Equilibrium: Vestibular System, Balance & Vestibular Pathway

Sense of equilibrium

Sense of balance that depends on information from vision, proprioceptors, and the vestibular system.

Three sources of information for equilibrium

Vision, proprioceptors in muscles and joints, and the vestibular system of the inner ear.

Role of vision in equilibrium

Provides visual information that helps the brain determine body position and movement.

Role of proprioceptors in equilibrium

Provide feedback from muscles and joints about body position.

Vestibular system

Structures of the inner ear, including the vestibule and semicircular canals, that provide information about head position and movement.

Vestibular sensation

Sensory information from the vestibular system concerning head position and movement.

Static equilibrium

Maintaining balance when the head and body are not moving but the head is tilted.

Dynamic equilibrium

Maintaining balance while moving.

Vestibule

Inner-ear region containing the utricle and saccule.

Utricle and saccule

Structures within the vestibule that monitor static equilibrium and linear acceleration.

Maculae

Sensory structures in the utricle and saccule containing receptor hair cells.

Hair cells of the maculae

Receptor cells containing stereocilia and one true cilium called a kinocilium.

Stereocilia

Hairlike projections of vestibular hair cells that bend in response to movement.

Kinocilium

Single true cilium on each vestibular hair cell.

Otolithic membrane

Gelatinous membrane in which the stereocilia and kinocilium of macular hair cells are embedded.

Otoliths

Calcium carbonate crystals contained within the otolithic membrane.

Effect of otoliths

Add weight to the otolithic membrane so gravity and acceleration can cause it to shift and bend the hair cells.

Neurons receiving signals from macular hair cells

Bipolar sensory neurons whose axons form the vestibular portion of the vestibulocochlear nerve.

Vestibular hair-cell neurotransmitter

Glutamate.

Effect of bending vestibular stereocilia

Changes the amount of glutamate released onto sensory neurons.

Utricle stereocilia orientation

Stereocilia are oriented vertically when the head is upright and stationary.

Utricle response

Stereocilia bend when the head tilts to either side or backward.

Saccule stereocilia orientation

Stereocilia are oriented horizontally when the head is upright and stationary.

Saccule response

Stereocilia bend in response to up-and-down movements.

Effect of tilting the head

Gravity pulls on the otolithic membrane and bends the stereocilia.

Bending stereocilia toward the kinocilium

Depolarizes the hair cell, increases glutamate release, and increases action-potential frequency.

Bending stereocilia away from the kinocilium

Hyperpolarizes the hair cell, decreases glutamate release, and reduces action-potential frequency.

How the brain determines head position

Maculae are paired, so depolarization and hyperpolarization occur together and the brain interprets the combination of signals.

Linear acceleration

Form of dynamic equilibrium involving movement in a straight line, such as riding in a car or elevator.

Role of inertia in linear acceleration

When movement begins, the hair cells move with the body while the endolymph and otolithic membrane lag behind.

Effect of beginning linear acceleration

Lagging endolymph and otolithic membrane bend the stereocilia and alter vestibular nerve activity.

Effect of stopping linear movement

Endolymph and the otolithic membrane continue moving briefly due to inertia, bending the stereocilia and altering glutamate release.

Vestibular activity after linear movement stops

Returns to baseline after the fluid and otolithic membrane stop moving.

Semicircular canals

Three structures enclosing semicircular ducts that detect rotational movements of the head.

Three semicircular ducts

Anterior, posterior, and lateral semicircular ducts.

Fluid inside semicircular ducts

Endolymph.

Rotational equilibrium

Form of dynamic equilibrium that maintains balance during rotational movements such as turning the head or spinning.

Structures detecting rotational equilibrium

Hair cells within the anterior, posterior, and lateral semicircular ducts.

Ampulla

Bulb-shaped enlarged region of a semicircular duct containing sensory receptors for rotational movement.

Crista ampullaris

Cluster of hair cells and supporting cells located within an ampulla.

Hair cells of the crista ampullaris

Contain stereocilia and one kinocilium.

Cupula

Gelatinous mass in which the stereocilia and kinocilium of crista ampullaris hair cells are embedded.

Function of the semicircular ducts

Register rotation of the head.

Utricle and saccule vs. semicircular ducts

Utricle and saccule detect head tilt and linear acceleration; semicircular ducts detect rotational movement.

Motion sickness

Nausea, dizziness, and sometimes vomiting that can occur during certain types of movement.

Cause of motion sickness

Mismatched sensory information from the visual and vestibular systems.

Motion sickness example

On a rocking boat, the vestibular system detects movement while the eyes may indicate that the body is stationary relative to the boat.

Ways the PowerPoint says motion sickness may be reduced

Antihistamines, a scopolamine patch, sitting in the front seat of a car, or looking at the horizon while on a boat.

Three types of responses needed to maintain equilibrium

Postural muscle movement, conscious awareness of head position or movement, and compensatory eye movements.

Vestibular pathway step 1

Action potentials propagate to the vestibular nuclei at the medulla-pons junction.

Vestibular nuclei

Brainstem nuclei at the medulla-pons junction that receive vestibular information and simultaneously relay it to several CNS regions.

Vestibular pathway destination for conscious awareness

Vestibular nuclei → thalamus → inferior parietal lobe.

Inferior parietal lobe vestibular function

Provides conscious awareness of head position and movement.

Vestibular pathway destination for eye movement

Vestibular nuclei send information to cranial nerve nuclei that coordinate eye movement.

Cranial nerve nuclei involved in vestibular eye responses

Oculomotor, trochlear, and abducens nuclei.

Eye-movement cranial nerves involved in vestibular responses

CN III, CN IV, and CN VI.

Vestibulocochlear reflex

Reflex described by the PowerPoint that allows the eyes to remain fixed on an object while the head is moving.

Vestibular pathway destination for balance and posture

Vestibular nuclei send information to the cerebellum and spinal cord.

Cerebellum and spinal cord vestibular function

Coordinate muscle movements that maintain balance in response to head movement.

Example of a vestibular postural response

Tilting the head backward to look up requires contraction of torso and lower-limb muscles to prevent falling backward.

Three major destinations from the vestibular nuclei

Thalamus/inferior parietal lobe, cranial nerve nuclei controlling the eyes, and cerebellum/spinal cord.

Complete conscious vestibular pathway

Vestibular receptors → vestibular portion of CN VIII → vestibular nuclei → thalamus → inferior parietal lobe.

Special senses integration

Special sensory information is integrated with other brain regions to form a coherent picture of the environment.

Cranial nerve carrying olfactory information

Olfactory nerve pathways carry information to the olfactory bulb and then olfactory tract.

Cranial nerves carrying gustatory information

Facial (VII), glossopharyngeal (IX), and vagus (X) nerves.

Cranial nerve carrying auditory and vestibular information

Vestibulocochlear nerve (VIII).

Cranial nerve carrying visual information

Optic nerve (II).

Special sense that initially bypasses the thalamus

Olfaction.

Role of the thalamus in special senses

Relays special sensory information to the appropriate primary sensory cortex, except for olfaction.

Primary sensory cortices

Areas where conscious awareness of special sensory information occurs.

Role of the frontal lobe in special-sense integration

Integrates sensations to create an overall understanding of the situation.

Role of the limbic system in special-sense integration

Integrates sensory information with past experiences and emotional responses.