Ch.15 (pt.3) 36

UMHB BIOL 2340 Human Anatomy & Physiology I

Nervous System (Chapter 15)

15.8 Sound Detection

  • Hearing is defined as the reception of an air sound wave that is converted to a fluid wave.

  • This conversion ultimately stimulates mechanosensitive cochlear hair cells, which then send impulses to the brain for interpretation.

Properties of Sound

  • Definition: Sound is described as a pressure disturbance that consists of alternating areas of high and low pressure.

  • Sound is produced by the vibration of an object and propagated through molecules of a medium such as air.

Creation of Sound Waves

  • Sound waves are created when an object moves:

    • Air molecules displaced by the object are pushed forward into the adjacent area, resulting in an increase of air molecules in that area, creating high pressure (compression).

    • As the object returns to its original position, the area it leaves behind has fewer air molecules, leading to low pressure (rarefaction).

  • These alternating areas of compression and rarefaction are referred to as sound waves.

Movement and Propagation

  • The kinetic energy of the vibrating object is transferred to nearby air molecules, which in turn transfer it to others.

  • Wave energy tends to decline with both time and distance.

  • Sound waves move outward in all directions due to the compression and rarefaction caused by the vibrating object.

Visualization of Sound Waves

  • Sound waves can be illustrated visually as an S-shaped curve or sine wave, with:

    • Compressions represented as crests.

    • Rarefactions represented as troughs.

Sound: Source and Propagation

  • Figure 15.28a illustrates how a struck tuning fork creates alternating areas of compression and rarefaction in surrounding air molecules.

    • The areas of high pressure correspond to compressed molecules.

    • The areas of low pressure correspond to rarefaction.

    • Wavelength: The distance between two consecutive crests in a sound wave.

    • Amplitude: Related to the height of the crests of the wave.

Frequency and Amplitude of Sound Waves

  • Sound can be characterized by two main physical properties: frequency and amplitude:

    • Frequency:

    • Defined as the number of waves that pass a given point in a given time frame.

    • Measured in hertz (Hz), where 1 Hz equals one wave per second.

    • The human hearing range is between 20 to 20,000 Hz, with heightened sensitivity from 1500 to 4000 Hz.

    • Pitch: The perception of frequency; higher frequencies correspond to higher pitches.

    • Most sounds encountered in daily life comprise variations of different frequencies.

    • Tone: A sound at a singular frequency, such as a tuning fork.

    • Wavelength:

    • The distance between consecutive crests; shorter wavelengths relate to higher frequencies.

  • Amplitude:

    • Defined as the height of the crests of the sound wave.

    • Perceived as loudness, with subjective interpretation of sound intensity.

    • Measured in decibels (dB):

    • Scale ranges from 0 dB (threshold of hearing) to 120 dB (threshold of pain).

    • A normal conversation occurs around 50 dB.

    • Prolonged exposure above 90 dB can lead to severe hearing loss.

  • Figure 15.29a illustrates how frequency and amplitude correspond to pitch and loudness respectively:

    • High frequency (short wavelength) results in a high pitch, while low frequency (long wavelength) leads to a low pitch.

  • Figure 15.29b demonstrates the relationship between amplitude and loudness:

    • High amplitude equates to loud sounds; low amplitude denotes soft sounds.

Transmission of Sound to Internal Ear

  • Pathway:

    • Sound waves first enter the external acoustic meatus and strike the tympanic membrane (eardrum), causing it to vibrate.

    • The intensity of sound influences the degree of vibration.

    • Vibrations are transferred to the auditory ossicles (malleus, incus, stapes), amplifying vibrations to approximately 20 times as the tympanic membrane is about 20 times larger than the oval window.

  • Scala Vestibuli:

    • The stapes rocks back and forth against the oval window with each vibration, causing wave motions in the perilymph fluid located in the scala vestibuli.

    • These waves travel up to the round window, resulting in its bulging into the middle ear cavity.

  • Helicotrema Path:

    • Waves with frequencies lower than the threshold of human hearing are transmitted through the helicotrema into the scala tympani to the round window.

  • Basilar Membrane Path:

    • Sounds within the hearing range enter the cochlear duct and vibrate the basilar membrane at a specific location corresponding to the sound frequency.

  • Figure 15.30 illustrates the pathway of sound waves through the auditory structures, including the temporal dynamics of vibration and transmission through the cochlea.

Resonance of the Basilar Membrane

  • Definition: Resonance refers to the movement of different areas of the basilar membrane in response to specific frequencies of sound.

  • The structure of the basilar membrane varies along its length:

    • Fibers close to the oval window are shorter and stiffer, resonating with high-frequency waves.

    • Fibers near the cochlear apex are longer and floppier, resonating with lower-frequency waves.

  • Age-related degeneration may result in a loss of shorter fibers, leading to diminished ability to hear high-pitched sounds.

