Static Equilibrium

The organs of static equilibrium are located within the vestibule, a bony chamber between the semicircular canals and the cochlea. More specifically, the membranous labyrinth inside the vestibule consists of two expanded chambers—a utricle and a saccule. The larger utricle communicates with the saccule and the membranous portions of the semicircular canals; the sac-cule, in turn, communicates with the cochlear duct (fig. 12.18).

The utricle and saccule each has a small patch of hair cells and supporting cells called a macula (mak'u-lah) on its wall. When the head is upright, the hairs of the macula in the utricle project vertically, while those in the saccule project horizontally. In each case, the hairs contact a sheet of gelatinous material (otolithic membrane) that has crystals of calcium carbonate (otoliths) embedded on its surface. These particles add weight to the gelatinous sheet, making it more responsive to changes in position. The hair cells, which are sensory receptors, have nerve fibers wrapped around their bases. These fibers are associated with the vestibular portion of the vestibulocochlear nerve.

Gravity is the stimulus that causes these hairs to respond. This usually occurs when the head bends forward, backward, or to one side. Such movements tilt the gelatinous mass of one or more maculae, and as the gelatinous material sags in response to gravity, the hairs projecting into it bend. This action stimulates the hair cells, and they signal the nerve fibers associated with them. The resulting nerve impulses travel into the central nervous system by means of the vestibular branch of the vestibulocochlear nerve, informing the brain of the

Shier-Butler-Lewis: I III. Integration and I 12. Somatic and Special I I © The McGraw-Hill

Human Anatomy and Coordination Senses Companies, 2001

Physiology, Ninth Edition

Medial geniculate body of thalamus

Midbrain

Midbrain

Medial Geniculate Body

Medial geniculate body of thalamus

Auditory cortex (temporal lobe)

Thalamus

Cochlear nuclei

Vestibulocochlear nerve

Pons

Superior olivary nucleus

Medulla oblongata

Figure 12.17

The auditory nerve pathway extends into the medulla oblongata, proceeds through the midbrain to the thalamus, and passes into the auditory cortex of the cerebrum.

Auditory cortex (temporal lobe)

Thalamus

Pons

Superior olivary nucleus

Cochlear nuclei

Vestibulocochlear nerve

Medulla oblongata

Figure 12.17

The auditory nerve pathway extends into the medulla oblongata, proceeds through the midbrain to the thalamus, and passes into the auditory cortex of the cerebrum.

head's position. The brain responds to this information by sending motor impulses to skeletal muscles, and they may contract or relax appropriately to maintain balance (figs. 12.19 and 12.20).

The maculae also participate in the sense of dynamic equilibrium. For example, if the head or body is thrust forward or backward abruptly, the gelatinous mass of the maculae lags slightly behind, and the hair cells are stimulated. In this way, the maculae aid the brain in detecting movements such as falling and in maintaining posture while walking.

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.

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