INTRODUCTION


Higher center for hearing is in the temporal lobe. The fibers of the auditory pathway (Vestibulocochlear Nerve, VIII Cranial Nerve) terminates in the temporal lobe.


RECEPTORS


 The hair cells in the organ of Corti are the receptors of the auditory sensations.
Two types of hair cells outer and inner hair cells. Each hair cells are innervated by afferent and efferent nerve fibers.

FIRST ORDER NEURONS  

The bipolar cells from the spiral ganglion situated in the modiolus of cochlea represent the first order neurons.
These neurons enter the medulla oblongata. After entering the medulla oblongata, the fibers divide into two groups which end on the ventral cochlear nucleus and dorsal cochlear nucleus of the same side in the medulla oblongata. 

SECOND ORDER NEURONS 

The neurons of the dorsal and ventral cochlear nuclei in the medulla oblongata form the second-order neurons of the auditory pathway.
The axons of the second order neurons pass through a complex pathway, which runs in four different directions. 

First Group - Nerve fibers cross the midline and run to the opposite side to form trapezoid body go to the superior olivary nucleus.  

Second Group- Second group neurons terminate at the superior olivary nucleus of the same side via trapezoid body of the same side.  

Third Group - It runs in the lateral lemniscus of the same side and terminates in the nucleus of lateral lemniscus of the same side.  

Fourth Group - It runs into the reticular formation, crosses the midline as intermediate trapezoid fibers and finally joins the nucleus of lateral lemniscus of the opposite side.  

THIRD ORDER NEURONS 

Third order neurons are in the superior olivary nuclei and nucleus of the lateral lemniscus.
The third order neurons end in the medial geniculate body which forms the subcortical auditory center.  Fibers from the medial geniculate body go to the temporal cortex, via internal capsule as auditory radiation. 


The fibers of auditory radiation are involved in the reflex movement of the head in response to auditory stimuli.


CORTICAL AUDITORY CENTERS

It is present in the temporal lobe of the cerebral cortex.  The auditory areas are-

  1. area 41- It is the primary area situated in the superior temporal gyrus. 
  2. area 42 -It is the primary area situated in the superior temporal gyrus. 
  3. Wernicke area-  It is a secondary auditory area situated posterior to area 41 and area 42. 
  4. Area 22- It is known as audito psychic area and is related to analysis and interpretation of sounds.


FUNCTIONS OF CORTICAL AUDITORY CENTERS 

  •  Perception of auditory impulses (Area 41 & Area 42).

  • Analysis of pitch and intensity of sound. (Wernicke’s area)

  • Determination of source of the sound. (Wernicke’s area)

MECHANISM OF HEARING

Acoustic energy, in the form of sound waves, is channeled into the ear canal by the pinna. Sound waves strike the tympanic membrane, causing it to vibrate like a drum, and changing it into mechanical energy. The malleus, which is attached to the tympanic membrane, starts the ossicles into motion. (The middle ear components mechanically amplify sound). The stapes moves in and out of the oval window of the cochlea creating a fluid motion. The fluid movement within the cochlea causes membranes in the Organ of Corti to shear against the hair cells. This creates an electrical signal which is sent via the Auditory Nerve to the brain, where sound is interpreted!