PHYSIOLOGY OF HEARING

1. Conductive part.
2. Sensory-neural part.

1. The Conductive part:

• It transmits sound as mechanical vibrations.
• It consists of: the External ear and the Middle ear.

• The External Ear:
• The auricle collects the air-borne sound vibrations
• The external auditory canal transmits the air-borne sound vibrations to the tympanic membrane.
• When the air-borne sound vibrations reach the tympanic membrane, it vibrates and converts the air-borne vibrations to mechanical vibrations.

• The Middle Ear:
• The ossicles have two functions
  

• Transmition of the sound waves from the tympanic membrane to the oval window.
• Amplification of the sound waves by two mechanisms:

1. The areal ratio between the vibrating area of the tympanic membrane and the area of the oval window, is 17 : 1
2. The lever ratio between the arm of malleus and the arm of incus, which is 1.3
   

• The eustachian tube ventilates the middle ear to equalize the middle ear and atmospheric pressure. This is important for free movements of the tympanic membrane and ossicles.

2. The Sensory-neural part

• It transmits sound as electrical impulses.
• It consists of the Cochlea and Cochlear nerve.

• The Cochlea:
  Converts the mechanical sound vibrations to electrical impulses by vibration of the footplate of stapes in the oval window ≫ vibration of the cochlear fluids≫ vibration of the basilar membrane≫ stimulation of the cochlear hair cells which lie on the basilar membrane (in the organ of Corti) ≫conversion of the mechanical sound vibrations to electrical impulses.

17

Figure 10: The The lever action between malleus and incus.

• The Cochlear Nerve
  It transmits the electrical impulses to the cochlear nuclei (in the brainstem) ≫ to the higher hearing centers in the brain (auditory area in the temporal lobe), which perceives them as meaningful sounds.

Category: Physiology Notes