Zap-PoofPosted: May 17, 2011
If the job of the brain is to process information and generate thoughts, feelings and behaviour, how does it work? The brain is made of two kinds of highly specialized cells, neurons and glia. Neurons communicate with each other to process information. Glia, broadly speaking, are support cells, helping neurons function. It is the communication between the neurons that is primarily responsible for what the brain does. Neurons have a very specific structure (Figure 1) which is related to how they function. The bushy dendrites take in information from other neurons. The architecture of the dendrites is carefully organized to allow the neuron to get input from where it needs it, and is referred to as its dendritic tree. Information travels through the neuron in the form of small electrical signals called membrane potentials, these can be either excitatory, making the cell more likely to generate a signal of its own, or inhibitory, making it less likely. These signals travel down the dendrites and through the cell body. At the base of the cell body (the axon hillock) all these membrane potentials are summed together, if the resulting signal excites the cell above a set threshold the then the neuron will fire a signal of its own called an action potential which travels down the axon (the difference between an action potential and a membrane potential is related to their electrical properties). The myelin wrapped around the axon acts like an insulator so the electrical signal can bounce between the nodes of Ranvier, and travels much faster than it would otherwise.
While schematic neurons, like the one here, are a convenient way to see all the parts of a neuron different types of neurons actually all look slightly different and their shape is determined by their function. Neurons that gather input from all over the brain will have immense dendritic trees and small nearly invisible axons (Figure 2). The pyramidal motor neurons which carry the information used to signal the muscles to move have small trees, but very large axons needed to transmit information down the body (Figure 3). Typically neurons which transmit information out to the body are called motor neurons, those which carry information from the body to the brain are called sensory neurons, and those within the brain and spinal cord which process and transfer information are referred to as interneurons.
Signalling between neurons involves first a burst of electricity and then a puff of chemicals called neurotransmitters, so it is referred to, imaginatively, as electrochemical signalling. When the electrical signal reaches the axon terminals, they release a small burst of neurotransmitters. Individual neurons are separated by a small gap called a synapse. The neurotransmitters cross the synapse and bind to special receptors on the dendrites of the next neuron which is what creates the membrane potentials. The combination of a neurotransmitter and a receptor is what determines whether a signal is inhibitory or excitatory each one can bind to multiple receptors, meaning that each transmitter can cause excitation or inhibition by binding to different receptors. Different receptor types occur in different parts of the brain, which help control the effects of neurotransmitters, and determine the function of the different brain regions.