Question

In the human ear, how do different hair cells respond to different frequencies of sound? a. Waves of pressure move through the fluid in the cochlea. b. Hair cells are "sandwiched" between membranes. c. Receptor proteins in the stereocilia of each hair cell are different; each protein responds to a certain range of frequencies. d. Because the basilar membrane varies in stiffness, it vibrates in certain places in response to certain frequencies.

Short answer

Different hair cells in the human ear respond to different frequencies of sound because the basilar membrane varies in stiffness, allowing it to vibrate in certain places in response to specific frequencies. This mechanism is known as the "tonotopic organization" of the cochlea. Therefore, the correct answer is (d).

Step-by-step solution

Option a: Waves of pressure move through the fluid in the cochlea

The waves of pressure are caused by the vibration of the stapes bone that pushes against the oval window of the cochlea. This pressure results in the movement of fluid inside the cochlea, which is essential for the transmission of sound. However, this option does not explain how different hair cells respond to different sound frequencies.

Option b: Hair cells are "sandwiched" between membranes

Hair cells that are responsible for detecting vibrations in the cochlea are situated between two membranes known as the tectorial membrane and the basilar membrane. While these cells are important for the detection of sound, this option does not provide an explanation for the different responses to various frequencies.

Option c: Receptor proteins in the stereocilia of each hair cell are different; each protein responds to a certain range of frequencies

The stereocilia are hair-like extensions from hair cells that contain receptor proteins inside them. These proteins are known as mechanotransduction channels, which help in the conversion of mechanical stimuli into electrical signals. Although there are different types of receptor proteins in hair cells, it is not the primary reason for their frequency-specific response.

Option d: Because the basilar membrane varies in stiffness, it vibrates in certain places in response to certain frequencies

The basilar membrane is a gradient structure that changes its stiffness from one end of the cochlea to the other. This gradient allows the basilar membrane to vibrate at different locations depending on the frequency of the sound, thus enabling hair cells to respond to different sound frequencies. This mechanism is also known as the "tonotopic organization" of the cochlea and explains how different hair cells respond to different sound frequencies. Based on our exploration, the correct answer is: d. Because the basilar membrane varies in stiffness, it vibrates in certain places in response to certain frequencies.

Key concepts

Cochlea Fluid Dynamics

Understanding the human ear's ability to perceive sound involves exploring the complex process that occurs within the cochlea, a snail-shaped structure within the inner ear. Sound waves traveling through the ear canal ultimately cause the eardrum to vibrate, transmitting these vibrations through the middle ear bones to the fluid-filled cochlea.

The cochlea's main function hinges on fluid dynamics. When the stapes bone pushes against the oval window of the cochlea, it generates waves of pressure in the cochlear fluid. These waves activate a remarkable series of events critical for hearing. This pressure movement is essential, as it transports the energy of sound deep into the cochlea, setting the stage for the transformation of mechanical waves into electrical signals that the brain interprets as sound.

It's the motion of the fluid within the cochlea that initiates the response from the hair cells, serving as the bridge between the mechanical and neuronal aspects of hearing. Despite its pivotal role, it is important to acknowledge that the fluid dynamics alone do not explain the ear's ability to distinguish different sound frequencies. This discernment of pitch is achieved through the interplay with other specialized structures within the cochlea, such as the basilar membrane and the stereocilia of hair cells.

Stereocilia Receptor Proteins

Atop each hair cell in the cochlea, there is an array of tiny, hair-like structures known as stereocilia. These structures bend in response to the movement of the cochlear fluid. The bending action is crucial, as it opens tiny channels at the tips of the stereocilia, allowing ions to rush in and create an electrical signal.

The receptor proteins located within these channels are the unsung heroes of the hearing process. They are sensitive to the stereocilia’s movement, and different types of receptor proteins vary in their response to certain frequencies. This variance in protein sensitivity adds a layer of complexity to how we interpret sounds of different pitches. However, it's not just the presence of various receptor proteins that affords us our detailed auditory perception; the stereocilia's organization plays into the mechanics of frequency differentiation, an aspect vastly enhanced by the tonotopic organization of the basilar membrane.

Basilar Membrane Tonotopic Organization

The tonotopic organization of the basilar membrane is the key to understanding how our ears can distinguish a wide range of sound frequencies. The basilar membrane is not uniform in its thickness or width; it is narrower and stiffer at the base near the oval window and becomes wider and more flexible toward the apex of the cochlea.

This gradation in stiffness creates a spatial map for sound frequencies along the membrane: high-frequency sounds best vibrate the stiffer base, while low-frequency sounds peak in the flexible apex. Hair cells sitting atop the basilar membrane are fine-tuned to resonate with specific frequencies depending on their location along this tonotopic map.

The elegant design of the basilar membrane's varying stiffness allows for precise vibrational response patterns, leading to the activation of specific sets of hair cells. This remarkable system enables the cochlea to function almost like a piano, with each hair cell responding like an individual key tuned to a particular note. Ultimately, it is this tonotopic organization that explains why different hair cells in the cochlea are activated by different sound frequencies.