NBIO 401

Fall 2013

Vestibular Anatomy & Function Class 18 – Monday, November 4, 2013 Robinson

Objectives: -Be able to describe the location of the vestibular labyrinth and the orientation of the semicircular canals relative to the skull and to each other. -Be able to describe the neural pathway from receptors in the vestibular labyrinth to the vestibular nuclei. -Be able to explain the structural and cellular basis for the transduction of rotational and linear acceleration of the head by vestibular receptors in the semicircular canals and otoliths.

VESTIBULAR SYSTEM Overview – The vestibular system senses rotational and linear acceleration of the head. It is the sense that tells us when we are turning even when we have our eyes closed. It is the sense that tells us when the elevator in which we are riding is moving up or down. In addition, and probably much more importantly, the vestibulospinal system constantly contracts and relaxes muscles in our legs and trunk to maintain our upright and stable posture. The drive to the vestibulospinal system originates in the vestibular labyrinth, a sensory apparatus in the inner ear immediately posterior and dorsal to the cochlea. It is called the labyrinth because it consists of hollow tubes and chambers that are all interconnected. The drawing to the right depicts the left labyrinth. The three hoop-like tubes are called the semicircular canals. They are part of the labyrinth that senses head rotation. The two rugby ball-shaped chambers to which the tubes connect are the utricle and saccule, together called the otoliths. The otoliths sense linear acceleration, tilt direction, and the direction of gravity. The areas in blue above show the membranes of the cochlea and the vestibular labyrinth. These membranes are inside bony structures of the same shape.

-Class 18, page 1-

NBIO 401

Fall 2013

Semicircular Canals – The picture below and to the right shows the membranes that, together, form the membranous labyrinth. They line channels in the bone, called the boney labyrinth. Both the canals and otoliths are filled with a fluid called endolymph. During head rotation the position of the endolymph inside the canals lags the position of the canals. This is because the endolymph is not rigidly coupled to the canals that surround it. The motion of a canal relative to that of the

endolymph inside it deflects a structure inside each canal. This structure is called the cupula. It occludes the canal in a region where the canal’s diameter is enlarged. This enlarged area is called the ampulla. The drawing to the left shows the canal, the enlarged section (the ampulla), and the cupula occluding the canal in the ampulla. The processes of hair cells are imbedded in the cupula. When endolymph moves relative to the canal, the movement deflects the cupula. This deflection bends the processes. Depending on the direction of the bend, this excites or inhibits the nerve fibers receiving input from the hair cells. The actual size of cupula deflection necessary to drive the vestibular fibers is quite small. For example, a 500°/sec head movement, roughly the fastest you can turn your head, deflects the cupula only about 2°. -Class 18, page 2-

NBIO 401

Fall 2013

The canals are oriented in planes that are ~90° from one another. The plane of one pair of canals (one canal on each side of the skull) is oriented roughly horizontally. These are called the horizontal canals but they actually tilt upward ~30° toward the front of the skull. Another pair of canals, called the anterior canals, are oriented vertically and tilted laterally toward the front of the skull. The third pair, called the posterior canals, are oriented vertically and tilted laterally toward the back of the skull. (Note canal labels on right side of drawing above.) Head rotation in any direction causes a unique pattern of excitation and inhibition of fibers carrying information from the canals. Otoliths – The otoliths consist of two hollow chambers continuous with the semicircular canals. They are called the saccule and utricle. They each have a patch of sensory epithelium, called a macula, on one of their inside walls. In these patches are hair cells whose processes are covered by a gelatinous material called the otolithic membrane. This thick membrane has calcium carbonate particles, called otoconia or otoliths, embedded in it (picture A to the right). It is these crystals that give the saccule and utricle their common name of “the otoliths”. These embedded crystals are very dense and they make the density of the otolithic membrane higher than that of the endolymph surrounding it. Thus, when the head accelerates along a line or tilts to change its orientation to gravity, the otolithic membrane shifts relative to the bone supporting the maculae. This shift bends the hair cells whose processes are embedded in the otolithic membrane. -Class 18, page 3-

NBIO 401

Fall 2013

Each cell has several processes that are arranged in an orderly way so that the longest process is on one side of the cell and the shortest is on the other. As illustrated below in picture B, bending the processes of these cells toward the longest process, or kinocillium, depolarizes the cell which, in turn, increases the firing rate of the fiber receiving input from that cell. Bending the hairs the away from the kinocillium inhibits the cells. There are hair cells imbedded in the in the otoliths that are oriented in every direction. Thus acceleration or tilt in any direction excites a particular group of these cells. The saccule senses accelerations or tilt in the anterior-posterior direction. The utricle senses accelerations or tilt in the vertical direction.

How do the vestibular signals generated by the semicircular canals and otoliths reach the brain? – As the drawing (of the cupula in the semicircular canal) below and to the left shows, the peripheral processes of neurons receive input from the hair cells in the cupula of the semicircular canals and the maculae of the otoliths. The cell bodies of these processes are in a peripheral ganglion near the labyrinth. This ganglion is called the vestibular ganglion or Scarpa’s ganglion. Its location is illustrated in the picture below to the right where red arrows point to the ganglion. (For our purposes in this course there is one vestibular ganglion though, in reality, it is made up of two different ganglia.) As we described in the lecture on descending motor tracts, the central process of neurons in the vestibular ganglion terminate on the vestibular nuclei in the medulla.

-Class 18, page 4-

NBIO 401

Fall 2013

The drawing below is from the second lecture on cranial nerve nuclei. It shows the VIIIth nerve entering a transparent brainstem. The VIIIth nerve contains both auditory and vestibular axons. The auditory axons, which originate in the spiral ganglion in the cochlea, terminate more laterally in the superior cochlear nucleus. The vestibular axons that originate in Scarpa’s ganglion travel past the cochlear nucleus into the more medial vestibular nuclei.

The drawing below shows a fiber-stained transverse section of the most rostral medulla. (Note the caudal end of the pontine nuclei on the bottom of the section.). In the dorsolateral part of the section we see the cochlear nucleus laying on top of the inferior cerebellar peduncle and the vestibular nuclei just medial to the inferior cerebellar peduncle.

-Class 18, page 5-