Balance

===Lecture Details===
Ramesh Rajan; Week 6 MED1022; Physiology
===Lecture Content===
Vestibular system maintains a stable image, allows spatial orientation. It is critical for adaptive interaction, involved in motion sickness, dizziness, nausea. Otolith organs detect head translation in three planes; self motion or gravity. Semicircular canals detect rotation on three axes: self motion. Otolith organs are the saccule and utricle, which are located in the macula and respond to gravitation or linear acceleration. The semicircular canals are horizonal, rostral and caudal vertical. They are located in the ampulla and look at rotational acceleration.
Damage may result in imbalance, circling, head tilt, vertigo and nausea. Is important for falls in elderly- 1/3 of 65+ fall per year. Reduced reaction time is most commonly associated cause of falls. 
The vestibular system is located in the cochlea, in the bony labryinth (cochlea, vestibule, semicircular canals). Membranous labyrinth is suspended in the bony labyrinth. In the bony labyrinth is perilymph (similar to extracellular fluid), in membranous labyrinth is endolymph (similar to intracellular fluid) which is continuous with the endolymph of the cochlea. Endolymph has a density greater than water, secreted by stria vascularis of the cochlear duct, absorbed by endolymphatic sac and moves freely within canal in response to head rotation.
The vestibular canals contain hair cells which stick up into the endolymph which possess stereocilia. Beyond the tallest stereocilia is the kinocilium. Bending the stereocilia in one direction causes excitation, in the other hyperpolarisation (inhibition).
The semicircular canals are at 90 degrees to each other. Lateral SCC is tilted up 30 degrees so the two other are also tilted 30. Each canal has a coplanar mate, points in the opposite direction, each posterior is paired with anterior. Hair cells are oriented in opposite directions in coplanar mates. Each canal is enlarged at one end to form the ampulla which contains crista ampullaris, receptor hair cells rest on the crista. Each canal has continuous circle of fluid interrupted at the cupula; cilial cells of HCs are embedded at cupula, forms a seal across the ampulla but is compliant enough to allow displacement of endolymph and ciliae. As the head moves, acceleration causes the fluid to lag behind due to inertia. There is motion of the head and fluid movement in the opposite direction; displaces the cupula and bends stereocilia- with continued motion, head catches up. SCCs therefore signal transient events (changes in acceleration).
Bending stereocilia towards KC opens K channels (bathed in endolymph which is high in K), there is a K influx down the gradient- towards kinocilia is excitation, away is inihbition (K moves out of HC, Ca moves in, depolarisation). Ca influx causes release of glutamate to vestibular neurons to produce AP. Coplanar pairing has opposite depolarisations in response to movement. Brain compares this input.
Otoliths are located in the vestibule of the bony labyrinth. Saccule does vertical movement, utricle does horizontal. It exerts a tonic influence on body musculature, reinforces muscle tone and excites contractions necessary to maintain equilibrium. Necessary for static, postural, tonic neck and righting reflex (also called static labryinth). Each contains a macula which is sensory epithelium in otolith organs. Macula is lined with hair cells, there is a striola in the centre which all HCs have their kinocilia oriented towards. Stereocilia project into gelatinous layer of otolithic membrane. Upper layer of membrane is fibrous, resting on top of this membrane are otoliths. Tilting of gravity allows pull of gelatinous otolithic membrane. Sustained tilt results in sustained signalling. Transient tilt such as in acceleration has transient signal.
Saccular macula is mainly in the vertical plane (mostly activated when the head is tilted to the side), utricular is mainly in the horizontal plane (mostly when head is bent forward or backwards).
Vestibular-ocular reflex is to maintain a stable image on the retina; vestibular-cervical and vestibular-spinal are to maintain stable head and body position.
===Readings===
Guyton 55, Cortical and brainstem control of motor function 692-97 (11)