HEARING AND THE VESTIBULAR SYSTEM

Know the basic properties of sound, the stimuli of hearing system.

Sound is a mechanical wave of compression (high pressure) and rarefaction (low pressure) propagating through an elastic medium

Loudness (volume): directly related to the amplitude (max pressure difference of the wave)

• measured in decibels, 30dB= whisper, Normal speech= 60, Jet engine = 130dB

Pitch: directly related to the frequency (number of cycles per second measured in Hz) how high it low a sound is

Timbre/Quality: the unique sound or “character” of a sound that lets you tell what made the sound, even with the same note (piano vs violin)

Know the basic structures of the ear critical for sound perception.

1. Outer ear: (auricle: collects sound waves) and external auditory canal collect and direct sound waves toward the tympanic membrane (eardrum). (Outer ear collects sound and sends it to ear drum)

2. tympanic membrane vibrates when sound hit it. The auditory tub connects the middle ear to the pharynx and helps equalize pressure, The ossicles (malleus, incus, stapes) amplify the vibrations 15-20 times and transfer them to the oval window of the cochlea. ( The middle ear vibrates, amplifies the sound, and sends it into the inner ear.)

3. Inner ear: The cochlea spiral shaped structure with three compartments: scala vestibuli, cochlear duct, and scala tympani. The cochlear duct contains fluid with a high K+ concentration. (The cochlea turns sound vibrations into signals the brain can understand.)

4. Sound → Pinna → Auditory canal → Eardrum → Ossicles → Oval window → Cochlea → Organ of Corti → Brain

Know how the loudness and the pitch of sounds are perceived.

Pitch perception: how high or low a sound is. The basilar membrane is organized so different parts response to different frequencies : the location of where the basilar membrane moves tells the brain the pitch

• base: narrow/stiff responds best to high frequency (high pitched sounds

• Apex: wide/flexible; responds best to low frequency

Loudness perception: loudness depends on how much the basilar membrane moves. Louder sounds cause more bending of the sterocilla, which causes more neurotransmitters release and a higher frequency of action potentials in afferent neurons

Louder sound → more basilar membrane movement → more stereocilia bending → more neurotransmitter → more action potentials

Know the basic structures of the vestibular system, its functions, and how it works.

The vestibular system detects head position and motion to maintain posture, balance, and visual stability

• Semicircular Canals (rotation)

  • Function: detect annual acceleration ( nodding, head shaking)

  • Head rotation → endolymph moves → cupula bends → stereocilia bend → signal is sent to the brain.

2. Otolith Organs (linear movement/ tilt): The utricle and saccule detect linear acceleration and head tilt relative to gravity.

• Utricle: Detects horizontal movement and tilt.

• Saccule: Detects vertical movement and acceleration.

Movement/tilt → otoliths shift → stereocilia bend → signal to brain.

3. Main functions: vestibule-Ocular Reflex: keeps your vision stable when your head moves by making your eyes move in the opposite direction

4. Postural control: sends signals to spinal motor pathways to help maintain balance and upright posture

Spatial Perception: Sends information through Cranial Nerve VIII to the brainstem, cerebellum, and parietal cortexso your brain knows your head and body position in space

• Semicircular canals = rotation (structure)

• Utricle = horizontal (structure)

• Saccule = vertical (structure)

• What the vestibular system does :

• VOR = keeps vision stable

• Postural control = keeps you balanced