VI Neuroanatomy and Neurophysiology

• External Ear:

  • Auricle composed of elastic cartilage; lobule lacks cartilage
  • Auricle funnels sound waves into external acoustic meatus
  • External acoustic meatus begins as elastic cartilage, then becomes a canal in temporal bone; lined with skin, hairs, sebaceous & ceruminous glands
  • Cerumen (earwax) helps trap insects or debris
  • Sound waves make tympanic membrane vibrate; sound energy then transferred to ossicles
  • Tympanic membrane is the boundary between the external and middle ear
• Middle Ear:

  • Air-filled cavity with eardrum laterally and a bony wall with oval window and round window medially
  • Pharyngotympanic tube links middle ear with nasopharynx
  • Eardrum vibrates only if pressure on both sides is equal
  • Ossicles (malleus, incus, stapes) transmit vibration of eardrum to oval window, setting fluids of inner ear into motion
  • Tensor tympani and stapedius muscles contract to protect hearing receptors by limiting ossicle vibration in response to loud noises
• Internal Ear:

  • Bony labyrinth contains vestibule, cochlea, semicircular canals
  • Filled with perilymph (similar to CSF) and potassium-rich endolymph
  • Perilymph and endolymph conduct sound vibrations and respond to mechanical forces linked to changes in body position and acceleration
  • Vestibule contains utricle and saccule, which monitor head position and contain equilibrium receptors called maculae
  • Semicircular canals define two-thirds of a circle and monitor angular movements of the head
• • Cochlea is a spiral, bony chamber containing the cochlear duct and spiral organ (organ of Corti; hearing receptor)
• Sound:

  • Sound is a pressure disturbance originating from a vibrating object and propagated by molecules of the medium
  • Composed of compression and rarefaction areas creating sound waves
  • Frequency of sound waves is perceived as pitch; loudness is perceived as amplitude
  • Human hearing frequency range is 20-20,000 Hz; most sensitive to 1500-4000 Hz
  • Intensity of sound measured in decibels (dB); normal conversation is ~50 dB, prolonged exposure to 90 dB considered dangerous for hearing loss
• Sound Transduction:

  • Inner hair cells are key for sound reception
  • Movement of basilar membrane causes bending of hairs, opening mechanically gated cation channels, releasing NT glutamate to excite cochlear nerve
  • Outer hair cells amplify motion of basilar membrane and protect inner hair cells
  • Auditory pathway involves impulses from cochlea passing to spiral ganglion, cochlear nuclei, superior olivary nuclei, inferior colliculi, and auditory cortex
• Balance:

  • Balance organs monitor head movements for orientation and balance in space
  • Balance receptors located in semicircular canals and vestibule (utricle and saccule)
  • Maculae in utricle and saccule contain equilibrium receptors responding to gravity and head movements
  • Crista ampullaris in semicircular canals stimulated by rotational movements and changes in velocity
  • Equilibrium pathway to the brain involves direct pathways to the brainstem for reflexive responses