Balance and Equilibrium - Labyrinthine Reflexes - Expected Results
EQUILIBRIUM (from your Silverthorn text): sensory information from proprioceptors in muscles and joints
and photoreceptors in our eyes are linked with our sense of equilibrium in the inner ear to help coordinate our
balance through coordinated muscle action mediated by cerebellum. The primary sense of equilibrium is
mediated through hair cells located in the fluid-filled vestibular apparatus and semicircular canals of the
inner ear. These receptors respond to changes in rotational, vertical, and linear acceleration. The primary
stimulus is gravity and acceleration.
The special sense of equilibrium has 2 components:
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a dynamic component that tells us about our movement through space and a
static component that tells us if our head is displaced from it normal upright position.
The three semicircular canals sense rotational acceleration in various directions, and the otolith
organs tell us about linear acceleration and head position
The receptor (hair cells) works like the hair cells in the Organ of Corti. When the cilia bend in one direction,
they depolarize, and when they bend in the opposite direction, they hyperpolarize.
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As the head turns, the bone structure of the skull and the membranous labyrinth move, but the
endolymph with the canals cannot keep up because of inertia. The drag of the fluid bends the cupula and
its hair cells in the direction opposite to the way the head is turning.
For an analogy, think of pulling a paintbrush (the crista attached to the wall of the canal) through sticky
wet paint (the endolymph) on a board. If you pull the brush to the right, the drag of the paint on the
bristles bend them back to the left. In the same way, the inertia of the fluid in the semicircular canal
pulls the cupula and the cilia of the hair cells to the left when the head turns right.
If the rotation continues, the rotation of the endolymph finally catches up. Then if the head rotation stops
suddenly, the fluid has built up momentum and cannot stop as fast. The fluid continues to rotate in the
direction of the head rotation, leaving the person with a turning sensation. If the sensation is strong
enough, the person may throw his/her body in the direction opposite the rotation in a reflexive attempt to
compensate for the apparent loss of equilibrium.
The otoliths are arranged to sense linear forces.
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The maculae of the utricle are horizontal when the head is in its normal upright position. If the head tips
back, the otoliths in the gelatinous mass slide backward owing to gravity acting on the dense particles.
The cilia of the hair cells bend and set off an action potential.
The maculae of the saccule are oriented vertically when the head is erect and are sensitive to vertical
forces, such as dropping downward in an elevator. The brain analyzes the pattern of which hair cells are
sending action potentials (signals) and which are not (hyperpolarized) in order to compute direction of
movement.
The hair cells of the vestibular apparatus stimulate primary sensory neurons in the vestibular nerve. Those
neurons either synapse in the vestibular nuclei of the medulla or run without synapsing to the cerebellum.
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Labyrinthine Reflexes - Expected Results
1. Stimulation of lateral semi-circular canals (detect “no” motion) = head bent forward 30°, right rotation.
o Rotary Feeling: spinning to the right
o Rotatory Nystagmus (not seen because subject is spinning, eyes closed): fast-right, slow-left.
o Postrotatory Feeling: spinning to the left
o Postrotatory Nystagmus: (seen after spinning stops and subject opens eyes): fast-left, slow-right
o Postrotatory Compensation: Subject leans to the right
2. Stimulation of anterior semi-circular canals (detect “yes” motion) = head on right shoulder, right
rotation.
o Rotatory Feeling: falling backward
o Rotatory Nystagmus (not seen because subject is spinning, eyes closed): fast-up, slow-down
o Postrotatory Feeling: falling forward
o Postrotatory Nystagmus: (seen after spinning stops and subject opens eyes) Fast-down, slow-up
o Postrotatory Compensation: Subject leans to back
3. Stimulation of posterior semicircular canals (detect side-to-side/carwheeling motion) = head forward,
between knees, right rotation.
o Rotatory Feeling: cartwheeling to the right
o Rotatory Nystagmus (not seen because subject is spinning, eyes closed); Fast-diagonally
downward to the right, slow-diagonally upward to the left.
o Postrotatory Feeling: cartwheeling to the left
o Postrotatory Nystagmus: (seen after spinning stops and subject opens eyes): fast diagonally
downward to the left; slow-diagonally upward to the right
o Postrotatory Compensation: lean to the right
4. If the direction of rotation is reversed, all results are reversed.
QUESTIONS:
1. How might your sense of balance and equilibrium be effected in outer space in a “weightless”
environment? Could you sleep comfortably upside in space? on earth?
2. Think about what you have just learned and suggest a cause for “car sickness” and/or “sea sickness.”
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