The Saccule and Utricle

Each of these chambers has a 2 by 3 mm patch of hair cells and supporting cells called a macula.34 The macula sac-culi lies nearly vertically on the wall of the saccule, and the macula utriculi lies nearly horizontally on the floor of the utricle (fig. 16.18a).

Each hair cell of a macula has 40 to 70 stereocilia and one true cilium called a kinocilium.35 The tips of the stere-ocilia and kinocilium are embedded in a gelatinous otolithic membrane. This membrane is weighted with calcium carbonate-protein granules called otoliths36 (fig. 16.18b), which add to the density and inertia of the membrane and enhance the sense of gravity and motion.

Figure 16.18c shows how the macula utriculi detects tilt of the head. With the head erect, the otolithic membrane bears directly down on the hair cells and stimulation is minimal. When the head is tilted, however, the weight of the membrane bends the stereocilia and stimulates the hair cells. Any orientation of the head causes a combination of stimulation to the utricules and saccules of the two ears. The brain interprets head orientation by comparing these inputs to each other and to other input from the eyes and stretch receptors in the neck.

The inertia of the otolithic membranes is especially important in detecting linear acceleration. Suppose you are sitting in a car at a stoplight and then begin to move. The heavy otolithic membrane of the macula utriculi briefly lags behind the rest of the tissues, bends the stere-ocilia backward, and stimulates the cells. When you stop at the next light, the macula stops but the otolithic membrane keeps on going for a moment, bending the stereocilia forward. The hair cells convert this pattern of stimulation to nerve signals, and the brain is thus advised of changes in your linear velocity.

If you are standing in an elevator and it begins to move up, the otolithic membrane of the vertical macula

3saccule = little sac; utricle = little bag

4macula = spot

5kino = moving

Saladin: Anatomy & I 16. Sense Organs I Text I © The McGraw-Hill

Physiology: The Unity of Companies, 2003 Form and Function, Third Edition

Chapter 16 Sense Organs 607

Chapter 16 Sense Organs 607

Structure Macula Utricle And Saccule

Figure 16.18 The Saccule and Utricle. (a) Locations of the macula sacculi and macula utriculi. (b) Structure of a macula. (c) Action of the otolithic membrane on the hair cells when the head is tilted.

Figure 16.18 The Saccule and Utricle. (a) Locations of the macula sacculi and macula utriculi. (b) Structure of a macula. (c) Action of the otolithic membrane on the hair cells when the head is tilted.

sacculi lags behind briefly and pulls down on the hairs. When the elevator stops, the otolithic membrane keeps going for a moment and bends the hairs upward. The macula sacculi thus detects vertical acceleration.

Essentials of Human Physiology

Essentials of Human Physiology

This ebook provides an introductory explanation of the workings of the human body, with an effort to draw connections between the body systems and explain their interdependencies. A framework for the book is homeostasis and how the body maintains balance within each system. This is intended as a first introduction to physiology for a college-level course.

Get My Free Ebook


Responses

  • alighiero
    What is the orientation of macula of utricle and saccule of ear?
    2 years ago

Post a comment