VESTIBULAR SYSTEM PERIPHERAL RECEPTORS They are of two types 1. Cristae They are located in the ampullated ends of the three semi-circular ducts. These receptors respond to angular accel-eration. 2. Maculac They are located in otolith organs (ie, utricle and sac cule). Macula of the utricle lies in its floor in a horizontal plane. Macula of the saccule lies in its medial wall in a vertical plane. They sense position of head in response to gravity and linear acceleration. A. Structure of a Crista (Fig. 27) It is a crest-like mound of connective tissues on which lie the sensory epithelial cells. The cilia of the sensory hair cells proj ect into the cupula, which is a gelatinous mass extending from the surface of crista to the ceiling of the ampulla and forms a water tight partition, only to be displaced to one or the other side like a swing door, with movements of endolymph. The gelatinous mass of cupula consists of polysaccharide and con tains canals into which project the cilia of sensory cells Hair cells are of two types (Fig. 2.8) type I cells are flask shaped with a single large cup-like nerve terminal surrounding the base: type II cells are cylindrical with multiple nerve terminals at the base. From the upper sur-face of each cell project a single hair, the kinocilium and a number of other cilia, the stereocilia. The kinocilium is thicker and is located on the edge of the cell. Sensory cells are surrounded by supporting cells which show microvilli on their upper ends. B. Structure of a Macula macula consists mainly of two parts: (i) a sensory neu-oepithelium, made up of type 1 and type II cells, similar Hair cells Crista ampullaris Kerspersed supporting calls Ho substance of cupul Vestib medial, come fro Supporting cell Nerve chalice (Type I cel Fig. 2.8 Sensory hair cells of the vestibular organs type 1 (left) and type right to those in the crista; (ii) an otolithic membrane, which is made up of a gelatinous mass and on the top of it the crystals of calcium carbonate called otoliths or otoconia (Fig. 2.9). The cilia of hair cells project into the gelatinous layer. The linear, gravitational and head tilt movements cause displacement of otolithic membrane and thus stim ulate the hair cells which lie in different planes. VESTIBULAR NERVE Vestibular or Scarpa's ganglion is situated in the lateral part of the internal acoustic meatus. It contains bipolar cells. The distal processes of bipolar cells innervate the sensory epithelium of the labyrinth while its central pro cesses aggregate to form the vestibular nerve. CENTRAL VESTIBULAR CONNECTIONS The fibres of vestibular nerve end in vestibular nuclei while some go to the cerebellum directly Vestibular nuclei are four in number-the superior. medial, lateral and descending. Afferents to these nuclei come from: 1. Peripheral vestibular receptors (semicircular canals, utricle and saccule) 2. Cerebellum 3. Reticular formation 4. Spinal cord 5. Contralateral vestibular nuclei Thus, information received from the labyrinthine recep-tors is integrated with information from other somatosensory systems. Efferents from vestibular nuclei go to: 1. Nuclei of CN III, IV, VI via medial longitudinal bundle. It is the pathway for vestibulo-ocular reflexes and this explains the genesis of nystagmus. 2. Motor part of spinal cord (vestibulospinal fibres). This coordinates the movements of head, neck and body in the maintenance of balance. 3. Central, which is made up of nucles and fibre tracts in the central nervous system to integrate vestibular im-pulses with other systems to maintain body balance. 4. Cerebellum (vestibulocerebellar fibres). It helps to coor-dinate input information to maintain the body balance. 5. Autonomic nervous system. This explains nausea, vomiting, palpitation, sweating and pallor seen in ves-tibular disorders (e.g. Ménière's disease). 