20.12: Equilibrium and Balance
The inner ear assumes dual functionalities of auditory perception and equilibrium maintenance. The vestibule is the organ responsible for balance. This organ contains mechanoreceptors, specifically hair cells, endowed with stereocilia, which aid in deciphering information regarding the position and motion of our heads. Two intrinsic components, the utricle and saccule, help perceive head position, while the semicircular canals track head movement. Neurological messages initiated in the vestibular ganglion are sent to the brainstem and cerebellum via the vestibulocochlear nerve.
The macula, a tissue found in the utricle and saccule, consists of surrounding support cells encircling the hair cells. The stereocilia, extensions of the hair cells, are embedded in a gelatinous substance called the otolithic membrane. This membrane is topped with a layer of calcium carbonate crystals or otoliths. The otoliths render the otolithic membrane heavy, causing it to move independently from the macula during head movements. In the event of a head tilt, the otolithic membrane, which slides over the macula, is influenced by gravity. This movement subsequently results in the bending of the stereocilia, causing specific hair cells to depolarize while others hyperpolarize. The brain decodes the precise head position through the depolarization pattern of hair cells.
The vestibule extends to form three ring-like structures, the semicircular canals. These are arranged in different planes: one horizontally and the other two vertically at approximately 45 degrees relative to the sagittal plane. Each canal base is connected to a dilated region called the ampulla, which houses hair cells that respond to rotational head movements. The stereocilia of these hair cells extend into a structure called the cupula, located on top of the ampulla. When the head rotates in a plane corresponding to a semicircular canal, the fluid inside the canal lags, causing the cupula to deflect oppositely to the head movement. The semicircular canals encompass several ampullae arranged both horizontally and vertically. This arrangement allows the vestibular system to decipher the direction of diverse head movements within three-dimensional (3-D) space.
Common disorders of the vestibular system:
Motion sickness:
The fundamental cause of motion sickness resides in the complex interactions between our sensory systems. The vestibular system in the inner ear plays a crucial role in maintaining balance and spatial orientation. During travel, the visual input may not align with the signals from the vestibular system, causing a sensory conflict that leads to the symptoms of motion sickness. This disparity between visual and vestibular inputs triggers neurochemical changes within the brain. Specifically, the neurotransmitter histamine is released in higher amounts, contributing to the onset of motion sickness symptoms.
Several factors can influence the onset of motion sickness. Vehicle type is one such factor; some people may be more susceptible to car motion sickness, while others may experience it more frequently on boats or airplanes. The route of travel can also contribute to the severity of motion sickness. For example, winding roads or turbulent flights may trigger more intense symptoms. Individual susceptibility is another key factor. Some people are naturally more prone to motion sickness than others, and various genetic and environmental factors can influence this.
Symptoms of motion sickness can include nausea, vomiting, dizziness, and headaches. These symptoms can significantly impact an individual's daily life, particularly if they need to travel frequently for work or personal reasons.
Benign paroxysmal positional vertigo (BPPV):
Benign paroxysmal positional vertigo (BPPV) is the most common vestibular disorder. It occurs when tiny calcium particles clump up in the inner ear, causing brief episodes of mild to intense dizziness. Diagnosis is typically through the Dix-Hallpike test, which involves observing involuntary eye movements as the patient's head is moved into specific positions. Treatment often involves maneuvers to move the calcium deposits out of the canal they're affecting.
Vestibular neuritis:
Vestibular neuritis, an inner ear inflammation usually caused by viral infections, leads to sudden, severe vertigo, nausea, and imbalance. Diagnosis typically involves ruling out other causes of these symptoms. Treatments usually focus on alleviating the symptoms and may include medications, vestibular rehabilitation therapy, and lifestyle changes.
