Monday, November 8, 2010

I can't hear you!

How about ears... Shall we start on the inside and work our way out?

There are essentially 3 major parts to the ear. The inner, the middle and the outer. Wow, crazy naming there eh? Lets start at the middle. This is where all the good stuff is (the tiniest bits of your ear, the microscopic bits that actually allow you to hear). The inner ear is essentially formed from the semicircular canals, the vestibule (saccule and utricle) and the cochlea.
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The cochlea looks like a snail shell and holds the cochlear duct (in which the organs of sound, the organ of corti are contained). 
 If you cross section this area you get many round windows (with scala vestibuli and scala tympani both holding perilymph separated by the scala media that has endolymph and houses the organ of corti). 
The organ of corti house hair cells lightly covered by a tectoral membrane. Vibration of this membrane deflects the hairs and the electrical impulses created by this deflection are passed along Cranial Nerve VIII (CN VIII = vestibulocochlear, or cochlear nerve) to the brain. This is what we and other animals "hear".

Moving outwards from this section is the vestibule of the inner ear, which forms the Vestibular system (imagine that!). This is made of the saccule and utricle respectively. This area houses the macula and otoliths ("ear stones") which are calcium carbonate crystals (not stones at all which I found a little disappointing). The macula and otoliths allow for recognition of static balance (balance when the head is still). The macula/ otoliths in the saccule recognize balance when the head is positioned horizontally and the ones in the utricle recognize balance when the head is held upright. So when the head is moved forwards and backwards (in respect to upright position with gravity) the Utricle recognizes what's going on, alternately when the head moves from side to side in the same situation the Saccule "knows" where you are. Together they can also recognize linear acceleration and deceleration (if you start to run or leap up and down).

Basically this works by the otoliths pulling on the hair cell = a nervous signal is created and a pulse is sent down CN VIII and your brain does the rest. If there is no pull there is no signal sent. So essentially the movement of the head is what creates the deflection of the hair cells and results in a signal being sent to the brain to let it know what's going on. Move your head = move your hair (cells) = lets your brain know you're moving!

Also in this area is the cristae and cupola. This apparatus allows for recognition of circular movement of the head (rotational acceleration and deceleration). It works just like the macula and otoliths (minus the stones which weight the gelatinous mass (so to speak). These cupola are found in all ampullae (where the semicircular canals meet the utricle) and in these positions allow for recognition in all planes of orientation (granted, being in the presence of 1g gravity is key here). Again movement of the head results in the cupola bending the hair projections from the hair cells and yet more information is sent down the CN VIII about your movements. This is the best sort of big brother there is.

Here's a cool cover of what Nasa says about your (inner) ear in space http://www.nasa.gov/audience/forstudents/9-12/features/F_Human_Vestibular_System_in_Space.html

So now for the Semicircular canals. What do those do? Well along with the Utricle they house the endolymph and cupola that allow for recognition of circular movements (housed in the ampulla of the semicircular canals where it meets the utricle).

Now for the clinical aspect.. Basically if something goes wrong here, you're deaf..... permanently.

Moving on: the mechanics of hearing. This involves the middle ear, tympanic membrane and covers the transmission of sound waves to the inner ear (the actual sense organ).

Mechanics of Hearing
  1. Ear Drum vibrates,
  2. Ossicles vibrate
  3. The stirrup pushes the oval window in and out
  4. Perilymph moves
  5. Pressure waves transmit from scala vestibula and scala tympani to the endolymph of scala media
  6. The basilar membrane deflects (hair bundles bend and hair cells convert the mechanic vibration stimulus to an electrochemical signal
  7. This signal travels to the brain via CNVIII (auditory nerve in this image)
Now this breakdown of the mechanics of how you hear included some structures not yet mentioned. So here they are.
First: the Ear Drum (or tympanic membrane), now depending on who you talk to this is part of the middle ear or the outer ear (basically it's the marker of the boundary between them). It's where the sound transmits from the outside of an animals body to the processes that allow them to "hear".

The bones (ear bones or ossicles) that allow this process to happen are as follows: Malleus (the hammer), Incus (the anvil) and Stapes (the stirrup). These bones are linked together by synovial joints and transmit the movement of the ear drum to the inner ear.

Also in this area are the spaces (which as our anatomy professor is fond of pointing out, all have names). The tympanic bulla is the round space (can be seen as is housed by a round projection off the base of the skull) and houses... well... apparently nothing. It is just the space. However if it does get something in it (like infection) it can drain via the Eustachian tube to your pharynx. Yummy. It also allows for air to be equalized on either side of the tympanic membrane (or ear drum) which would explain that "popping of your ears" during pressure changes.

A few things can happen clinically here.
1) Otitis media: a middle ear inflammation or infection. Pus can collect in the bulla (shows up on radiographs). You'll also see your dog or cat tilting their heads towards the affected side. This can happen because of viruses or perforation of the ear drum amongst many other possibilities. It can happen to anyone.

2) Horner's syndrome: This is a neat one (at least I think so). It usually stems (in relation to this position of the anatomy) from prolonged middle ear infections. Essentially the inflammation and build up of pus put pressure on a nerve that is completely unrelated to hearing -> The Facial nerve (CNVII) or the sympathetic fibres of the cervical ganglion. Basically you can get the symptoms from damage to the facial nerve in other areas as well but we are talking about the middle ear here!
The damage to CNVII results in facial paralysis, constriction of the iris, drooping of the upper eyelid and the displacement of the nicitating membrane over the eye on the affected side.

Sooooo finally the OUTER EAR!!! This is the bit that we all see (and that dogs usually love to have itched for them). This includes the Pinna (outer ear/auricular/chondral cartilage) to funnel sound towards the external auditory meatus (the ear canal) which carries sound through the annular cartilage (a ring that links the cartilaginous parts to the internal bony meatus of the external ear).... now if that's not a mouthful I don't know what is!.... 

All of this ends on the tympanic membrane (see I told you it's either inner or outer ear depending on which direction you come from).

The pinna is the cute part of the ear... Well maybe not in Lulu's case here.... But usually it's considered the cute bit!






And they come in all shapes and sizes....


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