Vestibular System (PHYSIO 10) PDF

PHYSIO 10 - THE VESTIBULAR SYSTEM​ In vertebrates the vestibular system is considered the “inertial guidance system”. It comprises 5 sensory organs in the internal ear. The vestibular system is used to monitor ourselves in space/environment. The system can monitor our movement in terms of linear an...

PHYSIO 10 - THE VESTIBULAR SYSTEM​ In vertebrates the vestibular system is considered the “inertial guidance system”. It comprises 5 sensory organs in the internal ear. The vestibular system is used to monitor ourselves in space/environment. The system can monitor our movement in terms of linear and angular acceleration (2 degrees of movement possible under gravitational force) of the head, being the location of the sensory organ (inner ear). The system is capable of understanding both movements of the head alone or of the head together with the whole body. Some possible functions of the vestibular system: -​ Keeping the eyes still when head moves (walking) without we’d have blurred vision of the world while moving. -​ Maintaining upright posture the system allows to maintain the centre of gravity in a specific area allowing us not to fall down under the action of the gravitational force. -​ Perception of our own movement done with the collaboration of proprioception -​ Perception of space around us (by providing a measure of the gravitational force) 1.​ Anatomy The main peripheral component of the vestibular system is an elaborate set of interconnected chambers, the labyrinth, that is continuous with the cochlea. Receptors of the vestibular system are located in the labyrinth of the inner ear. The inner ear is therefore composed of: -​ the cochlea, dedicated to the acoustic system -​ the labyrinth, dedicated to the vestibular system. It is buried deep in the temporal bone and consists of : o​ the two otolith organs ▪​ the utricle ▪​ the saccule o​ three semicircular canals ▪​ superior ▪​ posterior ▪​ lateral The utricle and saccule are specialized primarily to respond to translational movements of the head and static head position relative to the gravitational axis (i.e., head tilts), whereas the semi- circular canals, as their shapes suggest, are specialized for responding to rotations of the head. The position with respect to the vertical and the horizontal axis of the semicircular canal and the maculae are crucial to understand the function. The cochlear and vestibular spaces are actually joined and the specialized ionic environments of the vestibular end-organ are much like those of the cochlea. The membranous sacs within the bone are filled with fluid (endolymph) and are collectively called the membranous labyrinth. Between the bony walls (the osseous labyrinth) and the membranous labyrinth is another fluid, the perilymph, which is similar in composition to cerebrospinal fluid. The vestibular hair cells are located in the utricle and saccule and in three jug-like swellings called ampullae, located at the base of the semicircular canals next to the utricle. As in the cochlea, tight junctions seal the apical surfaces of the vestibular hair cells, ensuring that endolymph selectively bathes the hair cell bundle while remaining separate from the perilymph surrounding the basal portion of the hair cell. a.​ Semicircular canals In the labyrinth there are 3 semicircular canals: superior, posterior and lateral. These are tubes shaped as a semicircular structure ending up with an enlarged portion called ampulla, where the receptors are host. Tubes are filled with fluid, the endolymph. Receptors are ciliated cells located at the base of the structure together with cells providing support to them (supporting cells). The basilar portion of the ciliated cells is connected with afferent fibres belonging to the vestibular nerve (type II receptors – not proper neurons but cells synapsing with afferent fibres). On the top of the receptors is a gelatinous mass called cupola at the base of which cilia are embedded. Cupola almost completely fills the ampulla, it is not fixed on the ceiling, hence it can be moved on the 2 sides with a certain viscosity and inertia. b.​ Maculae Maculae are located in the saccule and the utricle.  -​ The macula of the utricle lies mainly in the horizontal plane on the inferior surface of the utricle and it plays an important role in determining orientation of the head when the head is upright. -​ The macula of the saccule is located mainly in a vertical plane and signals head orientation when the person is lying down. These receptors consist of row of ciliated cells, supported by sustaining cells. Afferent fibres are connected to the basilar region of the receptors. Cilia are embedded in a gelatinous mass, in which many small calcium carbonate crystals called statoconia are embedded: they have a specific gravity two to three times the specific gravity of the surrounding fluid and tissues so that their weight bends the cilia in the direction of gravitational pull. On top of the layer small solid movable masses are seated, called otoliths.  