  • Figures 15.31-1 & -2 describe the structure and function of the basilar membrane in separating various frequencies of sound.

Sound Transduction

  • Inner Hair Cell Excitation:

    • The movement of the basilar membrane deflects the stereocilia (hair-like structures) of inner hair cells.

    • These hair cells possess microvilli containing stereocilia, which bend at their bases.

    • The longest stereocilia connect to shorter ones via tip links; when tip links are pulled, they open ion channels.

  • Mechanism of Depolarization:

    • When stereocilia bend toward the tallest hair, K+ and Ca2+ ion channels in the shorter stereocilia open, allowing ions to flow into the cell.

    • This generates a receptor potential potentially leading to neurotransmitter release, which can trigger action potentials in afferent neurons of the cochlear nerve.

    • Conversely, if stereocilia bend away from the tallest hair, the tip links relax, closing ion channels and leading to the cell's repolarization or hyperpolarization.

  • Figure 15.32 visualizes the mechanical gating of ion channels in hair cells due to the movement of the basilar membrane.

Auditory Processing

  • Perception of Pitch: Impulses from hair cells are interpreted by the brain as specific pitches based on their position along the basilar membrane.

  • Detection of Loudness: The brain interprets increased loudness as a higher frequency of action potentials resulting from larger deflections of hair cells.

  • Localization of Sound: Depends on the relative intensity and timing of sound waves arriving at both ears. If sound reaches one ear before the other, the brain infers the sound's direction based on timing differences.

15.10 Maintenance of Equilibrium

  • Definition: Equilibrium refers to the body's response to head movement through sensory input from the inner ear, eyes, and stretch receptors.

  • Vestibular Apparatus: This structure includes equilibrium receptors positioned in the semicircular canals and vestibule:

    • Vestibular receptors monitor static equilibrium.

    • Semicircular canal receptors monitor dynamic equilibrium.

The Maculae

  • Definition: Maculae are sensory receptor organs located in the walls of the saccule and utricle, responsible for monitoring static equilibrium.

    • Role: Detects head positioning in space and facilitates control of posture.

    • Responds to linear acceleration forces but not rotation.

Anatomy of a Macula

  • Each macula consists of a flat epithelial patch with hair cells and supporting cells:

    • Hair cells possess stereocilia and a kinocilium.

    • Their stereocilia are embedded in a jelly-like membrane called the otolith membrane, which is studded with otoliths (tiny stones).

    • The otoliths increase the membrane's weight and inertia, enhancing its efficacy in detecting motion.

Orientation & Response

  • Utricle Maculae:

    • Horizontal orientation with vertical hairs.

    • Sense changes along the horizontal plane (e.g., head tilting).

    • Responds to forward/backward movements.

  • Saccule Maculae:

    • Vertical orientation with horizontal hairs.

    • Sense changes along the vertical plane (e.g., up/down movements like elevator acceleration).

Activating Receptors of a Macula

  • Hair cells continuously release neurotransmitters; however, acceleration or deceleration alters the quantity released, adjusting action potential (AP) frequency to the brain.

  • The dense otolith membrane lags behind the movement of the hair cells, resulting in detectable changes in cell signaling based on head positioning.

    • Bending hairs toward kinocilia leads to depolarization and increased neurotransmitter release, while bending hairs away induces hyperpolarization and reduced neurotransmitter release.

The Cristae Ampullares

  • Receptor Function: Crista ampullaris (crista) manifests as small elevations within the ampulla of each semicircular canal, responding primarily to rotational acceleration and deceleration.

  • Major stimuli detected include rotational (angular) movements, allowing cristae to capture head rotation across three spatial planes.

  • Structure: Each crista contains supporting and hair cells that extend into a gel-like mass called the ampullary cupula.

Activation of Crista Ampullaris Receptors

  • Bending of hair structures in the cristae is followed by depolarization, which leads to rapid impulse transmission towards the brain. In contrast, bending in the opposite direction results in hyperpolarization.

  • The opposing axes of hair cell orientation in complementary semicircular ducts yield complementary responses: one ear experiences depolarization while the other undergoes hyperpolarization.

Equilibrium Pathway to the Brain

  • Equilibrium information transmits to reflex centers in the brainstem:

    • This allows rapid reflexive responses to maintain balance and prevent falls.

  • Impulses from activated vestibular receptors travel to the vestibular nuclei in the brainstem or the cerebellum.

  • There are three modes of input for balance and orientation: vestibular receptors, visual receptors, and somatic receptors (skin, muscle, and joints).

Clinical – Homeostatic Imbalance

  • Motion Sickness: This condition arises when sensory inputs are mismatched, causing conflicting information; common symptoms include excess salivation, pallor, rapid deep breathing, and profuse sweating.

  • Treatment: Antimotion drugs that depress vestibular input can mitigate symptoms of motion sickness.