Myelin structure

The myelin sheath is wrapped around the axon to form concentric layers or lamellae, which are best seen in transverse section (Figure 8.2). Longitudinally along axons, consecutive myelin sheaths are separated by nodes of Ranvier, the highly specialized areas of naked axonal membrane where action potentials are propagated (see Chapter 8.4). The myelin sheath between nodes is therefore called

Structure The Myolin Sheath

Outer mesa x on

Figure 8.2 The myelin sheath in transverse section. The myelin sheath is seen to be formed by multiple layers of compacted myelin lamellae that spiral around the axon. The inner mesaxon and outer mesaxon are cytoplasmic ridges which pass along the length of the myelin sheaths, and are contiguous with the cell body (see Figure 8.4). This basic structure of the myelin sheath is the same in both PNS and CNS

Outer mesa x on

Figure 8.2 The myelin sheath in transverse section. The myelin sheath is seen to be formed by multiple layers of compacted myelin lamellae that spiral around the axon. The inner mesaxon and outer mesaxon are cytoplasmic ridges which pass along the length of the myelin sheaths, and are contiguous with the cell body (see Figure 8.4). This basic structure of the myelin sheath is the same in both PNS and CNS

the internode. Complex axo-glial junctions between the terminal ends of the myelin sheath (paranodal loops) and the axolemma at the paranode are important in the separation of Na+ and K+ channels that are respectively clustered at the node of Ranvier and under the myelin sheath in the juxtaparanode. A major difference between PNS and CNS nodes is that myelinating Schwann cells extend microvilli to fill the nodal gap, whereas oligodendrocytes do not have microvilli and the function of providing perinodal processes is fulfilled by astrocytes and NG2-glia (Figure 8.3). Perinodal processes play an important role in stabilizing axo-glial interactions at the node and in ion homeostasis. In addition, the PNS node is completely covered by the basal lamina, which is absent in the CNS.

Myelin is one of the most complex cellular structures in the brain. If unwrapped, the myelin sheath would appear as an extraordinarily large trapezoid or spadelike extension of the glial cell plasmalemma; a single sheet in Schwann cells (Figure 8.4) and multiple sheets in oligodendrocytes (Figure 8.5). The myelin sheets are continuations of the cell membrane, in which most of the cytoplasm is extruded so that the apposing phospholipid bilayers are fused to form the lamellae of compacted myelin. The fusion of apposed cytoplasmic interfaces of the plasma membrane forms the major dense line of the myelin sheath, whilst fusion of the outer interfaces of the plasma membrane forms the minor dense line or intraperiod line, as seen in transverse section under the electron microscope (Figure 8.6). There is a ridge of cytoplasm around the edge of the compacted myelin sheath and isolated strands of cytoplasm within the compacted myelin - called

Schwann Cells Longtudinal

Figure 8.3 The myelin sheath in longitudinal section. Myelin sheaths are separated along the length of axons by nodes of Ranvier, the sites of axonal action potential propagation. The myelin sheath between two nodes comprises the compacted internodal myelin sheath (see Figure 8.2); the internodal axolemma is relatively undifferentiated. The terminals of the internodal myelin sheath form paranodal loops ('para' from the Greek for 'alongside'), which are the lateral cytoplasmic ridges of the myelin sheath, stacked one upon the other as the myelin lamellae spiral around the axon (refer to Figures 8.4 and 8.6). The paranodal loops are very tightly apposed to the axolemma, with which they form highly complex junctions. These paranodal axoglial junctions are critical for separating the axolemmal Na+ (at nodes) and K+ channels (at juxtaparanodes; 'juxta' from the Latin for 'next to'), which are the basis of the action potential (see Figure 8.10). The cytoplasm of the paranodal loops is contiguous with that of the inner and outer mesaxons and the cell body (see Figure 8. 4). These elements are the same in the CNS and PNS. The PNS and CNS differ in that myelinating Schwann cells extend microvilli to fill the nodal gap, and the Schwann cell basal lamina completely covers PNS nodes and internodes to form a tube along the axon. Oligodendrocytes do not form a basal lamina, which is therefore absent from CNS nodes. Neither do oligodendrocytes form microvilli, and this function is performed by the perinodal processes of astrocytes and NG2-glia

