![]() The Coulomb force prevents the ions from diffusing across in their entirety. Diffusion of K + and Cl – thus creates the layers of positive and negative charge on the outside and inside of the membrane. In its resting state, the cell membrane is permeable to K + and Cl –, and impermeable to Na +. But the cell membrane is semipermeable, meaning that some ions may cross it while others cannot. As discussed in Molecular Transport Phenomena: Diffusion, Osmosis, and Related Processes, free ions will diffuse from a region of high concentration to one of low concentration. This thin membrane separates electrically neutral fluids having differing concentrations of ions, the most important varieties being Na +, K +, and Cl – (these are sodium, potassium, and chlorine ions with single plus or minus charges as indicated). Figure 2 illustrates how a voltage (potential difference) is created across the cell membrane of a neuron in its resting state. The most important of these are the Coulomb force and diffusion. ![]() The method by which these electric currents are generated and transmitted is more complex than the simple movement of free charges in a conductor, but it can be understood with principles already discussed in this text. The number of interconnections can be far greater than shown here. Signals in the form of electric currents reach the cell body through dendrites and across synapses, stimulating the neuron to generate its own signal sent down the axon. A neuron with its dendrites and long axon. Signals may arrive from many other locations and be transmitted to yet others, conditioning the synapses by use, giving the system its complexity and its ability to learn.įigure 1. (See Figure 1.) Signals arrive at the cell body across synapses or through dendrites, stimulating the neuron to generate its own signal, sent along its long axon to other nerve or muscle cells. Nerve cells, properly called neurons, look different from other cells-they have tendrils, some of them many centimeters long, connecting them with other cells. It is one aspect of bioelectricity, or electrical effects in and created by biological systems. Nerve conduction is a general term for electrical signals carried by nerve cells. The sheer number of nerve cells and the incredibly greater number of connections between them makes this system the subtle wonder that it is. ![]() Third, nerves transmit and process signals within the central nervous system. Second, nerves carry messages from the central nervous system to muscles and other organs. First, nerves carry messages from our sensory organs and others to the central nervous system, consisting of the brain and spinal cord. These are representative of the three major functions of nerves. In later chapters, we make use of these structures and their dynamics to construct model neuronal networks.Electric currents in the vastly complex system of billions of nerves in our body allow us to sense the world, control parts of our body, and think. In this chapter, we consider idealised mathematical models of these processes and methods for their analysis, which now include a spatial aspect. These structures allow neurons to communicate in different ways, and to transmit information to other nerve cells, muscle, or gland cells. Axons and dendrites make contact with each other at axo-dendritic synapses, and dendro-dendritic synapses are also possible. The cell body or soma, contains the nucleus and cytoplasm, the axon extends from the soma and ultimately branches before ending at nerve terminals, and dendrites are branched structures connected to the soma that receive signals from other neurons. However, the classical notion of a neuron is of a specialised cell with a body, an axon, and dendrites. Chapter 2 and chapter 3 considered single neuron models that idealised the nerve cell as a point or patch of cell membrane in which voltage is the same everywhere.
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