The Cell Body

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The cell body of the neuron, also referred to as the soma or perikaryon, can range in size from 5 to 100 mm. The neuron cell body is like that of any other cell with the usual complement of organelles: a nucleus, usually with a prominent nucleolus, which contains the DNA of the cell, rough and smooth endoplasmic reticulum (RER and SER), free poly-ribosomes, a well-developed Golgi apparatus, mitochondria, and lysosomes (Fig. 4). Neurons are particularly rich in mitochondria, which produce cellular energy in the form of ATP, in part because the maintenance ofthe ionic gradients described earlier is an energy-dependent process. Cytoplasmic proteins are synthesized by free ribosomes in the cytoplasm, whereas those destined for insertion into membranes or for secretion are synthesized by ribosomes on the RER and further processed by the Golgi apparatus. In many neurons, the RER is arranged into parallel stacks close to the nucleus. These stacks stain prominently with a class of basic stains called Nissl stains, e.g., cresyl violet and toluidine blue, and for that reason are sometimes referred to as Nissl bodies (Fig. 4). An interesting feature of neurons is that they do not contain stores of glycogen, as do many types of cells. Thus, they are reliant on a constant supply of blood to meet their energy needs. However, astrocytes, the most numerous type of glia, do contain glycogen stores and are intimately associated with both neurons and blood vessels, suggesting that they provide some metabolic support to the neuron.

The cell body is the trophic center of the cell. It performs the bulk of protein synthesis for the neuron and contains the nucleus. In situ hybridization studies show that most mRNA species are confined to the cell soma. Dendrites and axons separated from their soma cannot survive and will eventually degenerate.

Somata Stain Neuron
Figure 3 Schematic drawing of a neuron showing the major compartments and features.

However, neurons are also dependent for survival upon certain molecules, called growth factors, obtained from their targets via their axons and transported back to the cell soma. Especially during development, a neuron that fails to establish contacts with an appropriate target will degenerate.

When brain tissue is stained with a technique that identifies the cell bodies, e.g., a Nissl stain, the density of neuronal somata is very low compared to other tissues like muscle or skin. The bulk of nervous tissue is not composed of cell bodies in close apposition or extracellular space, but rather is composed of the intermingling of dendrites, axons, dendritic spines, and glial elements, which is called neuropil. Under the electron microscope, the neuropil appears as a dense conglomerate of small profiles (Figs. 4, 5, and 10).

B. Dendrites

One or more dendrites (Gr: "tree" or "branch") extend from the cell soma with a gradual tapering. Dendrites can extend for tens to hundreds of microns in length and can branch extensively, usually at acute angles and

Figure 4 Electron micrograph of a motor neuron in the rat spinal cord showing the transition between the soma and a primary dendrite. The soma is the area surrounding the nucleus which gradually tapers into a dendrite. Numerous synaptic contacts onto the plasma membrane are visible (arrows). A group of myelinated axons is visible next to the soma.

Figure 4 Electron micrograph of a motor neuron in the rat spinal cord showing the transition between the soma and a primary dendrite. The soma is the area surrounding the nucleus which gradually tapers into a dendrite. Numerous synaptic contacts onto the plasma membrane are visible (arrows). A group of myelinated axons is visible next to the soma.

with subsequent diminishment in dendritic diameter. There is often no clear morphological distinction between proximal dendrites and the soma; the same organelles, with the exception of the nucleus, are found

Figure 5 Electron micrograph of a cross section of a Purkinje cell dendrite in the chicken cerebellum with a single spine emerging from the dendritic shaft via a thin spiny neck. Small-caliber axons are visible coursing on either side of the dendrite.

in cell bodies and proximal dendrites, although the amounts of rough endoplasmic reticulum and Golgi apparatus diminish and eventually disappear from more distal dendrites (Fig. 4). In more distal dendrites, the only organelles found with regularity are mitochondria, the smooth endoplasmic reticulum, and multivesicular bodies (Fig. 5). The smooth endoplas-mic reticulum is able to uptake, sequester, and release calcium, and dendrites express high levels of SER proteins involved in intracellular calcium regulation (Fig. 6). In some neurons, an expansion of the SER into cisterns closely apposed to the plasma membrane is seen. This expansion is very characteristic of Purkinje neurons and spinal motor neurons and is called the hypolemmal cisternae.

Microtubules are the most prominent and abundant cytoskeletal element in dendrites and are aligned along the long axis of the dendrite, whereas intermediate filaments are less common. Although microtubules are found in the soma, axon, and dendrites, microtubules in each region are distinguished by the proteins associated with them. For example, dendrites are distinguished by high levels of MAP2 (microtubule-associated protein, type 2) compared to the soma and to axons. These microtubule-associated proteins appear to be important in determining the stability and arrangement of microtubules. Staining for MAP2 is often used as a method for identifying a neuronal process as a dendrite.

Figure 6 Purkinje neurons in the chicken cerebellum stained for the ryanodine receptor, a calcium channel located in the membranes of the smooth enodplasmic reticulum. When activated, the channel opens and releases calcium from stores inside the SER. Many other neurons are present in this brain region, but they do not stain for this particular protein.

Figure 6 Purkinje neurons in the chicken cerebellum stained for the ryanodine receptor, a calcium channel located in the membranes of the smooth enodplasmic reticulum. When activated, the channel opens and releases calcium from stores inside the SER. Many other neurons are present in this brain region, but they do not stain for this particular protein.

Proteins and other constituents required for dendritic maintenance and functioning are transported into the dendrite from the soma by an active mechanism at the rate of about ~ 20 mm/day. Whereas identifiable RER and the Golgi apparatus are almost never seen in distal dendrites, ultrastructural studies have shown that polyribosomes, some in close association with membranous cisterns, are found throughout the dendritic tree. Many of the polyribosomes are found near synaptic contacts. With the development of high-resolution in situ hybridization techniques, researchers discovered that a small number of mRNA species are also found localized in dendrites. These mRNAs are specifically targeted to dendrites by signal sequences contained within their 3'-untranslated regions and often encode proteins important for signal transduction. The most well-studied dendritic mRNA is that encoding the a form of calcium/calmodulin-dependent protein kinase II (CamKII), which is found in high levels in the dendrites of pyramidal neurons in the hippocampus. CaMKII is a multifunctional enzyme important in signal transduction at the synapse. Neurons grown in culture have been demonstrated to support protein synthesis in their dendrites, and some evidence exists that this also occurs in vivo. Researchers have hypothesized that these targeted mRNAs support protein synthesis that is activated in response to local activation of synapses and, thus, may be related to synaptic plasticity or the modulation of synaptic efficacy.

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