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| "The human nervous system is probably the most intricately organized aggregate of matter on Earth. A single cubic centimeter of the human brain may contain well over 50 million nerve cells, each of which may communicate with thousands of other neurons in information-processing networks that make the most elaborate computer look primitive. These neural pathways control our every perception and movement and enable us to learn, think, and be conscious of ourselves and our surroundings." |
The most striking differences between humans and other animals are in the size and the complexity of our brains. With our big brains we have acquired a rich culture, which far exceeds that of any other species in scope and complexity. We have developed science to understand how and why an immensity of things and processes work, including those of our own brain. At the start of the twenty-first century neuroscientists are increasingly able to explain the functions of brain in molecular terms.
To understand how the brain works we first must consider what the brain does. This can be broken down into three basic functions: (1) take in sensory information, (2) process information between neurons, and (3) make outputs. The neurons that take in information from the environment are called sensory neurons. These are specialized to respond to a particular stimulus, such as light, heat, chemicals, or vibration - anything you might encounter from outside, or even inside, the body. The processing within the brain can range from a knee-jerk reaction - which takes place entirely in the spinal cord - to the strategy adopted by a master chess player. In humans, we usually call this "thinking." The output is most often a body movement, which results from the action of motor neurons. The brain is the link between the outside world and behavior, and is thus crucial for survival. These three basic functions are shared by organisms from humans down to invertebrates like Caenorhabditis elegans, a nematode that doesn't even have a true "brain" but a collection of about three hundred neurons. (See the Genetics of Development unit.)
But how does the individual neuron work to carry out these tasks? Neurons' unique systems capabilities arise from their cellular ability to communicate with one another very rapidly, using both electrical and chemical communication. Keep in mind, however, that the neuron is not the only type of cell in the brain. The neuron may be the star of the show but there are other supporting players. Indeed, neurons constitute only a small fraction of cells in the brain. For every neuron there are about ten to fifty supporting cells, called glial cells, in the brain. The word "glial" means glue, and these cells are the "glue" of the nervous system. They perform many vital tasks, including removing dead neurons and debris, releasing critical growth factors to neurons, and acting as insulating material for the neurons.
The incredibly complex ways in which brains function exemplify the importance of cell-cell interactions. Below we discuss the chemical and electrical means by which neurons communicate, and describe how various therapeutic and recreational drugs alter these processes at the molecular level. We then turn to the molecular nature of memory and learning. Finally, we describe recent studies that demonstrate that new neurons are being produced continuously in us.