Schwann cell covering

Figures 16.2 and 16.3 also show that each nerve axon has its own membranous covering, often called the nerve membrane, tightly surrounded by a myelin sheath called a Schwann cell covering. The myelin is not continuous along the fiber. The interruptions are the nodes of Ranvier, which are of great importance for nerve functioning.

Nerve cells (neurons) are easily excitable cells that produce electrical signals and can react to such signals as well. Their structure is markedly different from that of other types of cell. Numerous branching processes project from their cell body (soma). Neurons are able to receive signals via dendrites and to pass them on via axons. The axons, which can be up to 1 m long, are usually surrounded by Schwann cells, which cover them with a lipid-rich myelin sheath to improve their electrical insulation. 

The neuronal cytoskeleton provides the axon with mechanical support and is directly involved in the transport of materials from the cell body towards the synapse (anterograde transport) and in the opposite direction (retrograde transport). Axons are generally covered (insulated) with a myelin sheath, which is formed by oligodendrocytes (in the CNS) or Schwann cells (PNS). 

The white matter is a covering for the nerve fiber known as the myelin sheath. Myelin is a sphingolipid produced by oligodendrocytes (Schwann cells). It acts as a special type of insulation which allows for a much more rapid transmission of the nerve signal. The myelin cover is interrupted periodically. This "bare" area is called a node of Ranvier. It is very important to rapid nerve conduction. See Figure 21. 

Neurolemma—Protective nerve covering made by Schwann cells. 

Fig. 6.3 Adult neuromuscular junction with the three cells that constitute the synapse the motor neuron (i.e. nerve terminal), muscle fibre and Schwann cell. The motor neuron from the ventral horn of the spinal cord innervates the muscle. Each fibre receives only one synapse. The motor nerve loses its myelin to terminate on the muscle fibre. The nerve terminal, covered by Schwann cell, has vesicles clustered about the membrane thickenings, which are the active zones, towards its synaptic side and mitochondria and microtubules towards its other side. A synaptic gutter, made up of primary and many secondary clefts, separates the nerve from the muscle. The muscle surface is cormgated, and dense areas on the shoulders of each fold contain acetylcholine receptors. The sodium channels are present at the clefts and throughout the musele membrane (from Martyn 2005, p 863 copyright Elsevier)...

---