2.​ Stimulation a.​ In the Semicircular canals Cilia are arranged from tall to small. Kinocilium is the tallest cilium. The stimulus is the deflection of the cilia, which can be inhibitory or stimulatory: -​ movement of the cilia towards the kinocilium leads to a depolarisation (stimulation) -​ movement of the cilia away from the kinocilium leads to a hyperpolarisation (inhibition) -​ when the receptor is at rest cilia not bent Receptors have a background discharge, so that the information conveying to the brain is either negative or positive (this happens only in case of a pre-existent background discharge because if a receptor is completely at rest it is not discharging, and therefore it cannot decrease its discharge). Resting/background discharge nerve impulses -​ increased impulse frequency = depolarization of receptor potential = excitation -​ decreased impulse frequency = hyperpolarization of receptor potential = inhibition 1)​ Mechanoelectrical transduction The depolarising current is an inward potassium current, due to the peculiar properties of the fluid (endolymph) filling the tubes and hence surrounding the cilia of the receptors. The movement of the cilia mechanically opens or closes the K+ channels, where certain proteins behave as springs. When the stereocilia are stretched by movement toward the kinocilium, K+ channels are opened and depolarisation occurs. When the stereocilia are compressed (moving away from the kinocilium) a hyperpolarisation of the hair cells occurs. The background discharge is a constant entrance of K+ currents. The discharge of the fibres connected to the receptors: -​ increases when increasing the K+ inward current (depolarising event) -​ decreases when there is a decreased K+ inward current (hyperpolarising event).  2)​ Architecture matters The kinocilium is always oriented in one direction at rest, all the groups of cilia of a cell are oriented similarly in all the cells of the same ampulla. Cilia of the different cells deflect either on one direction or the opposite (linear movement only). When there is a stimulation, all the cilia will move towards the kinocilium, when there is an inhibition all of the cilia move away from the kinocilium. All of the receptors will be inhibited in one direction and all of them will be excited in the opposite one. Receptors are therefore always oriented in a single direction. b.​ In the maculae In the maculae there is a background discharge as well. In both sacculus and utricle there is a sort of line dividing the structure in 2 functional subsectors, called striola. -​ In the sacculus the kinocilia of the different cells are oriented away from the striola, -​ in the utricle the kinocilia of the different cells are oriented towards the striola. The orientation of the cilia depends on what their relationship is with the ground, due to the force of gravity. Due to the organisation, the movement in a certain direction will interest some cilia, depending on the orientation. A movement in a given direction will lead to a different stimulation or inhibition, some receptors will be strongly inhibited, some strongly activated, some will have an intermediate situation and some will not be interested at all. The intensity of the activation or inhibition of the receptors depends on the direction of the stimulus. From the observation of the arrangement of the receptors in the maculae it is possible to deduce that there is a stimulation with a higher range of movement  3.​ Mechanical stimulation The adequate stimulus for both maculae and the ampulla is a mechanical one. As cilia of the receptors are embedded in the gelatinous structure, the mechanical displacement of the gel leads to a movement of the cilia. The mechanical stimulation comes from the movement of the fluid, coming in turn from the gravitational force. a.