Lecture Two Orienting Question

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Lecture Two - Orienting Question
Are two muscles needed for the normal functioning
of the mammalian middle ear?
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Main function is to transform pressure variations
from those that occur in air to saline fluid in
cochlea. Guinan and Peake measurements Included
test of linearity. For single sinusoid input
amplitude linearly related to output amplitude
(every day meaning of linearity). Extra
restriction that has to pass through origin.
Guinan and Peake's tests in anaesthetized cats
(relaxes muscles) shows linear
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Does the system have to have three ossicles and
two muscles to do pressure transformation?

• Evidence from birds and reptiles (columnella) and
  from ossicle replacement surgery that suggests
  this might not be necessary.
• What other functions might this arrangement
  serve?

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Protective role - protection from loud sounds

• The two muscles might pull the ossicles apart and
  protect them.
• Evidence for Stapedius contracts in response to
  sound.
• Therefore, stapedius might have this role. Be
  careful not to overstate it
• Contraction after stimulated
• Muscle tissue takes time to contract
• Doesn't come in until sounds very loud (i.e.
  damage done).

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Tensor tympani What causes it to contract?

• Swallow
• Yawn
• Smelling salts
• Some consider responds to acoustic stimulation

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Eustachian tube unfolding (Zemlin) Odd
anatomical structure

• Spindles and how they control contraction
  (Merton). Tensor tympani got none (Malmfors
  Wersall, 1960), stapedius has (Blevins, 1963).
• Golgi tendon organs (GTO's) as all-or-none cut
  out mechanism. Blevins 1963 don't get in tensor
  tympani because fibres too thin.
• Lack of effect of nerve block (Wersall, 1958).
• Can't condition (Simmons, Galambos Rupert).

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Eustachian tube unfolding (Zemlin) Histochemical
structure

• 5. Lot of fat (Eggston Wolff - man Kobayashi -
  most mammals).
• 6. Low enzyme activity - Anderson

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Rationales that have been given to explain these
oddities

• Wersall expressed surprise at lack of spindles
  and lack of effects of nerve block.
• Blevins belittled absence of GTOs on basis of
  compariosn of other non-weight bearing muscles -
  but they probably have spindles.
• Justification for odd histochemistry
• Fat cushions and protects muscle (Malan 1934)
• Low enzyme actvity reflects need for slow
  contraction.

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Adaptationist fallacy

• All the preceding may appear alright on their own
  but they are not very parsimonious.
• Simpler to assume tensor tympani has no function
  - remember evolved from gill arches and other
  evolutionary paths don't lead to this structure
  (birds and reptiles).
• To show this, you can show that what happens in
  the tensor tympani parallels what happens in
  other muscle tissue that has poor innervation and
  atrophies

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Can't condition - Vrbova and Gutman Spindle loss
- Zelena and Hnik Reduction of fibre diameters -
Fernand and Young Denervated muscle atrophies -
fat one sign (Gutman and Zelena) Low enzyme
activity in denervated muscle (Bass) and
dystrophied muscle (Furakawa) There are also
parallels with other characteristics not
discussed up to now Fast fatigue (Moller) Low
level of spontaneous activity (Wersall)
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In the light of these observations, we will now
revisit the proposed functions

• Terkildsen - considers has an acoustic function.
  Have to be very loud. When tensor tympani
  transected, acoustic functions are not affected.
• von Bekesy - need two muscles to stop three
  ossicles separating.
• Joint between malleus and incus stiff in most
  mammals (Moller, Dallos)
• Tensor tympani absent in a species of bat
  (Henson)
• Should be attached to malleus or incus but in
  harbour seal, attached to stapdius (Ardouin)
• Misurya - opens Eustachian tune when the muscle
  goes slack - but no spindles etc. Tensor tympani
  and tensor veli palatini continuous (Lupin).
  Relayed activity, tvp does have spindles
• Could it have some function around birth (get rid
  of fluid in middle ear through Eustachian tube)?
  But properties we have equated with atrophy have
  been observed in utero.