Anaphy Review for Midterms - Nervous System (Myelination ; Myelin Sheath)

What do myelinated axons do?

• the myelin sheath electrically insulates the axon of a neuron

• increases the speed of nerve impulse conduction

what do you call axons not covered with a Myelin sheath?

Unmyelinated Axons

the 2 types of Neuroglia that produce Myelin Sheath

• Schwann Cells (PNS)

• Oligodendrocytes (CNS)

When do Schwann cells start producing myelin sheaths around axons?

during fetal development

Each Schwann cell wraps how much of an axon’s length?

1 millimeter (1 mm) ; by spiraling it many times

the myelin sheath contains how many layers of Schwann cells?

100

• The outer nucleated cytoplasmic layer of the Schwann cell

• encloses the Myelin sheath

Neurolemma (sheath of Schwann)

A neurolemma is found only around axons in the _____.

PNS (Peripheral Nervous System)

How does a neurolemma aid in regeneration when an axon is injured?

by forming a regeneration tube that guides and stimulates regrowth of the axon

• gaps in the myelin sheath

• appear at intervals along the axon

nodes of Ranvier

Each Schwann cell wraps how many axon segments between 2 nodes?

1 axon segment

In the CNS, the oligodendrocyte puts forth about how many processes that spiral around CNS axons?

15 broad, flat processes

Is a neurolemma present in the axons of the CNS?

No

Are Nodes of Ranvier present in CNS axons?

Yes, but they are fewer in numbers compared to those in the PNS

compared to the axons in the PNS, what is the percentage of regrowth displayed by the axons in the CNS?

little regrowth after injury

the axons in the CNS display little regrowth compared to those in the PNS due to what?

• absence of a neurolemma, and;

• an inhibitory influence exerted by the oligodendrocytes on axon regrowth

why does an infant’s response is not as rapid and coordinated compared to that of an older child / adult?

• amount of myelin increases from birth → maturity

• presence of myelin increases speed of nerve impulse conduction

• loss or destruction of myelin sheaths around axons

• may result from disorders, medical treatments

Demyelination

Disorders that can cause demyelination

• Multiple Sclerosis

• Tay - Sach’s disease

Medical treatments that may cause Demyelination

• Radiation Therapy

• Chemotherapy

what are the effects of an episode of Demyelination?

deterioration of affected nerves