Myelin sheath

Nerve cells have a long, thin, flexible fibre that transmits impulses.These impulses are electrical signals that travel along the length of the nerve.

Most nerve fibres are surrounded by an insulating, fatty sheath called myelin, which is essential for rapid impulse transmission.

The myelin sheath is an extension of the cell in which most of the cytoplasm is extruded and apposed phospholipid bilayers of the plasmalemma are fused to form the lamellae of compacted myelin.

The myelin membranes originate from, and are part of, Schwann cells in the PNS and oligodendrocytes in the CNS.
Myelin sheath

Glial cells of the brain

During embryological development certain cells derived from neural epithelium do not develop as neurons but as cells that occupy space between neurons.

These cells have been called neuroglia or glial cells from the suggestion they were the neural glue that held the neuron population. Glial cells are small and very numerous. In fact there are about 10 times more glial cells than neurons.

Originally, the glial cells were thought to function only as supportive tissue for the intricate neuronal matrix of the brain. While glial cells do play an important supportive role, it is now known that glial cells may play an even more dynamic, interactive, and regulatory role in brain function.

Each type of glial cell serves a very specific function. Some common glial cells are astroglia, radial glia, and oligodendrocytes, Astroglia are star-shape cells that transfer energy molecules and oxygen from the blood to the neurons and carry waste products from the neuron to the blood stream.

Radial glia are specialized astrocytes that play a critical role in brain development- they provide a ‘pathway’ for developing neurons to follow in the way to the final destination.

Researchers have found that the number and density of glial cells in key parts of the brain are often substantially reduced in people with depression.

They are generally divided into the microglia and microglia. The microglial cells function in phagocytosis of neuronal debris.
Glial cells of the brain

Neurons

The neuron is the main type of cell in the nervous systems that, in association with neuroglial cells, mediates the information processing that underlies nervous function.

The neurons are the cells specialized to carry on the rapid communications required for coordinated function both within the organism and between the organism and its environment.

They are highly asymmetric cells, with processes that often extend a considerable distance from the cell body.

The exploitation of two characteristics of protoplasm - irritability and conductivity – permits these cells to react to various stimuli in a fraction of second and to transmit the excitation to another location.

In addition some nerve cells possess a secretary capability that enables integration of function through localized and selective effects.

A neurons’ cell body holds its nucleus and most organelles. Cytoplasmic extensions from the cell body allow neurons to detect stimuli, recover signals and signal other cells. Information flows from sensory neurons, to interneurons, to motor neurons.
Neurons