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Schwann Cells are the primary glial cells of the peripheral nervous system (PNS). They are also known as Neurilemma Cells or Neurolemmocytes. Schwann cells are named after British Zoologist, Theodore Schwann (1839), who also discovered the ‘plasma membrane’.
- Schwann cells are derived from neural crest cells that emigrate from the neural tube and migrate into the periphery.
- They are essential for the regeneration of neurons of the PNS.
- They are similar to a type of neuroglia called Oligodendrocytes.
- Schwann cells are classified into two types namely Myelinating and Non-myelinating Schwann Cells.
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Key Terms: Schwann Cells, Axons, Myelin Sheath, Regeneration, Oligodendrocytes, Neurons, Peripheral Nervous System, Myelination
What are Schwann Cells?
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Schwann cells are the primary glial cells responsible for the maintenance and regeneration of the motor and sensory neurons of the peripheral nervous system (PNS).
- These cells are essential for the insulation (myelination) and supply of nutrients to individual nerve fibers (axons) of the PNS neurons.
- Non-myelinating Schwann cells are essential for the supply of nutrition and cushioning effects to non-myelinated axons.
- Schwann cells create the myelin sheath around the neuronal axons.
- They differentiate from cells of the neural crest during the development and formation of the embryo.
- These cells are similar to oligodendrocytes in their function of providing protection and insulation to axons.
Schwann Cells
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Structure of Schwann Cells
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The structure of Schwann Cells can be described as follows:
- The plasma membrane of schwann cells contains a high amount of lipids and produces the myelin sheath in myelinated axons.
- The sheath does not stick to the contours of the body and is not continuous.
- Each myelinating schwann cell occupies almost 1mm of an axon, which makes it around 1000 schwann cells per meter of axon duration.
- The gaps between adjoining Schwann cells are known as Nodes of Ranvier.
- During the process of peripheral nerve regeneration, the schwann cells release 9-O-acetyl GD3.
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Myelin Sheath
Myelin is a protective layer or sheath made up of protein and fatty substances that forms around nerves, including those in the brain and spinal cord. The myelin sheath allows the quick and efficient transmission of electrical impulses along the nerve cells.
Functions of Schwann Cell
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The nervous system of the vertebrates utilizes the myelin sheath for protection and the reduction of membrane capacitance in the axon.
- During a process called saltatory conduction, the action potential jumps from node to node which increases the conduction velocity to ten times, without increasing the axonal diameter.
- Schwann cells are the primary nervous system’s (PNS’s) counterparts of the central nervous system’s oligodendrocytes.
- However, they are different from oligodendrocytes as myelinating Schwann cells provide protection to only one axon.
- Thus, such an arrangement allows the saltatory conduction of action potentials along with the repropagation at the Nodes of Ranvier.
- Therefore, myelination increases the speed of conduction and saves energy.
- Non-myelinating schwann cells are also essential for neuronal survival and the preservation of axons.
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Regeneration
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Schwann cells play a crucial role in promoting nerve regeneration. These cells myelinate numerous axons in the peripheral nervous system (PNS) and form nerves. If a nerve is damaged, the Schwann cells help in digesting the axons (phagocytosis).
- Afterward, they aid in regeneration by forming a type of tunnel, called the Büngner band, that goes toward the target neurons.
- The tunnel acts as an endoneurial tube and guides the track toward regenerating axons.
- Ideally, the stump of the damaged axon will sprout, and those sprouts go through the Schwann-cell “tunnel” at the rate of 1mm/day.
- The rate of regeneration gradually decreases with time.
- Schwann cells have been connected to preferential motor reinnervation due to their ability to aid axon regeneration.
- The regeneration of axons is ineffective without the support and guidance of Schwann cells.
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Schwann Cells vs Oligodendrocytes
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Schwann cells help in the protection and preservation of axons. Oligodendrocytes are a type of neuroglia that are responsible for providing protection and insulation to axons.
- One single oligodendrocyte may coil across 50 axons, with each individual axon receiving around 1m of myelin sheath; whereas Schwann cells can wrap over only one axon.
- The process of saltatory transmission of action potentials occurs at the Nodes of Ranvier between Schwann cells and oligodendrocytes, which increases impulse speed.
Myelination
Nervous system of vertebrates such as mammals, birds, reptiles, amphibians, and fish is dependent on the process of myelination or the formation of myelin sheath by Schwann cells for the following reasons:
- For providing insulation to axons during fetal development.
- For decreasing cell membrane capacitance in the axon.
Myelin
Metabolic Support
Oligodendrocytes provide trophic (nutritional) aid to nerve cells (IGF-1), as well as generate the following:
- Glial cell line-derived neurotrophic factor (GDNF)
- Insulin-like growth factor-1
- Brain-derived neurotrophic factor (BDNF)
Furthermore, the “Lactate Shuttle Hypothesis” states that oligodendrocytes can help in the supply of metabolites to neurons. On the other hand, satellite oligodendrocytes or perineuronal oligodendrocytes are different from oligodendrocytes in their function as
- They do not add insulation to axons because they are not connected to neurons through myelin sheaths.
