The notion that the nervous system is made up of individual cells, rather than being a syncytium, is attributed to Santiago Ramon y Cajal.
Heinrich Waldeyer first used the term 'neuron' about 1891.
Most
neurons have
several parts:
i. cell body (soma, perikaryon)-contains the
nucleus and many of the cell's organelles for synthesis of macromolecules.
ii. dendrite(s)-usually several come off/attach to the soma
iii. axon-usually only one comes off the soma, but it may have branches (collaterals) distally
The pictures demonstrate light and
electron micrographs
of multipolar neurons.
On the far right the photomicrograph emphasizes the several
processes that come off the cell body of a neuron, which is in the center
of the picture.
The light micrograph at the left shows the typical appearance of
multipolar neuronal cell bodies when stained with basic dyes. The
nucleus is pale; often a heavily stained nucleolus is evident within
it. Nissl bodies are prominent in the cytoplasm of the soma and
proximal parts of dendrites.
Stacks of rough endoplasmic reticulum are evident in the cytoplasm. They are what constitute Nissl bodies at the light microscopic level.
This neuron was labeled with a marker using the technique of retrograde axonal transport . The location of that marker is represented by the black granules, many of which localize in the vicinity of the Golgi apparatus.
Cell bodies of unipolar neurons
form spinal ganglia, most cranial nerve sensory ganglia, and the trigeminal mesencephalic
nucleus.
A light micrograph of unipolar neurons in a spinal ganglion is shown at
right. A darkly-stained nucleolus is evident within the nucleus of at
least a dozen of the neurons. Each of the neuronal cell bodies is
surrounded by lots of cells whose nuclei stain darkly.
An electron micrograph of a bipolar neuron from a vestibular ganglion is shown. A dark nucleolus is present in the nucleus of the neuron and clumps of rough endoplasmic reticulum are faintly visible in the cytoplasm.
The cytoplasm of a satellite cell immediately surrounds the neuronal soma; its nucleus is at about a 9 o'clock position at the neuron's perimeter. A few myelinated fibers are also present in the picture.
i. Purkinje cell - single layer of large cell bodies in the cerebellar cortex, between the molecular (left) and granular (right) layers.
ii. pyramidal cell - named on the basis of the cell body's shape. It is one of the two major neuronal types in the cerebral cortex. It has a large apical dendrite, which extends vertically from the top of the pyramid, and basal dendrites, which come off horizontally at the base of the pyramid. The axon also typically extends from the base.
iii. granule cell of dentate gyrus in the hippocampus (a particular region of the cerebral cortex) - dendrites extend from one end of the cell while the axon comes off from the other.
iv. motor neuron (motoneuron) -lamina IX of the spinal cord and certain cranial nerve motor nuclei.