​ Semicircular canals Semicircular canals provide information about the angular velocity/acceleration of the head: -​ Turning -​ Tilting -​ Whole body rotation -​ Active/passive locomotion There are 2 labyrinths with respect to the midline – one per each ear. These systems are sensitive to the movements of the head, specifically the head. The head can undergo only 3 types of movement: -​ rotation on the horizontal axis, -​ flexion/extension (up and down) -​ tilting on the coronal axis (right and left). The labyrinth is arranged to have one of the 3 semicircular canals on the exact plane of each possible movement of the head. Whenever the head moves the movement is recorded. The head can be moved on its own or as a consequence of the movement of the whole body. In the semicircular canal is mainly measured the rotational movement, meaning the angular acceleration and velocity. The labyrinth is tuned on acceleration (fast movements) but it can monitor the velocity (i.e. constant or gradual changes in velocity) as well, given that everyday life movement are not going that fast for enough time to be monitored by the vestibule: the vestibule itself is used as an estimator of the velocity. No movement of the head can escape the detection of the semicircular canals. When the head is kept straight (chin aligned to the ground), the orientation of the horizontal canal is not perfectly parallel to ground and the maculae are also almost perpendicular to the ground. It is easier to approximate and assume that, when the head is kept straight: -​ Maculae – the saccule is perpendicular to the ground and the utricle is parallel to it. -​ Semicircular canals – lateral, superior and posterior: one monitoring flexion/extension, one tilting and one rotation. Note: The saccule and utricle lie at 90 degrees to each other. Thus, with any position of the head, gravity will bend the cilia of one patch of hair cells, due to the weight of the otoconia to which they are attached by a gelatinous layer. This bending of the cilia produces afferent activity going through the VIII nerve to the brainstem. There are three pairs of semicircular ducts, which are oriented roughly 90 degrees to each other for maximum ability to detect angular rotation of the head. Each slender duct has one ampulla. When the head turns, fluid in one or more semicircular ducts pushes against the cupula and bends the cilia of the hair cells. Fluid in the corresponding semicircular duct on the opposite side of the head moves in the opposite direction. The semicircular canals of the two ears are arranged three-dimensionally in planes so that the activity of one semicircular canal is coupled with the activity of the functionally correspondent contralateral semicircular canal (i.e. the one situated on the same plane). The canals need to be lying on the same plane in order to work together to detect the same movement: -​ Right anterior and left posterior are coupled, they lie on the same plane. -​ Left anterior and right posterior are coupled, they are on the equivalent plane. Therefore the movements that can be detected by the semicircular canals are in the roll, pitch and yaw axis (= all possible movement of the head).  1)​ Inertia of the fluid. The semicircular canals detect rotational or angular acceleration or deceleration of the head (such when starting or stopping, spinning, somersaulting, or turning the head). Every time the head is moved, the bone moves and the fluid, due to its inertia, moves with a delay, representing the time needed to win the inertia. Inertia is an undirect measure of movement, it is necessary to monitor the inertia to monitor the movement, hence a structure needs to detect the inertia. The cupola is stuck on the ceiling, it has some freedom of movement: -​ when standing still cupola perfectly still and the cilia are arranged in line without any deflection. -​ when the head turns right fluid, due to inertia, does not follow the right movement of the head immediately, but it can be said that it’s flowing in the opposite direction The consistence of the gel is such that every time we move in one direction, the inertia of the fluid acts on the gel and bend it in the opposite direction: when the rotation on the right occurs the cupola bends to the opposite direction; then the cupola comes back to be still when the fluid starts moving, hence when the inertia is won (after the delay). This means that the stimulation occurs in the transient phase, when the movement has just happened, because of the bending of the cupola. If the movement is then constant, the motion of the fluid will be constant, as well as the inertia since it has already been won, and the cupola will then go back to the original position. The receptor hair cells of each semicircular canal are situated on top of a ridge located in a swelling at the base of the canal. The hairs are embedded in an overlying, caplike, gelatinous later, the cupula, which protrude into the fluid within this swelling. Hair cells bend on one side or the other depending on the direction of the rotation. An external force is pushing in a specific direction. The inertia of the fluid applies a force on the cupola with a direction opposite to that provided by the external one. The opposing force leads the cupola to bend in the opposite direction of the movement. When the resistance of the inertia is won, hence it is overcome by the stiffness of the cupola, the fluid will then move. Therefore, the system (semicircular canal in particular) is