- They reject neurons and control extracellular fluid.
Clinical Significance of Schwann Cells
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Examples of neuropathies involving Schwann Cells are:
- Schwannomatosis
- Leprosy
- Zika Virus
- Guillain-Barré syndrome (GBS)
- Charcot-Marie-Tooth disease (CMT)
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Things to Remember
- Schwann cells are the primary glia cells of the Peripheral Nervous System.
- They are also referred to as neurilemma cells or neurolemmocytes.
- Schwann cells are divided into two types namely Myelinating and Non-myelinating.
- Schwann cells play an important role in promoting nerve regeneration.
- They provide protection and reduce membrane capacitance throughout the axon.
Previous Years’ Questions
- Schwann cell is found around… (JIPMER - 2005)
- Cell theory was formulated by…
- Myelin sheath is formed by… (UP CPMT - 2010)
- Which of the following is not the cell of areolar tissue…
- Myelin of nerve fibres of central nervous system is produced… (AMUEEE - 2009)
- In a myelinated neuron, two adjacent myelin sheaths are…
- Myelinated nerve fibres are present in spinal and cranial nerves…
- Myelin sheath is produced by… (NEET - 2017)
- Which of the following statement is correct about the node of… (Rajasthan PMT - 2007)
- Omnis cellula was stated by… (AMUEEE - 2015)
Sample Questions
Ques. What are Schwann Cells? (2 Marks)
Ans. Schwann Cells are the primary glial cells in the peripheral nervous system (PNS) that are responsible for the maintenance and regeneration of the motor and sensory neurons. These cells are otherwise known as neurilemma cells or neurolemmocytes.
Ques. How do Schwann cells help in promoting nerve regeneration? (2 Marks)
Ans. Schwann cells are integral for the process of nerve regeneration.
- They myelinate several axons in the peripheral nervous system (PNS) and form nerves.
- They help in digesting the axons (phagocytosis) if the nerve is damaged.
- They also help in regeneration by forming a type of tunnel, called the Büngner band, that goes toward the target neurons.
Ques. Give the examples of neuropathies concerning Schwann cells. (2 Marks)
Ans. The neuropathies concerning Schwann cells are as follows:
- Schwannomatosis
- Leprosy
- Zika Virus
- Guillain-Barré syndrome (GBS)
- Charcot-Marie-Tooth disease (CMT)
Ques. What is Myelin? (2 Marks)
Ans. Myelin is a protective layer or sheath made up of protein and fatty substances that forms around nerves, including those in the brain and spinal cord.
Ques. Who was Theodore Schwanns ? (1 Marks)
Ans. Schwann Cells were named after British Zoologist Theodore Schwann (1839). He also discovered the concept of ‘plasma membrane’.
Ques. Write a short note describing the structure of Schwann cells. (3 Marks)
Ans. Schwann cells contains a high amount of lipid in their plasma membrane and it produces the myelin sheath in myelinated axons.
- The sheath is not continuous and does not adhere to the body's lines.
- As about 1 mm of an axon is occupied by each myelinating Schwann cell, there are roughly 1000 Schwann cells per meter of axon length.
- The nodes of Ranvier are the spaces between adjacent Schwann cells.
- The Schwann cells emit 9-O-acetyl GD3 during the regeneration of peripheral nerves.
Ques. How are Schwann Cells and Oligodendrocytes similar? (2 Marks)
Ans. Schwann cells are similar to oligodendrocytes in their function of providing protection and insulation to axons.
Ques. How is the myelin sheath formed by Schwann cells? (2 Marks)
Ans. The plasma membrane of the Schwann cells, containing high amounts of lipids, helps in the formation of the myelin sheath. The cholesterol present in the plasma membrane of the Schwann cells is a requisite for assembling the myelin sheath.
Ques. Describe the process of saltatory transmission in Schwann cells and oligodendrocytes. (3 Marks)
Ans. Schwann cells are the primary glial cells of the peripheral nervous system (PNS) that help in the protection and preservation of axons.
- The neuroglia known as oligodendrocytes are in charge of insulating and safeguarding axons.
- At the nodes of Ranvier, between Schwann cells and oligodendrocytes, the process of saltatory transmission of action potentials increases impulse speed.
Ques. What is a Büngner Band? (2 Marks)
Ans. Schwann cells can aid in regeneration by forming a type of tunnel, called the Büngner band, that goes toward the target neurons. The tunnel acts as an endoneurial tube and guides the track toward regenerating axons. Ideally, the stump of the damaged axon will sprout, and those sprouts go through the Schwann-cell “tunnel” at the rate of 1mm/day.
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