Hair Cells and Semicircular Canals Function

Vestibular Pathways

• Medial vestibulospinal tract (MVST) is a medial descending motor system that only extends to the cervical spine and is important for controlling head and neck positions (vestibulo-collic reflexes).
• The largest vestibular nucleus is the Superior Vestibular Nucleus (SVN), followed by the Inferior Vestibular Nucleus (IVN).

Medial Longitudinal Fasciculus (MLF)

• MLF is a pathway that connects the SVN and MVN to the oculomotor nucleus, trochlear nucleus, and abducens nucleus.
• MLF is responsible for controlling eye muscles and reflexes, particularly the vestibulo-ocular reflex (VOR).
• VOR adjusts eye movements to compensate for changes in head positions, ensuring clear vision during head movements.

Vestibular Nuclei Outputs

• Thalamus: projects to the cortex, transmitting information about head orientation and movement.
• Cerebellum: receives ipsilateral projections from the vestibular nuclei, contributing to balance and coordination of movements.
• Cortex: receives conscious perception of head orientation and movement via the thalamus.

Vestibulo-Ocular Reflex

• Eyes move in the opposite direction of head movement (same direction as cupula).
• Eye movements are adjusted for changes in head position to maintain a stable line of sight.

Nystagmus

• Continual vestibular stimulation leads to nystagmus (beating eyes).
• Frenzel glasses: remove visual fixation during vestibular examination, allowing for observation of nystagmus.
• Electronystagmography (ENG): measures changes in the electrical field within the eye to detect nystagmus.
• Video-nystagmography/Vide-oculography: uses infrared sensors to record eye movements and measure nystagmus.

Measuring Nystagmus

• Rotating chair test: tests horizontal canal function by rotating the patient in a chair.
• Head impulse test (HIT): tests canals in all planes, stimulating the acoustic nerve with cold or warm water or air.

Hair Cells and Vestibular Function

• Hair cells are like a tap that leaks a small amount of neurotransmitter onto the 8th nerve afferents.
• Bending the cilia either increases (depolarization) or decreases (hyperpolarization) the leak, affecting impulse frequency.
• Excitation of hair cells: pushing towards the kinocilium increases impulse frequency.
• Inhibition of hair cells: pushing away from the kinocilium decreases impulse frequency.

Semicircular Canals and Otolith Organs

• Semicircular canals detect angular acceleration.
• Otolith organs (utricle and saccule) detect linear acceleration and effects of gravity due to movement.
• Maculae of the organs detect linear acceleration and head tilt, transmitting information to the vestibular nerves.

Vestibular Nuclei

• Lateral Vestibular Nucleus (LVN): Deiters' nucleus, controls balance and extensor tone in limbs, extends throughout the spinal cord.
• Medial Vestibular Nucleus (MVN): arises from the medial vestibular nucleus with additional contributions from the inferior vestibular nucleus.