capable of monitoring the fast transient phase of the movement. After the transient phase has been completed, there is not a tonic stimulation, meaning that the stimulus provided during the transient phase will no longer be provided when the movement is stopped and the neurons go back discharging at the background discharge. Information to the brain is also provided in a quiet state (when there is no movement) through the background discharge: if all of the 6 semicircular canals provide a background discharge, the brain knows that the head is in a steady position (straight up head). Summarising: The cupola is a gelatinous structure with a certain stiffness, it is seated on the hair cells, their specialisations are embedded in its base. The organ is placed in the ampulla of the semicircular canals, where the fluid filling the space is the endolymph, and this has a certain inertia. The cupola is capable of some degrees of movement, thanks to its contact with the ceiling without being fixed to it. When no forces are provided, the cupola is still, cilia are not bent and the fibres fire at the background discharge. When an external force (e.g., movement of the head) is provided, the inertia of the fluid acts on the cupola by pushing in the opposite direction of the movement of the head and as a consequence of the movement of the canal. When the cupola is pushed, and hence bent, the cilia undergo a bending that stimulates the opening of K+ channels providing a stimulation. In the case of a constant motion the cupola goes back to its original orientation only when the inertia is won, it is overcome by the stiffness of the gelatinous structure itself, the fluid starts moving in the direction of the movement of the head and the stimulation is no longer provided -> the system is interested in the fast transient phase.​ The sensors are capable of monitoring both the initial phase of the movement and its final phase. When the motion is stopped, the fluid goes on moving pushing towards the cupola in the opposite direction. In this case the cilia are deflected in the opposite direction. Semicircular canals are phasic receptors, and as all of them (apart from some exceptions – touch receptors) are interested in both onset and offset of the stimulation – important when the stimulus is firstly provided and when it is removed. Acceleration/deceleration of head in any direction causes fluid movement in at least on of the semicircular canals because of their 3D arrangement: -​ no turning no sensation: everything still, background discharge -​ start of turn sensation of turning as moving fluid deflects hairs: depolarization and excitation -​ constant rate turn no sensation after fluid accelerates to same speed as tube wall: no more discharge -​ turn stopped sensation of turning in opposite direction as moving fluid deflects hair in opposite direction it monitors the transient phase (= the acceleration) other than the movement itself (it doesn0t mind constant movement). 2)​ Acceleration and velocity The semicircular canals detect changes in the rate of rotational movement of the head. The angular acceleration is related to the angular velocity, in order for the system to be stimulated by an acceleration, this should be persisted for at least 25 seconds, which is very unusual in nature. In life-normal-head-movements (animal movements of humans) the acceleration phases are very fast and not enough to induce a cupule displacement (acceleration due to cars, airplanes the vestibule would be stimulated differently – the acceleration is felt). In the image, the vestibule stimulation in the specific profile of velocity and acceleration is seen. It can be noted how the modulation of the discharge (depending on firing rate and time) parallels more the profile of the velocity rather than the acceleration. This is similar to the muscle spindle -> profile of discharge parallels both acceleration and velocity. 3)​ Opposite responses in the 2 labyrinths Semicircular canals are functionally paired: each one of them works with the respective in the opposite ear lying on the same plane. For the same movement they perform an opposite effect: one is hyperpolarised (inhibited) and the other depolarised (excited). Indeed, the orientation of the kinocilium in the 2 of them is opposite. This is a way for the brain, as usual, to check for a confirmation. In the image – Schematic of the horizontal semicircular canals complete with hair cells sporting stereo- and kinocilia. Direction of a leftward head rotation is indicated as are the relative fluid movements in the canals. The effect of the fluid movement on the two hair cells is indicated by the bold upward and downward arrows. This is very important, not related to the lecture (about the visual system) It is the same that happens for