Neuron ultrastructure

 

  

  Neuron

The general plan of a multipolar neuron shows an enlarged cell body containing cellular organelles.  It has a specialized cell membrane which under the transmission electron microscope is triple layered. This triple layered membrane has an outer limiting membrane which is denser than inner limiting membrane that limits the cytoplasm directly.  The total thickness of the triple layered membrane is 90 1000A. Between the two membranes is the intermembranous clear zone.  The cell membrane of the cell body extends into the axon and dendrities. The cell membrane covering of the axon is further augmented by the myelin sheath in the myelinated neurons through the process of myelinization which occur early in neuroorganogeny. This process of myelinization in the peripheral nervous system is carried out by the agency of Schwann cels and in the central nervous system by the agency of oligodendrocytes.

          At the nodes of Ranvier, myelin is absent. Myelin is also deficient at some cleafts called the Schmidt Lantermann clefts.  At synapses the cell membranes become thickened for impulse transmission. This is at the junctional synaptic membranes.  The nucleus of the perikaryon which is the cell body of the neuron is usually large and prominent and also centrally placed excepting in pathological conditions in which it may become eccentrically placed. The nucleoplasm is filled with a proteinaceous continuum containing diffuse chromatin. A mucleolar organizer region is recognizable under the transmission electron microscope and this region contains considerable RNA. The region also produces RNA that are transported across the nuclear pores into the cytoplasm of the pericaryon. A dense region is recognisableat the periphery of the nucleolus called the perinuleolar satellite. It is found in great abundance in the female and it is therefore homologous to the Barr body found in cells of other parts of the body representing the resting X chromosome of the female.

          The endoplasmic reticulum of the neuron is not very different from what is found in other cells except that it has considerable number of Nissl substances within it and under the light microscope this represents a form of peculiarity. But under the transmission electron microscope, they have been recognized as aggregations of ribosomes. The endoplasmic reticulum therefore represents an extensive membranous vacuolar system which are structurally and functionally related to the Golgi complexes.

          The Golgi complexes are placed around the nucleus in most neurons and paraneurons, i.e., they are circumnuclear. They are however placed in differing locations in many instances. They are found in abundance in the presynaptic junctions presumably to assemble and package synaptic neurotransmitters for export across the synaptic membranes.  Nissl substances are special types of chromatophilic structures in the neuron which are basophilic and are related to the endoplasmic reticulum structurally and functiaonally an dhave been shown to be ribosomal particles. They contain considerable RNA as one will expect of ribosomal particles and have special histochemical properties. The nissl substance does not exist in the neuron beyond the axon hillock meaning it is not present in the axon.

Other organellesin the neuron include

Neurofilaments which are the special types of microfilaments found in the neuron

Microtubules which exist in the cell body and the axon

Pigments are also found in the nerve cell as follows

1)           nuromelanin found in areas of the nervous system like the substantia nigra.

2)           Lipofuschin

Mitochondria are found in abundance in the nerve cell. They contain enzymes for glycolytic biochemical pathways and also enzymes for oxidative phosphorylation. These enzymes include succinate dehydrogenases and other dehydrogenases most of which can bd demonstrated histochemically. More mitochondria are found in synaptic terminals for obvious reasons in the aid of the production and export of synaptic transmitters.

          Lysosomes are found also in nerve cells. They contain hydrolytic enzymes which are demosntratable by histochemical reactions like acid phosphatase, etc.  Nerve cells are usually active metabolically. They produce neurotransmitters. Some nerve cells produce secretions that are endocrine or at best paracine in function. The secretions are produced in the pericaryon and are then transported through the axon to the toledendria where they can be transported out of the nerve cell into the synaptic membranes. Their process of transportation is via the somatoaxonoal flow and this flow can be bidirectional in nature.
 

 

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Somato-axonal flow

 

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