Course: PG- Pathshala Paper number: 13 Physiological Biophysics Module number M23: Posture and Movement Regulation by Ear Principal Investigator: Co-Principal Investigator: Paper Coordinator: Content Writer: Content Reviewer: Dr. Moganty R. Rajeswari, Professor AIIMS, New Delhi Dr. T. P. Singh, Prof., AIIMS, New Delhi Dr. K. K. Deepak, Prof. & HOD, Physiology AIIMS, New Delhi Dr. Navdeep Ahuja, Sr. Resident AIIMS, New Delhi Dr. Renuka Sharma, Professor VMMC & SJH, New Delhi Objectives: 1. To understand the functional anatomy of the vestibular system 2. To understand the mechanism of transduction 3. To learn about various reflexes mediated by vestibular system 4. To learn about the disorders of vestibular system Section 1: Introduction The Human Ear has mainly two functions i.e. hearing and maintenance of balance and posture. Hearing is the function of cochlea whereas maintenance of balance is the function of vestibular apparatus. Maintenance of posture and movement of limbs is a complex task because i) Movements and posture have to take into account the gravitational force and centrifugal forces acting on the body. ii) The relative position of limbs and head with respect to body has to be calculated. iii) All this has to be calculated quickly in real time. So, regulation of posture and movement needs inputs from vestibular apparatus, visual inputs from eyes, somatosensory information from limbs and proprioceptive information from the joints. Finally, all these inputs are integrated in brainstem mediating various reflexes, as well as cerebellum for precise movements and motor learning. There is a role of basal ganglia and cortex as well, but in this module, the function of vestibular apparatus would be described in detail. The vestibular apparatus consists of semicircular canals and utricle and saccule. Semicircular canals are responsible for detection of rotational motion, utricle detects horizontal acceleration and saccule detects vertical acceleration. The organs responsible for transduction for the linear and rotational motions are cristae ampullaris and maculae. The hair cells present in these organs act as transducers which convert the mechanical energy of motion into action potentials of the vestibular nerve and hence, transmit the information regarding the extent and direction of motion to the central nervous system. The module involves description of functional anatomy of these organs, their functions in different reflexes required for posture and movement and finally some applied aspects. Section 2: Functional anatomy
2.1 Ear and the vestibular apparatus Ear consists of three parts i.e. Outer ear, middle ear and inner ear. Outer ear is mainly responsible for conduction of sound from external environment to the tympanic membrane. The tympanic membrane is connected to the manubrium of malleus which transfers the vibrations of the sound waveform from outer ear to middle ear. Malleus in turn, transfers the vibrations to incus, followed by stapes and then through oval window to the cochlea. Inner ear consists of Bony labyrinth and membranous labyrinth. Bony labyrinth is a series of bony channels which contains the cochlea and the vestibular apparatus. Bony labyrinth is filled with perilymph, which is a fluid resembling the extracellular fluid and is high in N + and low in K +. The membranous labyrinth consists of cochlea and the vestibular apparatus, and is filled with endolymph, a fluid resembling intracellular fluid, rich in K +. Vestibular apparatus consists of three semicircular canals and utricle and saccule. Next section deals with the detailed functional anatomy of vestibular apparatus. Figure 1: Schematic diagram showing the overall structures of ear Chittka L, Brockmann A (2005) Perception Space The Final Frontier. PLoS Biol 3(4): e137. https://doi.org/10.1371/journal.pbio.0030137 2.2 Semicircular canals There are three semicircular canals in the inner ear on one side. They are connected to utricle on one end and have a dilated portion on the other end, which is called ampulla. All three semicircular canals have different orientations. The Horizontal canal is also called lateral, anterior is also called superior and third one is posterior canal. Each canal detects the angular acceleration in orthogonal axes. Bilaterally, both sets of semicircular canals are in mirror symmetry. Hence, three pairs of canals detect angular acceleration in three different axes. Each of the semicircular canals have dilated portion in the end which is called crista ampullaris. Each crista is the organ which transduces mechanical movement into action potentials. 2.3 Crista Ampullaris Crista ampullaris consists of hair cells embedded in the epithelial layer, its hair bundles project into a gelatinous diaphragm called cupula. The hair cells are the specialized cells which contain hair bundles extending outwards in the cupula. The hair bundles consist of one large kinocilium and few stereocilia of progressively decreasing length. The stereocilia are attached to one another by tip links which are made up of cadherin protein. Cupula extends along the length of the ampulla. The cupula is displaced
