Lecture 9

Audio recording started: 12:04 PM Wednesday, February 04, 2009

 
 

Lecture 9 – More Senses:

 
 

•    The Ear – has 3 closely linked sensors 

o    1.) The Semi-circular canals - detect rotational acceleration

•    They are filled with Endolymph

•    The Capulla (a gelatinous membrane that seals off the Crista Ampularis) sits on top of the Crista ampularis

•    There are hairs that protrude into the gelatinous mass of the Capulla. 

•    When we turn our head, the Endolymph pushes on the Capulla (billows like a sail) and pushes the hair cells to Distortion, and that is what it is detecting

•    Since Semi-circular canals lie in each plane of space, the anterior on one side is the same plane as the posterior on the other side. 

o    2.) Utricle and Sacule – detects head position and linear acceleration

•    They are at right angles to each other

•    Utricle – front to back and side to side – detects head position and linear acceleration

•    Sacule – front to back and up and down - detects head position and linear acceleration

•    The gelatinous mass is sitting on top of the Macula.  There are Calcium carbonate stones called Otoliths (which are acting as weights) sitting on here and they shift in response to gravity or acceleration, and they pull the membrane back and change the hair cells.

•    The induced bending of the cilia (shifting the membrane back and forth) is doing the detection of the motion. 

 
 

• Cochlea – detects Sound
  
  • Hair cells:
    
    • Vestibular Hair Cells-The hair cells are embedded in the Macula and in the gelatinous membrane of the Macula. 
      
      • The hair cells have clumps of Cilia on them (these are Vestibular Hair cells)
        

  There is a single very large cilia called the Kinocilium, this cannot depolarize, it cannot function, other than it is there.  
  

  •    The small away you move from the Kinocilium, the smaller the cilia (the Steriocilia) become shorter and shorter.
  

  •    The rest of the cilia are called Steriocilia – there are rows of the Seriocilia moving away from the Kinocilium, with each row getting smaller and shorter and shorter.  The tallest cilia will be the Kinocilium. 
  

  •    There are Cross-links between the Sterocilia, this is from 
  

  •    The Cross-links make sure that everything bends in the same direction at the same time. 
  

  •    There are Tip-links between the Steriocilia – from the tip of a small Sterocilia to the tip of the next highest one.
  

  •    The Tip-links open channels in the side of the larger cilia
  

  •    When the Sterocilia (the hair cells that are doing the electrical event) bend towards the Kinocilium there is a depolarization of the afferent nerve. 
  

  •    When the Stereocilia bend away from the Kinocilium, there is a hyperpolarization of the afferent nerve. 
  

  •    The hair cells rain out Neurotransmitters, and you are just increasing or decreasing the amount of Neurotransmitter over time
  

  •    The whole thing is considered a hair cell because it has the hairs sticking out of it. 
  

  •    The hairs sticking out of the cell are bathed in Endolymph, and the rest of the hair cell is being bathed in Perilymph. 
  

  •    Endolymph – is high in Potassium, and is much like intercellular fluid.
  

  •    Perilymph – is like extracellular fluid. 
  

  •    When the Sterocilia bends towards the Kinocilium (due to a depolarization) and opens the Distortion channels (a potassium channel), potassium is flowing into the cell. This is very unique because this is the first place where Potassium has a higher concentration gradient outside the cell than inside of the cell. Potassium is going into the cell. This is the only place in the human body where you find this.
  

  •    There is a high level of K in the Endolymph
  

  •    The Potassium (came in from the Endolymph) is going to cause a depolarization and cause the voltage gated Calcium channels to let Calcium into the cell.   The Calcium will flow into the cell from the Perilymph – this influx of Calcium increases the Neurotransmitter output.  The Neurotransmitter will bind to the Potassium channel and the Potassium will go out into the Perilymph.
  

  •    Support cells will pass this to other support cells, and then it will work its way back into the Endolymph and the cycle will restart again
  

  •    Hair cells don't fire action potentials, they just increase and decrease their Neurotransmitter output. Their afferents will fire action potentials.  
  

