PSYC 367 Lecture 10

What are the 3 major planes of the human body?

• Median (sagittal) - divides body into left and right

• Transverse (axial) - divides body into upper and lower

• Frontal (coronal) - divides body into front and back

What plane is associated with the X-axis?

Frontal (coronal) plane

What plane is associated with the Y-axis?

Median (sagittal) plane

What plane is associated with the Z-axis?

Transverse (axial) plane

What direction does the X-axis point?

Back to front, out of nose

What direction does the Y-axis point?

Side to side, out of left ear

What direction does the Z-axis point?

Bottom to top, out of head

What is pitch rotation?

Rotation around Y-axis, on sagittal plane, eg: nodding head yes

What is yaw rotation?

Rotation around Z-axis, on transverse plane, eg: shaking head no

What is roll rotation?

Rotation around X-axis, on frontal plane, eg: tilting ear toward shoulder

What are the 3 directions of translation?

X-axis - forward and backward, eg: plane taking off; Y-axis - side to side, eg: sliding sideways; Z-axis - up and down, eg: elevator

What are the 2 directions of tilt when upright?

• Pitch - tilting head forward or backward

• Roll - tilting head left or right

Why is there no yaw tilt when upright?

Yaw rotation occurs around Z-axis, which is aligned with gravity, so head orientation relative to gravity does not change

What is the difference between rotation, translation, and tilt?

• Rotation - movement around an axis

• Translation - linear movement along an axis

• Tilt - change in head orientation relative to gravity

What are the 5 vestibular organs in each inner ear?

• 3 semicircular canals - anterior, posterior, horizontal

• 2 otolith organs - utricle and saccule

What is the ampulla?

Swelling at the base of each semicircular canal near the utricle

What is the crista?

Sensory epithelium inside ampulla containing approximately 7k hair cells

What is the cupula?

Jelly-like structure containing hair cell stereocilia, surrounded by endolymph

What is the effective physical stimulus for semicircular canals?

Angular (rotary) acceleration - change in rate of head rotation

What axis of rotation is each semicircular canal maximally sensitive to?

An axis perpendicular to its plane

What rotation is the horizontal semicircular canal sensitive to?

Yaw

What rotation is the anterior semicircular canal sensitive to?

Pitch

What rotation is the posterior semicircular canal sensitive to?

Roll

What is the macula?

Sensory region in each otolith organ containing hair cells

How do the utricle and saccule differ in their macula?

• Utricle - approximately 30k hair cells, horizontal macula

• Saccule - approximately 16k hair cells, vertical macula

What is the otolithic membrane?

Gelatinous membrane containing hair cell stereocilia

What are otoconia?

Heavy calcium-carbonate crystals covering otolithic membrane

What is the striola?

Central region around which hair cells are arranged in macula

How are kinocilia arranged around the striola?

• Utricle - point toward striola

• Saccule - point away from striola

What are the effective physical stimuli for otolith organs?

• Linear acceleration - changes in rate of straight-line movement

• Tilt - changes in head orientation relative to gravity

What movements is the utricle sensitive to?

X and Y linear acceleration, pitch and roll tilt

What movements is the saccule sensitive to?

X and Z linear acceleration, pitch tilt

How does transduction occur in semicircular canals?

Head rotates → endolymph initially moves opposite relative to canal due to inertia → cupula deflects → stereocilia bend → hair cell receptor potentials change → vestibular nerve firing changes

What happens when stereocilia bend toward the kinocilium?

K+ channels open → hair cells depolarize → glutamate release increases → vestibular nerve AP firing increases

What happens when stereocilia bend away from the kinocilium?

K+ channels close → hair cells hyperpolarize → glutamate release decreases → vestibular nerve AP firing decreases

Do vestibular hair cells fire action potentials?

No, they produce graded receptor potentials and synapse with vestibular nerve fibres, which fire APs

Why is the high spontaneous firing rate of vestibular nerve fibres important?

Allows firing to increase above or decrease below spontaneous levels, enabling coding of opposite movement directions

How is direction coded in semicircular canals?

Through push-pull pairs - one canal excited while paired canal inhibited; Brain compares firing rates to determine direction of rotation

What are the 3 push-pull pairs of semicircular canals?

Left and right horizontal - yaw; Left anterior and right posterior - pitch and roll components; Right anterior and left posterior - pitch and roll components

What happens in semicircular canals when head turns right?

Right horizontal canal - depolarization, increased glutamate and vestibular nerve firing; Left horizontal canal - hyperpolarization, decreased glutamate and firing

How is amplitude coded in semicircular canals?

Greater angular acceleration → greater cupula deflection → more stereocilia bending → larger changes in receptor potentials and vestibular nerve firing rates

How do push-pull pairs code amplitude?

One canal increases firing above spontaneous while paired canal decreases below spontaneous; Greater difference between excitation and inhibition = greater angular acceleration

How does transduction occur in otolith organs?

Linear acceleration or tilt → otoconia shift → otolithic membrane moves → stereocilia bend → hair cell receptor potentials change → vestibular nerve firing changes

How do otoconia respond to linear acceleration versus tilt?

• Linear acceleration - otoconia lag behind due to inertia, shifting opposite to acceleration

• Tilt - gravity causes otoconia to shift in direction of tilt

How is direction coded in otolith organs?

Through hair cell orientations around striola and push-pull responses

• Same movement excites some hair cells and inhibits others

• Opposite movement reverses pattern

What is the push-pull response in otolith organs?

Movement in one direction maximally excites hair cells on one side of striola and inhibits cells on opposite side; Opposite movement reverses excitation and inhibition

How is amplitude coded in otolith organs?

Greater linear acceleration or tilt → greater otolithic membrane displacement → more stereocilia bending → larger changes in receptor potentials and vestibular nerve firing rates

What happens to firing rates during larger versus smaller otolith movements?

Larger movement - greater increase in excited fibres and decrease in inhibited fibres; Smaller movement - smaller firing rate changes

What do semicircular canals primarily respond to?

Changes in rotational velocity rather than sustained constant velocity

What happens in semicircular canals when rotation begins?

Endolymph lags behind → cupula deflects → stereocilia bend → vestibular nerve firing changes → brain detects rotation

What happens during constant rotational velocity?

Endolymph eventually moves with canal → cupula returns to resting position → vestibular nerve firing returns toward spontaneous rate → brain no longer receives strong signal of continued rotation

What happens when rotation suddenly stops?

Endolymph continues moving due to inertia → cupula deflects in opposite direction → vestibular nerve firing signals opposite rotation even though head has stopped

What causes alcohol-induced head spinning?

Alcohol enters endolymph and changes its density → cupula and endolymph no longer have same density → gravity deflects cupula without head rotation → stereocilia bend → vestibular nerve firing changes → false spinning sensation

What is the oculogyral illusion?

Visual disorientation and apparent movement after rapid body spinning

How does the oculogyral illusion occur?

Person spins → endolymph deflects cupula → during constant rotation cupula returns toward resting position → person stops but endolymph continues moving → cupula deflects opposite direction → brain perceives opposite rotation

What is the oculogravic illusion?

Illusion of backward body tilt and visual elevation during forward acceleration

How does the oculogravic illusion occur?

Forward acceleration → otoconia lag behind → otolithic membrane shifts → same displacement as backward tilt → brain interprets acceleration as backward tilt, eg: plane takeoff

Why can't otolith organs distinguish forward acceleration from backward tilt?

Both cause similar otoconia displacement and stereocilia bending, producing similar vestibular signals