the off centre on surround in the eye. In this case it is not only a confirmation, the organisation is needed because; if the perception is grounded only on the ON-centre/OFF-surround, when the light is flashed in the surround there is not a very clear discharge of the fibres, there is either a background discharge or a tiny inhibition – there will never be a clear idea of how much light is in the surround. The OFF-centre/ON-surround in this case are needed to provide information about the light stimulating the surround. This is the only way to analyse contrast. Indeed depending on the light being flashed, either the whole surround region or just in a small spot, the OFF-centre/ON-surround change their discharge and provide information on the amount of light flashed, the ON-centre/OFF-surround does not care because when the light is in the surround it is inhibited. Bending the hairs in one direction increases the rate of firing in afferent fibres within the vestibular nerve, whereas bending in the opposite direction decreases the frequency of action potentials in these afferent fibres.  4)​ Possible other stimulations Semicircular canals can also be stimulated by all the possible factors able to change the viscosity or the stiffness of the cupola: -​ thermal stimulation -​ alcohol changes the physical properties of the cupola that becomes more sensitive (dizziness due to alcohol is due mainly to maculae) -​ otoliths detaching from the maculae These situations lead to an abnormal stimulation. b.​ Otolith organs The otolith organs provide information about the position of the head relative to gravity (that is, static head tilt) and detect changes in the rate of linear motion in any direction with respect to gravitational force (moving in a straight line regardless of direction). The brain has to compute which is the resultant force of gravity depending on the movement is being performed, furthermore brain monitors gravitational force even when we are not moving. Otoliths are those free to be submitted to gravitational force. The hairs of the receptor hair cells in these sense organs also protrude into an overlying gelatinous sheet. Many tiny crystals of calcium carbonate, the otoliths (“ear stones”), are suspended within the gelatinous layer, making it heavier and giving it more inertia than the surrounding fluid. Considering as tilting of the macula as seen in the picture. The macula is subjected to a linear acceleration. Gravity force pulls down the otoliths, these are attached to the gelatinous layer, in turn attached to the sustaining cells and to the receptors. Otoliths falling in the direction of the force will make the gel move, and given that the cilia are embedded in the gel, they will move. Otoliths are sitting on the gel but are not completely attached, for this reason a shear force can move them (they are free to move with respect to the macula that is attached to the bone).  1)​ Stimulus: The stimulation is once again a mechanical one. As the head moves, the maculae will tilt depending on the direction of motion. Upon movement in a given direction, the macula tilts, the otoliths being seated on the gel, are falling towards the same direction of the force applied by the motion (gravitational), moving the gel and in turn deflecting the cilia. The deflection of the cilia represents the mechanical stimulation for the receptors. Motion of head -> otoliths move -> bend the gel -> move the cilia. 2)​ Orientation of maculae Standing still on ground upright position -​ Saccule is orthogonal to the ground and the utricle is parallel. -​ The body weight is contrasted by the contact force. -​ The utricle is not stimulated because lined parallel to the ground – no tilting of the otoliths. -​ Otoliths in the saccule (dash line) are vertically oriented with respect to the position of the body, they are constantly driven towards the ground by gravity (constant bending) when standing. -​ The saccule is a clear example of tonic receptors because is subject to a continuous stimulation – it does not provide a background discharge, but an actual excitation (more than background). The brain in this situation receives information from the saccule and none from the utricle, therefore it knows what the position of the head is and in turn of the body. The brain always asks for confirmation. Descending phase of a jump -​ Utricle and saccules are not stimulated, gravity force acts on both in the same way. -​ All the body in the descending phase of a jump undergoes the same force, because no contact force is present. -​ Body an otoliths fall together under the gravity force no stimulation -​ The brain knows that we are moving because there is no stimulation, all of the body is under gravity force. Ascending in a lift -​ The situation of standing still