Point 13 Intro Receptors Pathway Deficits Eyes Eye Defic. Overview Problems Contents Anatomy

SEMICIRCULAR CANALS The three (on each side) membranous semicircular canals lie within the bony labyrinth and contain endolymph. As shown in two of the drawings below, the canals, one horizontal and two vertical, lie in three planes that are perpendicular to each other. The HORIZONTAL or lateral canals on the two sides lie in the same plane, while the plane of each anterior canal is parallel to that of the posterior canal of the opposite side. The horizontal semicircular canals communicate at both ends with the utricle, which is a large dilation of the membranous labyrinth. The vertical canals (anterior and posterior) communicate with the utricle at one end, and join together at the other end (the common canal communicates with the utricle). At one end of each semicircular canal is a dilation called the ampulla (L., little jar). Each ampulla contains a crista (crista ampullaris; ridge), which is a transversely oriented ridge of tissue. The upper surface of the crista contains ciliated sensory hair cells that are embedded in a gelatinous material called the cupula (L., little tube). These ciliated sensory hair cells contain vesicles that possess neurotransmitter. When the neurotransmitter is released from the hair cell, the peripheral process of a cell in the vestibular ganglion is turned on. Interestingly, the hair cells release transmitter even when they are not stimulated, so the axons in the vestibular nerve are always firing at a baseline rate. Each hair cell of the crista possesses several shorter stereocilia and a single tall kinocilium at one margin of the cell. It has been shown that deflection of the stereocilia TOWARDS the kinocilium results in an INCREASE in the firing rate of the vestibular fiber associated with the hair cell, while deflection AWAY from the kinocilium results in a DECREASE in the firing rate of the vestibular fiber. Now let's consider how the stereocilia and kinocilia in the cristae are deflected by movement of the head. To do this I will discuss what happens in the RIGHT horizontal semicircular canal. As mentioned above, both ends of the horizontal canal communicate with the utricle and contain endolymph. However, only the ROSTRAL portion of the canal has the ampulla, crista and hair cells. Most important, the hair cells in this region are polarized, such that the kinocilia lie on the utricular side of the ampulla. To better understand the effect of turning your head upon the cupula and hair cells in the RIGHT horizontal semicircular canal, imagine an innertube-shaped vessel made of transparent material and filled with water. On one outer wall is attached a rubber flap that is bent if the water (i.e., the endolymph) swirls around in the tube. If the tube is moved along a linear axis (linear acceleration), the water will move with the tube because it is incompressible. In this case the rubber flap will not bend. If the tube is rotated about an axis through its center (angular acceleration), the water will lag behind, at first, because of its inertia. Thus, the outer walls will initially move relative to the water, and the rubber flap will bend. Eventually, frictional forces between the tube and the water will cause the water to "catch up," eliminating the relative movement; elastic forces of the rubber flap then return it to a vertical position. If the tube is now stopped, the water will tend to keep moving for a while, and the rubber flap will be bent in the direction opposite to that at the beginning of rotation. Consider if the rubber flap is the cupula and hair cells. As mentioned earlier, the hair cells are polarized such that the kinocilia in the right and left horizontal semicircular canals lie on the utricle side. Rotation of the head to the RIGHT will result in stimulation of the hair cells in the crista of the RIGHT horizontal semicircular canal and inhibition of the hair cells in the LEFT horizontal semicircular canal. Stimulation of the hair cells in the RIGHT horizontal semicircular canal will result in an increase in the number of action potentials in the RIGHT vestibular nerve which causes increased firing of cells in the RIGHT vestibular nuclei. This is easy, RIGHT HEAD ROTATION---RIGHT HORIZ. SEMICIRC. CANAL---RIGHT VESTIBULAR NERVE---RIGHT VESTIBULAR NUCLEI TURNED ON OR TUNED UP. All of this is in response to ANGULAR ACCELERATION of the head to the RIGHT, the stimulus needed to turn on the hair cells of the RIGHT horizontal semicircular canal. UTRICLE AND SACCULE While semicircular canals respond to angular acceleration in specific directions, hair cells in the utricle and saccule respond to linear accelerations. The utricle and saccule are saclike structures that contain a patch of sensory hair cells called the macula (L., spot). The hair cells in the macula, which are similar to those in the cristae, are embedded in the otolith (ear stone) membrane, a gelatinous structure that contains a large number of hexagonal prisms of calcium carbonate called otoconia (ear dust). Since the density of the otoconia is greater than the surrounding endolymph, the otolith membrane will be displaced by the force of gravity or other linear accelerations. Such displacement bends the stereocilia and, depending on the polarity of the cell, either causes an increase or a decrease in the number of impulses in the associated vestibular fiber.