Structure which responds to rotational movement

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. By comparing the relative movements of all six semicircular canals, the vestibular system can establish movement in any direction within three-dimensional space.

Skip to main content. Chapter 8: The Nervous System. Search for:. Figure 8. Anatomy of the Ear. The outer ear is the auricle and ear canal through to the tympanic membrane.

The middle ear contains the ossicles and is connected to the pharynx by the auditory tube. The inner ear is the cochlea and vestibule which are responsible for hearing and equilibrium, respectively. This work by Cenveo is licensed under a Creative Commons Attribution 3. The stereocilia of the hair cells are bent because they are embedded in the gelatinous cupula.

Shearing of the hair cells opens potassium channels, as discussed at the beginning of the auditory section See Figure Then, press PLAY to watch the reaction to head movement.

There are three pairs of semicircular ducts, which are oriented roughly 90 degrees to each other for maximum ability to detect angular rotation of the head. Each slender duct has one ampulla. When the head turns, fluid in one or more semicircular ducts pushes against the cupula and bends the cilia of the hair cells. Fluid in the corresponding semicircular duct on the opposite side of the head moves in the opposite direction.

The basic transduction mechanism is the same in the auditory and vestibular systems See Figure A mechanical stimulus bends the cilia of the hair cells. Fine thread-like tip links connect to trap doors in the adjacent cilium. Hair cells in the vestibular system are slightly different from those in the auditory system, in that vestibular hair cells have one tallest cilium, termed the kinocilium.

Bending the stereocilia toward the kinocilium depolarizes the cell and results in increased afferent activity. Bending the stereocilia away from the kinocilium hyperpolarizes the cell and results in a decrease in afferent activity.

The semicircular ducts work in pairs to detect head movements angular acceleration. A turn of the head excites the receptors in one ampulla and inhibits receptors in the ampulla on the other side.

Then press PLAY to watch the reaction to head movement. Begin by pressing "expand" to show details from the horizontal semicircular ducts on both sides of the head.

Beneath the ampullae are new details, which highlight the orientation of the stereocilia in both cristae and their outputs. The kinocilia are oriented in the direction of the ampullae ampullo fugal within the ducts on both sides.

The two sides are mirror images. There is a constant low level of ionic influx into the body of the hair cells, so there is a steady-state receptor potential and a spontaneous low-level discharge of afferent activity. These neutral neurophysiological properties are shown in graphs below each ampulla. By pressing the "play" button you will see an animation of this.

A constant low level of spontaneous activity keeps all the muscles slightly and equally contracted, causing the eyes to look straight ahead.

When the head turns, inertia causes the fluid to move more slowly than the head, generating relative fluid motion in the semicircular duct in the opposite direction of the head turn. This moving fluid, shown by arrows in the lumens of the semicircular duct, bends the hair cells on both sides of the head. Because the two sides are mirror images, the stereocilia are bent toward their kinocilium on one side and away from their kinocilium on the other side.

Shearing of the stereocilia toward the kinocilium causes a depolarization of the receptor potential and an increase in afferent action potentials. There is an opposite effect on the other side — a decrease in afferent activity. These counteracting bilateral changes in afferent activity affect the vestibular and occulomotor nuclei.

The ampullo fugal movement of fluid on the patient's right reader's left causes an increase in afferent activity shown in green for "go" in the inset. This has a positive effect on the right medial and superior vestibular nuclei, which in turn stimulate the ipsilateral occulomotor and contralateral abducens nuclei. There are exactly opposite effects on the other side shown in red for "stop" in the inset.

The result of these combined counteracting effects is a smooth movement of the eyes toward the left, keeping the visual field stable as the head turns. Press "expand" to see the utricle at the top of Figure These two similar organs lie against the walls of the inner ear between the semicircular ducts and the cochlea. The receptors, called maculae meaning "spot" , are patches of hair cells topped by small, calcium carbonate crystals called otoconia.

The saccule and utricle lie at 90 degrees to each other. Thus, with any position of the head, gravity will bend the cilia of one patch of hair cells, due to the weight of the otoconia to which they are attached by a gelatinous layer.

This bending of the cilia produces afferent activity going through the VIIIth nerve to the brainstem. Activate Figure The utricle is most sensitive to tilt when the head is upright. The saccule is most sensitive to tilt when the head is horizontal. Unlike the semicircular ducts, the kinocilia of hair cells in the maculae are NOT oriented in a consistent direction.

These movements of the head around an axis are referred to as rotational acceleration, and can be contrasted with linear acceleration, which involves movement forward or backward.

The semicircular canals are filled with a fluid called endolymph, which is similar in composition to the intracellular fluid found within neurons. When the head is rotated, it causes the movement of endolymph through the canal that corresponds to the plane of the movement.

The endolymph in that semicircular canal flows into an expansion of the canal called the ampulla. Within the ampulla is a sensory organ called the crista ampullaris that contains hair cells , the sensory receptors of the vestibular system. Hair cells get their name because there is a collection of small "hairs" called stereocilia extending from the top of each cell.

Hair cell stereocilia have fine fibers, known as tip links, that run between their tips; tip links are also attached to ion channels. When the stereocilia of hair cells are moved, the tip links pull associated ion channels open for a fraction of a millisecond. This is long enough to allow ions to rush through the ion channels to cause depolarization of the hair cells.

Depolarization of hair cells leads to a release of neurotransmitters and the stimulation of the vestibulocochlear nerve. The hair cells associated with the semicircular canals extend out of the crista ampullaris into a gelatinous substance called the cupula , which separates hair cells from the endolymph. When the endolymph flows into the ampulla, however, it causes the distortion of the cupula, which leads to movement of hair cells.

This prompts stimulation of the vestibulocochlear nerve, which transmits the information about head movement to the vestibular nuclei in the brainstem as well as to the cerebellum. The vestibular system uses two other organs, known as the otolith organs, to detect linear acceleration, gravitational forces, and tilting movements. There are two otolith organs in the vestibular labyrinth: the utricle and the saccule.