12.4 Nervous Tissue: Glial Cells

As with myelin sheaths formed by oligodendrocytes in the CNS, this allows for

faster propagation of action potentials

You may find it helpful to imagine that an oligodendrocyte is like a latex glove with fluid added to it. Each finger of the glove is wrapped numerous times around the axon of different neurons, and the fluid is pushed into the hand of the glove. Remember, myelination in both the PNS and CNS allows for

faster propagation of action potentials

Satellite cells are

flattened cells arranged around neuronal cell bodies in a ganglion that physically separate cell bodies from their surrounding interstitial fluid. They both electrically insulate the cell body and regulate the continuous exchange of nutrients and waste products between neuron cell bodies and their environment.

Ependymal cells have slender processes that branch extensively to make contact with other

glial cells in the surrounding nervous tissue.

Glial cell tumors, termed

gliomas

Myelin mainly consists of the plasma membrane of these glial cells and contains a large proportion of lipids and a lesser amount of proteins. The high lipid content of the myelin gives an axon a distinct,

glossy-white appearance and serves to effectively insulate an axon

Most cases of GBS are preceded by an acute, flulike illness, although no specific infectious agent has ever been identified. The condition in rare instances may follow an

immunization

Microglial cells wander through the CNS and replicate in response to an

infection

Guillain-Barré syndrome (GBS) is a disorder in which

inflammation causes loss of myelin from the peripheral nerves and spinal nerve roots

Myelin is the

insulating covering around the axon that consists of repeating concentric layers of plasma membrane of glial cells

Astrocytes help maintain an optimal chemical composition of the

interstitial fluid (fluid around cells) within the brain.

Myelination

is the process by which part of an axon is wrapped with myelin

gliomas

may be either relatively benign and slow-growing or malignant (capable of metastasizing or spreading to distant sites).

GBS is characterized by

muscle weakness that begins in the distal limbs, but rapidly advances to involve proximal muscles as well (ascending paralysis)

Neurolemmocytes are elongated and flattened cells wrap around and insulate axons within the PNS to form a

myelin sheath

The cytoplasmic extensions of the oligodendrocyte wrap repeatedly around a portion of each axon where plasma membrane layers of the oligodendrocyte form the

myelin sheath

The extensions of oligodendrocytes wrap around and insulate axons within the CNS to form a

myelin sheath

The overlapping inner layers of the plasma membrane form the

myelin sheath

In the CNS, an oligodendrocyte wraps around a small part of multiple axons, forming a

myelin sheath (but no neurilemma is formed)

In the PNS, the neurolemmocyte ensheathes only one small part of a single axon to form both a

myelin sheath and a neurilemma

A myelin sheath surrounds most axons. In the PNS, neurolemmocytes form both a

myelin sheath and neurilemma in a series of sequential stages

A neurolemmocyte in the PNS can myelinate only a 1-millimeter portion of a single axon. Thus, if an axon is longer than 1 millimeter (and most PNS axons are), it takes many neurolemmocytes to

myelinate the entire axon

The myelinating a PNS axon process is similar to what would happen if you were to take a balloon with a small amount of water in it and wrap it numerous times around a pencil. The balloon is wrapped over and over around a section of your pencil and the part of the balloon containing water is pushed to the outside. The wrapped layers of balloon represent the myelin sheath, and the external portion of the balloon with the water represents the

neurilemma

The periphery of the neurolemmocyte contains the cytoplasm and nucleus and is called the

neurilemma (lemma = husk)

The axons in many of the nerves in the body have hundreds or thousands of neurolemmocytes along their entire length. The gaps between the neurolemmocytes are called

neurofibril nodes, or nodes of Ranvier

Glial cells are also referred to

neuroglia

The neurolemmocyte starts to encircle a 1-millimeter portion of an axon. As the neurolemmocyte continues to wrap around the axon, the cytoplasm and nucleus of the neurolemmocyte are squeezed to the periphery of the

neurolemmocyte (the outside edge)

Myelination is completed by

neurolemmocytes in the PNS and by oligodendrocytes in the CNS

Glail cells far outnumber

neurons -glial cells account for roughly half the volume of the nervous system

Note that no neurilemma is formed as the CNS neurons are myelinated. Neurofibril nodes are also located between adjacent

oligodendrocyte "wraps

What are the end of The ends of astrocyte processes called?

perivascular feet

Microglia represent the smallest percentage of CNS glial cells with some estimates of their prevalence as low as 5%. They are classified as

phagocytic cells of the immune system

A tumor that originates within the organ where it is found is called a

primary tumor

Multiple sclerosis (MS) is

progressive demyelination of neurons in the central nervous system accompanied by the destruction of oligodendrocytes.

Even though GBS appears to be an immune-mediated condition, the use of steroids provides little if any measurable improvement. In fact, most people

recover almost all neurologic function on their own with little medical intervention

Astrocytes help direct the development of neurons in the fetal brain by secreting chemicals that

regulate the formation of connections between neurons

Two types of glial cells are found in the peripheral nervous system (PNS). These specialized glial cells are?

satellite cells and neurolemmocytes

Astrocytes

(as′trō-sīt; astron = star) exhibit a starlike shape due to projections from their surface -its also a neuron

As a result of the autoimmune system attacking the oligodendrocytes disorder, the conduction of action potentials is disrupted, leading to impaired

sensory perception and motor coordination. Repeated inflammatory events at myelinated sites cause scarring (sclerosis), and in time some function is permanently lost.

What is myelin sheath and what does it allow?

This insulation allows for faster propagation of action potentials.

Occupy the space of dying neurons

When neurons are damaged and die, the space they formerly occupied is often filled by astrocytes that replicate through cell division.

