Anatomy and Pathways of Hearing

Anatomy of the Ear

• The Organ of Corti is responsible for the transduction of sound waves into neural activity.
• The Organ of Corti is situated in the cochlea.
• The Organ of Corti is composed of hair cells that are responsible for transducing vibrations into neural signals.

Central Auditory Pathways

• Auditory signals travel from the cochlea to the brain via the auditory nerve.
• The auditory nerve synapses in the cochlear nucleus.
• Most axons cross over to the opposite side of the brain.
• They then synapse in the inferior colliculus, after which they travel to the medial geniculate nucleus in the thalamus.
• The thalamus is the final relay station before the auditory cortex.

Auditory Cortical Areas

• The primary auditory cortex is located in the temporal lobe.
• It receives information from both ears, but mostly from the contralateral ear.
• The auditory association cortex surrounds the primary auditory cortex.
• There are two streams of auditory information flow: the dorsal stream and the ventral stream.
• The dorsal stream is responsible for sound localization, while the ventral stream is responsible for processing complex sounds.

What is Sleep?

• Sleep is a state of reduced consciousness characterized by:
  • Reduced responsiveness to external stimuli
  • Reduced motor activity
  • Altered brain activity

The Study of Sleep

• Researchers study sleep by measuring brain activity and muscle activity.
• Brain activity is monitored using an electroencephalogram (EEG).
• Muscle activity is monitored using an electromyogram (EMG).
• Eye movements are monitored using an electrooculogram (EOG).

The EEG

• There are different types of brain waves observed on the EEG:
  • Alpha activity (8-12 Hz) - relaxation.
  • Beta activity (13-30 Hz) - arousal.
  • Delta activity (less than 3.5 Hz) - deepest stage of sleep.
• During sleep, the brain wave activity becomes more synchronized.
• The waves become slower as sleep progresses from stages 1-4.
• During REM sleep, the brain activity resembles the awake state.

Non-REM Sleep

• Stages 1-4 of sleep are called "non-REM sleep".
• Stages 1-2 are transitional phases.
• Stages 3-4 are called slow-wave sleep, characterized by low-frequency delta activity.
• During slow-wave sleep, breathing is light and even, muscle tone persists, and arousal is difficult.

REM Sleep

• REM sleep occurs approximately every 90 minutes throughout the night.
• It is characterized by:
  • Increased respiration and blood pressure.
  • Rapid eye movements.
  • Loss of muscle tone (paralysis).
  • Vivid and emotional dreams.

Cycles of Sleep

• There are cycles of REM and non-REM sleep.
• Each sleep cycle lasts approximately 90 minutes.
• Each sleep cycle contains 20-30 minutes of REM sleep.
• Over the course of a night (8 hours), there are 4-5 periods of REM sleep.

Explore the anatomy of the ear, including the Organ of Corti and its role in sound transduction. This quiz will also cover the central auditory pathways from the cochlea to the brain, and the auditory cortical areas involved in processing sound. Test your knowledge on these crucial topics in auditory physiology.