  •    The Voltage gated Calcium channels kick out Calcium, which binds to a Potassium channel, which kicks the Potassium out into the Perilymph. 
  

  •    The Potassium Distortion channel is pulled open by the Tip-link
  

  •    Endolymph is formed by dark cells. 
  

  •    Potassium is picked up in the Perilymph by support cells, and it is passed along to the dark cells, and then recycled into the Endolymph. 
  

  o    In the Semi-circular canals, the Kinocilium of the Crista Ampularis all face the same direction. 
  

  •    In the Lateral Semi-circular canals – the kinocilia face towards the utricle. 
  

  •    In the Posterior and Anterior Semi-circular canals – the kinocilia face away from the utricle. 
  

  •    In the utricle and sacule, the macula will have hair cells that go up into the gelatinous membrane. 
  

  •    There is an area in the utricle and sacule that does not have any hair cells coming out, and this is called the Striola. 
  

  •    The ridge of Otoliths is above this Striola. This is not the snowy ridge that sits above it. 
  

  •    In the Utricle the Striola is C-shaped 
  

  •    In the Sacule it is S-shaped or hook-shaped. 
  

  •    In the Utricle the Kinocilia face toward the Striola (the Kinocilia will be on both sides of the C)
  

  •    In the Sacule the Kinocilia face away from the Striola (the Kinocilia will be on both sides of the S)
  

  •    There are 2 types of Vestibular hair cells in the Macula of the Utricle and the Sacule. 
  

  o    Type I hair cells – it only connects to 1 afferent neuron, and it sits in a cup, and the axon comes up and wraps right around this cup.  This is a sensory axon. 
  

  •    There is Efferents going down and end on the outside of the cup, but they never actually touch the hair cell.
  

  •    The Afferent neuron is firing Tonally (it is always firing, and it just changes its firing rate based upon how much Neurotransmitter is being released; it speeds the firing up and down). 
  

  o    Type II hair cells – hair cells which directly connect to both efferents and afferents neurons; we have a bunch of efferents coming to it, and we have a bunch of afferents leaving it. 
  

  •    Type II - These cells fire Phasically (they have to be stimulated with enough Neurotransmitter to get a firing rate), they are NOT firing tonally 
  

   
   

   
   

• Cochlea and your Hearing: 
  
  • Sound is comprised of air vibrations. 
    

  o    The air vibrates and vibrates the Tympanic membrane, what we refer to as the eardrum. 
  

  o    The Tympanic membrane vibrates the Ear ossicles. 
  

  o    The Ear ossicles (the Stapes) vibrates the Oval window. 
  

  o    The Round window opens to the air filled middle ear. 
  

  o    See drawing (straightened cochlea). 
  

  •    The role of the Round window is that you cannot compress a fluid.  This is the only way the vibration can come in. 
  

  •    If one part pushes inward, something has to push out. 
  

  •    The Round window pushes out. 
  

  •    The vibrations are entering into the Perilymph and the vibrations get the Scala media to move up and down. (The Scala media has the vibrations going to it)
  

  •    As it moves up and down it causes a lateral movement of the Tectorial membrane. 
  

  •    The hair cells are below the Tectorial membrane
  

  o    There are 2 types of hair cells in the Cochlea:
  

  •    1.) Outer hair cells – there are 3 rows of them, and they stick up into the Tectorial Membrane
  

  •    2.) Inner hair cells – there is 1 row of them, and they don't stick up into the Tectorial Membrane
  

  •    Sometimes you can get up to five rows of outer hair cells in some regions of the Cochlea. 
  

  •    There are 3 rows of Outer hair cells, and they don't have a Kinocilium.  All the rows have Sterocilia.
  

  •    Within each row the Stereocilia are the same size/height. But the rows get shorter and shorter, arranged from tallest to smallest
  

  •    There are also Cross-links and Tip-links present. 
  

