Nerve Cell and Tissue

Nerve cell and Tissue

The organs of the nervous system, like other organs are composed of various tissues. The brain and spinal cord for example – contain nerve tissue, blood, and various connective tissues. 

The main organs of nervous system – the brain and spinal cord, which make up the portion called the central nervous system  and the peripheral nerves which connect the central nervous system to other body parts and constitute the peripheral nervous system.

Nervous Tissue

The nerve tissue of the brain and spinal cord consists of masses of nerve cells or neurons which are the structural and functional units of the nervous system. These cells are specialized to react to physical and chemical occurring in their surrounding.

They also function to conduct nerve impulses to other neurons and to cells outside the nervous system.

Neurons commonly contain inclusions of a golden brown substance called lipofuscin, which is thought to be a by-product of lysosomal activity. This pigment tends to accumulate in neurons and may become so abundant that the nuclei and other organelles are pushed to the sides of the cell bodies. 

Near the center of the cell body there is a large, spherical nucleus with a conspicuous nucleolus. This nucleus, however, does not undergo mitosis, and consequently mature neurons are incapable of reproduction.
 

Two kinds of nerve fibers, called dendrites and axons, extend from the cell bodies of most neurons. Although a neuron usually has many dendrites, it has a single axon.

Dendrites are usually relatively short and highly branched. These fibers, together with the membrane of the cell body, provide the main receptive surfaces of the neuron to which fibers from other neurons communicate. Often the dendrites process tiny, thorn like spines (dendritic spines) on their surfaces, which serve as contact points of other nerve fibers.

Axon

The axon, which usually arises from a slight elevation of the cell body, is a slender, cylindrical fiber with a nearly smooth surface and uniform diameter. It is specialized to conduct nerve impulses away from the region of the cell body. Many mitochondria, microtubules and neurofibrils, occur within its cytoplasm. Although it begins as a single fiber, the axon may give off branches called collaterals, and it may have many fine branches at its terminal end.  

Cytoplasm

Cytoplasm is formed continuously in the neuron cell body, and some of it flows along the axon to the end. This movement of cytoplasm (axoplasmic flow) is thought to supply the axon with substances that it cannot synthesize.
 

Many axons are enclosed in sheaths formed by other cells. Axons of peripheral nerves, for example – commonly have sheaths composed of Schwann cells, which are tightly wound around the nerve fibers like miniature jellyrolls. As a result, the axons are coated by many layers of cell membranes that have little or no cytoplasm between them. 

These membranes, largely composed of a lipid-protein (lipoprotein) called myelin, form myelin sheaths on the outsides of the nerve fibers. In addition, the portions of the Schwann cells that contain cytoplasm and nuclei remain outside the myelin sheaths. These parts comprise a neurolemma, or neurolemmal sheath, which surrounds the myelin sheath.
 

Axona that possess myelin sheaths are called myelinated (medullatedd), while those that lack these sheaths are unmyelinated. Groups of myelinated fibers are responsible for the white matter in the brain and spinal cord. In this instance, however, the myelin is produced by certain neuroglial cells (oligodendrocytes) rather than by Schwann cells. Furthermore, nerve fibers in the brain and spinal cord lack neurolemmal sheaths. The gray matter of these parts consists of unmyelinated nerve fibers and neuron cell bodies.