Anatomy and Physiology of

Balance

Vestibular Hair Cells

• Type I

(aka inner)

• Type II

(aka outer)

With Kinocilium

The Semicircular Canals

• posterior canal shares plane with contralateral anterior canal.

• horizontal canals share plane.

Stimulated by Angular Acceleration

• greatest when fulcrum is within head

• induces relative motion of endolymph

• crista is displaced by fluid motion

Responses of the Cristae

• All kinocilia are oriented in the same direction

• Crista in each pair of canals respond inversely to each other

The Otolithic Organs

Saccule: roughly

vertical orientation,

responds to acceleration components within saggital plane

Utricle: horizontal (+ 30 deg.) orientation

Excitation Patterns in the Utricle

STRIOLA

Posterior

Anterior

Medial

Cranial Nerve VIII

Vestibular Portion of C.N. VIII

superior division: utricle, anterior part of saccule, and horiz & anterior canals

inferior division: posterior part of saccule, and posterior canal

• to vestibular nuclei

• to cerebellum

Vestibulocochlear Nerve

• Exits temporal bone near its medial edge.

• Enters lateral face of brainstem at the level of the lower pons.

• Synapsing in (Cochlear and) Vestibular Nuclei

Responses of Vestibular Neurons:

• To changes in acceleration, but onset and fade slowly

• For most normal head movements firing rates are in phase with head VELOCITY.

Other inputs to vestibular nuclei:

• Cerebellum: primarily inhibitory

• Spinal cord

• Pontine reticular formation

• Contralateral vestibular nuclei

From the Vestibular Nuclei:

• Vestibulo-Oculomotor Pathways:

– Direct: to oculomotor nuclei.

– Indirect: via reticular formation to oculomotor nuclei (III IV and VI)

• Vestibulo-Spinal Pathways:

– Lateral V-S-throughout spinal cord

– Medial V-S-cervical & thoracic

– Reticulospinal tract-via brainstem reticular formation

Median Longitudinal Fasciculus

• A tract linking Vest. Nuclei to nuclei of CN III, IV, & VI;

• Supports conjugate eye movement during movement of the head.

• Continuous with the medial vestibulospinal tract.

• The mlf runs near midline ventral to ventricle IV and the periaqueductal gray matter of the midbrain

In the brainstem

• Vestibular inputs undergo integration

• Integrated signal is combined with original (velocity driven) signal

• Processing to reset spatial map for eye musculature

Integration & “Leaky” Integration

The VOR

Central Nervous System Will Adapt to Peripheral Damage

Eye Movements

• Saccades—rapid shift in gaze

• Pursuit—stabilize image of moving object

• Fixation—stabilize image of still object

• VOR—stabilize image during head motion

• OKN—backup for when VOR decays to cont’d head rotation

• Vergent movements—change depth of focus

Pause cells inhibit

Burst Neurons

which stimulate:III & VI (horizontal)

or

III & IV (vertical)

Saccades

VisualSystem

VestibularSystem

Somato-Sensation

Compare, Select& Combine Senses

SENSORY INPUTSSENSORY INPUTSVisionVision

VestibularVestibularSomatosensorySomatosensory

SENSORY INPUTSSENSORY INPUTSVisionVision

VestibularVestibularSomatosensorySomatosensory

Somato-Sensation

Compare, Select& Combine Senses

SOMATOSENSORY SOMATOSENSORY RECEPTORSRECEPTORSSOMATOSENSORY SOMATOSENSORY RECEPTORSRECEPTORS

JointsJointsPositionPosition

KinesthesiaKinesthesia

MusclesMusclesLengthLengthTensionTension

SkinSkinTouchTouch

PressurePressure

VisualSystem

Compare, Select& Combine Senses

CentralCentralOrientationOrientationNavigationNavigation

PeripheralPeripheralMotion-sensitiveMotion-sensitive

Body-swayBody-sway

VISUAL VISUAL RECEPTORSRECEPTORS

VestibularSystem

Compare, Select& Combine Senses

Semi-circular canalsSemi-circular canalsOrientationOrientationNavigationNavigation

Utricle & SacculeUtricle & SacculeHorizontal & verticalHorizontal & vertical

acceleration & decelerationacceleration & deceleration

VESTIBULAR RECEPTORSVESTIBULAR RECEPTORS

Selection based onAvailability AccuracyValue for the task at hand

Processing of inputs from the periphery

SENSORY ORGANIZATIONSENSORY ORGANIZATION

Compare, Select& Combine Senses

Determination of Body Position

Visual Vestibular Somatosensory

Functional Balance:

Navigating in our

Environment

A Quiz

Vestibular Pathways Quiz