E-Brain
E-Brain is a mathematical model of brain by which one can investigate brain's functions. We are developing methods to simulate reactions of the brain to photic, chemical, electrical, and mechanical stimuli using computers. Inter-disciplinary works have to be done in order to develop these methods. The brain is a complex system composed of neurons, their supporting cells, and blood vessels. The mass of adult brain is about 1.4kg. In the brain, there are gray matter and white matter. Since white matter is rich in fibers of neurons, population of cell bodies of neurons in the gray matter is much larger than that in the white matter.  Inside the skull, the brain floats in cerebro-spinal fluid, a clear liquid that is made at the central chambers called ventricles in the brain. The brain is covered by several membranes called pia, arachnoid, and dura. The dura, a tough, thick, yellowish white membrane, directly contacts the skull. Just inside the dura, the arachnoid, that has many mesh-like small fibers called trabecula, exists. The pia, a clear, very thin layer, directly contacts the brain tissue. The neurons are the cells that can process information using chemical and electrical reactions. A neuron has a long process called axon and shorter ones called dendrites. These processes can have branches. The branches of the axons are called axon collaterals and those of dendrites are called dendrite collaterals. The end of the axon is called axon terminal. Neurons can make junctions each other through thin gaps called synapses. At a synapse the axon terminal of a neuron and another neuron's process exchange signals using molecules called neurotransmitters. The neurotransmitters are released from the axon terminal and received by another cell's process. When a dendrite receives the neurotransmitters the electrical potential (voltage) at its surface changes. The change of the voltage runs through the dendrite for the body of the neuron. The change of the voltage stores up at the neuron's body. When the stored change of the voltage is large enough, an impulse of the voltage is triggered at the base of the axon and it runs down the axon toward its terminal. The impulse is called action potential. When the action potential reaches the axon terminal, the neurotransmitters are released so that they can reach the surface of another neuron's dendrite through the synapse. Neurons and other cells are covered by a plasma membrane made of lipid bilayer; a double layer of fatty acids. Molecules including ions cannot overpass the plasma membrane. Thus the membrane insulates the cell from its outside. Proteins called ion channels and ion pumps that floats on the plasma membrane control the concentration of ions inside the cell. Some ion channels works like a one-way filter; they allow only a specific type of ion to pass. Other type of ion channels are controlled by the voltage surrounding them; at rest they are closed, but they open when the voltage increases above a limit, then they close as time goes by. After the closure, the gate will be inactive for a short time period. During the period, the gate will not open even if the voltage increases above the limit so that the action potential will not go back toward the cell body. As implied by their name, ion pumps work like pumps; they pump ions against the gradient of their concentration so that the difference of the concentration between the inside and the outside of the neuron can be kept.  The neurotransmitters are made at the nucleus in the neuron's cell body. Thus they have to be sent to and stored in the axon terminal so that they can be ready to be released when action potentials reach the axon terminal. The neurotransmitters are packed into small containers called vesicles that are made of lipid bilayer. Then they are sent to the axon terminal by a linear-motor-like mechanism called axonal transport. In the axonal transport mechanism motor molecules and tube-like railroad structures play important roles. Kinesin and dynein are the motor molecules to which the vesicles are attached. They can run on microtubules, the tube-like structures in the axon made of tubulin molecules. In the brain tissue neurons are surrounded by the supporting cells; they are called glia cells. They are divided into several classes; oligodendrocytes, astrocytes, and microglias. The oligodendrocytes have many process. The end of these processes tightly wrap the axons so that they provide the axons with electrical and chemical insulation from their environment. These tight insulations are called myelins. These insulation contribute to increase of the speed of action potential. The astrocyte also have many processes; some of these are attached to the neurons and some are attached to the blood vessels. Astrocytes work as barriers between the blood vessels and the neurons. Thanks to the barrier neurons are kept from direct contact with chemicals that come from the blood through the vessels. The microglias are immune cells in the brain.
Ion Channels of Excitable Membranes
Copyright (C) 2006 BioDyn, Inc. All rights reserved. Email: admin@biodyn.us URL: http://www.biodyn.us/e-brain.html