biolpsy_09

Sound Waves

• We perceive a repetitive pattern of local increases and decreases in air pressure as sound.

• A single alternation of compression and expansion of air is called one cycle.

• The length of these cycles defines the pitch of the sound.

• The amplitude of the vibration defines the loudness of the sound.

Hearing

Auditory Mechanisms

• Auditory stimuli are mechanical waves.

• These waves are transduced by neurons located in the inner ear.

• Sound waves travel from the tympanic membrane through the ossicles to reach the inner ear.

Cochlea and Organ of Corti

• The cochlea houses the organ of Corti, which contains neurons responsible for signal transduction.

• Sound transmitted to the cochlea generates mechanical waves in the fluid surrounding the organ of Corti.

• The basilar membrane's tautness changes gradually, allowing different frequencies of sound to move distinct parts of the membrane more effectively.

Hair Cells Functionality

• Movement of the basilar membrane causes hair cells to be pulled away from the tectorial membrane.

• Inner hair cells are responsible for producing the signals propagated in the auditory system.

• Outer hair cells adjust the sensitivity of inner hair cells by altering the stiffness of the basilar membrane.

Signal Processing

• The movement of the basilar membrane bends the processes (stereocilia) of inner hair cells, opening ion channels in their membranes.

The Auditory Pathway

• Auditory signals travel through the following pathway:

  • Cranial Nerve VIII (vestibulocochlear)

  • Cochlear nucleus

  • Superior olivary complex

  • Inferior colliculus

  • Thalamus (medial geniculate nucleus)

  • Auditory cortex

Auditory Cortex

• Several levels of the auditory pathway display tonotopic organization.

• Example: Primary auditory cortex is organized in a manner that corresponds to different sound frequencies.

Localization of Sound Sources

• Sound localization relies on intensity and time differences between the ears.

• Onset disparity: Latency difference in sound arrival at each ear.

• Ongoing phase disparity: Difference in the sound wave peaks and troughs reaching each ear.

Jeffress Model of Sound Localization

• Describes how sound localization occurs in the auditory brainstem of birds through coincidence detection.

• Neurons receive simultaneous inputs from both ears, where differences in timing lead to localization of sound sources.

Deafness

• Most cases of deafness arise from issues in the middle ear or cochlea, including its neural projections.

• Less common are lesions in auditory areas of the brain that lead to hearing difficulties.

Cochlear Implants

• Cochlear implants function by detecting sound with a microphone and stimulating the auditory nerve.

Balance

• Equilibrium is governed by the inner ear, specifically the semicircular canals and utricle/saccule.

• Signals from these structures propagate to the vestibulocochlear nucleus and various brain regions (e.g. cerebellum, motor cortex).

Taste

• Taste receptors are mainly located on the papillae of the tongue, but also in the mouth and pharynx.

• Different tastants activate specific receptors: sweet, sour, bitter, salty, and umami.

• Taste receptors are classified as chemoreceptors.

• Taste processing follows a pathway where peripheral receptors transmit signals toward the thalamus, which then relays them to the cortex.

Smell

• Smell is mediated through chemoreceptors, with receptor cells sending signals to the same glomerulus in the olfactory bulb.

• Olfactory information is communicated directly to the pyriform cortex without thalamic intervention.

Odor Encoding

• Each odorant can activate multiple receptor types, and each receptor can respond to various odorants, indicating a combinatorial coding approach.

Olfactory Receptors

• Olfactory receptors are G protein-coupled receptors that signal through secondary messengers to open ion channels, rather than being ion channels themselves.

Conclusion