The body electric

***Short one for the holidays!  Also full of oversimplified neurophysiology, not designed to be used as an actual source for teaching or research or anything.  I'm just a med student, not a teacher***

So, like most med students, I hated physics.  Sorry physics professors, that’s just the way it is: we all take your classes because we have to, forget it all, cram some of it back in for the MCAT, forget it all again.  No wonder we get really cranky when Neuroscience time comes.  For the not medically or biologically inclined out there: your nervous system is all about electricity.  Everything you know about the world around you, all your memories, feelings, sight, sound, touch, and taste – it all at some point in your body gets boiled down to an electrical current running through you.  I don’t say this to denigrate the breadth and depth of the human experience – I say it because it amazes me.  It’s a miracle – a full scale major marvel of the world that we are able to experience everything the way we do.  

It’s the holiday season and one of my favorite things to do is to listen to Christmas music – full rich brass quartets and eight part church choirs rendering the classis (with descents and harmony, of course).  To hear this music the sound waves have to travel from the speakers through the air, through my ear, eardrum, bones of the ear to a tiny snail shell organ – the cochlea.  Inside this is a membrane so perfectly tuned that when the vibrations hit this fluid-filled canal it causes vibration in a small area – an area that differs depending on the pitch.  So the soprano descent causes vibration in a different place than the tuba.  That vibration disturbs some cells, actually pulls open channels in their cell membranes to allow ions to flow in, converting music to a tiny electric current.  And no matter which cells, no matter how loud the sounds, the size of the current (amplitude for the nerds out there) never changes.  All the richness of sound, the variations of timber, pitch, temp, and volume are coded by which little cells light up and how often they light up.  Their little electrical signals travel to the brain stem, to be organized, categorized, sorted and perceived.  Incredibly, they pass through multiple nerves carefully placed just so for any point in space two cells will light up at the same time (one from each ear) and your brain knows what it means when they do – this is how you can find that band playing in the shopping mall.  Even more incredibly, when we hear even the simplest of musical sounds – a lone pianist for example – there’re are actually a whole group of tones.  When I play a note on a piano it sounds different from the same not played on a harp.   Again, we have to deal with physics, but this is because each instrument actually produces a specific set of extra pitches with each tone played – the harmonics.  This is how we learn to recognize different types of sound.   But somehow we know which pitches are the central ones and which are just harmonics.  Somehow the nerves all line up and we know where the sounds is coming from, somehow the pitches trigger the right piece of the membrane and connect to the right cells which our brain knows to perceive as music.  The fact that this works all day, every day is incredible to me.