by the flow of endolymph which results in movement of hair bundles and transduction of mechanical stimulus. 2.4 Utricle and Saccule The otolithic organs, utricle and saccule detect linear acceleration and the relative position of head with that of the body. The organ present in utricle and saccule is called macula. Otolithic organs contain a gelatinous membrane, called otolithic membrane, in which the hair bundles project. The otolithic membrane contains otoconia, also called as ear dust. The hair bundles are arranged in different axes which converge towards striola. Due to this arrangement, utricle can detect linear acceleration in all the horizontal directions. In saccule, almost similar arrangement is present in vertical direction. Figure 2: Schematic diagram showing the vestibular apparatus Section 3: Mechanism of detection of motion by vestibular apparatus 3.1 Hair cell transduction When the hair bundles get displaced in either direction, the tip links get stretched like a spring and open up or close the ion channels attached to them. If the shorter stereocilium moves towards the longer one, the ion channel opening time is increased and vice versa. The opening up of the ion channels causes the movement of cations inside the hair cells and cause their depolarization. The potassium and calcium are the ions which move inside the cell to cause depolarization. After some time, the molecular motors shift the attachment site of tip links, and hence depolarization ceases. Figure 3: Schematic diagram showing mechanism of mechanotransduction by hair cells
3.2 Detection by Utricle and Saccule Utricle detects the changes in head tilt with respect to gravity or horizontal linear acceleration. Gravity causes displacement of the cupula, so does the linear acceleration. Due to relative displacement of cupula in tilt or acceleration, the hair cells get bent and action potentials are produced/inhibited due to depolarization/hyperpolarization caused by the mechanism previously described. The vestibular nerve supplying the otolithic organs have a high baseline firing rate. This enables the information transfer of both depolarization and hyperpolarization by increasing or decreasing the firing rate of the vestibular nerve. The increase or decrease of nerve impulses generated are proportional to the level of tilt or the acceleration. The increase or decrease of nerve impulses may be sustained or may be temporary depending on whether the tilt/acceleration is sustained or transient. The phenomena have been studied in monkeys where action potentials could be recorded from the vestibular nerve while the head was tilted. For more information, the references given at the end of the module may be looked into. While utricle detects the changes of tilt/acceleration in horizontal direction, saccule does so in vertical direction. But, in reality, the movements of the head and body are complex and may have components of both horizontal and vertical axes. So, a combination of the impulses generated by both utricle and the saccule is sent to the central nervous system which indicates the actual direction of tilt/acceleration. 3.3 Detection by Semicircular Canals The semicircular canals have ampula at their ends which contains the crista ampullaris. Similar to the nerve signals of the otolithic organs, crista also has a high baseline firing rate. When the head is rotated with a angular acceleration, the crista is deformed. There are three pairs of semicircular canals. So, in case of angular acceleration in one axis, one of the cupula gets displaced to ipsilateral side while the cupula of opposite side gets displaced to the other side. So, there is depolarization in one of the sets of hair cells and hyperpolarization in the other set. In case of sustained angular motion, initially there is an angular acceleration which causes increase of impulse firing rate in one side and decrease in other side. However, after some time, the angular velocity becomes constant and cupula comes back to its original position. This causes the change in nerve firing to cease and it returns to the baseline condition. However, whenever the head stops moving, the cupula is displaced in the opposite direction due to inertia of motion and impulses are fired in opposite manner.
