chapter 12: neuroglia, myelination, nerve fibers

what are neuroglial cells?

support cells of the nervous system

neuroglial cells protect what and help what?

protect neurons and help them function

neuroglial cells bind what and form what?

bind neurons together and form framework for nervous tissue

neuroglial cells guide…?

migrating neurons to their destination during development and regeneration

mature neurons of neuroglial cells are….

are covered with glial cells in nearly all places not in synaptic contact with another cell

what does no synaptic contact of mature neurons with glial cells ensure?

that neurons only contact at synapses and prevents the unwanted signaling

what are the neuroglia types?

4 only in the CNS neuroglia and 2 only in the PNS

what neuroglia are found in the CNS?

oligodendrocytes, ependymal cells, microglia, and astrocytes

what are the neuroglia of the PNS?

satellite cells and schwann cells

what do oligodendrocytes form?

they form myelin sheaths in the CNS that speed signal conduction

oligodendrocytes have what processes?

they have arm like processes that wrap around nerve fibers

what is the function of oligodendrocytes?

function is similar to that of schwann cells in the PNS

where do the ependymal cells lie?

they line the internal cavities of the brain and spinal cord

what is the function of ependymal cells?

to secrete and circulate cerebrospinal fluid (CSF)

what role do microglia serve as?

serve as the role of the immune system of the brain

where are microglia located?

they wander through the CNS looking for debris, damage, and pathogens

what neuroglia collectively check over the entire brain multiple times a day?

the microglia

microglia do what in areas of damage?

they proliferate

what function do astrocytes have?

they’re functionally diverse

what are the most abundant glial cells in the CNS?

the astrocytes

where are astrocytes located?

they cover the brain surface and most nonsynaptic regions of neurons in the gray matter

what do the extensions (perivascular feet) of astrocytes do?

they contact blood capillaries and stimulate them to form a seal called the blood-brain barrier (BBB)

what is the blood-brain barrier (BBB)?

a selective barrier that prevents unwanted materials from leaving the blood and entering fluid around the CNS

astrocytes convert what to what?

convert glucose from blood to lactate and supply this to neurons for energy

how do astrocytes regulate chemical composition of fluid around neurons by….?

absorbing excess neurotransmitters and ions

where are satellite cells located?

they surround the neurosomas in ganglia of the PNS

what do satellite cells provide around the soma?

electrical insulation

what do satellite cells regulate of the neurons?

chemical environment

where are schwann cells located?

they wind repeatedly around a nerve fiber

what do schwann cells produce?

they produce a myelin sheath (similar to the ones produced by oligodendrocytes in the CNS)

what do schwann cells assist in?

the regeneration of damaged fibers

what does the myelin sheath do for the axon?

it insulates the axon and increase the speed of signal

the myelin sheath consists of what?

the plasma membrane of glial cells (mostly lipids (80%) rest is protein)

where is there no myelin sheath at birth?

because it forms during infancy and adolescence

the process of myelination differs between

the CNS and PNS

myelination starts in the PNS when what spirals?

schwann cells spiral around a single nerve fiber (up to 100 layers)

what is the neurilemma?

thick, outermost coil of the myelin sheath located in the PNS

what does the neurilemma contain?

the nucleus of schwann cell and most of its cytoplasm

what is the endoneurium?

the fibrous CT around a schwann cell

what structures are key players in nerve regeneration?

the endoneurium and the neurilemma

what begins the process of myelination in the CNS?

when the oligodendrocyte myelinates several nerve fibers in its immediate vicinity

nerve fibers in the CNS do not have

the neurilemma or the endoneurium

since the nerve fibers in the CNS have no neurilemma or endoneurium, what does that mean?

it means that they cannot regulate CNS fibers

what is a gap between segments?

the myelin sheath gap

what are internodes of the myelin sheath?

myelin-covered segments of an axon

what is the trigger zone of the myelin sheath?

the axon hillock and the initial segment

why is the trigger zone important?

it plays an important role in initiating a nerve signal

many CNS and PNS nerve fibers are

unmyelinated

In the PNS, schwann cells hold 1 to 12 small nerve fibers in what?

surface grooves

in the surface grooves of the schwann cell, the membrane does what?

folds once around each fiber that is held in the groove

an “unmyelinated” fibers helps control?

the environment around the axon

why is control of the environment around the axon important?

vital for maintaining resting membrane potential of the axon

the speed of nerve signal depends on what two factors?

diameter of the fiber and presence or absence of myelin

larger fibers have more

surface area and faster signals

the myelin sheath increases

signal speed

what do slow signaling fibers supply?

the gastrointestinal tract where speed is less important

what do fast signaling fibers supply?

the skeletal muscles where quick responses are more important

if all fibers were maximized speed, then the nervous system would

be very large and have few nerves

the regeneration of nerve fibers only occurs in what nervous system?

the PNS

why can’t you replace dead nerves?

because neurons don’t divide

what may regenerate in the PNS?

damaged axons

what will encourage axon growth with growth factors in the PNS?

schwann cells

what problems can occur with regeneration?

slow regrowth, some nerve fibers connect with the wrong target cells, and some nerve cells die during regeneration

brain tumors are usually composed of what types of cells?

glial cells

why does chemotherapy usually not work as well on brain tumors?

due to the blood-brain barrier, only radiation and surgery

what is cancer?

uncontrolled cell division