Ductus Reuniens

sound shadow wavelength less than order of head diametersound direction localized intensity and localization of sound wavelength more than order of head diameter -sound direction not localized

Ductus Reuniens

phase difference calculated as: (d.sin XIX) degrees (d.sin L/v) msec, where d = metres; v =sound velocity (metres/sec)

phase differences in sound localization

Vestibular Apparatus Image

The Vestibular Apparatus

The vestibular apparatus is part of the labyrinth in the inner ear. It evolved partly from the lateral line organ, which in fish enables them to orientate in water and sense changes in water movement. The organ differentiated into the cochlea, which is exteroceptive, and senses air pressure changes, and the vestibular apparatus, which is proprioceptive, and senses head movements. Unlike the cochlea, the vestibular apparatus does not depend on the external ear. The vestibular apparatus is bilateral; each consists of three semicircular canals and two small vesicles, the saccule and utricle. These organs are oriented at different angles to each other, and together can sense changes in angular and linear velocity of the head.

Each semicircular canal is a looped tube, filled with endolymph, that communicates with the cochlear endolymph through the ductus reuniens. The anterior, horizontal (also called lateral) and posterior semicircular canals are oriented essentially orthogonally, representing the three dimensions of space. Each canal has a swelling, the ampulla, into which projects a crest, which is covered with a layer of hair cells. The cilia of the hair cells are embedded in a jelly-like cupula, a sort of flap that swings to and fro with movements of the endolymph in the canal. The bending of the cupula excites the cilia of the hair cells, which results in their discharge. The canals respond to rotation of the head; when the head turns, the fluid lags behind the head movement and pushes back onto the cupula, which bends. The cilia in a given crest are all oriented in the same direction. Turning the head to the right will fire cells in the right horizontal canal, and inhibit those in the left horizontal canal and vice versa.

The sensory epithelium of the vestibular apparatus consists of supporting cells and of sensory ciliated hair cells that are innervated by branches of the vestibular nerve. Hair cells are either type I or type II. The cell types are similar in both the semicircular canals and the otolith organs. Type I are goblet-shaped cells which are innervated by larger diameter afferent nerves whose endings are chalice-shaped, and which usually respond to stimulation with a phasic discharge. Type II cells are cylindrical, with smaller diameter afferent and efferent nerve endings at the base of each cell. The afferents respond, typically, with a tonic discharge. The cells are supplied by the vestibular ganglion (Scarpa's ganglion), which lies at the base of the external auditory meatus. Both cell types have stereocilia embedded in a cuticle; the stereocilia, of which there may be around 60-100, are relatively stiff, and increase in size toward a long, terminal kinocilium, which is a more complicated structure than are the stereocilia. Bending or deformation of stereocilia in the direction of the kinocilium increases ionic permeability of the cell, similar to that caused in cochlear hair cells, and causes excitation of the cilia. This results in release of a neurotransmit-ter, probably glutamate. Bending away from the kinocilium inhibits the stereocilia. The only major difference between vestibular and cochlear hair cells appears to be a permanent basal activity.

In the saccule and utricle there are patches of hair cells, the saccular and utricular maculae. These hair cells are covered by a jelly-like mass that contains otolith calcium crystals (see p. 270).

posterior semicircular duct.

ampullae.

saccule endolymphatic duct posterior semicircular duct.

endolymphatic duct

Ductus Reuniens

ampullae.

saccule ductus reuniens to cochlea apex of cochlea ductus reuniens to cochlea apex of cochlea vestibular apparatus

Apec Cell Orientation

plane of horizontal canal and utricle naso-occipital plane plane of horizontal canal and utricle naso-occipital plane otolith membrane perilymph supporting cell plane of saccule vestibular orientation otolith membrane perilymph supporting cell

Utricular And Saccular Macula
section through saccular macula

stereocilia.

supporting cell.

nerve calyx.

afferent fiber.

myelin sheath.

hair cells

Cell Corti Types Calyx Hair

type I type II

kinocilium cuticle efferent nerve ending endolymphatic duct type I type II

Kinocilium

cupula

.type I hair cell .type II hair cell cupula

.type I hair cell .type II hair cell

Orientation of Hair Cells

Hair cells fire off when their stereocilia are bent in the direction of the kinocilium. This phenomenon makes possible the sensing of head orientation and the initiation of appropriate postural and visual responses. The hair cells are oriented in the labyrinth of the ear through the orientation of the canals and of the otolith organs, the saccule and utricle. In the canals, the hair cells, which are the sensory receptors, are situated in the crista ampullaris, and in the otolith organs they are situated in the maculae.

The macula is a plate-like structure, which roughly follows the shape of the organ. It holds the sensory epithelium, which consists of the hair cells and supporting cells. On the macula, the hair cells are embedded in the otolith membrane, a gelatinous substance that contains otoconia, which are crystals of calcium carbonate. In both the saccule and utricle, the macula is roughly bisected by a striola, where the concentration of overlying otoconia is densest. In the utricle, the hair cells are polarized so that the kinocilium is always nearest the striola. This effectively divides the hair cells on each side of the striola into two morphologically opposed groups of sensory cells. Note that in the saccule, the stereocilia are polarized away from the striola. Since in both otolith organs the macula is curved, this enables a wide variety of hair cell alignments.

The otolith organs do not respond to the rotation of the head. Instead, they respond to tilting of the head and to its linear acceleration. These movements of the head displace the crystalline otoconia, due to the difference in density between the otoconia and the endolymph that surrounds them. The movement of the oto-conia in turn moves the underlying gelatinous matrix on the macula. This results in the movement of the stereocilia, and depolarization of the hair cell, if the direction of movement is towards the kinocilium. If the movement is away from the kinocilium, this results in hyperpolari-zation of the hair cell. Hair cells situated on one side of the striola will be depolarized, while those on the other side will be hyperpolarized. The macula is curved, which greatly increases the versatility of the otolith organs in terms of their response to different degrees of tilt. Note that afferent fibers of the saccular and utricular nerves, which are branches of the vestibular nerve, each innervate hair cells of a relatively small area of the macular sensory epithelium. This greatly increases the acuity of the directional signal.

The otolith organs lie in different planes, essentially at right angles to each other, which makes possible the detection by the brain of many different directions of acceleration. Nevertheless, the brain is unable to distinguish between head tilting and linear acceleration purely through afferent inputs from the hair cells, because they respond only to displacement of the stereocilia in response to movement of the otoconia. Awareness of changes in the position of the head is provided by afferent inputs from the semicircular canals, and from the eyes.

ion channels open+

direction of endolymph ion channels open+

direction of endolymph ion channels open++++

ion channels open++++

direction of endolymph ion channels closed

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