Schmidt-Lanterman incisures - which are important for intracellular transport from the cell body to the myelin sheath (see Chapter 8.2.3). When wrapped around the axon, the inner and outer cytoplasmic ridges (or mesaxons) appear to 'corkscrew' around the axon, and the lateral cytoplasmic ridges build up to form the paranodal loops which are apposed to the axolemma by complex axo-glial junctions. Compacted myelin therefore comprises multiple fused phospholipid bilayers and it is these lipids that provide the insulating properties of the sheath. Like other cell membranes, myelin has a high electrical resistance and low capacitance, which is essential for rapid saltatory conduction from node to node.

Schwann Cells Structure

Figure 8.4 Myelinating Schwann cells. There is a 1:1 relationship between a myelinating Schwann cell and the axon it myelinates. Consecutive Schwann cells are separated by nodes of Ranvier and the myelin sheaths between nodes are internodes; the internodal length is directly related to axon diameter and determines the speed of conduction. Each myelin sheath is a large trapezoid sheet of compacted myelin that is wrapped around the axon; the thickness of the myelin sheath determines the insulating properties of the sheath and is directly related to axon diameter (the g-ratio defines the relationship axon diameter (D):the number of myelin lamellae - myelin sheath thickness, which is a constant - 1:10 - in PNS and CNS). Hence, large diameter axons have thicker and longer myelin sheaths (as great as 1000 |xm for the largest axons) and conduct action potentials must faster than small diameter axons (see Figure 8.9); this relationship is the same in the PNS and CNS. The sheet of compacted myelin is an extension of the cell plasmalemma (see Figure 8.6), and is completely surrounded by a ridge of cytoplasm. The inner and outer cytoplasmic ridges form the inner and outer mesaxons, respectively (see Figure 8.2), and the lateral ridges form the paranodal loops, which stack upon each other when the sheet of myelin is wrapped around the axon (see Figure 8.3). In the myelin sheaths around larger diameter axons, strands of cytoplasm called Schmidt-Lantermann incisures extend into the compacted myelin. The cytoplasm of all these elements is contiguous with that of the cell body and forms the intracellular pathway along which all of the myelin products are transported from the cell body and targeted to the myelin sheath, over hundreds and potentially thousands of microns

Schwann Cells Structure

Figure 8.5 Oligodendrocytes:

A. In contrast to Schwann cells, oligodendrocytes myelinate multiple axons. The ratio of oligo-dendrocyte:axons within the unit varies from 1:1 in type IV oligodendrocytes, 1:5 in type III oligodendrocytes, and 1:30 in type I/II oligodendrocytes. On average, oligodendrocytes myelinate 10 or more axons within approximately 10-30 |xm of the cell body; they do not produce consecutive myelin sheaths along the same axon.

B. Each myelin sheath is a large sheet of compacted myelin, with ridges of cytoplasm that are connected to the oligodendrocyte cell body by connecting processes (see Figure 8.4). As in the PNS, there are strict positive relationships between axon diameter and myelin sheath dimensions, and there is also a strict negative relationship between axon diameter and the ratio of oligodendrocyte:axons; type I/II oligodendrocytes myelinate numerous small diameter axons with short, thin myelin sheaths, and type III/IV oligodendrocytes myelinate a small number of large diameter axons with long, thick myelin sheaths. Consequently, there is a sharp demarcation in the volume of myelin supported by type I/II and type III/IV oligodendrocytes, which respectively support myelin volumes of approximately 500 |xm3 and 30 000 |xm3; oligodendrocytes and Schwann cells that myelinate the largest diameter axons support as much as 150 000 |xm3 of myelin

