Nervous System and Neurons

Lipofuscin is the name given to finely granular yellow-brown pigment granules composed of lipid containing residues of lysosomal digestion. Lipofuscin or lipochrome is a wear and tear pigment since the number of its granules within the neurons increases with age. These granules are end products of intracellular peroxidation and polymerisation of unsaturated fatty acids. The precise function of lipofuscin is unknown.

An axon is a long branching cell structure that is unique to nerve cells. Axons are responsible for carrying information from the nerve cells to all the other cells of the body. The axon carries nerve signals away from the soma and also carries some types of information back to it. Interference with signals as they travel through the axons has been identified as a cause of certain degenerative neurological disorders.

It is the insulating envelope of myelin that surrounds the core of a nerve fiber or axon and that facilitates the transmission of nerve impulses formed from the cell membrane of the schwann cell in the peripheral nervous system, and from oligodendroglia cells. If the axons have myelin sheaths then the pathway appears bright white because myelin is primarily lipid. If nearly  all of the axons lack myelin sheaths or are unmyelinated then the pathway will appear a darker beige colour, which is generally called grey. It is the myelin sheath that gives the whitish colour to the areas of white matter.

It is the innermost layer of connective tissue in a peripheral nerve forming an interstitial layer around each individual fiber outside the neurilemma. It is the delicate connective tissue surrounding nerve fibres within a bundle.

Ribosomes are nonmembrane bounded particles where protein synthesis occurs. A nissl body also known as nissl or tigroid substance is a large granular body found in neurons. These granules are rough endoplasmic reticulum (RER) with rosettes of free ribosomes and are the site of protein synthesis.

The myelin sheath layer has a cellular covering known as the neurilemma or the schwann cell sheath. The neurilemma is essential for regeneration of nerves. It is present only in the peripheral nervous system. There is no neurilemma in the central nervous system and this may be one reason why injuries to the brain and spinal cord do not heal very well.

Schwann cells are peripheral nervous system structures that wrap some neuronal axons to form an insulating coat known as the myelin sheath. Oligodendrocytes and schwann cells indirectly assist in the conduction of impulses,  as myelinated nerves can conduct impulses quicker than unmyelinated ones.

They are wrapped in a fatty layer called myelin which insulates the axons and allows them to conduct signals quickly,  much like rubber insulation does for electrical wires. The type of fat in myelin makes it look white, so myelin dense white matter takes on a white hue as well. Thus the white matter in the brain gets its colour from a large number of myelinated nerve cells.

Blood brain barrier is formed by specific nueroglial cells called astrocytes. These astrocytes are of two types-fibrous astrocytes and protoplasmic astrocytes. The fibres of these astrocytes form a network with capillaries of blood vessels that supply blood to brain. This network of fibres forms a barrier between blood medium and brain tissue for the purpose of maintaining homeostasis.

The axon hillock is a specialized domain of the neuronal cell body from which the axon originates. A nissl body also known as nissl or tigroid substance is a large granular body found in neurons but it cannot be seen in the axon or axon hillock.

Each fasciculus is surrounded by a thick layer of connective tissue called perineurium.

Merkel cells or discs are expanded tip tactile receptors. They are responsible for giving steady-state that allow one to determine continuous touch of objects against the skin.

Advances in the study of electrical activity in nerves include devices like microelectrode, cathode-ray oscilloscope, etc.

Medulla oblongata is a part of autonomic nervous system. It controls all the involuntary actions like breathing, respiration etc.

Spinal cord is the elongation of medulla oblongata.