Mouth and throat, face, and the five senses
Frank J. Dye in Human Life Before Birth, 2019
To perceive vibrations propagated through the air, natural selection has provided us with a high-fidelity, stereophonic receiver, comprising our ears and brain. Sound waves, gathered into our external ear canals by the pinnae, cause first our eardrums and then our middle ear ossicles to vibrate, setting in motion fluid movement in the cochlea of the inner ear. This movement causes stereocilia in the cochlea to bend, resulting in a train of nerve impulses being sent (via cranial nerve VIII) to the brain, which interprets the impulses as sound. In addition, the inner ears possess systems of semicircular canals. These canals allow us to sense our position in space, and consequently maintain balance as we go about our various activities, by detecting fluid motion in three axes (x, y, and z) (Figure 17.8).
Anatomy and Physiology of Balance
R James A England, Eamon Shamil, Rajeev Mathew, Manohar Bance, Pavol Surda, Jemy Jose, Omar Hilmi, Adam J Donne in Scott-Brown's Essential Otorhinolaryngology, 2022
The semicircular canals are orientated so that they provide complementary information about angular movement. Anti-clockwise head rotation (turning your head to the left) increases the nerve-fibre firing rate of the left lateral semicircular canal and decreases the firing rate of fibres from the right lateral semicircular canal. The perception of head rotation is the net result of differential firing rates between these complementary lateral semicircular canals (see Figure 2.4). The superior and posterior canals of opposing ears lie in the same plane. Therefore, with the head rotated 45° to right, head flexion results in an increased fibre firing rate of the right superior semicircular canal and decreased firing rate of the left posterior semicircular canal. With the head rotated 45° to right, head extension causes reversal in this pattern of activation, between the right superior and left posterior semicircular canals.
Examination of the Nervous System
John W. Scadding, Nicholas A. Losseff in Clinical Neurology, 2011
The semicircular canals are the sensors of dynamic changes (angular acceleration) of head position, while the otolith organs, the utricle and saccule, are the sensors of linear acceleration and gravity changes (including head tilt). The hair cell is the basic sensory cell. In each semicircular canal, there is a mound of hair cells, the ampulla, with a divider, the cupula. These hair cells either increase or decrease their firing rate depending on the direction of fluid displacement. In the otolith organs, the hair cells are situated in the maculae and are covered by a crystal-laden gelatinous membrane. These crystals are particles of calcium carbonate, the otoconia. Movements of the head by tilting or by linear acceleration may displace the otoconia and stimulate the otolith hair cells.
Video head impulse testing in patients with benign paroxysmal positional vertigo
Published in Acta Oto-Laryngologica, 2020
Semicircular canals are activated during rapid head movements and otolithic signal is created to provide information to central structures for the direction of motion and the orientation of the body [6]. Compensatory eye movement occurs, opposite in direction to the head movement and is generated to stabilize the image on the target in co-ordination of movement and balance. This is critical to maintain the retinal image for body stability during high velocity head motion. Reflex eye velocity delay occurs in patients with vestibular dysfunction which means significant retinal slip between the position of the target on the retina and the desired position on the fovea. Video head impulse test allows to study the vestibular function of the semicircular canal separately by recording head and eye velocity in the plane of head movement and provides earlier and much information than other tests [7]. It enables to analyze the VOR gain and gain asymmetry and also detects overt or covert saccades.
Dynamic visual acuity in benign paroxysmal positional vertigo
Published in Acta Oto-Laryngologica, 2018
Vestibulo-ocular reflex (VOR) abnormality is not expected in patients with BPPV as long as they do not have any associated inner ear problem since no organic pathology specific to BPPV is reported up to now. However, in view of recent studies, testing the VOR to understand the inner ear function and to analyze how well those patients use their vestibular function in daily life is worth to investigate. Semicircular canals react to head movements. The otolithic signal is created in response to motion, which plays an important role in perception of orientation and the direction of motion [4]. Compensatory eye movement occurs after linear acceleration of the head, opposite in direction to the head movement, and is generated to stabilize the image of the target in co-ordination with movement and balance. Thus, vision is maintained during high-velocity head motion. The impact of dysfunction on the VOR in patients with chronic vestibular problems is associated with inability to have a clear image of the target on the retina during head movements, resulting in blurred vision [4]. This condition is due to a distorted phase relationship between the eye and head movement.
Video head impulse test relevance in the early postoperative period after cochlear implantation
Published in Acta Oto-Laryngologica, 2019
Roseli Saraiva Moreira Bittar, Eduardo Sato, Douglas Josimo Silva Ribeiro, Jeanne Oiticica, Signe Schuster Grasel, Raquel Mezzalira, Robinson Koji Tsuji, Ricardo Ferreira Bento
Our results suggest that vHIT is a specific vestibular test for horizontal semicircular canals and may not be indicated in all patients. Chen et al. [19] suggest a greater risk of horizontal canal impairment. It has been reported that CI electrode insertion may damage osseous spiral lamina, basilar membrane and vestibular sensors as saccule, utricle and semicircular canals [9]. Therefore, a suitable angular VOR gain does not mean absence of vestibular impairment. Other vestibular sensors may be damaged such as saccule and utricle. When the vestibular end organs of implanted patients are examined postmortem, patterns that can be attributed to implantation include saccular membrane distortion as well as fibrosis, calcification, and ossification [20]. These changes are most often observed in the saccule, which is not surprising given the anatomic proximity to the round window and cochlea, but have also been noted with less frequency in the utricle and semicircular canal [20]. This proximity can make them more vulnerable to surgical trauma during electrode insertion and drilling. [12]. Trauma of these structures may be considered in case of instability or imbalance, but not when vertigo is the main complaint. In our study vHIT allowed to identify horizontal semicircular canal impairment after CI surgery when vertigo was the main symptom, but not instability or imbalance. In these cases, cervical and ocular VEMP are the best choice to investigate the affected organs.
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