The organ of Corti's hair cells are affected by shear forces when the basilar membrane oscillates due to pressure waves entering through the oval window into the scala vestibuli. These shear forces open mechanoreceptor K+ channels, leading to a change in membrane conductance that influences the release of neurotransmitters onto cochlear nerve fibers.

12.
Hair cells near the base of the cochlea are most responsive to high-frequency sounds, while cells near the apex are activated by low-frequency sounds. This tonotopic arrangement is also observed in central auditory structures such as the cochlear nuclei, superior olivary complex, inferior colliculus, medial geniculate nucleus, and primary auditory cortex.

13.
Processing of auditory information at various locations within the central auditory pathway contributes to tasks such as sound localization, frequency and intensity analysis, and speech recognition.

14.
The vestibular apparatus, found in the inner ear, consists of three semicircular canals (horizontal, anterior, and posterior) and two otolith organs (utricle and saccule) on each side. These structures convert angular and linear accelerations of the head, respectively. The three semicircular canals are mutually orthogonal, enabling them to detect angular head acceleration around any axis of rotation.

15.
Each semicircular canal contains sensory hair cells with cilia that extend into a cupula, blocking the cross-section of the endolymph-filled canal. Angular head acceleration displaces the endolymph and cupula, resulting in the bending of the cilia. If the stereocilia bend towards the kinocilium, the hair cell depolarizes, leading to an increase in firing rate in the afferent fiber.

16.
In the otolith organs, the cilia project into an otolithic membrane. Acceleration or changes in head position relative to gravity cause displacement of the otolithic membrane due to the mass of the otoliths. This displacement alters the firing patterns of the hair cells, depending on their orientation.

17.
Central vestibular pathways involve afferent connections to the vestibular nuclei and the cerebellum. The brain detects activation of vestibular afferent fibers as head acceleration or changes in position, which is then relayed via the vestibular nuclei to pathways involved in generating compensatory eye movements, neck movements, and adjustments to posture.

18.
Taste buds consist of chemoreceptor cells arranged around a taste pore. They are located on different types of papillae on the tongue, as well as in the pharynx and larynx. Five taste-receptor cell types detect the elementary qualities of taste: salty, sweet, sour, bitter, and umami. The perception of complex flavors involves the patterned activity of multiple taste receptor classes and their correlation with accompanying olfactory input.

19.
Afferent taste fibers synapse in the nucleus of the solitary tract. Thalamic relay occurs through a portion of the ventroposterior medial nucleus, which then projects to the taste-receiving areas located in the S1 cortex and the insula.

20.
Odors are detected by olfactory chemoreceptor cells, which continually regenerate in the olfactory mucosa. These cells are neurons equipped with various G protein-coupled receptors that enable the detection of hundreds of odor molecules.

21.
Individual olfactory axons project to specific olfactory glomeruli corresponding to each stimulus type within the olfactory bulb. They synapse on the dendrites of mitral cells, which, in turn, have reciprocal synapses with inhibitory interneurons. This synaptic organization within the glomerulus facilitates stimulus adaptation and contrast enhancement.

Source: Squire L, Berg D. Fundamental Neuroscience. 4th ed. New York: Academic Press; 2012.
When the basilar membrane oscillates in response to pressure waves introduced into the scala vestibuli at the oval window, the stereocilia of the hair cells of the organ of Corti are subjected to shear forces, which open mechanoreceptor K+ channels. This results in a membrane conductance change that modulates the release of neurotransmitters on to cochlear nerve fibers.
12.
High-frequency sounds best activate the hair cells near the base of the cochlea, and low-frequency sounds activate cells near the apex. Such a tonotopic organization is also present in central auditory structures, including the cochlear nuclei, superior olivary complex, inferior colliculus, medial geniculate nucleus, and primary auditory cortex.
13.
Auditory processing at many sites in the central auditory pathway contributes to sound localization, frequency and intensity analysis, and speech recognition.
14.
The vestibular apparatus is part of the inner ear. It includes three semicircular canals (horizontal, anterior, and posterior) and two otolith organs (utricle and saccule) on each side. These transduce, respectively, angular and linear accelerations of the head. The three semicircular canals are mutually orthogonal, so they can resolve angular acceleration of the head about any axis of rotation.
15.
In each semicircular canal, there are sensory hair cells whose cilia extend into a cupula, which blocks the cross-section of the endolymph-filled canal. Angular head acceleration displaces the endolymph and the cupula, bending the cilia. If the stereocilia bend toward the kinocilium, the hair cell is depolarized, which causes an increase in the firing rate in the afferent fiber.
16.
In the otolith organs, the cilia project into an otolithic membrane. Acceleration of the head, as with linear movement, or change in position in relation to gravity displaces the otolithic membrane (because of the mass of the otoliths) and changes the firing patterns of the hair cells, depending on their orientation.
17.
Central vestibular pathways include afferent connections to the vestibular nuclei and the cerebellum. Activation of the vestibular afferent fibers is detected by the brain as head acceleration or position change and is relayed via the vestibular nuclei to pathways that mediate compensatory eye movements, neck movements, and adjustments to posture.
18.
Taste buds contain chemoreceptor cells arranged around a taste pore. Taste buds are located on several kinds of papillae on the tongue and in the pharynx and larynx. Five types of taste-receptor cells detect the five elementary qualities of taste: salty, sweet, sour, bitter, and umami. Complex flavors are signaled by the patterned activity of multiple classes of taste receptor and by central correlation with accompanying olfactory input.
19.
Afferent taste fibers synapse in the nucleus of the solitary tract. The thalamic relay is via a part of the ventroposterior medial nucleus to the taste-receiving areas located in the S1 cortex and the insula.
20.
Odors are detected by olfactory chemoreceptor cells, which are continuously regenerated in the olfactory mucosa. These cells are true neurons that are endowed with a wide array of G protein–coupled receptors that enable the detection of hundreds of odor molecules.
21.
Individual olfactory axons project to olfactory glomeruli, specific for each stimulus type, in the olfactory bulb. They synapse on the dendrites of mitral cells, which have reciprocal synapse with inhibitory interneurons. This synaptic organization in the glomerulus underlies stimulus adaptation and contrast enhancement.
Squire L, Berg D. Fundamental Neuroscience. 4th ed. New York: Academic Press; 2012.
Upon completion of this chapter, the student should be able to answer the following questions: 1.
What is a motor neuron, and how are α and γ motor neurons different?
2.
What is a motor unit? How does the “size principle” apply to the orderly recruitment of motor units?