BIO 261 Chapter 14

Semicircular canal activation

Canal hair cells respond to angular acceleration. They do not respond to constant velocity. The displacement of endolymph occurs during changes in velocity. So here there is a quick increase in firing, and then a decrease in firing in response to acceleration and deceleration, respectively.

The Otolith Organs

Hair cells of the utricle and saccule are embedded in a gelatinous layer called the otolithic membrane. Attached to the top of the membrane are solidified calcium crystals called otoconia. When head displacement occurs in the translational or tilting manner, the otoconia are pulled by gravity, which pulls on the attached otolithic membrane, deflecting the embedded vestibular hair cells, and either inhibiting or depolarizing them depending on their orientations. Gravity and acceleration forces are "pulling" on the crystals. When you tilt your head, gravity pulls the crystals towards the ground. When you accelerate in a linear plane, the crystals are pulled in the opposite direction.

Semicircular canal activation

Importantly, the hair cells of the crista ampullaris are all oriented in one direction. (as opposed to the mirrored polarity in the otolith organs). Endolymph moving through the canals will reach the ampulla and push the cupula in that direction. If it pushes towards the kinocilium, depolarization. If it pushes away from the kinocilium, inhibition.

Vestibular Hair Cells

Similar to auditory hair cells in that these are layered stereocilia of increasing height. The tallest projection is slightly different structure, and is called a Kinocilium. Surrounded by K+ rich environment Connected by tip links. As they are deflected towards the kinocilium, tip links pull open mechanically-gated K+ channels and the cells depolarize. Hair cells synapse onto vestibular nerve neurons.

Head Movement for Semicircular Canals

The 3 different type of head "turning" that cause angular acceleration. Each will move endolymph through an appropriately oriented canal. The movement of the endolymph is in the opposite direction as the head movement. Ex: as I move my head to the left, endolymph is flowing to the right through the canals. A closer look at horizontal head movement. The ampulla of the horizontal canals are oriented such that the kinocilium are on the ANTERIOR aspect of the cupula. If I turn my head to the left: Right Ear - endolymph flows posteriorly through the ampulla, inhibiting the hair cells Left Ear - endolymph flows anteriorly through ampulla, exciting the hair cells

The Structures

The Otolith Organs •Utricle and the Saccule •Linear Movement and tilt •Sensory organ: Macula Semicircular Canals •3 canals. •Angular Movement •Sensory organ: Crista ampullaris •These structures are organized in a specific way that allow them to measure movement in these directions.

The Inner Ear

The Vestibular Apparatus is composed of: 2 Otolith Organs •The Utricle •The Saccule 3 Semicircular Canals •Superior •Posterior •Horizontal These both project into the vestibular aspect of CN 8 (The "Vestibulocochlear Nerve")

Orientation of Stereocilia and Kinocilia

The specificity of movement detection is achieved through different orientations of hair cell sterocilia and kinocilium in our different vestibular organs. By orienting hair cells in a specific pattern, each of these structure will only respond to one type of head movement. Other movements will not depolarize or inhibit the hair cells. Otolith Organs respond to translational and tilting (falling) movements. Semicircular canals respond to angular acceleration (head turning) movements. The anatomy of each of these sensory organs and the orientation of the hair cells allows for detection of movement in all directions

Semicircular Canals

The structure is composed of: The circular canal The ampulla The cupula The crista ampullaris The hair bundle The crista ampullaris in the canals is similar to the macula in the otolith organs. The canals are only filled with endolymph. NO HAIR CELLS IN THE CANALS. The 3 different canals detect angular acceleration in 3 different orientations (x, y, and z).

Utricle vs. Saccule

Utricular Macula: •Curved orientation in the horizontal plane, with a "coronal" displacement at the anterior end. •This helps detect: side to side, front to back, and falling to the side. Saccular Macula: •Fairly linear orientation in the sagittal plane, with a "dorsal" displacement at the anterior end. •This helps detect: up and down, and falling front or back. Remember that the orientations of the hair cells dictate which stimuli these hair cells can respond to. Utricle = detect horizontal movement and lateral tilts Saccule = detect vertical movement and forward/backward tilts

Hair Cell Activation Preference

Vestibular hair cells have a tonic level of activity When deflected towards the kinocilium, they will increase their firing pattern throughout the entire tilt (excited). When deflected away from their kinocilium, they will decrease their firing pattern throughout the entire tilt (inhibited).

Vestibular Hair Cells

While the key with the auditory system is where in the cochlea the hair cells were deflected, the key with the vestibular system is the direction the hair cell is deflected. When there is head movement, it causes the bending of stereocilia and kinocilium in one direction. Movement of the stereocilia toward the kinocilium opens the channels, depolarizing the hair cell. Movement away from the kinocilium will close the channels, hyperpolarizing the cell.

Vertigo

• Benign Paroxysmal Positional Vertigo. BPPV occurs when tiny otoconia crystals break off. These can travel throughout the otolith organs and semicircular canals, deflecting hair cells. •The American Academy of Neurology recommend a series of specific head and body movements to move the crystals out of the canals and otolith organs so that your body can break down free crystals.

What happens after hair cell activation?

•Ascending Pathway •From the vestibular nuclei, there are bilateralprojections to the ventral posterior thalamus. •From thalamus into the "Vestibular Cortical System" - No single region. - Collection of multiple parietal regions for integrating the info.

Descending projections

•Cerebellar Pathway •The vestibular nucleus and the cerebellum have dense bilateral connections. •These project down the spinal cord as the vestibulospinal tracts for balance correction, gate correction, etc. •Medial Vestibulospinal à trunk muscles for postural control •Lateral Vestibulospinal à distal muscles for gate correction, breaking fall, etc.

Control of the Eyes

•Vestibulo-Occular Reflex (VOR) •Allows you to keep your gaze while your head moves •Direct connections between the vestibular nuclei and the CN nuclei controlling extraocular eye muscles. •It is critical that angular head movement only excites one side of the head and inhibit the other. Based on that pattern, the eyes will move one way or the other.