neurons

In the CNS, neurons are topographically organized either as aggregates (nuclei, ganglia) or as elongated columns or layers (such as the intermediolateral gray column of the spinal cord or the six-layered cerebral cortex).

Functional domains are located in many of these anatomically defined regions (such as the hypoglossal nucleus of the medulla for motor fibers of the twelfth cranial nerve; calcarine cortex of the occipital lobe for primary visual cortex).

In addition, as a further dimension of anatomic-functional specificity, some cortical and subcortical neurons and their projections are arranged somatotopically (such as motor and sensory homunculi).

Neurons vary considerably in structure and size throughout the nervous system and within a given brain region. With conventional histologic preparations, an anterior horn neuron in the spinal cord has a cell body (perikaryon) that is about 50 μm wide, a relatively large and somewhat eccentrically placed nucleus, a prominent nucleolus, and abundant Nissl substance; the nucleus of a granule cell neuron of the cerebellar cortex is about 10 μm across, and its perikaryon and nucleolus are not readily visible by light microscopy.

Electron microscopic study reveals further variability among neurons in cytoplasmic content and the shape of the cells and their processes.

Characteristic ultrastructural features common to many neurons include microtubules, neurofilaments, prominent Golgi apparatus and rough endoplasmic reticulum, and synaptic specializations.

Despite these shared structures, axon length may vary greatly (hundreds of microns for interneurons versus a meter for an upper motor neuron).

Immunohistochemical markers for neurons and their processes commonly used in diagnostic work include neurofilament protein (NF), NeuN, and synaptophysin.

Subtypes

 motoneurons

See also

 neuronal migration
 neuronal connectivity

Videos

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 Histology of the brain (by Washington Deceit)

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See also

 Nervous system

• central nervous system
• peripheral nervous system