Overview
This lecture explains how the cochlea and brain distinguish between sounds of different frequencies through specialized hair cells and auditory mapping.
Sound Frequency and the Cochlea
- The main difference between sounds like a bass drum and a bee’s wings is their frequency.
- The cochlea is responsible for processing and distinguishing sound frequencies.
- Humans can detect frequencies from 20 Hz to 20,000 Hz.
Basilar Tuning in the Cochlea
- The basilar membrane inside the cochlea contains hair cells that respond to specific frequencies.
- The base of the cochlea detects high-frequency sounds; the apex detects low-frequency sounds.
- Sound waves traveling in the cochlea stimulate hair cells tuned to their frequency.
Signal Transmission to the Brain
- Activated hair cells generate action potentials that travel via the auditory nerve.
- The auditory nerve transmits frequency-specific signals to the brain.
- The primary auditory cortex in the brain receives and organizes these signals by frequency.
Tonotypical Mapping
- Different parts of the primary auditory cortex correspond to different cochlear hair cells and specific sound frequencies.
- This mapping allows the brain to distinguish between various sounds and prevents all hair cells from firing simultaneously.
Key Terms & Definitions
- Frequency — the number of sound wave cycles per second, measured in hertz (Hz).
- Cochlea — a spiral-shaped organ in the inner ear that processes sound.
- Basilar Membrane — the membrane inside the cochlea lined with frequency-sensitive hair cells.
- Hair Cells — sensory cells on the basilar membrane that detect specific sound frequencies.
- Basilar Tuning — the organization of hair cells by frequency sensitivity along the basilar membrane.
- Primary Auditory Cortex — brain region that receives and decodes auditory information from the cochlea.
- Tonotypical Mapping — the orderly mapping of sound frequencies both in the cochlea and in the auditory cortex.
Action Items / Next Steps
- Review diagrams of the cochlea and primary auditory cortex to visualize basilar tuning and tonotypical mapping.
- Study the process by which sound waves are converted to neural impulses in the cochlea.