Orientation Lecture

Ampulla

Dilation at the end of each semicircular duct. Embedded in the Ampulla is the sensory organ of that semicircular duct

Structure of the vestibular organ

It is a membranous labyrinth consisting of three perpendicular semicircular canals, a utricle, a saccule. The semicircular canals detect angular acceleration or rotation. The utricle and saccule detect linear acceleration.

Inputs to the Vestibular Nuclei

1. Primary Vestibular Afferents via Vestibular (Scarpa's) Ganglion a. Utricle b. Saccule c. Semicircular ducts 2. Cerebellum (via juxtaresiform body) a.Coordinates intended with actual movement 3. Spinal Cord (positioning of body) 4. Contralateral Vestibular Nuclei

Nystagmus

An initial rotation of the head causes the eyes to move slowly in the opposite direction to maintain visual fixation. When the limit of eye movement is reached, the eyes rapidly snap back (nystagmus), then slowly move again. The direction of the nystagmus is defined as the direction of the fast (rapid eye) movement. Therefore, the nystagmus occurs in the same direction as the head rotation. If your head is stationary and objects are moving in front of it (such as in a car with telephone poles and trees going by). Your eyes stay fixated on the object and then they quickly flick back to the center and find another object that they fixate on. This motion is called Nystagmus and is named for the direction of the rapid component A corresponding thing happens when you rotate. Your eyes pick an object, track it and then they flip, pick another object and track and so on. In Ballet, if the dancer does a bunch of spins they hold their head fixated on one object to help prevent dizziness. So if you spin around and around and around your eyes will show a lot of herky jerky motion. This is because when you stop the fluid moves around in the semicircular ducts and this is interpreted as the beginning of another motion

Steps in vestibular system transduction-angular acceleration

During counterclockwise (left) rotation of the head, the horizontal semicircular canal and its attached cupula also rotate to the left. Initially, the cupula moves more quickly than the endolymph fluid. Thus, the cupula is dragged through the endolymph, as a result, the cilia on the hair cells bend. If the stereocilia are bent toward the kinocilium, the hair cell depolarizes (excitation). If the stereocilia are bent away from the kinocilium, the hair cell hyperpolarizes (inhibition). Therefore, during the initial counterclockwise (left) rotation, the left horizontal canal is excited and the right horizontal canal is inhibited. After several seconds, the endolymph "catches up" with the movement of the head and the cupula. The cilia return to their upright position and are no longer depolarized or hyperpolarized. When the head suddenly stops moving, the endolymph continues to move counterclockwise (left) dragging the cilia to the opposite direction. Therefore, if the hair cell was depolarized with the initial rotation, it now will hyperpolarize. If it was hyperpolarized with the initial rotation, it now will depolarize. Therefore, when the head stops moving the left horizontal canal will be inhibited and the right horizontal canal will be excited.

Vestibuloocular Reflex

Gaze can stay fixed on an object while the head is moving. Called: "Doll's Eye Movement". Rotation of the Head to the Left both eyes reflexively turn right: Left horizontal semicircular duct: 1. Increased contraction of the right lateral rectus & left medial rectus via MLF from left vestibular nuclei 2. Inhibition of left lateral rectus and right medial rectus 3. Right horizontal semicircular duct has a complimentary effect of less firing You can test this by having someone hold up their finger - stare at the finger while shaking your head back and forth - the eyes should stay fixed on the finger without moving themselves. Or you can move your finger rapidly and have someone follow the finger with their eyes. Note that the eyes will always lag behind the finger movements.

Otolithic Membrane of the Saccule

Resembles a honeycomb like structure. In some you can see the sterocilia sticking out. Thus, anytime this membrane moves over the stereocilia it can cause a deflection. Remember that stereocilia have a directon of high sensitivity and a direction of low sensitivity which is 90 degrees from the direction of high sensitivity. This means for the entire organ you need groups of stereocilia that are 90 degrees to each other and then some that are in between. The Utricle and Saccule sense the stereocilia in different planes Saccule Macula 1. Acceleration in the sagittal plane 2. Tilt beginning from a head -sideways position. It may elicit muscles that control posture. Utricle Macula 1. Acceleration in the horizontal plane 2. Tilt beginning from a head-upright position It may elicit eye movements as a response.

Macula

The Utricle and Saccule have a region within them called a Macula which is the sensory part. Macular stereocilia penetrate into gelatinous membrane. Gelatinous material with embedded calcium carbonate crystals. These are called Otoconia. Stuck on top of the gelatinous layer, you literally have these little very small rocks in your ear. These are much denser than the fluid. This is like having a plate of jello with rocks on top of the jello, eventually the rocks are going to move the jello out of shape

What are the canals filled with?

The canals are filled with endolymph and are bathed in perilymph.

What doe the hair cells of the cupula do?

The hair cells sense the deflections of the Cupula. Deflections of the Cupula in one direction increases the afferent firing frequency. This occurs towards the Utricle in the horizontal duct and away from the Utricle in anterior and posterior ducts. Deflection of the Cupula in opposite direction decreases the firing rate.

What are the receptors?

The receptors are hair cells located at the end of each semicircular canal. Cilia on the hair cells are embedded in a gelatinous structure called the cupula. A single long cilium is called with kinocilium; smaller cilia are called stereocilia.

Outputs from Vestibular Nuclei

To ipsilateral and contralateral: 1. Motor Nuclei of Extraocular Muscles: This is so that your eyes can move with your head b. Oculomotor Nucleus (via MLF) c. Trochlear Nucleus (via MLF) d. Abducens Nucleus 2. Thalamus, then sensory cortex 3. Cerebellum 4. LVST - Lateral Vestibulospinal Tract: Postural compensation 5. MVST - Medial Vestibulospinal Tract: Stabilizes head position and coordinates eye movement 6. Contralateral Vestibular Nuclei 7. Reticular Formation --> vagus --> hurl

Vestibular system

detects angular and linear acceleration of the head Reflex adjustments of the head, eyes, and postural muscles provide a stable visual image and steady posture.