is enhanced. -​ In this case more stiffness is needed to move the otoliths leading to a higher deformation and in turn a higher stimulation. These movements are all parallel to the gravity force. Linear movement – moving inside a car -​ A linear movement is one not along the gravity force. -​ Maculae in this case are submitted to the resultant vector of the gravity force depending on your position with respect to the direction of movement. -​ The result in moving in a direction not parallel to the gravity force is the bending of both utricles and saccules due to the tangential component of gravity. Tilting the ground floor -​ This is a linear movement with a certain angle with respect to the gravity force. Maculae, thanks to their arrangement, are capable to detect whatever movement and position that the body assumes with respect to the ground. In the matrix are different possibilities of movement (in any direction) and given the arrangement of cells in the macula (different orientation towards the striola), there will always be a group of receptors that is highly stimulated. Therefore whatever is the force applied there will always be some receptors that are stimulated. In this way the brain has a double validation system: considering both pairs and also the fact that the brain is capable to understand which of the receptors are activated and which not in a single macula. These informations are used to keep the body not falling on the ground, the brain is always informed about the position of the body with respect to the gravitational force. The sensor does not distinguish between the movement of the body and that of the head alone, the discharge in the 2 cases are exactly the same. In order to distinguish between the movement of the head alone and that of the body together with the head is proprioception. There is an integration with proprioceptive information received from the neck, specifically the cervical spine and associated muscles: -​ Vestibule information without any proprioceptive information of movement from the neck movement of head and body. -​ Vestibule information + proprioceptive information of motion from the neck movement of head alone. Although the actions of the vestibular organs may be separated conceptually and experimentally, actual human movements generally elicit a complex pattern of excitation and inhibition in several receptor organs in both labyrinths.  4.​ Key points The CNS thus receives different patterns of neural activity depending on head position with respect to gravity. The utricle hairs are also displaced by any change in horizontal linear motion (such as moving straight forward, backward, or to the side). The hair cells of the utricle detect horizontally directed linear acceleration and deceleration, but they do not provide information about movement in a straight line at constant speed. The saccule functions similarly to the utricle, except that it responds selectively to tilting of the head away from a horizontal position (such as getting up from bed) and to vertically directed linear acceleration and deceleration. Depending on the movement there can also be the stimulation of the semicircular canals together with that of the maculae. a.​ Vestibular system summary The vestibular nerve fibres have a tonic activity (background), thus receptors are not needed to reach threshold in nerve cells which are already active, but the modulate their activity. This is useful to signal both “positive” (excitation of receptors) and “negative” (inhibition of receptors) events. Receptors in semicircular canal on the same plane work together and signals coming from coupled canals are integrated at central level. 5.​ Vestibular function Signals arising from the various components of the vestibular apparatus are carried through the vestibulocochlear nerve to the vestibular nuclei (a cluster of neuronal cell bodies in the brain stem) and to the cerebellum. Depending on the nucleus of the complex of vestibular nuclei, the output of the information is targeted to different functions and aims: -​ some fibres can be projected towards the eye to control the ocular muscles -​ some fibers descend to provide innervation to body muscles depending on the medial or the lateral vestibulospinal system… The cerebellum receives a high concentration of this information, which with the spinocerebellar tract receives also proprioceptive information. Here, the vestibular information is integrated with input from the eyes, skin surface, joints, and muscles to give rise to compensatory reflexes for: -​ The maintaining balance and desired posture: posture is the desired position, either still or in movement. -​ controlling the external eye muscles so that the eyes remain fixed on the same point, despite movement of the head (the retina cannot receive ‘moving’ information) -​ perceiving motion and orientation – knowledge on where we are going, segments that are moving, the direction of motion…  6.