Neuroscience
Online

Numerous variations of the "model" neuron described above exist. An important modification, which occurs especially in receptor neurons, involves the designation of a neuronal process as a dendrite or as an axon. Classically, the axon has been identified as the myelinated or unmyelinated process that transmits signals away from the cell body. The classical view of the dendrite is that of an unmyelinated tube of cytoplasm which carries information toward the cell body. However, this distinction does not hold for ALL neurons. Some cells have a myelinated process that transmits signals toward the cell body. Morphologically the "dendrite" and the "axon" may, therefore, be indistinguishable. Neither the position of the cell body nor the presence or absence of myelin is always a useful criterion for understanding the orientation of the neuron. The region of impulse initiation is more reliable guide to understanding the functional focal point of the cell. This region is analogous to the initial segment of the model neuron, discussed above. Routinely the fiber or process, which contains the initial segment or trigger zone, is referred to as an axon. Note, as shown in Figure 8.8, the trigger zone does not have to be immediately adjacent to the cell body.