Vestibular apparatus - Part 2 (Vestibular pathway) #mbbs #physiology

Summary

This video explores the vestibular pathway and the mechanism of stimulation of the semicircular canals. The vestibular pathway begins with hair cells in the inner ear, sending signals through afferent fibers to the vestibular division of the eighth cranial nerve. The video details how these signals are processed in the brainstem, cerebellum, and spinal cord, contributing to balance and posture. The second part discusses how the semicircular canals detect angular acceleration, explaining how head movements and inertia stimulate or inhibit the canals, helping maintain dynamic equilibrium and coordinate eye movements during head motion.

Takeaways

• 😀 The vestibular pathway starts from the receptors, which are hair cells, and their afferent fibers form the vestibular division of the 8th cranial nerve.
• 😀 The first order neurons' cell bodies are located in the vestibular ganglion (also called Scarpas ganglion).
• 😀 The vestibular division of the 8th cranial nerve has ascending and descending parts, terminating in different areas of the central nervous system.
• 😀 Ascending fibers of the 8th cranial nerve terminate in the vestibulo-cerebellum (flocculonodular lobe) of the cerebellum.
• 😀 Descending fibers of the 8th cranial nerve terminate in the vestibular nucleus in the brainstem, which sends signals to multiple CNS regions.
• 😀 The vestibulo-ocular tract connects the vestibular nucleus to the 3rd, 4th, and 6th cranial nerve nuclei, responsible for the vestibulo-ocular reflex.
• 😀 The vestibulo-thalamocortical fibers transmit information to the thalamus, which relays it to the cerebral cortex.
• 😀 The vestibulospinal tract connects the vestibular nucleus to alpha motor neurons in the spinal cord, crucial for maintaining posture.
• 😀 The vestibulorubral and vestibuloreticular tracts transmit information from the vestibular nucleus to the red nucleus and reticular formation.
• 😀 The mechanism of semicircular canal stimulation involves detecting head movement through angular acceleration and the movement of endolymph, which triggers depolarization or hyperpolarization of hair cells.

Q & A

What are the main components of the vestibular pathway?

The vestibular pathway includes receptors (hair cells), the vestibular division of the eighth cranial nerve, the vestibular ganglion (Scarpas ganglion), the vestibular nucleus, and various fibers that project to different parts of the central nervous system such as the cerebellum, thalamus, spinal cord, and brainstem.

What is the role of the vestibular ganglion?

The vestibular ganglion, also known as Scarpas ganglion, contains the cell bodies of the first-order bipolar neurons, which are part of the vestibular division of the eighth cranial nerve.

Where do the ascending fibers of the vestibular division of the eighth cranial nerve terminate?

The ascending fibers of the vestibular division of the eighth cranial nerve terminate in the cerebellum, specifically in the vestibulocerebellum (also called the flocculonodular lobe).

What is the vestibulo-ocular reflex, and what fibers are involved in it?

The vestibulo-ocular reflex helps stabilize the gaze by moving the eyes in the opposite direction of head movement to maintain fixation on an image. The fibers involved are from the vestibular nucleus that project to the third, fourth, and sixth cranial nerve nuclei.

How does the vestibular pathway help in maintaining posture?

The vestibular pathway, specifically through the vestibulospinal tract, terminates at the alpha motor neurons in the spinal cord. These neurons help maintain posture by controlling the tone of proximal and axial muscles.

What is the function of the vestibulo-thalamocortical fibers?

The vestibulo-thalamocortical fibers relay information from the vestibular nucleus to the thalamus and then to the cerebral cortex, contributing to the conscious perception of balance and spatial orientation.

How are the fibers of the vestibular pathway distributed to different areas of the central nervous system?

The fibers of the vestibular pathway are distributed to various CNS regions including the cerebellum (vestibulocerebellar tract), the brainstem (via the medial longitudinal fasciculus for ocular control), the thalamus (vestibulo-thalamocortical fibers), the spinal cord (vestibulospinal tract), and the red nucleus and reticular formation (vestibulorubral and vestibuloreticular tracts).

What happens during the initial movement of the head in relation to the semicircular canals?

At the beginning of head movement, the semicircular canals move in the direction of the head, but the endolymph within the canals lags behind due to inertia. This causes the endolymph to move in the opposite direction, stimulating the hair cells in the crista ampullaris and initiating a signal.

Why does the left semicircular canal get inhibited during head movement to the right?

During head movement to the right, the endolymph in the left semicircular canal moves in the opposite direction to the canal's movement. This results in the stereocilia moving away from the kinocilium, causing hyperpolarization and inhibiting the left semicircular canal.

What happens when the movement of the head stops in relation to the semicircular canals?

When the head stops moving, the endolymph continues to move due to inertia. This causes the stereocilia in the semicircular canals to move away from the kinocilium, inhibiting the canal that was initially stimulated (e.g., right semicircular canal), while stimulating the opposite canal (e.g., left semicircular canal).