Relationship between stimuli and receptors for static equilibrium

Anatomy and Physiology

The saccule is a bed of sensory cells situated in the inner ear. The saccule translates head The vestibular system is important in maintaining balance, or equilibrium. The difference between them is that the utricle is more sensitive to horizontal acceleration, whereas the saccule is more sensitive to vertical acceleration. The Utricles and Saccules Contain Hair Cells That Respond to Static Forces of The macula is the receptor found in the utriculus and saccule, which are of the vestibular neurons that are in synaptic relationship with the base of the hair cells. of the saccule may be more sensitive to vibrational stimuli and loud sounds. we spend our lives in an ocean of sensory stimuli: light gravity electrical All sensory receptors are “connected to” our CNS by way of sensory monitors relationship of external to internal environment .. static equilibrium. → orientation wrt.

Decompression reverses both the direction of endolymph movement and the turning of the head and eyes. The hydrodynamic concept was proved correct by later investigators who followed the path of a droplet of oil that was injected into the semicircular canal of a live fish. At the start of rotation in the plane of the canal, the cupula was deflected in the direction opposite to that of the movement and then returned slowly to its resting position. At the end of rotation it was deflected again, this time in the same direction as the rotation, and then returned once more to its upright stationary position.

These deflections resulted from the inertia of the endolymph, which lags behind at the start of rotation and continues its motion after the head has ceased to rotate. The slow return is a function of the elasticity of the cupula itself.

relationship between stimuli and receptors for static equilibrium

These opposing deflections of the cupula affect the vestibular nerve in different ways, which have been demonstrated in experiments involving the labyrinth removed from a cartilaginous fish.

The labyrinth, which remained active for some time after its removal from the animal, was used to record vestibular nerve impulses arising from one of the ampullar cristae.

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When the labyrinth was at rest there was a slow, continuous, spontaneous discharge of nerve impulses, which was increased by rotation in one direction and decreased by rotation in the other. In other words, the level of excitation rose or fell depending on the direction of rotation. The deflection of the cupula excites the hair cells by bending the cilia atop them: Electron-microscopic studies have shown how this polarization occurs.

The hair bundles in the cristae are oriented along the axis of each canal. For example, each hair cell of the horizontal canals has its kinocilium facing toward the utricle, whereas each hair cell of the superior canals has its kinocilium facing away from the utricle.

In the horizontal canals, deflection of the cupula toward the utricle—i. Deflection away from the utricle causes hyperpolarization and decreases the rate of discharge. In superior canals these effects are reversed. Detection of linear acceleration: The left and right utricular maculae are in the same, approximately horizontal, plane and, because of this position, are more useful in providing information about the position of the head and its side-to-side tilts when a person is in an upright position.

The saccular maculae are in parallel vertical planes and probably respond more to forward and backward tilts of the head. Both pairs of maculae are stimulated by shearing forces between the otolithic membrane and the cilia of the hair cells beneath it.

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The hair bundles of the macular hair cells are arranged in a particular pattern—facing toward in the utricle or away from in the saccule a curving midline—that allows detection of all possible head positions. These sensory organs, particularly the utricle, have an important role in the righting reflexes and in reflex control of the muscles of the legstrunk, and neck that keep the body in an upright position. The role of the saccule is less completely understood.

Some investigators have suggested that it is responsive to vibration as well as to linear acceleration of the head in the sagittal fore and aft plane.

Human ear - The physiology of balance: vestibular function |

Of the two receptors, the utricle appears to be the dominant partner. There is evidence that the mammalian saccule may even retain traces of its sensitivity to sound inherited from the fishes, in which it is the organ of hearing.

Disturbances of the vestibular system The relation between the vestibular apparatus of the two ears is reciprocal. When the head is turned to the left, the discharge from the left horizontal canal is decreased, and vice versa.

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Normal posture is the result of their acting in cooperation and in opposition. When the vestibular system of one ear is damaged, the unrestrained activity of the other causes a continuous false sense of turning vertigo and rhythmical, jerky movements of the eyes nystagmusboth toward the uninjured side.

