functions of SCC + otolith organs

Functions of SCC

Provide sensory input abt head velocity

Enable vestibulo-ocular reflex for eye movement that matches velocity of head  → vision stays clear when head is moving

Features of SCC

Perpendicular arrangement  → each canal is perpendicular to other 2 canals  → within one ear

Viscous endolymph

Features of scc 2

Co-planar pairing of semi-circular canal

→ endolymph of co-planar pair displaced in opp directions w/ respect to ampulla 

→ neural firing increases in SCC on side that the head is turning towards 

→ neural firing decreases in SCC of the other side 

→ bwt 2 ears 

Features of SCC 3

Direction of excitation 

SCC is excited by head motion towards that canal in appropriate plane 

→ excitation in one canal + inhibition in paired canal

Purpose of perpendicular arrangement

Allows sensing of movement in all planes  

Planes: Yaw: right + left / pitch: up + down/ roll )

Purpose of co-planar pairing of SCC

Aligned w/ optimal pull of ocular muscles 

→ effective + fast vestibulo-ocular reflexes

3 co-planar pairs: 

1. Horizontals 

2. Left anterior- Right posterior 

3. Right anterior - Left posterior

Purpose of viscous endolymph

Resistance to movement created 

→ cupula deflected 

→ neural firing altered

Advantage of co-planar pairing

1. Provide sensory redundancy in event of unilateral loss → info abt movement in the plane will be provided to brain by contralateral pair

2. Allow brain to ignore changes in neural firing occurring on both sides simultaneously 

3. Assists in compensation of overload

Ewald’s first law

Eye + head movement always occur in the same plane as canal being stimulated

Ewald’s 2nd law

• Ampullopetal: flow towards ampulla 

• Ampullofugal: flow away from ampulla 

• Inertia causes endolymph movement to lag behind head movement → I.e. left head turn causes right relative movement of  endolymph 

In horizontal SCC:

Ampullopetal flow causes greater stimulation than ampullofugal flow

Ewald’s third law

In vertical SCC: ampullofugal flow causes greater stimulation

Otolith function

 senses linear acceleration in all 3 planes in upright position 

Utricle: horizontal / Saccule: vertical 

→ orientation of head relative to gravity

Roles + organs responsible for input of vestibulo-ocular reflex

Roles	Organs responsible for input
Maintaining stable + clear vision in quick/ non-constant head movements  → image of interest steady on fovea of retina in head movements  → both stationary + moving targets  2 components	SCC + otolith

angular componentof VOR + action provided + organs responsible

Action produced	Organs responsible
Compensatory eye movement in opp direction to head rotation	SCC

Linear component of VOR action produced + organs responsible

Action produced	Organs responsible
Eyes move in opp direction to head movement	Otolith  Linear horizontal: utricle  Linear vertical: saccule

Mechanism of VOR

In presence of stimuli: excitatory signal to extraocular muscles on one side and inhibitory on the other

Roles + organs responsible for input of vestibulospinal reflex

Roles	Organs responsible for input
Stabilises head on body + maintains upright posture Lateral vestibulospinal tract: antigravity postural motor activity  Medial vestibulospinal tract:  Ongoing postural changes + head righting in response ( SCC controlled )  Reticulospinal tract: balance reflexes	SCC < otolith

Roles + organs responsible for input of vestibulocolic reflex

Roles	Organs responsible for input
Acts on neck musculature to stabilize head	SCC + otolith