Anatomy & Physiology I and II

Anatomy & Physiology Study Guide , a snail shell, is a spiral-shaped, bony chamber that contains the cochlear duct of the membranous labyrinth. Receptors within the cochlear duct provide the sense of hearing. The duct is sandwiched between a pair of perilymph-filled chambers. The entire complex makes turns around a central bony hub, much like a snail shell. The walls of the bony labyrinth consist of dense bone everywhere except at two small areas near the base of the cochlear spiral. The round window is a delicate, membranous partition that separates the perilymph of the cochlear chambers from the air- filled middle ear. Collagen fibers connect the bony margins of the opening, known as the oval window, to the base of the stapes. Equilibrium The semicircular ducts convey information about rotational movements of the head. Sensory receptors in the semicircular ducts respond to rotational movements of the head. These hair cells are active during a movement but are quiet when the body is motionless. For example, as you rotate your head to the left, receptors stimulated in the semicircular ducts inform you of how fast the movement is, and its direction. The saccule and the utricle provide information about your position with concerning gravity. If you stand with your head tilted to one side, these receptors communicate your head’s angle and whether your head tilts forward or backward. The saccule and the utricle are also triggered by sudden speed. When your car accelerates from a stop, the saccular and utricular receptors give you the impression of increasing speed. Pathways for Equilibrium Sensations Sensory fibers from these ganglia form the vestibular branch of the vestibulocochlear nerve (VIII). These fibers innervate neurons within the pair of vestibular nuclei at the boundary between the pons and the medulla oblongata. The two vestibular nuclei have four functions. Hearing The receptors of the cochlear duct provide a sense of hearing that enables us to detect the quietest whisper yet remain functional in a noisy room. The receptors responsible for auditory sensations are hair cells similar to those of the vestibular complex. However, their placement within the cochlear duct and the organization of the surrounding accessory structures shield them from stimuli other than sound. In transporting vibrations from the tympanic membrane to the oval window, the auditory ossicles convert pressure fluctuations in the air into much greater pressure fluctuations in the perilymph of the cochlea. These fluctuations stimulate hair cells along the cochlear spiral. The frequency of the perceived sound is determined by which part of the cochlear duct stimulated. The intensity (volume) of the perceived sound is determined by howmany of the hair cells at that location are stimulated. The Hearing Process The process of hearing can be divided into six basic steps: • St p 1: Sound waves arrive at the tympanic membrane: Sound waves enter the external acoustic meatus and travel toward the tympanic membrane. • Step 2: Movement of the tympanic membrane results in displacement of the auditory ossicles: The tympanic membrane provides a surface for the collection of sound, and it vibrates in ©2018 Achieve Test Prep Page 198 of 367 The cochlea