Sensory systems: The vestibular system

Lecture

Vestibular System

Processes sensory information underlying motor responses to and perceptions of: Self-motion, Head position, and Spatial orientation relative to gravity

• Helps to stabilize gaze, heads, and posture

• Key component in postural reflexes and eye movements

• “Sixth sense” critical to autonomic behaviors and to perception

3 axes of angular acceleration

Roll (x-axis), Pitch (y-axis), and Yaw (z-axis)

Labyrinth

Converts physical motion that arises from the effects of gravity and from linear and rotational accelerations of the head into neural impulses.

Works similar to the cochlea

2 otolith organs (utricle and saccule)

Respond to linear accelerations of the head and static head position relative to gravitational axis (head tilts)

The distribution of hair cells and orientation of stereocilia in the utricle and saccule is continuous to encode all possible directions

→Utricle: horizontal movements

→Saccule: vertical movements

Gelatinous layer (otholithic membrane) in which small crystals sit

3 semicircular canals

Specialized for responding to rotations of the head → encode head rotations

Located in the Labyrinth (Superior, Posterior, and Horizontal)

Vestibular hair cells

Located in the utricle, saccule, and the 3 ampullae and work like auditory hair cells

Some transduction channels are open in the absence of stimulation → vestibular nerve fibers have considerable spontaneous activity

Hair cells in the 3 organs (semicircular canals, saccule, utricle) are selective for certain directions

Encoding Head Tilts

Hair bundle movement occurs:

→ tonically in response to tilting of the head

→ transiently in response to acceleration

Relative change from tonic firing is important

ampulla

The bulbous expansion at the base of each semicircular canal

Crista

The ampulla of each canal houses the sensory epithelium, the ____, that contains the hair cells

cupula

Hair cells extend out of the crista into a gelatinous mass, the ____

→ Viscous barrier through which endolymph cannot circulate

When the head rotates, fluid in the canal distorts the ___

Semicircular Canals Encoding

• Semicircular canals encode head rotations → transmit information by increasing or decreasing their firing rate

• Acceleration → maximum firing rate (cupula deflected)

• Deceleration → minimum firing rate (cupula deflected in opposite direction)

• Constant velocity → firing rate returns to baseline (cupula undeflected)

Vestibular inputs

Involved in 2 major classes of reflexes:

→ Helping to maintain equilibrium and gaze during movement

→ Maintaining posture

vestibulo-ocular reflex

• Eye movements that counter head movements → maintains gaze

• Cell bodies in the vestibular nerve ganglion (Scarpa’s ganglion)

• Central process projects to the ipsilateral vestibular nuclei

• Medial vestibular nucleus → contralateral abducens nucleus + ipsilateral abducens nucleus (inhibitory neurons)

Output → eye movement opposite direction of head movement (turning head left → right eye movement)

spontaneous nystagmus

When one canal is damaged, ___ ___ occurs, due to the tonic firing of fibers from the opposite canal

physiological nystagmus

Rotating the head elicits ___ ___ which consists of a slow eye movement counter to the direction of head turning → due to relative differences in firing

vestibulospinal reflex and vestibulocervical reflex

Descending projections through the vestibular nuclei to the spinal cord underlie the ___ ___ (VSR) and the ___ ___ (VCR) which maintain balance/posture of the body and head, respectively

Patients have problems with balance and gait.

Vetral posterior nucleus

Superior and lateral vestibular nuclei project to the ___ ___ ___ of the thalamus

Cold water

This water causes slow movement of the eyes towards the side of the irrigated ear

Mimics rotational head movements away from the irrigated ear

Decreased firing rate on this side

Warm water

This water mimics endolymph movement induced by turning the head to the irrigated side

Increased firing rate on this side