In our inner ear sits two sets of remarkable sensors.
One uses the smallest joints in the human body.
- Malleus to incus. Incus to stapes.
- "The hammer, anvil & stirrup."
Tiny joints that amplify tiny vibrations in the air
25x amplification to hear those vibrations
Vibrations we hear as sound.
👂Ears with 3D balance The other sensor uses 3x bony labyrinths to balance us.
Detecting movement in a 3-dimensional world.
3x semi-circles at roughly right angles.
Mirrored opposites in each ear.
Fluid filled canals with a bunch of hair cells.
Head movement one way accelerates fluid in the same direction.
But the mirror image canals on each side of our head have opposite effects.
Like a mini tsunami, fluid in one ear pushes against hair cells one way.
Hair cells depolarise.
Sparks fly. Fluid in the other ear pulls on the hair cells.
Signals decrease.Our brain reads both inputs
=> ‘Head is moving that-a-way’.Even with our eyes closed, we can tell.
Reflexes between our ears and eyes keep our vision stable.
Even as our head moves all over the place.
Our vision stays fixed.
We stay upright.
🍷Why does our head spin after a few drinks? Alcohol plays havoc. As it does with most things...
Blood alcohol goes up after a first drink.
Drawing water into the blood by osmosis...
...and sucking water
out of our semi-circular canals.
Less water causes head spinning vertigo in one direction.
Later in the evening, alcohol diffuses out of our blood...
...and
into our semi-circular canals. Drawing water along with it.
More water causes head spinning vertigo spins in the other direction.
I’m not sure I’ve ever paid enough attention :)
👀 Eyes, and why we get travel sick...When our eyes signal different to our ears, our brain signals something is wrong.
Look at the horizon and your eyes AND ears signal
‘We’re jiggling about’.Read a book that’s also jiggling about in the car...
...and our eyes can
NOT see the jiggling about.
Ears 👂 => Jiggle
✅ Eyes 👀 => No jiggle
❎Brain 🧠 =>
‘I feel sick’. Most of the time, with no alcohol or motion, our eyes and ears co-ordinate beautifully.
💪 Muscle sensorsOur third feedback system, our proprioception, makes a strong case to be our sixth sense.
- Close your eyes
- Put your hand in the air
- Don't touch anything
=> We know where our hand is! That bit of physiology still amazes me...
Joint receptors detect position.
Golgi tendon organs (at either end of a muscle) detect load.
Muscle spindles detect stretch.
Together these sensors form an all over body network.
A network that, not only tells us where our limbs are in space, but how fast they're moving and how much load they're carrying.
Muscle spindles trigger protective stretch reflexes.
Reflexes that don’t need the brain.
Over-stretch a muscle
=> Spindle fires
=> Action potential
sparks => Spinal cord relay
=> S
parks straight back
=> Same muscle pulls
=> Over-stretch corrected.
Phew!A tendon hammer tap to the knee, ankle, elbow and wrist triggers the classic tendon reflexes.
- Relax your bent knee over the edge of a bench or chair.
- Dangle your foot freely
- Firmly tap your patella tendon just below the kneecap.
Tendon stretches
=> Reflex arc activates
=> Quadriceps muscle contracts
=> Our foot kicks out.
Those reflexes are fun. But they're also good tests on the examination couch to check that our nerves are working.
But their real importance is in everyday life.
Reflexes that trigger all the time. Without us knowing. Keeping us upright. Keeping us moving forward.
Eyes, ears and muscle sensors keeping us upright. Keeping us moving forward.
