Midterm 1 - Audition & Olfaction Flashcards
Sound Pathway
(Process Overview)
- Sound
- Ear canal
- Tympanic membrane (eardrum) vibrates
- Ossicles (stapes) press against membrane behind oval window (opening to inner ear)
- Air transferred to liquid medium in cochlea
- Organ of Corti
- Shearing of basilar and tectorial membrane (where hairs are anchored)
- Mechanoreceptors open
- K+ enters
- Membrane depolarizes
- Release of NTs
- 8th CN (auditory nerve)
Ear Anatomy
(Overview)
Organ of Corti Unrolled & Place Code
Organ of Corti
- Within cochlea and contains tectorial membrane (where hair cells are anchored), basilar membrane (what vibrates), hair cells, mechanoreceptors, CN#8
Shearing Effect
(Visual)
Shearing Effect
When the tympanic membrane vibrates, there is a “shearing effect” between the basilar and tectorila membranes. This results in hair cells getting pulled. If they get pulled far enough, their mechanoreceptor will open and K+ will rush in, causing depolarization.
Cranial Nerve # 8
Acoustic/Vestibulocochlear
The axons leaving the hair cells form a bundle of axons that go to the brain. Even though we have a greater number of outer hair cells, the majority of the axons are from inner hair cells (which are closer to CN#8).
Mechanoreceptor
The ion channels in the tectorial membrane open as a result of the shearing effect (as opposed to a ligand binding).
When the mechanoreceptors open, K+ rushes in and causes depolarization.
2 Different Ways of Conveying Sound Messages to the Brain
- Frequency
- Place Code
Frequency
(conveying of sound message to the brain)
AKA “Rate Coding”
If there’s a low frequency (pitch measured in Hz), the hair cells will fire in synchrony with the low tone.
Place Code
As the cochlea is laid out, it goes from high tones to low tones. The brain can tell the tone of the sound based on where it comes from in the organ of corti.
Tonotopy Based on Place Code
The auditory cortex maintains the frequency specificity of the cochlea. Organized as a tonotopic map in MGN and A1.
Auditory Pathway
(Starting with CN # 8)
(Visual)
Auditory Pathway
(Verbal)
- The auditory nerve makes a synapse in the cochlear nucleus in the brain stem.
- The projections from the cochlear nucleus mostly cross and synapse in the superior olive.
- Axon collaterals then go to the inferior colliculus of the midbrain. Main information however goes directly to the thalamus.
- The inferior colliculus projects to the thalamus (medial geniculate nucleus).
- The medial geniculate projects to primary auditory cortex in the superior temporal lobe.
Inferior Colliculus
- Midbrain
- Directly below superior colliculus
- Allows you to start orienting head in a particular direction if we hear an alarming sound (you turn your head towards sound).
Primary Auditory Cortex
(A1)
On the upper bank of the temporal lobe, just below the lateral fissure.
Pleasure of Sound & Recruitment
Because we have a tonal map, neurons in cochlea, MGN and A1 can all fire at the same frequency. Then they will “recruit” other neurons around them which will start firing at the same frequency.
When huge populations of neurons in the brain fire together, it’s very pleasurable.
If sound enters left ear, where does it go?
Ends up in A1 of the right side.
We know that sound information crosses by performing dichotic listening tasks.
Vestibular System
- Balance
- You can tell which way your head is going by the way fluid moves in the semi-circular canals.
- Part of the auditory system.
Olfactory Pathway
- Raw olfactory molecules attach to mitral cells (in olfactory bulb) that are sitting in the cribriform plate.
- Most of the axons of the mitral cells go straight into cortex (pyriform cortex, entorhinal cortex) via CN # 1. NOT TO THALAMUS.
- Raw olfactory information goes straight to the pyriform and entorhinal cortices (have something to do with emotion and memory; could be primary olfactory cortex?). There is no filtering of raw olfactory information b/c it doesn’t go to the thalamus.