Figure 4: Schematic diagram showing movement of cupula causing mechanotransduction in semicircular canals Section 4: Vestibulo-Ocular reflex 4.1 Mechanism Vestibular apparatus helps in fixation of gaze to one point while the head and hence eyes, are rotating. The neural connections of the vestibular nucleus are responsible for this reflex. The horizontal semicircular canals detect the motion of the head in the horizontal plane and send the impulses to the vestibular nucleus. The medial and the lateral vestibular nuclei send the impulses to the contralateral abducens nucleus and to the ipsilateral oculomotor nucleus via medial longitudinal fasciculus. Consider an example, the head starts rotating towards the left side. This will cause an increased firing in the left horizontal semicircular canal. This will cause excitation of the neurons in the contralateral abducens nucleus which causes contraction of the right lateral rectus and moves the right eye towards right side. Also, it sends excitatory information to the left oculomotor nucleus supplying the left medial rectus and will cause the movement of left eye towards the right side. Hence, both the eyes move towards right in a leftward movement of the head and hence, stabilize the gaze. Figure 5: Pathway of Vestibulo-ocular reflex
.Koen / Wikimedia Commons / CC-BY-SA-3.0 / GFDL 4.2 Nystagmus In case of continued movement of the head towards one side, the movement of opposite side may be restricted by the bony orbit. In that case, after a certain period, eyes rotate to the side of movement with very high velocity as a corrective response which is called the fast phase of the nystagmus. Because of continued rotation, eyes again start moving in opposite direction to stabilize the gaze which is called the slow phase. These two phases combined are called nystagmus. Nystagmus can be elicited by continuous movement of head in normal individuals or it may be pathological in certain diseases. Section 5: Vestibulo-cervical and Vestibulo-spinal reflexes and role of higher centers 5.1 Vestibulo-Cervical reflex: This reflex maintains the position of the head in case of tilting of the body. In case of falling, the angular acceleration can cause the head to be dorsiflexed which can activate extension of upper limbs and flexion of lower limbs and maintain a protective posture. The neural pathway involves the medial vestibular nucleus which send the connections to the cervical spinal cord neurons. 5.2 Vestibulo-Spinal reflexes: It helps in maintaining the upright posture of the body. The connections include the medial and lateral vestibular nuclei sending projections to spinal neurons particularly supplying the extensor muscles and inhibits the flexor muscles. Hence, in response to any tilt, it activates the extensor muscles and maintains the posture of the body. In normal conditions, vestibulospinal reflexes are under inhibitory control by higher centers. In case of transection at mid brain level, decerebrate rigidity happens which is the result of excessive activation of extensor muscles by this reflex. It has been seen that in lesions of vestibular system, decerebrate rigidity decreases. 5.3 Higher centers: Superior and lateral vestibular nuclei also send information to the higher cortices via postero-lateral nuclei of thalamus. This helps in conscious perception of motion of the body and
cortex integrates the information coming from vestibular apparatus, somatosensory and proprioceptive information. Section 6: Applied Aspects 6.1 Video Nystagmography and Calorimetry In disorders of vestibular apparatus, symptoms such as vertigo and nystagmus happens because of altered vestibular reflexes. To assess the function of vestibular apparatus calorimetry is done to stimulate it and the response of nystagmus is tested objectively by video nystagmography. In calorimetry, warm or cold air/water is injected into the external auditory meatus. It increases/decreases the temperature of middle ear and then inner ear. When the temperature of membranous labyrinth is altered, convection currents are set up in the semicircular canals. These convection currents cause activation of crista and induces the nystagmus. Cold air causes nystagmus in opposite direction and warm air causes nystagmus in same direction. The nystagmus thus elicited is recorded by infrared camera which records the movement of pupil of the eye. The movement of eyes is assessed for adequate and symmetrical movement. Any lesion of vestibular apparatus or the nerves supplying it may cause inadequate movment of eyes in response of calorimetry. 6.2 BPPV (Benign Paroxysmal Positional Vertigo) This is a benign and transient disorder of otolithic organs. Sometimes, the otoconia get deposited and cause persistent activation of otolithic organ thereby eliciting a false sensation of rotational motion which is perceived as vertigo to the patient. It can cease spontaneously or might require movement of head in particular sequence to reposition the otoconia. Summary: Ear has two main functions i.e. Hearing and Maintenance of balance. Hearing is the function of cochlea whereas vestibular apparatus functions to regulate posture and body movements. Vestibular apparatus consists of semicircular canals which detect the angular acceleration, utricle which detects the horizontal linear acceleration and saccule which detects the vertical linear acceleration. The macula and crista, which are the organs responsible for performing these functions, have hair cells with hair bundles on them. The hair bundles have ion channels which increase their opening time when the hair bundles are deflected by the mechanical movement of the fluid around them whenever there is motion be it angular or linear motion. The influx of ions through these channels cause their depolarization/hyperpolarization and neurotransmitters are released. Neurotransmitters generate the action potentials in the vestibular nerve which is the afferent nerve for the vestibular apparatus. Vestibular nerve sends signals to the vestibular nuclei which mediate vestibulo-ocular reflex, vestibulo-cervical and vestibulo-spinal reflexes and hence, regulate posture and movement.