In terms of the volume of plasmalemma supported, oligodendrocytes and Schwann cells are probably the largest cells in the body, with lengths of up to 1 mm. The dimensions of the myelin sheath vary directly with axon diameter: the number of lamellae and internodal length vary approximately from 10-100 and 100-1000 ^m, respectively. The volume of myelin supported by an individual cell can be estimated as the internodal length (L) by the width, which is a function of axon circumference (kD), lamellar thickness (d), and the number of times the

Cell

Cell body body

Cytop

Cell body

Cytop

Cytoplasm extruded

Cell membrane

(phospholipid bilayer)

Apposed cell membranes

Myelin sheath

Cytoplasm extruded

Cell membrane

(phospholipid bilayer)

Extracellular faos

Cytoplasmic faoM (major darĀ» Una)

Cytoplasmic faoM (major darĀ» Una)

Extracellular face

Figure 8.6 Myelin structure. The myelin sheath is an extension of the cell in which most of the cytoplasm is extruded and apposed phospholipid bilayers of the plasmalemma are fused to form the lamellae of compacted myelin (see Figure 8.7). Envisage the myelin sheath as a partly inflated balloon - the air in the balloon represents the cytoplasm, and the rubber of the balloon represents the phospholipid bilayer of the plasmalemma, with inner (cytoplasmic) and outer (extracellular) faces. When the air/cytoplasm is extruded, the balloon deflates and the inner/cytoplasmic faces become apposed. Now, envisage the axon as a pencil, around which the deflated balloon is enwrapped, so that now the outer/extracellular faces become apposed to each other. This is analogous to the structure of the myelin sheath and to the process of myelin compaction. Production and compaction of myelin is a continuous process, whereby myelin components are inserted into the myelin sheath and cytoplasm is extruded. Gap junctions within the myelin sheath play important roles in water and ion transport and myelin compaction in both Schwann cells and oligodendrocytes. Knockout studies indicate a critical role for Cx32 in Schwann cells, whereas this is a function of both Cx32 and Cx47 in oligodendrocytes, in addition to the inward rectifying potassium channel Kir4.1. Oligodendrocytes are also rich in the enzyme carbonic anhydrase, which is important in ion and water transport in other tissues (such as the kidney and gut) and may play a similar role in extrusion of cytoplasm from the myelin sheath sheath is wrapped around the axon, i.e. the number of lamellae (N). Schwann cells have a 1:1 relationship with axons and internodal lengths as great as 1000 ^m, so that a cell myelinating a 10-15 ^m diameter axon supports a volume of myelin as great as 150000 ^m3, which would be visible to the naked eye. In oligodendrocytes, there is a sharp demarcation between type I/II oligodendrocyte units, which have a large number of small diameter axons with short, thin myelin sheaths, and type III/IV oligodendrocyte units, which have a small number of axons with long, thick myelin sheaths (see Chapter 3.2). Type I/II oligodendrocytes support approximately 500 ^m3 of myelin, whereas type III/IV oligodendrocytes, like Schwann cells, support 30000-150000 ^m3 of myelin, depending on axon diameter. The dimensions of the myelin sheath determine the conduction properties of the axons within the unit: the thickness of the myelin sheath determines the insulating properties and the length determines the speed of conduction. Hence, larger axons have longer and thicker myelin sheaths and conduct faster than thinner axons. It should be noted that the myelin sheath is a living structure and its production places a considerable metabolic load on myelinating cells; the turnover of phospholipids and myelin proteins is in the order of days to weeks.

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Responses

  • vittore
    Why is it important that the myelin sheath is in a phospholipid?
    5 years ago
  • Ernestina
    Why do oligodendrocytes have dark cytoplasm?
    4 years ago
  • Martina
    Is it the CNS or The PNs that has 1:1 relationship between mylenating cell and axon?
    4 years ago
  • robert whiteley
    What structure is formed by Schwann cellswrapping around an axon?
    4 years ago
  • Ferruccio
    How to rejuvenate phospholipid of schwann cells function?
    3 years ago
  • Malva Brown
    Is Myelin along the length of mammalian axons?
    4 months ago

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