In my quest to bring to you the 8th Cranial Nerve (Vestibulocochlear nerve), I made mention that it is important to have an idea of the Inner Ear, the Cochlea, the Vestibule, and the Semicircular canal. I created posts on the Inner ear, the Cochlea, and the Vestibule already, you can do well to go through my blog, to read about them, and I hope you enjoy them. Today, I will be talking about the Semicircular canals, after which in my next post, I will continue with the Vestibulocochlear nerve.
The Physiology of the Semicircular Canal
While the Vestibule deals with changes in static equilibrium, looking at linear head extension, and linear head flexion, vertically or horizontally, the semicircular canals is responsible for rotational equilibrium, picking from the x-axis, the Y-axis, and the Z-axis. In my previous post Physiology of the Vestibule (The Balancing Part of the Ear), I explained that the vestibule is made up of the saccule and the Utricle, while the Vestibular canal/apparatus is made up of the Vestibule (saccule and Utricle) and the semicircular canals. I also mentioned that the Cochlea is responsible for sound, and I explained that the vestibule is in the middle of the cochlea and the semicircular canal. The semicircular canals are the Anterior/Superior Semicircular canal, the posterior Semicircular canal and the Lateral Semicircular canal. The posterior semicircular canal can be found 56% posteriorly and laterally to the sagittal plane (an anatomical division that divides the body into left and right), the Anterior semicircular canal is 45% oriented anteriorlly and laterally to the Sagittal plane, while the lateral semicircular canal is about 25% oriented above the horizontal plane, helping them to pick up balance from all rotation..
To discuss a semicircular canal, I will start with the outer bony labyrinth which is made up of perilymph which is rich in sodium and low in potassium, and it has the inner membranous labyrinth which has an inner membrane filled with endolymph, which is rich in potassium and low in sodium. (I discussed this in my post on Vestibules). Each of the canals has an Ampulla, which has at its base the crystae ampullaris which is a sensory epithelium. In the Ampulla, overlying the crystae ampularis is a semicircular canal with jellylike mound known as the Cupula, which has hair-like cilia. The Cupula is embeded in the endolymph. Underneath the Cupula are the hair cells which have cilia which are stereocilia. Beneath the hair cells are nerve terminals which are connected to the scarpa's ganglion..
The Lateral semicircular canal detects movement of the head in a transverse plane (Left or right movement), the Anterior Semicircular canal detects movement of neck flexion or extension (When the head looks down and up in a sagittal plane), the Posterior Semicircular Canal detects rotational movement of the head.
When there is a rotation towards a direction, the endolymph around the canal of the direction of rotation pushes the crystae ampularis and the cupula as the semicircular canal rotates, causing the cupula to bend. The bending of the cupula can lead to an excitation or an inhibition. When the cupula bends causing the stereocilia to flow towards the kinocilium, thereby streching the tiplinks which opens the channels, allowing for the flow of Ca2+ and K+ into the cell, causing the cell to be electropositive (depolarized). When the Calcium ions goes into the cells, the synaptic proteins in the cell fuse to the cell membrane causing exocytosis to release glutamate which stimulate the affarent terminal, creating an action potential where information is sent to the central processes where it is sent to the central nervous system. When the Stereocilia doesn't flow towards the kinocelium, Ca2+, and K+ won't flow into the cell, and the cell will be hyperpolarized, which would not stimulate action potential.,(You can read more on this in my previous post Physiology of the Vestibule (The Balancing Part of the Ear))
When the hair cell is pushed towards the kinocilium, action potentials are stimulated through the peripheral processes, to the scapa's ganglion, after which the action potentials are sent to the central nervous system (the Vestibulocochlear nerve) through the internal acoustic meatus. In the vestibular nuclei complex the action potential goes to the medial vestibular nucleus where the action potentials are sent to abducens nerve nucleus, and the occulomotor nerve. The Abducens nerve send the action potentials to the lacteral rectus of the eye, and the occulomotor nerve stimulate the medial rectus of the eye. The semicircular canal plays an important part in the vestibulo-occular reflex, helping to maintain equilibrium and for gaze fixation in sight., .
Final Note
Let me quickly recap that the ear has the function of hearing as well as balancing. The Cochlea is responsible for hearing while the vestibular canal is responsible for balancing. The Vestibular canal is made up of the vestibule and the semicircular canal. The vestibule is made up of the Utricle and the Saccule. The Saccule, which is the nearest to the cochlea is responsible for linear vertical acceleration, while the Utricle which is near the semicircular canal is responsible for linear horizontal acceleration. The semicircular canal has three canals which are the anterior semicircular canal, the posterior semicircular canal, and the lateral semicircular canal. Each canals are connected via the ampulla. The semicircular canals are responsible for the balancing of rotational accelerations, as well as balancing sight and maintaining eye gaze.
Image 1 || Wikimedia Commons || Equilibrium and Semicircular Canals
Image 2 || University of Minnesota Pressbook || Vestibular Transduction