Furthermore, the particular structure of the membrane due to the presence of specific protein channels allows information to travel along the course of the axon. In fact, since the absence of ribosomes does not allow protein synthesis, all axon proteins originate in the soma. These structural differences translate into functional distinctions. First, no rough endoplasmic reticulum extends into the axon secondly, the composition of the axonic membrane (axolemma) is fundamentally different from that of the somatic membrane. Two notable features distinguish the axon from the soma (also referred to as perikaryon). Localization: retinal cells and olfactory system The branching processes, in turn, play a role of fundamental importance in neuroplasticity, for instance, in cognitive processes such as memory and learning.Īnatomically and based on the appearance of the protoplasmatic protrusions, neurons are classified into three groups:īipolar neurons. Shape: one axon and one dendrite that extend from the cell body in opposite directions. Axon extension and growth of new telodendrons (and synapses) are guided by several factors, including the nerve growth factor (NGF). These parts are called telodendron and continue with the synapse (synaptic knob or button) which represents the specialized structure that comes into contact with another neuron (soma, axon or dendrite), or muscle fiber. The terminal part of the axon and collaterals tapers progressively. These collaterals provide modulation and regulation of the cell firing pattern and represent a feedback system for the neuronal activity. A collateral branch is an axonal protrusion over10 micrometers in length. Of note, although each neuron has only one axon, bifurcations that are branches of the main axon can be present. Furthermore, axons generate and transmit all-or-none action potential, whereas dendrites produce depolarizing (below the threshold of the action potential) or hyperpolarizing (lowering the resting membrane potential) graded potentials. However, all these rules have exceptions. Dendrites usually receive signals, while axons typically transmit them. Axons are distinguished from dendrites by several characteristics including:įunctions. The axon is one of two types of protoplasmic protrusions of the neuronal soma. Moreover, they have demonstrated significant changes in the diameter along the single axon. The variation of the diameter has important functional implications since the speed of propagation of the impulse (i.e., action potential), besides being dependent on the presence of the myelin sheath, is directly proportional to the diameter of the axon. The diameter is variable as it ranges between 1 and 25 micrometers. Histological observation of axon shows a cylindrical structure, but recent 3D electron microscopy studies demonstrated that probably axon has not the shape of a perfect cylinder. Furthermore, compared to projecting neurons, interneurons usually have smaller soma, fewer organelles, and a reduced amount of cytoplasm (axoplasm). The longest axons of the human body are those that make up the sciatic nerve where the length can exceed one meter. In contrast, interneurons, that work within local circuits, have a short axonal terminal (up to several mm). Considering the functional distinction between projection neurons and interneurons, cortical projection neurons (CPNs), also termed as pyramidal neurons and spinal cord projection neurons (dorsal horn neurons), usually have long axons (from several mm and up to 1 m). Functionally, it carries electrical impulses and projects to synapses with dendrites or cell bodies of other neurons or with non-neuronal targets such as muscle fibers.Ĭoncerning length, the length of axons varies according to the function of the neuron. In size, the axon may represent over 95% of the total volume of the neuron. Axons are the elongated portion of the neuron located in the center of the cell between the soma and axon terminals. In 1860, the German anatomist Otto Friedrich Karl Deiters (1834-1863) described the basic structure of the nerve cell and identified two different protoplasmatic protrusions of the cell body that he termed as "axis cylinder," and "protoplasmatic processes," respectively axons and dendrites.
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