Schwann cells and oligodendrocytes can also associate with axons but not form a myelin sheath around the axon. In the central nervous system, oligodendrocytes form the sheath, and one oligodendrocyte can myelinate multiple neurons. In the peripheral nervous system, Schwann cells form the sheath around axons, and each Schwann cell forms the sheath for just one neuron. Nerve fibers are found in the peripheral nervous system and central nervous system. The nerve fiber consists of a neuron's axon and its myelin sheath, if present. Neurofibrils are important because they mediate slow and fast axonal transport, the method by which cytoskeletal elements and membrane-bound organelles move to and from the soma. These represent aggregates of microtubules and neurofilaments, and can be visualized by EM. Neurofibrils extend from the soma out into the dendrites.By identifying Nissl substance, you can easily distinguish the two processes. Nissl bodies, the equivalent of the rough endoplasmic reticulum in the neuron, are found only in the soma and dendrites of the neuron - never in the axon hillock or axon. Instead, you can use Nissl substance to make this easier. It is not always possible to distinguish dendrites from axons based on their shape and size alone.The axon hillock is a conical elevation of the cell body from which the single axon extends.The single axon tends to be longer - while the axon may split into multiple pathways, it typically originates from a single point, the axon hillock. Usually, several short dendrites extend from the cell body. You should begin by distinguishing the axon from the dendrites.There are a few key points to remember when you are viewing a neuron under the microscope: This is the effector portion of the neuron when an action potential reaches the terminal, the content of the vesicles is released and either excite or inhibit the next neuron.
At the end of the axon is the synaptic terminal, which is notable for its high concentration of vesicles containing neurotransmitters. Efferent signals flow down the axon in one direction, toward the terminal branches. The axon extends away from the soma and is the conductile portion of the neuron.
The soma also contains the nucleus and most of the organelles of the neuron, surrounded by the cytoplasm or perikaryon. The neuron will fire or not fire based upon the results of this summation. The cell body, also the soma, is the integrative portion of the neuron, where incoming signals from dendrites are summed together. The dendrites make up the receptive portion of the neuron, and receive most synaptic afferent inputs from upstream neurons. Neurons can be divided into four regions:
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