Sense of balance
The sense of balance or equilibrioception is the perception of balance and spatial orientation. It helps prevent humans and nonhuman animals from falling over when standing or moving. Equilibrioception is the result of a number of sensory systems working together: the eyes, the inner ears, and the body's sense of where it is in space ideally need to be intact.
The vestibular system, the region of the inner ear where three semicircular canals converge, works with the visual system to keep objects in focus when the head is moving. This is called the vestibulo-ocular reflex. The balance system works with the visual and skeletal systems to maintain orientation or balance. Visual signals sent to the brain about the body's position in relation to its surroundings are processed by the brain and compared to information from the vestibular and skeletal systems.
Vestibular system
In the vestibular system, equilibrioception is determined by the level of a fluid called endolymph in the labyrinth, a complex set of tubing in the inner ear.Dysfunction
When the sense of balance is interrupted it causes dizziness, disorientation and nausea. Balance can be upset by Ménière's disease, superior canal dehiscence syndrome, an inner ear infection, by a bad common cold affecting the head or a number of other medical conditions including but not limited to vertigo. It can also be temporarily disturbed by quick or prolonged acceleration, for example riding on a merry-go-round. Blows can also affect equilibrioreception, especially those to the side of the head or directly to the ear.Most astronauts find that their sense of balance is impaired when in orbit because they are in a constant state of weightlessness. This causes a form of motion sickness called space adaptation syndrome.
System overview
This overview also explains acceleration as its processes are interconnected with balance.Mechanical
There are five sensory organs innervated by the vestibular nerve; three semicircular canals and two otolith organs. Each semicircular canal is a thin tube that doubles in thickness briefly at a point called osseous ampullae. At their center-base each contains an ampullary cupula. The cupula is a gelatin bulb connected to the stereocilia of hair cells, affected by the relative movement of the endolymph it is bathed in.Since the cupula is part of the bony labyrinth, it rotates along with actual head movement, and by itself without the endolymph, it cannot be stimulated and therefore, could not detect movement. Endolymph follows the rotation of the canal, however, due to inertia its movement initially lags behind that of the bony labyrinth. The delayed movement of the endolymph bends and activates the cupula. When the cupula bends, the connected stereocillia bend along with it, activating chemical reactions in the hair cells surrounding crista ampullaris and eventually create action potentials carried by the vestibular nerve signalling to the body that it has moved in space.
After any extended rotation the endolymph catches up to the canal and the cupula returns to its upright position and resets. When extended rotation ceases, however, endolymph continues, which bends and activates the cupula once again to signal a change in movement.
Pilots doing long banked turns begin to feel upright as endolymph matches canal rotation; once the pilot exits the turn the cupula is once again stimulated, causing the feeling of turning the other way, rather than flying straight and level.
The HSCC handles head rotations about a vertical axis, SSCC handles head movement about a lateral axis, PSCC handles head rotation about a rostral-caudal axis. E.g. HSCC: looking side to side; SSCC: head to shoulder; PSCC: nodding. SCC sends adaptive signals, unlike the two otolith organs, the saccule and utricle, whose signals do not adapt over time.
A shift in the otolithic membrane that stimulates the cilia is considered the state of the body until the cilia are once again stimulated. E.g. lying down stimulates cilia and standing up stimulates cilia, however, for the time spent lying the signal that you are lying remains active, even though the membrane resets.
Otolithic organs have a thick, heavy gelatin membrane that, due to inertia, lags behind and continues ahead past the macula it overlays, bending and activating the contained cilia.
Utricle responds to linear accelerations and head-tilts in the horizontal plane, whereas saccule responds to linear accelerations and head-tilts in the vertical plane. Otolithic organs update the brain on the head-location when not moving; SCC update during movement.
Kinocilium are the longest stereocilia and are positioned at the end of the bundle. If stereocilia go towards kinocilium depolarization occurs causing more neurotransmitter, and more vestibular nerve firings as compared to when stereocilia tilt away from kinocilium.
Neural
First order vestibular nuclei project to IVN, MVN, and SVN.The inferior cerebellar peduncle is the largest center through which balance information passes. It is the area of integration between proprioceptive, and vestibular inputs to aid in unconscious maintenance of balance and posture.
Inferior olive nucleus aids in complex motor tasks by encoding coordinating timing sensory info; this is decoded and acted upon in the cerebellum.
Cerebellar vermis has three main parts: vestibulocerebellum, spinocerebellum , and cerebrocerebellum.
Flocculonodular lobe is a cerebellar lobe that helps maintain body equilibrium by modifying muscle tone.
MVN and IVN are in the medulla, LVN and SVN are smaller and in pons. SVN, MVN, and IVN ascend within medial longitudinal fasciculus. LVN descend the spinal cord within the lateral vestibulospinal tract and end at sacrum. MVN also descend the spinal cord, within the medial vestibulospinal tract, ending at lumbar 1.
Thalamic reticular nucleus distributes information to various other thalamic nuclei, regulating the flow of information. It is speculatively able to stop signals, ending transmission of unimportant info. The thalamus relays info between pons, motor cortices, and insula.
Insula is also heavily connected to motor cortices; insula is likely where balance is likely brought into perception.
The oculomotor nuclear complex refers to fibers going to tegmentum, red nucleus, substantia nigra, and cerebral peduncle. Nucleus of Cajal are one of the named oculomotor nuclei, they are involved in eye movements and reflex gaze coordination.
Abducens solely innervates the lateral rectus muscle of the eye, moving the eye with trochlear. Trochlear solely innervates the superior oblique muscle of the eye. Together, trochlear and abducens contract and relax to simultaneously direct the pupil towards an angle and depress the globe on the opposite side of the eye. The pupil is not only directed but often rotated by these muscles.
The thalamus and superior colliculus are connected via lateral geniculate nucleus. Superior colliculus is the topographical map for balance and quick orienting movements with primarily visual inputs. SC integrates multiple senses.
Other animals
Some animals have better equilibrioception than humans, for example a cat uses its inner ear and tail to walk on a thin fence.Equilibrioception in many marine animals is done with an entirely different organ, the statocyst, which detects the position of tiny calcareous stones to determine which way is "up".