Physiology of Balance and Vestibular Functions Prof Vajira

Physiology of Balance and Vestibular Functions Prof. Vajira Weerasinghe Dept of Physiology

Major functions n 1. The primary organ of equilibrium n 2. Major role in the subjective sensation of motion and spatial orientation n 3. Adjustments of muscle activity and body position to allow for upright posture n 4. Helps stabilise the eyes in space during head movements

Vestibular apparatus Semicircular canals n Utricle n saccule n

Vestibular apparatus n n Sense organs of the vestibular system are mechanoreceptors The semicircular canals are so arranged that they lie in planes orthogonal to one another

Semicircular canals n Semicircular canals sense angular acceleration n The three semicircular canals have swellings, called ampullae and within each ampulla is the sense organ, called the crista n In the cristae the hairs of the hair cells are embedded in a gelatinous mass, called the cupula, which extends across the ampulla

During movement n Rotational angular acceleration of the whole canal causes fluid to be left behind on account of its inertia n Fluid inertia during angular acceleration results in displacement of the cupula and bending of the sensory hairs n This is the adequate stimulus for exciting the hair cell

During movement of the cupula and its embedded hairs during rotation first in one direction and then in the opposite direction

Transduction in semicircular canals n the resultant bending of the cilia causes cell – excitation when the bending is toward the kinocilium – with a resultant increase in the firing frequency of vestibular nerve fibres – inhibition when away from the kino-cilium – with a resultant decrease in the firing frequency of vestibular nerve fibres

Transduction in semicircular canals n n the sensory cells exhibit a "resting discharge" modified (increased or decreased) depending on the direction in which the cupula is deflected depending on the direction of rotation n the cilia-kinocilium complexes of the hair cells in the semicircular canals are oriented in one direction n all the hair cells in each canal one of the three canals are maximally excited by the same direction of angular rotation

Semicircular canals n the neural output from ampulla represents the velocity at which the canal is being rotated over the range of normal head movements n the canal mechanism therefore preforms a mathematical integration of the input signal (the integral of acceleration = velocity) n due to the very small size of the canal (diameter of about 0. 3 mm), which results in a large increase in the viscous properties of the fluid

Semicircular canals n semicircular canal system acts as a precise angular (rotational) speedometer n its neural output is directly proportional to the angular (rotational) velocity of head movements n By combining the input from each of the three canals, the brain can create a representation of the vector which describes the instantaneous speed of head rotation relative to 3 D space : a 3 D speedometer!

Otolith organs (utricle and saccule) n The utricle and the saccule are two sac-like structures n each of which contains a specialized region (the macula) n which is made up of a ciliated sensory epithelium (the vestibular hair cells

n the hair cells in the vestibular system differ from those in the auditory system n Each vestibular cell has a number of thin stereocilia and one thicker longer kinocilium positioned at one end of the cell’s hair-bearing surface n Hair cells exhibit a constant "resting discharge activity" even in the absence of a stimulus n Thus, stimulation is sensed by the CNS as a change in this resting, "spontaneous" discharge rate

n The cilia which emerge from the hair cells are embedded in a gelatinous matrix (the otolith membrane) containing solid Ca. CO 3 crystals (the otoconia) which overlies the cells n During linear acceleration, the crystals (being denser than the surrounding fluid) will tend to be left behind due to their inertia

n resultant bending of the cilia causes cell n excitation when the bending is toward the kinocilium – with a resultant increase in the firing frequency of the afferent sensory fibres n inhibition when away from the kinocilium – with a resultant decrease in the firing frequency of the afferent sensory fibres

n Macula of the utricle lies in the horizontal plane – Respond to horizontal linear force n Macula of the saccule lies in the vertical plane – Respond to vertical linear force

n. Within each organ, the cilia-kinocilium complexes of hair cells are spatially arranged such that all possible directions of linear movement are represented

n Since they are sensitive to acceleration, the otolith organs detect – the direction and magnitude of gravity – transient linear accelerations due to movement

Central connections n The maculae and cristae are innervated by bipolar neurons of the vestibular ganglion n The central processes of these cells form the vestibular nerve which enters the brain stem at the cerebellopontine angle medial to the cochlear nerve n The vestibular nerve bifurcates into short ascending and long descending branches which are distributed to the vestibular nuclei

n Some vestibular nerve fibers continue without interruption to the ipsilateral cerebellar cortex and one of the deep cerebellar nuclei n Most primary vestibular fibers terminate differentially in the four main vestibular nuclei in the floor of the fourth ventricle n The vestibular nuclei give rise to secondary vestibular fibers which project to specific portions of the cerebellum, certain motor cranial nerve nuclei and to all levels of the spinal cord

Role of vestibular system

Role in posture n Vestibular postural reflexes – Tonic labyrinthine reflex – Labyrinthine righting reflex

Postural reflexes n tonic labyrinthine reflex n Stimulus: gravitational pull n Response: contraction of limb extensors n receptors: vestibular organs n (work through vestibulospinal tract) n labyrinthine righting reflex n Stimulus: gravitational pull n Response: attempt to maintain head level n receptors: vestibular organs Video

Vestibulo-ocular reflex n The is an important mechanism by which unblurred vision is made possible during the head movements that are generated during every day life n If the head is turned to the left, the balance of the afferent neural information from the two horizontal canals on each side of the head would cause the eyes turn to the right – opposite direction to head movement n The leftward head movement causes n This difference in activity between the left and right afferent fibers is responsible for generating an oppositely directed eye movement at the same velocity as the head movement n This eye movement "reflex" is very important n It allows us to keep our retina fixed on the same point in visual space both during and following naturally generated head movements – an increase in the activity of left horizontal canal hair cells and afferent fibers – a decrease in the activity in the horizontal canal hair cells and afferent fibers Video

Eye movements n Saccadic movement n Smooth pursuit movement

What is nystagmus? n Periodic rhythmic oscillatory movements of the eyeball

Various types of nystagmus Jerk nystagmus (with fast and slow phases) n Pendular nystagmus (like sine waves) n Horizontal, vertical or rotatory n Downbeat or upbeat n Pathological n – Vestibular disorder, cerebellar disorder n Physiological – Optokinetic

Optokinetic nystagmus

Physiological basis of nystagmus n Visual fixation n Vestibulo-ocular reflex n Neural integrator – cerebellum, ascending vestibular pathways, and oculomotor nuclei are important components of the neural integrator

Oculocephalic reflex (Doll’s head phenomenon, Doll’s manoevre) • In comatose or severely lethargic patients, the vestibulo-ocular reflex can be used to test whether brainstem eye movement pathways are intact • The oculocephalic reflex, a form of the vestibulo-ocular reflex, is tested by holding the eyes open and rotating the head from side to side or up and down • this test consists of the rapid rotation of the patient's head in a horizontal or vertical direction • the eyes move conjugately in the opposite direction of the head turn • vestibular nuclei and medial longitudinal fasciculi should be intact Doll’s head phenomenon

Caloric testing n Instilling cold or warm water into the external auditory canal can produce nystagmus n This produce the same movement of endolymph in the semicircular canals produced by rotations of the head n Cold water induces nystagmus away from the ear being irrigated n Water water induces nystagmus towards the ear being irrigated Video

Vertigo n Feeling of rotation n May be positional or non-positional n Causes – Inner ear disorders – Vestibular disorders – Brain stem lesions