Unmyelinated Axons

are surrounded by a neurolemmocyte but are not wrapped in a myelin sheath (b) An electron micrograph shows a myelinated axon and several unmyelinated axons.

MS is an autoimmune disorder, because the body's immune cells mistake the oligodendrocytes

as foreign and attack them

The cytoskeleton in astrocytes strengthens these cells to provide a

structural framework to support and organize neurons within the CNS

Because most mature neurons cannot undergo mitosis, primary CNS tumors typically originate in supporting tissues within the brain or spinal cord that have retained the capacity to undergo mitosis:

the meninges (protective membranes of the CNS) or the glial cells

An oligodendrocyte in the CNS, in comparison, can myelinate a 1-millimeter portion of many axons at the same time and not just one. Figure 12.8b shows oligodendrocytes myelinating portions of

three different axons

Not all axons are myelinated. Unmyelinated axons in the PNS are also associated with neurolemmocytes, which help to protect and support the axon. However, no myelin sheath covers them. Thus, the axon merely rests in a depressed portion of the neurolemmocyte, but its plasma membrane does not form repeated layers around the axon. In the CNS,

unmyelinated axons are not associated with oligodendrocytes

The MS disease usually affects young adults between the ages of 18 and 40. It is five times more prevalent in individuals of European descent than it is in African-Americans. Among the typical symptoms are

vision problems, muscle weakness and spasms, urinary infections and bladder incontinence, and drastic mood changes

Neoplasms resulting from unregulated cell growth, commonly known as tumors, sometimes occur

within the central nervous system (CNS)

When neurons are damaged and die, the space they formerly occupied is often filled by

astrocytes that replicate through cell division

The BBB strictly controls movement of substances from exiting the

blood and entering the nervous tissue in the brain

The perivascular feet and the brain capillaries together contribute to a

blood-brain barrier (BBB)

Astrocytes are the most abundant glial cell in the CNS and constitute over 90% of the nervous tissue in some areas of the

brain

Astrocytes have contact with both

capillaries (smallest blood vessels) and neurons

The choroid plexus helps produce

cerebrospinal fluid (CSF)

Ependymal cells and nearby blood capillaries together form a network called the

choroid plexus

Ependymal cells are

ciliated simple cuboidal or simple columnar epithelial cells that line the internal cavities (ventricles) of the brain and the central canal of the spinal cord

The cilia of ependymal cells help

circulate the CSF

cerebrospinal fluid (CSF) is a

clear liquid that bathes the external surfaces of the CNS and fills its internal cavities

astrocytes regulate potassium ion concentration by absorbing these ions to sustain a

constant potassium ion concentration that is critical to electrical activity of neurons

Microglia

(mī-krog′lē-ă; micros = small) are typically small cells that have slender branches extending from the main portion of the cell

Oligodendrocytes

(ol′i-gō-den′drō-sīt; oligos = few) are large cells with a bulbous body and slender cytoplasmic extensions or processes.

Four types of Glial cells that are found in the Central Nervous system

-Astrocytes -ependymal cells -microglia -oligodendrocytes

During development, what does glial celss do to help neurons?

-Glial cells form the framework that guides young, migrating neurons to their final destinations

Nervous System Disorders Affecting Myelin

-Multiple sclerosis MS -Guillain-Barré syndrome GBS

Glail cells are critial for the normal function at neural synapses because they both?

-maintain the anatomic structure of synapses and modifying transmission

Microglia protect the CNS against

-microorganisms and other potentially harmful substances -they do this by engulfing infectious agents and removing debris from dead or damaged nervous tissue that results from infections, inflammation, trauma, and brain tumors

How can the four types of Glial cells be distinguished?

-size -intracellular organization -presence of specific cytoplasmic process

Glial cells are

-smaller than neurons -capable of mitosis

Glial cells

-they support and protect neurons they also help to nourish them and provide organized, supporting scaffolding for all the nervous tissue -smaller than neurons -capable of mitosis -They do not transmit electrical signals - they do assist neurons with their function

The BBB protects the delicate neurons of the brain from

-toxins -but at the same time allows needed nutrients to pass through

Astrocytes nurture, protect, support, and guide neurons by what five factors

1) Help form the blood-brain barrier 2)Regulate interstitial fluid composition 3)Form structural support 4)Assist neuronal development 5)Occupy the space of dying neurons

Assist neuronal development

Astrocytes help direct the development of neurons in the fetal brain by secreting chemicals that regulate the formation of connections between neurons.

Regulate interstitial fluid composition

Astrocytes help maintain an optimal chemical composition of the interstitial fluid (fluid around cells) within the brain. For example, astrocytes regulate potassium ion concentration by absorbing these ions to sustain a constant potassium ion concentration that is critical to electrical activity of neurons.

Glial cells are found within both the

CNS and PSN

Neurolemmocytes are also called

Schwann cells

Blood-brain barrier (BBB)

Structure formed by capillary endothelial cells and astrocytes that regulates what can enter the interstitial fluid in the brain: helps prevent transport of harmful substances from the blood into the brain

Form structural support

The cytoskeleton in astrocytes strengthens these cells to provide a structural framework to support and organize neurons within the CNS.

Help form the blood-brain barrier

The ends of astrocyte processes are called perivascular feet: They both cover and wrap around capillaries in the brain. The perivascular feet and the brain capillaries together contribute to a blood-brain barrier (BBB). The BBB strictly controls movement of substances from exiting the blood and entering the nervous tissue in the brain. The BBB protects the delicate neurons of the brain from toxins, but at the same time allows needed nutrients to pass through

What does the perivascular feet do?

They both cover and wrap around capillaries in the brain