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
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.
organellesin the neuron include
Neurofilaments which are the special types of microfilaments found in
Microtubules which exist in the cell body and the axon
Pigments are also found in the nerve cell as follows
nuromelanin found in areas of the nervous system like
the substantia nigra.
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.