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===The Schwann Cell===

===The Schwann Cell===

*Myelination in the PNS is achieved by the Schwann cell, a derivative of neural crest cells, which bud off from the neuroepithelium at a very early stage of neurogenesis.  

Myelination in the PNS is achieved by the '''Schwann cell''', a derivative of neural crest cells, which bud off from the neuroepithelium at a very early stage of neurogenesis. During development, Schwann cells engage many small axons and as axonal diameter increases, Schwann cells eventually relate with only a single axon c.f [[Neurons - Anatomy & Physiology#Oligodendrocytes|oligodendrocytes]]. This single axon is enveloped in a trough by the Schwann cell processes that engulf it and as the processes come together, an inner '''mesaxon''' is formed. The leading-edge process continues to move over the axon forming a spiral. Myelination, an extremely complex molecular process, occurs when the cytoplasm within the process is extruded allowing the internal surfaces of the membrane to come together as the '''major dense line''', the outer membrane apposition constituting the intraperiod line. The alternating pattern of these two form the lamellae of compacted myelin. The myelin sheath is attached to, and is an integral part of the Schwann cell on which it is dependent for its maintenance.

*During development, Schwann cells engage many small axons.

*As axonal diameter increases, Schwann cells eventually relate with only a single axon c.f [[Neurons - Anatomy & Physiology#Oligodendrocytes|oligodendrocytes]].  

**This single axon is enveloped in a trough by the Schwann cell processes that engulf it.

A single Schwann cell forms a single myelin sheath or internode and there is a reasonably constant relationship between the myelin thickness and the internodal length, which in turn is associated with axon calibre. Large axons have long, thick myelin sheaths and therefore conduct more rapidly. The internodes do not abut one another but are separated by an exposed area of axon called the '''node of Ranvier'''. If the axons remain of small diameter, then a Schwann cell will continue to associate with many axons, although none of them are myelinated. Thus, ''even unmyelinated axons retain a Schwann cell ensheathment''. These non-myelinating Schwann cells are sometimes referred to as ''Remak cells.''

**As the processes come together, an inner mesaxon is formed.  

**The leading-edge process continues to move over the axon forming a spiral.  

*Myelination, an extremely complex molecular process, occurs when the cytoplasm within the process is extruded allowing the internal surfaces of the membrane to come together as the '''major dense line''', the outer membrane apposition constituting the intraperiod line.  

*The alternating pattern of these two form the lamellae of compacted myelin.  

*The myelin sheath is attached to, and is an integral part of, the Schwann cell on which it is dependent for its maintenance.

 

*A single Schwann cell forms a single myelin sheath or internode.

*There is a reasonably constant relationship between the myelin thickness and the internodal length, which in turn is associated with axon calibre.  

*''Large axons have long, thick myelin sheaths'', and therefore also conduct more rapidly.

*The internodes do not abut one another but are separated by an exposed area of axon called the '''node of Ranvier'''.

*If the axons remain of small diameter, then a Schwann cell will continue to associate with many axons, although none of them are myelinated.

**Thus, ''even unmyelinated axons retain a Schwann cell ensheathment''.  

**These non-myelinating Schwann cells are sometimes referred to as ''Remak cells.''

===Axoplasmic Transport===

===Axoplasmic Transport===