Question

myelin sheaths
most long nerve fibers are covered with a whitish, fatty material called myelin (mīē -lin), which has a waxy appearance. myelin protects and insulates the fibers and increases the ( 1 ) rate of nerve impulses. compare myelin sheaths to the many layers of insulation that cover the wires in an electrical cord; the layers keep the electricity flowing along the desired path just as myelin does for nerve fibers. axons outside the cns are myelinated by schwann cells, as just noted. many of these cells wrap themselves around the ( 2 ) in a jelly - roll fashion (figure 7.5, p. 231). initially, the membrane coil is loose, but the schwann cell cytoplasm is gradually squeezed from between the membrane layers. when the wrapping process is done, a tight coil of wrapped membranes, the myelin sheath, encloses the axon. most of the schwann cell cytoplasm ends up just beneath the outermost part of its plasma membrane. this part of the schwann cell, external to the ( 3 ) sheath, is called the neurilemma (nuri -lemmah,
euron husk\). because the myelin sheath is formed by many individual schwann cells, it has gaps, or indentations, called nodes of ranvier (rahn - vēr), at regular intervals (see figure 7.4).
as mentioned previously, myelinated fibers are also found in the central nervous system. oligodendrocytes form cns myelin sheaths (see figure 7.3d). in the pns, it takes many schwann cells to make a single ( 4 ) sheath, but in the cns, the oligodendrocytes with their many flat extensions can coil around as many as 60 different fibers at the same time. thus, in the cns, one oligodendrocyte can form many myelin sheaths. although the myelin sheaths formed by oligodendrocytes and those formed by schwann cells are similar, the cns sheaths lack a neurilemma. because the ( 5 ) remains intact (for the most part) when a peripheral nerve fiber is damaged, it plays an important role in fiber regeneration, an ability that is largely lacking in the central nervous system.

1 neurilemma 2 transmission 3 axon 4 myelin 5 myelin

1 transmission 2 axon 3 myelin 4 myelin 5 neurilemma

1 axon 2 myelin 3 myelin 4 neurilemma 5 transmission

1 myelin 2 neurilemma 3 transmission 4 axon 5 myelin

1 myelin 2 myelin 3 neurilemma 4 transmission 5 axon

Explanation:

Myelin sheaths increase the transmission rate of nerve impulses. Schwann cells wrap around the axon. The neurilemma is external to the myelin sheath. Many Schwann cells make a single myelin sheath in the PNS. The neurilemma plays a role in fiber - regeneration in the PNS.

Answer:

1. transmission
2. axon
3. myelin
4. myelin
5. neurilemma