Neuronal Excitability and Synaptic Transmission

Neurons are similar to the other cells of the body in that they are surrounded by a semi-permeable membrane that encloses the nucleus and the cytoplasm. The hypothesis that neurons are self-contained cells that serve as elementary functional units in the nervous system was first introduced by the pioneer neuroanatomist Ramon y Cajal at the turn of the last century. This notion became known as the neuron doctrine 1 or neuron theory. Conclusive evidence demonstrating that there is no cytoplasmic continuity between neighboring neurons was made available relatively recently with the help of the electron microscope. This device is capable of magnifying images by up to 100,000 times compared to the maximum magnification of 1-2,000 times of the conventional light microscope used by the early anatomists. Nerve cells, like the one shown in Figure 5 , vary widely in size and shape, but share a number of special features that differentiate them from other body cells. Each neuron consists of the soma that contains the nucleus, a number of thin filaments that branch like a tree (the dendrite), and a (usually) long process (the axon), which often has several branches, known as collaterals. Axons vary in length from a few millimeters to more than one meter in certain kinds of cells found in humans and other large animals.