When the vestibular hair cells of both inner ears are injured or destroyed, as can occur during treatment with the antibiotics gentamicin or streptomycinthere may be a serious disturbance of posture and gait ataxia as well as severe vertigo and disorientation. In younger persons the disturbance tends to subside as reliance is placed on vision and on proprioceptive impulses from the muscles and joints as well as on cutaneous impulses from the soles of the feet to compensate for the loss of information from the semicircular canals.

Recovery of some injured hair cells may occur. Routine tests of vestibular function traditionally have involved stimulation of the semicircular canals to elicit nystagmus and other vestibular ocular reflexes. There, it is sorted out and integrated with learned information contributed by the cerebellum the coordination center of the brain and the cerebral cortex the thinking and memory center.

The cerebellum provides information about automatic movements that have been learned through repeated exposure to certain motions. For example, by repeatedly practicing serving a ball, a tennis player learns to optimize balance control during that movement.

Contributions from the cerebral cortex include previously learned information; for example, because icy sidewalks are slippery, one is required to use a different pattern of movement in order to safely navigate them.

Processing of conflicting sensory input A person can become disoriented if the sensory input received from his or her eyes, muscles and joints, or vestibular organs sources conflicts with one another. For example, this may occur when a person is standing next to a bus that is pulling away from the curb.

The visual image of the large rolling bus may create an illusion for the pedestrian that he or she—rather than the bus—is moving. However, at the same time the proprioceptive information from his muscles and joints indicates that he is not actually moving.

Sensory information provided by the vestibular organs may help override this sensory conflict. In addition, higher level thinking and memory might compel the person to glance away from the moving bus to look down in order to seek visual confirmation that his body is not moving relative to the pavement.

Motor output As sensory integration takes place, the brain stem transmits impulses to the muscles that control movements of the eyes, head and neck, trunk, and legs, thus allowing a person to both maintain balance and have clear vision while moving.

Motor output to the muscles and joints A baby learns to balance through practice and repetition as impulses sent from the sensory receptors to the brain stem and then out to the muscles form a new pathway.

With repetition, it becomes easier for these impulses to travel along that nerve pathway—a process called facilitation—and the baby is able to maintain balance during any activity. This pathway facilitation is the reason dancers and athletes practice so arduously.

relationship between stimuli and receptors for static equilibrium

Even very complex movements become nearly automatic over a period of time. This also means that if a problem with one sensory information input were to develop, the process of facilitation can help the balance system reset and adapt to achieve a sense of balance again.

For example, when a person is turning cartwheels in a park, impulses transmitted from the brain stem inform the cerebral cortex that this particular activity is appropriately accompanied by the sight of the park whirling in circles.

With more practice, the brain learns to interpret a whirling visual field as normal during this type of body rotation. Alternatively, dancers learn that in order to maintain balance while performing a series of pirouettes, they must keep their eyes fixed on one spot in the distance as long as possible while rotating their body. Motor output to the eyes The vestibular system sends motor control signals via the nervous system to the muscles of the eyes with an automatic function called the vestibulo-ocular reflex VOR.

When the head is not moving, the number of impulses from the vestibular organs on the right side is equal to the number of impulses coming from the left side. When the head turns toward the right, the number of impulses from the right ear increases and the number from the left ear decreases.

The difference in impulses sent from each side controls eye movements and stabilizes the gaze during active head movements e. The coordinated balance system The human balance system involves a complex set of sensorimotor-control systems.

Its interlacing feedback mechanisms can be disrupted by damage to one or more components through injury, disease, or the aging process. Impaired balance can be accompanied by other symptoms such as dizziness, vertigo, vision problems, nausea, fatigue, and concentration difficulties. Thanks to VeDA, vestibular disorders are becoming recognized for their impacts on people's lives and our economy.

We see new diagnostic tools and research studies, more accessible treatments, and a growing respect for how life-changing vestibular disorders can be.

relationship between stimuli and receptors for static equilibrium

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