Head and Neck Surgery 2012, 147(3):535-537.

Benign paroxysmal positional vertigo

Benign paroxysmal positional vertigo, or BPPV, is a disorder caused by the breaking off of a piece of

from the otoliths. The otoconia floats freely in the inner ear fluid, causing disorientation and

. The disorder can be tested for using a

test, such as the Dix-Hallpike maneuver. This disorder can disrupt the function of the righting reflex as the symptoms of vertigo and disorientation prevent proper postural control. Treatment for the disorder includes

antihistamine Antihistamines are drugs which treat allergic rhinitis, common cold, influenza, and other allergies. Typically, people take antihistamines as an inexpensive, generic (not patented) drug that can be bought without a prescription and provide ...

s and

anticholinergic Anticholinergics (anticholinergic agents) are substances that block the action of the neurotransmitter called acetylcholine (ACh) at synapses in the central and peripheral nervous system. These agents inhibit the parasympathetic nervous sys ...

s, and the disorder often goes away without surgical removal of the free otoconia.

Ménière's disease

Ménière's disease Ménière's disease (MD) is a disease of the inner ear that is characterized by potentially severe and incapacitating episodes of vertigo, tinnitus, hearing loss, and a feeling of fullness in the ear. Typically, only one ear is affected initi ...

is thought to be a balance disorder involving fluid buildup in the inner ear. This can result from a number of factors, including head injury, ear infection, genetic predisposition, chemical toxicity, allergies, or syphilis.

Syphilis Syphilis () is a sexually transmitted infection caused by the bacterium '' Treponema pallidum'' subspecies ''pallidum''. The signs and symptoms of syphilis vary depending in which of the four stages it presents (primary, secondary, latent, a ...

can cause some patients to develop the disease later in life. The disease is characterized by pressure in the ears, ringing in the ears, and vertigo. It also causes nystagmus, or uncontrollable eye movements. There is no known treatment for the disorder, although symptoms can be treated. These include

water pills A diuretic () is any substance that promotes diuresis, the increased production of urine. This includes forced diuresis. A diuretic tablet is sometimes colloquially called a water tablet. There are several categories of diuretics. All diuretics in ...

to thin out ear fluid, eating a

low-salt diet A low sodium diet is a diet that includes no more than 1,500 to 2,400 mg of sodium per day. The human minimum requirement for sodium in the diet is about 500 mg per day, which is typically less than one-sixth as much as many diets "season ...

, and taking anti-nausea medication.

Other causes of righting reflex disorders

Vestibular and balance disorders can have a number of contributing factors. Dietary factors such as a high-salt diet, high

caffeine Caffeine is a central nervous system (CNS) stimulant of the methylxanthine chemical classification, class. It is mainly recreational drug use, used recreationally as a Nootropic, cognitive enhancer, increasing alertness and attentional perfor ...

intake, high sugar intake,

monosodium glutamate Monosodium glutamate (MSG), also known as sodium glutamate, is the sodium salt of glutamic acid. MSG is found naturally in some foods including tomatoes and cheese in this glutamic acid form. MSG is used in cooking as a flavor enhancer ...

(MSG) intake, dehydration, or

food allergies A food allergy is an abnormal immune response to food. The symptoms of the allergic reaction may range from mild to severe. They may include itchiness, swelling of the tongue, vomiting, diarrhea, hives, trouble breathing, or low blood pressur ...

can contribute to symptoms of vertigo and should be avoided in balance disorder patients. Other disorders can have symptoms of vertigo associated with them, such as

epilepsy Epilepsy is a group of non-communicable neurological disorders characterized by recurrent epileptic seizures. Epileptic seizures can vary from brief and nearly undetectable periods to long periods of vigorous shaking due to abnormal electrica ...

migraine Migraine (, ) is a common neurological disorder characterized by recurrent headaches. Typically, the associated headache affects one side of the head, is pulsating in nature, may be moderate to severe in intensity, and could last from a few hou ...

stroke A stroke is a disease, medical condition in which poor cerebral circulation, blood flow to the brain causes cell death. There are two main types of stroke: brain ischemia, ischemic, due to lack of blood flow, and intracranial hemorrhage, hemorr ...

, or

multiple sclerosis Multiple (cerebral) sclerosis (MS), also known as encephalomyelitis disseminata or disseminated sclerosis, is the most common demyelinating disease, in which the insulating covers of nerve cells in the brain and spinal cord are damaged. This ...

. Infectious diseases such as

Lyme disease Lyme disease, also known as Lyme borreliosis, is a vector-borne disease caused by the '' Borrelia'' bacterium, which is spread by ticks in the genus '' Ixodes''. The most common sign of infection is an expanding red rash, known as erythema ...

and

meningitis Meningitis is acute or chronic inflammation of the protective membranes covering the brain and spinal cord, collectively called the meninges. The most common symptoms are fever, headache, and neck stiffness. Other symptoms include confusion or ...

can also cause vertigo.

Righting reflex in animals

The righting reflex is not exclusive to humans. A well-known righting reflex in cats allows them to land on their feet after a fall. As a cat falls, it turns its head, rotates its spine, aligns its hindquarters, and arches its back to minimize injury."Cats: A Cat's Nine Lives." Video. National Geographic 2012. http://video.nationalgeographic.com/video/animals/mammals-animals/cats/cats_domestic_ninelives/ The cat reaches

free fall In Newtonian physics, free fall is any motion of a body where gravity is the only force acting upon it. In the context of general relativity, where gravitation is reduced to a space-time curvature, a body in free fall has no force acting on ...

to accomplish this, which is much lower than that of humans, and they are able to hit the ground in a relaxed body form to prevent serious injury. Bats, however, have a unique vestibular system anatomy. Their balance system, at an orientation 180 degrees opposite to that of humans, allows them to perform powerful feats of flight while hunting in the dark. This ability couples vestibular function with sensory echolocation to hunt prey. Forman S: "Bats with frikkin' lasers." The Brown Daily Herald. 10 March 2010. http://www.browndailyherald.com/bats-with-frikkin-lasers-1.2186630#.UKqC2rvK3B8 However, they lack a righting reflex similar to most mammals. When exposed to

zero-G Weightlessness is the complete or near-complete absence of the sensation of weight. It is also termed zero gravity, zero G-force, or zero-G. Weight is a measurement of the force on an object at rest in a relatively strong gravitational fie ...

, bats do not undergo the series of righting reflexes that most mammals do to correct orientation because they are accustomed to resting upside-down.Fejtek M, Delorme M, Wassersug R: Behavioral reactions of the bat Carollia perspicillata to abrupt changes in gravity. Uchu Seibutsu Kagaku. 1995, 9(2):77-81.

References

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