The big question for this segment is, in what way the nervous systems represent a new level of complexity? How complex are nervous systems? [MUSIC] The average adult human brain weighs 1.4 kilograms and is about the size of your fist. However it is responsible for every thought, every feeling, every taste, every action, every memory, every single thing you will ever do, think and feel throughout your entire life. The world we live in is extremely complex in itself. The brain continuously receives input from this complex world through senses that detect light, odors, sound, waves, temperature, pressure, vibration and so on. The brain converts the information from the outside world to construct the world that we see, feel, taste and hear. The brain determines what is more important for us at any given moment and that is what we perceive. It creates a conscious percept of the world inside our heads to allow us to operate in this complex world. All that we are is due to this amazing complexity of our brains. The average adult human brain has 86 billion neurons. If each neuron of a single human brain were laid end to end that could be wrapped around the Earth twice over. Of course, large numbers do not constitute complexity in itself. Each neuron can have as many 10,000 synapses connecting it functionally to other neurons. Therefore, a typical brain will have over 100 trillion connections or synapses. If we could map a connection every second, it would take more that 3 million years to map the human brain, and this is without considering the switches at each connection, nor its function. The neuronal communication further adds to the complexity of our brain. The most common form of communication between neurons involves chemical transmission via a vast array of chemicals and receptors. Different combinations of chemicals and receptors can result in either excitation or inhibition of the neuron. Simply put, they act as switches turning the neuron on or off, and increasing the incredible complexity of the neuronal architecture and function well beyond our imaginations. While complexity of the neuronal cells is quite overwhelming, neurons are not the only cells that make up our brains. Glial cells out number neurons somewhere in the range of 10 to 50 times over with conservative estimates at 60 billion cells in an adult human brain. Originally it was believed that glial cells were simply support cells, clearing away excess neurotransmitters, dead neurons and pathogens, and forming a layer of fat on neurons to speed up the flow of information. It is now known that they are also involved in facilitating connections between neurons which is vital for memory and thought processes. Unlike neurons, glial are also able to divide and reproduce themselves. Glial cells add exponentially to the complexity of the brain, and are intricately involved in the process of creating the world we know. Of course, the complexity of our nervous system does not simply involve essential nervous system of brain. The peripheral nervous system is also an amazingly intricate and fascinating system that allows us to interact with our world. In the cortex, alone, there are 100,000 miles of myelin covered nerve fibers. Nerve cells travel to the brain from the outer reaches of our skin, our muscles and our internal organs. We have millions of nerve endings in the outermost layer of our body that sense minute variations, such as vibrations, pressure, temperature and light. These nerves are sensitive to specific stimuli. They allow us to see, feel, smell and hear. And there are nerve cells that travel from our brain to all the muscles throughout our bodies. They control muscle contractions to allow us to stand, walk, grasp, hold, speak, run, swim and all the other actions we perform. They also control our heart rate, and breathing and other vital functions. Our peripheral nervous system is a complex network that is incomparable. An interesting question to ask is whether the complexity of our brains is what makes us human. Surprisingly, complexity does not seem to distinguish our brains from those of other animals. For example, some mammals, such as elephants, have larger brains, which are therefore more complex. Others have argued that it is the size of the brain in proportion to the body. This is known as the encephalization quotient, or EQ. However, small birds have a larger EQ than humans. So far, we do not know what it is about our brains that makes us human. But what we do know is the complexity of the brain and the nervous system is unimaginable. The mammalian brain is the most complex machine that ever existed. And given every brain is unique, there are currently over 7 billion different models functioning today. And I will leave you with a quote by the nuclear physicist George Pugh, that I think sums up the overall message. If our brains were simple enough for us to understand them, we'd be so simple that we couldn't. [MUSIC]
