Question

Trace (either draw a picture or write words and arrows) the path of sound from the outer ear to interpretation by the brain, detailing what each step in the pathway is generally responsible for. This does not have to be extremely detailed (e.g. cell types), but should show pathways and the function of specific nuclei & brain areas.

Answer 1

The ear is the main organ of hearing. It is divided into three parts beginninyfrom pinna and ending at the cochlea :

1. Outer ear : The outer ear consists of the pinna, ear canal and the tympanic membrane (ear drum).
The pinna is the outermost part of ear which directs the sound waves into the ear canal.
The ear canal directs the sound to tympanic membrane. It also secretes earwax that helps protect ears from infections.
When the sound hits Tympanic membrane it vibrates. These vibrations are passed on to the ear ossicles in the middle ear.  
2. middle ear : middle ear consists of the ear ossicles (maleus, incus and stapes), eustachian tube and round and oval window. The vibrations from the tympani membrane transfers to the malleus. Malleus transfer them to incus and then the stapes. Stapes rest on the oval window and causes it to vibrate.
The round window transfers the vibrations to inner ear and causes the fluid within the inner ear to move. The eustachian tube connects moddle ear yo nasopharynx and helps in pressure equalization and mucus drainage.
3. Inner ear : consists of the vestiules, semicircular canal and cochlea. Vestibules contains utricle and saccules that helps in maintaining equilibrium. Vestibule also contains endolymph that vibrates due to vibrations from inner ear. Endolymph passes these vibrations to the cochlea. Cochlea contains hair cells within organ of corti which moves due to vibration of tectorial membrane. As the hair cells move they generate electrical impulses that travel upto the auditory cortex within the brain through auditory nerve and gives auditory information to the brain.
Pathway of sound in ear : pinna--> external auditory canal--> tympanic membrane--> ear ossicles--> oval window --> basilar membrane--> hair cells in the organ pf corti--> auditory nerve.
Pathway of auditory nerve : organ of corti --> cochlear nuclei (1st order neuron) --> superior olivary complex (2nd order neuron) --> inferior colliculus (3rd order neuron) --> medial geniculate body in thalamus--> auditory cortex.