(From Dr. Glasser’s Lecture, 7 July 2000, by Brian Buschman)
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Photoreceptor cells sum in area 17 to a bar, edge or slit of light with a proper orientation in the excitatory part of the field. Any other stimulation in the receptive field of a given area 17 cell will cause inhibition of the given cell.
As you move deeper in the column of area 17 you get cells that have the same receptive field that sense a bar in the same orientation but in a different location. If you go to the next column you will sense the same bar in the same location but it will be rotated by 10o.
Ocular dominance is when one eye does more to drive the given cell then the other. These are columns that are arranged based on ocular dominance.
Hyper columns are based on a sum of ocular dominance columns and orientation columns.
Complex cells are cells where the orientation of the slit matters but the location within the field does not.
Hyper cells sense bars of light with a proper orientation moving with a proper direction through a certain excitatory portion of the receptive field.
The trichromatic theory says that color is sensed in three colors in the retina and relayed separately to the cortex to be assembled. The opponent theory says that color is actually sensed by taking things we know about perception and adding light. In the case of ganglion cells this might be a cell with a red center and a blue surround.
When we hear we perceive two things:
1) Tone (20-20,000 Hz)
2) Amplitude (120 db range)
The receptors are the hairs of the inner ear but for them to sense the sound we need to get the sound in to them. The sound is funneled through the external acoustic meatus to the tympanic membrane. The tympanic membrane vibrates the ossicles which are the three small bones of the inner ear. The stapes then vibrates the perilymph that is within the bony labyrinth. The resonant frequency of the ossicles is about 3kHz so that is the frequency that we can hear best at.
The bony labyrinth is filled with perilymph and inside it is the membranous labyrinth which contains endolymph. The bony labyrinth also contains the oval window which is the pressure release incase of a problem.
The sound passes through the cochlear duct that has 2½ turns and houses the auditory transducer, the “Organ of Corti.” It contains hair cells in inner and outer columns with the basilar membrane on the top of them and the tectorial membrane between them. The outer column of hairs is only one row thick and the inner row is three to five rows thick.
The basilar membrane starts out about 100mm thick near the base and gets thicker as it goes until it reaches about 500mm towards the apex. It senses higher pitches near the base and oval window and lower pitches further up.
As the basilar membrane vibrates it stretches the hair of the outer layer and causes them to vibrate the tectorial membrane. The tectorial membranes then vibrated the inner hair cells. The tension on the tectorial membrane also regulates the amplitude of the sound, to a degree.
The hair cells are innervated by spiral ganglion bipolar cells which send peripheral processes to the hair cells. There is better innervation to the inner hair cells then to the outer hair cells, therefore the inner hair cells do most of the conduction.
The sensations are carried by the cochlear division of CN VIII which goes into the upper medulla and ends at the dorsal and lateral cochlear nucli.
The part that goes to the dorsal cochlear nucli crosses and ascends in the lateral lemniscus to the inferior colliculi. Some fibers go in the brachium of the inferior colliculi to the medial geniculate nucleus (MGN) to areas 41 and 42 of the superior temporal gyrus.
Note: Fibers of the inferior brachium go into the MGN of the thalamus while fibers of the superior brachium bypass the thalamus to go into the pretectum an.
The part from the lateral cochlear nucleus goes bilaterally to the SON (Superior Olivary Nucleus). The fibers that cross are called the trapezoid body. They enter the lateral lemniscus to go into the inferior colliculi, MGN and areas 41 and 42. The SON functions in detecting the location of the sound in three space.
There is also a nucleus of the trapezoid body and a nucleus of the lateral lemniscus. Any of the fibers passing along either may synapse or not in either of these nucli. In light of this fibers that enter areas 41, 42 may be 5th or 6th order fibers.
The major projection from areas 41, 42 are to Wernicke’s area which is the auditory association cortex. It makes hearing into what we actually hear.
In a legion in area 41 you don’t localize sound well on the contralateral side. You may hear it but don’t know where it is.
Cortical fugal fibers provide input to the tectorial membrane to focus it to hear at specific frequencies. This is how you are able to tune-in or out a specific conversation.
Conductive hearing losses are associated with damage that blocks hearing up to the oval window.
Sensory hearing losses are losses associated after the oval window such as in the Organ of Corti or in the nerves.
You put a tuning fork on the mastoid process and hold it there until the patient says the sound is gone. You then hold it next to their ear to see if they can still hear it. The sound should still be heard because air conduction is better then bone conduction. If they are able to hear it longer when placed on the bone then they have a conductive hearing loss.
If you can tell they have a hearing loss of some type you first do the Rhind test to see if it’s a conductive hearing loss. You then do the Webber test which involves placing the tuning fork on the patient’s forehead and listening to see if the sound is localized to one side or another. The sound should be localized in the center. If they have a conductive hearing loss the sound will be localized to the same side as the legion. If they have a sensory hearing loss the sound will be localized to the contralateral side. To distinguish between the two is the reason you first do the Rhind test.
Menier’s is associated with endolymph drainage problems and results in vertigo, nausea and tinnitouss (ringing in the ear). Because this is a problem at the level of the hair cells it can be classified as a sensor hearing problem.
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