​ Central processing of vestibular system The 1° vestibular afferents enter the brain stem, bifurcate to then ascend and descend the brain stem to terminate within the 4 nuclei of the vestibular nuclear complex (inferior, medial, lateral and superior). The information is then sent to: -​ Visceromotor nuclei (via the reticular formation). -​ The extraocular motor nuclei (via the medial longitudinal fasciculus, MLF). -​ The cerebellum (via the inferior cerebellar peduncle). -​ The spinal cord (via the medial longitudinal fasciculus and lateral vestibulospinal tract). From the vestibular nuclei, specifically from the superior and the medial, a bilateral ascending projection arises, being part of the medial longitudinal fasciculus (MLF), the MLF-ascending branch. The MLF-ascending, then ends up to the oculomotor nerve nuclei (III, IV, VI). From the lateral and inferior vestibular nuclei an unilateral descending projection arises, the MLF-descending branch. a.​ Vestibulocerebellar connections Vestibular information travels in 2 parts of the cerebellum (the flocculonodular lobe first and then the vermis) playing 2 different functions.  b.​ Vestibulospinal connections Information from the vestibular nuclei can descend through 2 descending systems: -​ Medial vestibulospinal – terminates in the upper thoracic segments of the spinal cord. Neck muscles and proximal muscles of the upper limb are controlled. -​ Lateral vestibulospinal – terminates in the lumbar enlargement. Lower limb muscles are controlled (it can also stop in the neck but the medial system is mainly dedicated). Also extensors muscles of the trunk are controlled by descending fibres from both lateral and medial vestibular nuclei. The reticular formation is involved, then descends the spinal cord. Vestibular pathways to cortex The 2° afferents ascend in the lateral lemniscus or near the medial longitudinal fasciculus to reach the thalamus and then information reaching the cortex. From the ventral posterior inferior nucleus of the thalamus (via the posterior limb of the internal capsule), the information is then sent to:  -​ Inferior parietal lobule – area 5 and 7 (posterior parietal cortex), where information of proprioception, visual, and some acoustic are integrated. (This is where is located the representation of the body schema. The real representation of the body cannot be relied on the primary somatosensory cortex because it provides a sensory representation based on the homunculus (somatotopism). If it is wanted to be known where in the world we are and which the real dimensions are, it is necessary to rely on the posterior parietal cortex, that represents the body as it is.) -​ Posterior part of the insula near auditory cortex. -​ Primary somatosensory cortex - central sulcus near the face representation. 7.​ Summary Curiosity The semicircular canals of the huge blue whale are smaller than those of humans. The researchers found that in living cetaceans the semicircular canals are much smaller than in any other mammal of the same body size. In fact, the semicircular canals of the huge blue whale are smaller than those of humans. In general, cetaceans are more acrobatic than similarly sized land animals (imagine an elephant making the jumps of a similar-sized whale). This could be the result of the small canals, because the small size makes the canals less sensitive, preventing the animal from becoming dizzy (i.e. experiencing vertigo). 8.​ Clinical point – MENIERE’S DISEASE History of disease: -​ 1861 Prosper Meniere described a syndrome characterized by deafness, tinnitus, and episodic vertigo. He linked this condition to a disorder of the inner ear. -​ 1938 Hallpike and Cairns described the underlying pathology of Meniere’s disease as being endolymphatic hydrops but the precise etiology still remains elusive. Endolymphatic hydrops is a swelling of one of the tiny, fluid-filled compartments of the inner ear. In a normal inner ear, the fluid is maintained at a constant volume and contains specific concentrations of sodium, potassium, chloride, and other electrolytes. This fluid bathes the sensory cells of the inner ear and allows them to function normally. With injury or degeneration of the inner ear structures, independent control may be lost, and the volume and concentration of the inner ear fluid fluctuate with changes in the body’s fluid/blood. This fluctuation causes the symptoms of hydrops. Symptoms: -​ Periodic episodes of rotatory vertigo or dizziness -​ Fluctuating, progressive, low-frequency hearing loss low tones are not heard very well -​ Tinnitus -​ Fullness/pressure REAL PATHWAYS SEEN ON DELLAVIA LESSON