  •    Outer hair cells are V or W shaped, and the opening will face the inner portion of the spiral of the cochlea. 
  

  •    Inner hair cells are U shaped, but a flattened U-shape and the opening faces the inner portion of the spiral of the cochlea. 
  

  •    There are NO Type I and Type II cells 
  

  •    The inner hair cells only have afferent connections and they don't sit in a cup. Inner hair cells will have 10 or more synaptic connections, but all of them will be afferent.  
  

  •    Outer hair cells have both efferent and afferent connections.  (like Type II cells with no cillia)
  

  o    Inner hair cells are what we are hearing with
  

  o    Outer hair cells are for Amplification. 
  

  • These are what get damaged to cause deafness
    

   
   

   
   

  •    The Theories of Hearing: A human range of hearing is from 20Hz to 20,000Hz
  

  o    The Place Theory of Hearing (only works in the high notes well)
  

  •    The Basilar membrane that the Organ of Corti sits on is about 100um wide at the base, and about 500um wide at the apex. 
  

  o    The Organ of Corti is what you are hearing with, and it sits on a Basilar membrane 
  

  •    The tension is the same throughout its length, but it is stiffer at the base than at the apex. 
  

  •    This allows waves to travel one direction from base to apex. (there is no echoing and the wave cannot be returned back)
  

  •    If sound is vibrating the Oval window, we get a sigmoid (S) shaped wave in the vestibule, and it passes it all the way up towards the helicotremia, it will grow in size until it quickly disappears. 
  

  •    Where it peaks and dies, it what you hear as a tone. 
  

  •    If it peaks down close to the base, you hear a high note, if it peaks down towards the apex, you hear a low note. 
  

  •    Where the Maximum deflection occurs is where you hear the note. 
  

  •    There is a long flat part where the brain cannot discern the notes anymore. 
  

  •    Below 500Hz the peak of the traveling wave is too broad for pitch discrimination, so this theory does not work below 500Hz. 
  

  •    This theory is only good from about 20,000Hz to 500Hz. 
  

  •    It starts to fade out, as it gets closer to the 500Hz, and then dies. 
  

   
   

  o    The Phase Locking Theory of Hearing
  

  •    Below 500Hz the hair cells will vibrate at the same Hz frequency as the note/sound wave 
  

  •    Listing to the low notes because the hair cells are vibrating at the same frequency as the sound itself.  You are listening with the cells that are vibrating. 
  

  •    This is good from 20Hz to 500Hz. 
  

  •    We have this transition here b/c Phase Locking does not go above 500Hz.   At this 500Hz level the Absolute Refractory Period of the Afferent neurons prevent firing faster than 500 times a second. 
  

  •    The Place Theory is starting and isn't doing a good job of listening, and that's where the Transition Theory kicks in, which we call the Volley Theory of Phase Locking.
  

   
   

  o    The Volley Theory of Phase Locking
  

  •    Rows of hair cells, one hair cell fires, and then the next one up fires, until the first one can fire again. This is a Volley of firing. The brain analyzes and counts (counting how many hair cells that have gone by) the pattern of the Volley and interprets into sounds. 
  

  •    This transition from Phase and Place happens at 500Hz, switching to the Volley Theory, and that is doing a good job of hearing 
  

  •    The Volley Theory starts to phase out between 2,000Hz to 6,000Hz (this is variable between people)
  

  •    This is good from about 500Hz to 4,000Hz. 
  

  o    20Hz to 500Hz – only Phase Locking. 
  

  o    500Hz to 4,000Hz – both Volley theory and Place theory (with the Place increasing its ability as you move up further away from 500Hz towards 4,000Hz)
  

  o    4,000Hz to 20,000Hz – only the Place theory. 
  

  o    Most of the sensory input from the inner hair cells of the ear are sent to the opposite hemisphere of the cerebrum. If you are following the excitation of this, it will go to the opposite hemisphere. A small amount of information will be sent on the same side (Ipsolateral).  (Ex. This is useful for EEGs)                    End of Exam I Material
  

   
   

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