6. Vestibular nuclei of the opposite side. 7. Cerebral cortex (temporal lobe). This is responsible for subjective awareness of motion. PHYSIOLOGY OF VESTIBULAR SYSTEM Vestibular system is conveniently divided into: 1. Peripheral, which is made up of membranous laby-rinth (semicircular ducts, utricle and saccule) and vestibular nerve. SEMICIRCULAR CANALS They respond to angular acceleration and deceleration. The three canals lie at right angles to each other but the one which lies at right angles to the axis of rotation is stimulated the most. Thus, horizontal canal will respond maximum to rota tion on the vertical axis and so on. Due to this arrangement of the three canals in three different planes, any change in position of head can be detected. Stimulation of semicircular canals produces nystagmus, and the direction of nystagmus is determined by the plane of the canal being stimulated. Thus, nystagmus is horizontal from horizontal canal, rotatory from the superior canal and vertical from the posterior canal. The stimulus to semicircular canal is flow of endolymph. which displaces the cupula. The flow may be towards the cupula (ampullopetal) or away from it (ampullofugal), better called utriculopetal and utriculofugal. Ampullopetal flow is more effective than ampullofugal for the horizontal canal. The quick component of nystagmus is always opposite to the direction of flow of endolymph. Thus, if a person is rotatec to the right for sometime and then abruptly stopped, the endolymph continues to move to the right due to inertia (i ampullopetal for left canal), the nystagmus will be horizont and directed to the left (Fig. 2.10). Remember that nysta mus is in the direction opposite to the direction of flowe endolymph. In other words, the slow component of nystagr is in the direction of flow of endolymph. UTRICLE AND SACCULE Utricle is stimulated by linear acceleration and dece tion or gravitational pull during the head tilts. The sory hair cells of the macula lie in different planes am 3. Otolith use Noise Box Masking. 2.10 Rotation test. At the end of rotation to the night, semior cular canals (SCC) stop but endolymph continues to move to the nght, Le towards the left ampulls but away from the right, causing nystagmus to the left stimulated by displacement of otolithic membrane during the head tilts The function of saccule is similar to that of utricle as the structure of maculae in the two organs is similar, but experimentally the saccule is also seen to respond to sound vibrations. The vestibular system thus registers changes in the head position, linear or angular acceleration and decel-eration, and gravitational effects. This information is sent to the central nervous system where information from other systems-visual, auditory, somatosensory (muscles, joints, tendons, skin) is also received. All this informa-ion is integrated and used in the regulation of equilib ium and body posture. Cerebellum, which is also connected to vestibular end gans, further coordinates muscle movements in their te, range, force and duration and thus helps in the main-mance of balance. AINTENANCE OF BODY EQUILIBRIUM seful clinical approach to understand the physiology quilibrium is to imagine that the balance system (ves-Mar, visual and somatosensory) is a two-sided push pull system. In static neutral position, each side con-tes equal sensory information, ie. push and pull en of one side is equal to that of the other side. If one side pulls more than the other balance of the body is dis the nush and pull system, which la temporary change in the corrected by appropriate reflexes and motor outputs to the eyes (vestibulo-ocular reflex), neck (vestibulocervical reflex) and trunk and limbs (vestibulospinal reflex) to maintain new position of head and body, but if any com. ponent of push and pull system of one side is disturbed for a longer time due to disease, vertigo and ataxia will develop VERTIGO AND DIZZINESS Disorientation in space causes vertigo or dizziness and can arise from disorders of any of the three systems: ves tibular, visual or somatosensory. Normally, the impulses reaching the brain from the three systems are equal and opposite. If any component on one side is inhibited or stimulated, the information reaching the cortex is mis matched, resulting in disorientation and vertigo. The vestibular inhibition on one side (eg acute vestibular failure, labyrinthectomy, Ménière's disease, VIIIth nerve section) causes vertigo. Similarly, stimulation of labyrinth by thermal or rotational stimulus causes vertigo. Dizzi-ness can similarly result from the ocular causes, e.g. high errors of refraction or acute extraocular muscle paralysis with diplopia. Vertigo and its causes are discussed in detail in Chapter 7. MOTION SICKNESS It is characterized by nausea, vomiting, pallor and sweat-ing during sea, air, bus or car travel in certain susceptible individuals. It can be induced by both real and apparent motion and is thought to arise from the mismatch of in-formation reaching the vestibular nuclei and cerebellum from the visual, labyrinthine and somatosensory systems. It can be controlled by the usual labyrinthine sedatives. Simolifued