In my poetry and prose I like to talk a lot about how sensory input affects us. I am known to use the word cortex incessantly. Why? Because (aside from the fact that cortex is just objectively a badass sounding word) within various areas of cortex lies our entire experience of the world. Allow me to explain.
The cortex is the outer portion of the brain (the wrinkly looking part you see from the outside). The cortex is basically a large sheet of cells that are folded over into gyri (the wrinkles). It is made up of cell bodies. These cells (neurons) have long branches that send their information like wires transmit electricity. These branches (called axons) are wrapped in a white outer covering called myelin. Myelin is white, so the branches appear white and are called white matter. The cell bodies themselves contain very little myelin and are the grey matter. See? Pretty cool right?
So, what does the cortex do? Well, it receives and stores information. The type of information depends on which area of the cortex. There is a sensory cortex (for temp and touch sensation), visual cortex, auditory cortex, gustatory cortex (taste), and olfactory cortex (smell), among others. Now I know what you're thinking right about now. I sound like the Bubba Gump of brain. And that's fair. But stay with me, even if you are laughing at me. Trust me, I'm used to that anyway. Each of these areas receives appropriate information and stores it if necessary. Whether or not it's necessary is another matter for another time. Damn, shoulda said another grey matter.
One of the things I find most fascinating in neuroscience is signal transduction: how does a sensory stimulus get translated into neural code? I'm going to go through an example of this using sound. It may be helpful to watch the NIH video I have linked below.
Close your eyes. But like, don't nod off. Actually, go ahead, you work hard. Take a nap. Hell, take a thug nap because you're so gangsta. I'm sorry, it's been a long few weeks for me. Maybe I need a nap right along side you. Not physically. I love you all but I'm not spooning you. OK, so close your eyes and let me take you on a wonderful journey from soundwave to brainwave... (Ooh, that was badass I just made that up on the fly!)
OK, so you're in the middle of an auditorium alone. Now imagine that somewhere in the room, a person begins playing the flute. This kind of sounds like a horror story but I promise, it's not. So many incredible things happen when you hear this flute. Aside from knowing it's a flute and recognizing the melody, you can also discern with pretty good precision where that sound is originating from. This last ability stems from the fact that we have what is called coincidence detection. An area of the brain measures when the signal arrives from each ear and compares the time delay to determine the location of sound. Incredible right?
But what about the raw signal? How does it enter our brains? I'm so glad you asked. You know, I'm really starting to like you. You're definitely my people. You probably know that sound is a wave and that it causes your tympanic membrane (ear drum) to vibrate in the same frequency as each
sound. This vibration causes your three inner ear bones to move accordingly. They connect to another membrane on the snail-shaped cochlea. Then the magic really begins. The inside of the cochlea is cooler than Disneyland. I am aware that I am the biggest dork. But like, to be fair, you're reading this so... Anyway, lining the inside of the cochlea are hair cells. And they are some funky ass
cells if you ask me. These are actually nerve cells but are called hair cells because, well, they have hair-like structures that resemble the hair on that Beaker character from the Muppets. ------->
These cells are incredible. Their 'hairs' are arranged by length in a staircase orientation and they bend towards the tallest row of hairs. This physically opens channels and ions flow in and excite the cell, sending signals via the auditory nerve to the cortex. An amazing aspect of this is also the arrangement of these cells. The widest section of the cochlea contains hair cells sensitive to the highest notes, whereas the thinner part of the cochlea (think the inner section of a cinnamon roll) is sensitive to deeper, lower notes.
Soundwaves are expressed in hertz (Hz). Humans can hear sounds ranging from about 20 to 20,000 Hz. The human voice sits right around 100-200 Hz, well within the range of our abilities to hear. Though I question that when I'm telling my kids to do literally anything. Although side note, why do they never respond when I call yet they can hear me open a bag of chips or cookies from clear across the house. Eh, now we're entering psychology and straying from physiology. Anyway, things like dog whistles are about 25,000 Hz or higher, so we cannot hear them. Things we can hear are sent to the auditory cortex, where neurons there receive and process information.
It really is incredible when you think about it. In addition to the frequency of the sound, think of how many factors we are capable of discerning. We can tell many different instruments apart, even if they're playing the same notes. We can distinguish many different human voices. We can even detect inflection and emotion.
Our brains and associated sensory organs are just amazing, aren't they?
Any other senses you're interested in learning more about? Leave me a comment and I'll do my best. I mean, it's a chore for me to talk about the brain and senses, but I'll manage. Wink emoji.
XOXO
Your average neuroscientist,
Melinda
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