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Components of Vestibular System

The vestibular system includes semicircular canals (detect rotation) and vestibular sacs—utricle & saccule (detect gravity + head tilt).

Scenario: Patient feels dizzy when turning head quickly — which structure detects this?

Semicircular canals detect angular acceleration.

Information conveyed by semicircular canals

They detect rotation and angular acceleration of the head.

Scenario: Patient can’t tell when they turn their head left or right.

Semicircular canals are impaired.

Information conveyed by utricle and saccule

Utricle detects horizontal movement; saccule detects vertical movement and gravity.

Scenario: Patient can’t tell if they’re standing or lying down.

Utricle and saccule are impaired.

Cupula function

Bending of the cupula inside the semicircular canals triggers hair cells to fire.

Scenario: Head rotates → fluid pushes a structure → dizziness. What structure?

The cupula.

Otoconia definition

Calcium carbonate crystals that add weight to the gelatinous layer and detect gravity & tilt.

Scenario: Loose crystals cause vertigo (BPPV). What moved?

Otoconia.

Vestibular pathway

Hair cells → vestibular nerve → vestibular nuclei → cerebellum, eye muscles, spinal cord, thalamus.

Scenario: Eyes can’t stabilize during head movement. Which pathway is affected?

Vestibular → eye movement pathway.

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Cutaneous senses

Touch, pressure, vibration, temperature, and pain; receptors located in the skin.

Proprioception

Perception of body position; receptors in muscles & joints.

Kinesthesia

Perception of limb movement; receptors in joints & skin stretch.

Organic senses

Internal organ sensations (e.g., stomachache).

Scenario: Patient can’t sense limb position with eyes closed.

Proprioception deficit.

Merkel disks

Detect form and texture.

Scenario: Using fingertips to read Braille uses which receptors?

Merkel disks.

Pacinian corpuscles

Detect vibration.

Scenario: Feeling your phone vibrate uses what receptor?

Pacinian corpuscles.

Ruffini endings

Detect skin stretch and sustained pressure.

Meissner corpuscles

Detect fine touch and edge contours.

Free nerve endings

Detect pain, temperature, itch, and pleasurable touch.

Temperature receptors

TRP receptors responding to warm or cool.

Nociceptors

Pain receptors responding to extreme temperature, pressure, or capsaicin.

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Touch pathway (body)

Receptor → spinal cord (ipsilateral) → medulla (decussates) → thalamus (VPL) → primary somatosensory cortex.

Touch pathway (face)

Trigeminal nerve → trigeminal nucleus → thalamus (VPM) → cortex.

Scenario: A touch on the right arm is processed in which cortex?

Left somatosensory cortex.

Pain pathway

Receptor → spinal cord (immediate decussation) → spinothalamic tract → thalamus → cortex.

Scenario: Left leg burn is processed where?

Right somatosensory cortex.

Somatotopic organization

Body areas map onto cortex; more sensitive body parts have more cortical space.

Scenario: Which body part takes the largest cortical area?

Fingertips.

Columnar organization

Each column responds to the same body area but different sensations (pressure, heat, pain).

Cortical plasticity evidence

Using a body part frequently expands its cortical representation.

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Five basic tastes

Salty, sour, sweet, bitter, umami.

Taste transduction: salty

Na⁺ ions enter channels → depolarization.

Taste transduction: sour

H⁺ ions enter channels → depolarization.

Taste transduction: sweet

T1R2 + T1R3 receptors (G-protein gustducin).

Taste transduction: bitter

T2R receptors (gustducin).

Taste transduction: umami

T1R1 + T1R3 (glutamate binds).

Papilla structure

Contain taste buds with taste receptor cells.

Scenario: Where do taste molecules bind?

On taste cell cilia in taste pores.

Gustatory pathway

Cranial nerves VII, IX, X → solitary nucleus → thalamus (VPM) → gustatory cortex.

Scenario: Damage to CN VII affects taste on what part of tongue?

Anterior 2/3.

Other gustatory areas

Amygdala (emotion), hypothalamus (feeding behaviors).

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