Along with hearing, the inner ear is responsible for encoding information about equilibrium the sense of balance , which it does in the vestibule and semicircular canals , structures that are sometimes collectively referred to as the vestibular apparatus Fig. Several types of sensory receptors provide information to the brain for the maintenance of equilibrium.
The eyes and proproceptors in joints, tendons, and muscles are important in informing the brain about equilibrium. However unique receptors within the inner ear play a crucial role in monitoring equilibrium. There are two types of equilibrium: static gravitational equilibrium , which involves the movement of the head with respect to gravity and dynamic rotational equilibrium , which involves acceleration of the head in rotation, horizontal, and vertical movements.
Similar to the cochlea, the both the vestibule and semicircular canals use hair cells with stereocilia to detect movement of fluid, in this case, in response to changes in head position or acceleration. The information for static equilibrium and linear acceleration dynamic comes from the utricle and saccule within the vestibule.
The saccule and utricle each contain a sense organ, called the macula , where stereocilia and their supporting cells are found. These maculae plural are oriented 90 degrees to one another so that they respond to positions in different planes Fig.
The organs can respond to changes in position and acceleration because the tips of their stereocilia project into a dense otolithic membrane made up of a mixture containing granules of calcium and protein, called otoliths , translated in medical terminology — ear stones. When the head moves, gravity causes the stones to move. The movement of the stones within the membrane causes the stereocilia to bend, initiating action potentials in the vestibular nerve fibers that innervate them.
Bundles of stereocilia are arranged in various directions, so that any direction of inclination will depolarize a subset of the hair cells. How the body senses head position and the linear horizontal or vertical direction of acceleration is determined by the specific pattern of hair-cell activity across the maculae. The semicircular canals are three ring-like extensions from the vestibule and are mostly responsible for dynamic equilibrium.
One ring is oriented in the horizontal plane and two others are in the vertical plane. At the base of each semicircular canal, where it meets with the vestibule is an enlarged region known as the ampulla , which contains a hair-cell containing structure, called the crista ampullaris that responds to rotational movement. The stereocilia of the hair cells extend into the cupula , a membrane that attaches to the top of the ampulla Fig.
When the head rotates in a plane parallel to the semicircular canal, the fluid in the canal does not move as quickly as the head is moving. This pushes the cupula in the opposite direction, deflecting the stereocilia and creating a nerve impulse. Considering the semicircular canals on either side of the head, three orthogonal planes are defined, the horizontal plane with both horizontal canals, and two vertical planes 90 o to each other with the anterior canal from one side and the posterior canal from the other.
In each pair, deflection of the cupula on one side of the body causes depolarization of the hair cells while the same movement causes hyperpolarization of the hair cells on the other side of the body.
For example, when the head rotates to the right, the horizontal canals are active and the right side depolarizes while the left hyperpolarizes, indicating the direction of the movement. The study concluded that sound information coming through their hearing aids tended to be used by the participants as sensory reference points or landmarks to help maintain equilibrium.
Hearing aids can improve equilibrium by allowing you to hear many more sounds in the environment. The brain can get a better spatial vision and understand auditory signals.
Compared to visual information, which can only be gleaned in front of you, sounds enter you from all directions, and your sense of hearing is crucial to your understanding of your surroundings. Hearing explicitly allows the surroundings to be better understood, and you are less likely to be taken by surprise by something or someone around you. This awareness also improves your balance, and those with hearing aids have far fewer trips, slips drops, and accidents compared to those with untreated hearing loss.
Follow Us Facebook. Deep inside the ear, positioned just under the brain, is the inner ear. It also coordinates the timing and force of muscle movements initiated by other parts of the brain. Image sourced and used with permission from Centre for Neuro Skills. The external ear structures gather sound and direct it toward the eardrum. The middle ear chamber consists of an eardrum and three small ear bones.
These structures transmit sound vibrations to the inner ear fluid. The inner ear chamber labyrinth is encased in bone and filled with fluid endolymph and perilymph. This fluid bathes the delicate nerve endings of the hearing and the balance mechanism.
Fluid waves in the hearing chamber cochlea stimulate the hearing nerve endings which generate an electrical impulse. These impulses are transmitted to the brain for interpretation as sound. Movement of fluid in the balance chambers vestibule and three semicircular canals also stimulates nerve endings, resulting in electrical impulses to the brain, where they are interpreted as motion.
The human balance system is made up of four parts. The brain acts as a central computer receiving information in the form of nerve impulses messages from its three input terminals: the eyes, the inner ear, and the muscles and joints of the body.
There is a constant stream of impulses arriving at the brain from these input terminals. All three systems work independently and yet work together to keep the body in balance. The eyes receive visual clues from light receptors that give the brain information as to the position of the body relative to its surroundings.
The receptors in the muscles and joints are called proprioceptors. The most important ones are in the head and neck head position relative to the rest of the body and the ankles and joints body sway relative to the ground.
The inner ear balance mechanism has two main parts: the three semicircular canals and the vestibule. Together they are called the vestibular labyrinth and are filled with fluid. When the head moves, fluid within the labyrinth moves and stimulates nerve endings that send impulses along the balance nerve to the brain.
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