Foundations of Neuroscience I - Auditory and Vestibular Systems

Foundations of Neuroscience I - PSYC2101

Course Overview

• Topic: Systems Neuroscience II

• Focus Areas:

  • Auditory System

  • Vestibular System

Auditory System

Major Components

• Sensory Organ: Ear

  • Outer ear (Pinna, Tympanic Membrane)

  • Middle ear (ossicles: malleus, incus, stapes)

  • Inner ear (Cochlea, semicircular canals)

Sound Waves

• Amplitude: Pressure variations, measured in sound pressure level (SPL)

  • Low Amplitude: Quieter sounds

  • High Amplitude: Louder sounds

• Frequency: Cycles per second, measured in Hertz (Hz)

  • Human hearing range: 20 Hz - 20,000 Hz

Inner Ear Anatomy

• Cochlea: Fluid-filled structure with essential components

  • Oval window, round window, basilar membrane

  • Organ of Corti (contains hair cells)

• Basilar Membrane:

  • Base: Narrow & stiff (high frequency response)

  • Apex: Broad & floppy (low frequency response)

Hair Cells

• Types:

  • Inner Hair Cells: Major receptors for incoming sound information

  • Approximately 10 spiral ganglion fibers synapse with each

  • Outer Hair Cells:

  • More numerous, adjust tension of basilar membrane, amplify sound

  • One fiber synapses with multiple outer cells

Sound Transduction

• Vibration causes stereocilia to deflect against the tectorial membrane

• Depolarization Mechanism:

  • Ion channels open, leading to K extsuperscript{+} entry from endolymph

  • Resting potential of hair cells is ~ -70 mV

Central Auditory Pathway

• Sound processing from cochlea to auditory cortex

  • Cochlear nucleus ➝ Superior Olive ➝ Inferior Colliculus ➝ MGN (Medial Geniculate Nucleus) ➝ Auditory Cortex

• The superior olive is the first site of binaural convergence, vital for sound localization

Sound Localization

• Mechanisms:

  • Interaural Time Delay (ITD): Time difference in sound arrival between ears

  • Interaural Intensity/Level Difference (IID/ILD): Intensity variation in signals received by both ears

Vestibular System

Overview

• Function: Sense of balance and equilibrium

• Adjusts posture and coordinates head & eye movements

Components

• Vestibular Labyrinth: Contains otolith organs (utricle & saccule) and semicircular canals

  • Otolith Organs: Monitor gravity and linear acceleration, head tilts

  • Semicircular Canals: Sense head rotation and angular acceleration

Otolith Organs

• Macula: Sensory epithelium containing hair cells

  • Responds to gravity and linear motion

  • Hair cells have cilia projecting into a gelatinous mass embedded with calcium carbonate crystals (otoliths)

Semicircular Canals

• Fluid-filled structures that detect rotational movement

• Cupula: Gelatinous structure in which hair cells are embedded

Central Vestibular Pathways

• Neural Pathway: Axons from CN VIII (vestibulocochlear nerve) project to vestibular nuclei in the brainstem

• Further projections to thalamus and then to motor areas, influencing balance and posture

Summary

• Auditory system processes sound through a multi-step pathway involving both peripheral and central mechanisms

• Vestibular system plays a crucial role in maintaining balance through the integration of sensory information from the inner ear

• Both systems intricately connect to support auditory and spatial awareness in daily functions.