Inside the neuron

Histology
Despite the complexity of the nervous system, it consists of only two kinds of cells: neuroglia and neurons. Neuroglia serve to support and protect the neurons. Neurons are specialised for nerve impulse conduction and for all special functions, such as thinking, controlling muscle activity and regulating glands.

Neuroglia
The neuroglia or glia serve numerous support and protective functions for the neurons. Neuroglia are generally smaller than neurons and outnumber them by 5 to 50 times. There are six types of neroglia: astrocytes, oliodendrocytes, microglia, ependymal cells, neurolemmocytes (Schwann cells), and satellite cells. Neuroglia are a common source of gliomas (tumors) and it is estimated that gliomas account for almost half of brain tumors. Such tumors are highly malignant and grow rapidly.

Neurons
Structure:
Neurons have three distinct parts:
1) Cell body
2) Dendrites
3) Axon
The cell body contains a well-defined nucleus and nucleolus surrounded by a granular cytoplasm along with typical organelles such as lysosomes, mitochondria and a Golgi apparatus. It does not contain a mitotic apparatus. The inability of neurons to regenerate in adults is related to the absence of a mitotic apparatus.
Neurons have two kinds of cytoplasmic processes: dendrites and axons. Dendrites are usually short, thick, highly branched extensions of the cytoplasm. A neuron usually has several dendrites. They function to receive impulses and conduct them toward the cell body.

The second type of cytoplasmic process is the axon. It is a long, thin extension that sends impulses to another neuron or tissue. Axons vary in length from a few millimetres in the brain to a metre or more between the spinal cord and toes. Along the length of an axon, there may be branches called axon collaterals. The axon and it collaterals terminate by branching into fine filaments called axon terminals. The ends of axon terminals contain bulb-like structures called synaptic end bulbs, which contain sacs called synaptic vesicles that store chemicals called neurotransmitters. Neurotransmitters determine whether an impulse passes from one neuron to another or from a neuron to another tissue.

On to the formation of the myelin sheath...

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