Matthew L. Nathan

Transcription

You probably don't need to be told how important your brain is. After all, every single thing you experience, your thoughts and your actions, your perceptions and your memories are processed here in your body's control center. But if this already seems like a lot for a single organ to handle, it's actually only a small part of what the brain does. Most of its activities are ones you'd never be aware of, unless they suddenly stopped. The brain is made up of billions of neurons, and trillions of connections. Neurons can be activated by specific stimuli or thoughts, but they are also often spontaneously active. Some fire cyclically in a set pattern. Others fire rapidly in short bursts before switching off, or remain quiet for long periods until thousands of inputs from other neurons line up in just the right way. On a large scale, this results in elaborate rhythms of internally generated brain activity, humming quietly in the background whether we're awake, asleep, or trying not to think about anything at all. And these spontaneously occurring brain functions form the foundation upon which all other brain functions rely. The most crucial of these automatically occurring activities are the ones that keep us alive. For example, while you've been paying attention to this video spontaneous activity in your brain has been maintaining your breathing at 12 to 16 breaths a minute, making sure that you don't suffocate. Without any conscious effort, signals from parts of your brainstem are sent through the spinal cord to the muscles that inflate your lungs, making them expand and contract, whether or not you're paying attention. The neuronal circuits underlying such rhythmic spontaneous activity are called central pattern generators, and control many simple repetitive behaviors, like breathing, walking, and swallowing. Ongoing neural activity also underlies our sensory perception. It may seem that the neurons in your retina that translate light into neural signals would remain quiet in the dark, but in fact, the retinal ganglion cells that communicate with the brain are always active. And the signals they send are increases and decreases in the rate of activity, rather than separate bursts. So at every level, our nervous system is teeming with spontaneous activity that helps it interpret and respond to any signals it might receive. And our brain's autopilot isn't just limited to our basic biological functions. Have you ever been on the way home, started thinking about what's for dinner, and then realized you don't remember walking for the past five minutes? While we don't understand all the details, we do know that the ongoing activity in multiple parts of your brain is somehow able to coordinate what is actually a complex task involving both cognitive and motor functions, guiding you down the right path and moving your legs while you're getting dinner figured out. But perhaps the most interesting thing about spontaneous brain function is its involvement in one of the most mysterious and poorly understood phenomena of our bodies: sleep. You may shut down and become inactive at night, but your brain doesn't. While you sleep, ongoing spontaneous activity gradually becomes more and more synchronized, eventually developing into large, rhythmic neural oscillations that envelop your brain. This transition to the more organized rhythms of sleep starts with small clusters of neurons tucked in the hypothalamus. Despite their small number, these neurons have a huge effect in turning off brainstem regions that normally keep you awake and alert, letting other parts, like the cortex and thalamus, slowly slip into their own default rhythms. The deeper we fall into sleep, the slower and more synchronized this rhythm becomes, with the deepest stages dominated by large amplitude, low frequency delta waves. But surprisingly, in the middle of this slow wave sleep, the brain's synchronized spontaneous activity repeatedly transitions into the sort of varied bursts that occur when we're wide awake. This is the sleep stage known as REM sleep, where our eyes move rapidly back and forth as we dream. Neuroscientists are still trying to answer many fundamental questions about sleep, such as its role in rejuvenating cognitive capacity, cellular homeostasis, and strengthening memory. And more broadly, they are exploring how it is that brain can accomplish such important and complex tasks, such as driving, or even breathing, without our awareness. But for now, until we are better able to understand the inner workings of their spontaneous functioning, we need to give our brains credit for being much smarter than we ourselves are.