Lecture 1: Intro Terminology, Neurohistology

Question 1

The brain weighs approximately ___

Answer 1

3 lbs (1400 gm)

Question 2

The brain represents ____ of total body weight

Answer 2

2-3%

Question 3

The brain accounts for ____ of total energy consumption.

Answer 3

20%

Of this 20%, 60-80% supports communication among neurons and glia rather than the brain’s response to external stimuli.

Question 4

The brain contains several billion very fragile neurons which, if put end to end, would make a cable several thousand miles long

Answer 4

Estimated synaptic connections as high as a hundred trillion

Question 5

Approximately ____ of the human genome is involved in nervous system function

Answer 5

one half

Question 6

The brain directs all motor activity, somatic as well as autonomic

Answer 6

The brain directs all motor activity – voluntary as well as involuntary

Question 7

The brain receives far more _____ compared to ____ produced.

Answer 7

The brain receives far more sensory inputs compared to motor outputs produced.

Question 8

Approximately ____ Americans are afflicted with neurological, communicative, or behavioral disorders and the majority are _____.

Answer 8

Approximately 50 million Americans are afflicted with neurological, communicative, or behavioral disorders and the majority are chronic.

Question 9

Third leading cause of death

after heart disease and cancer

Answer 9

Cerebral vascular disease (stroke)

Question 10

____ of deaths occurring during the first year of life are caused by congenital malformations of the CNS.

Answer 10

40% of deaths occurring during the first year of life are caused by congenital malformations of the CNS.

Question 11

What disease is estimated to afflict one quarter of the population who live to age 85 years?

Answer 11

Alzheimer’s disease

Question 12

CENTRAL NERVOUS SYSTEM

Answer 12

Brain

Spinal Cord

Question 13

PERIPHERAL NERVOUS SYSTEM

Answer 13

12 pairs of cranial nerves -arise from brainstem

31 pairs of spinal nerves –arise from spinal cord

Ganglia

Axons that enter or leave the brain and spinal cord form nerves that are part of the PNS. This includes 12 pairs of cranial nerves arising from the brainstem and 31 pairs of spinal nerves arising from the spinal cord. Another structure found in the peripheral nervous system are ganglia.

Peripheral nerves are generally collections of axons on their way to or from places such as skin, muscle, or internal organs, accompanied by glial and connective tissue sheaths. Many of these axons have cell bodies that also reside in the PNS, and these somata are typically clustered in ganglia at predictable sites along the nerve

Question 14

GANGLIA

Answer 14

Ganglia = Greek for Tumor; anything gathered into a ball

Ganglia are structures located outside the CNS that contain neurons. An example is a dorsal root ganglion located immediately adjacent to the spinal cord.

Question 15

Dorsal Root ganglia

Answer 15

the small swellings located bilaterally at the level of each intervertebral foramen.

Question 16

NUCLEUS

Answer 16

NUCLEUS = Collection of neurons within brain or spinal cord (i.e., within the CNS)

Whereas a ganglion is a collection of neurons in the periphery, a nucleus is a collection of neurons within the CNS.

Specialized areas of gray matter.

Question 17

Myelin

Answer 17

the lipid membrane that surrounds the axons of neurons.

Myelin– plasma membrane of oligodendrocyte or Schwann cell that forms multiple wraps around segments of the axon.

Myelin extends for short distances. Gaps between wraps are called Nodes of Ranvier

Spiral wrappings of Schwann cell (PNS) or oligodendrocyte (CNS) membranes around axons, interrupted periodically by nodes of Ranvier. Myelin forms a low-capacitance insulating coating around axons, greatly increasing their conduction velocities by allowing saltatory conduction.

mammalian

Question 18

Dorsal

Answer 18

toward the top of the head

Question 19

ventral

Answer 19

toward the neck

Question 20

horizontal plane

Answer 20

transverse plane

the brain is cut from rostral to caudal for the forebrain which is dorsal to ventral for the brainstem.

Question 21

coronal plane

Answer 21

is a cut from dorsal to ventral for the forebrain

Question 22

sagittal plane

Answer 22

divides down the middle into two halves.

Sections taken adjacent to the mid-sagittal cut are referred to as parasagittal.

Question 23

The brain may be divided into 6 regions

Answer 23

. 4 regions are collectively referred to as the brainstem. Starting caudally these are: The Medulla which is a direct continuation of the spinal cord. The Pons (this is latin for Bridge as it has the appearance of which). the cerebellum (little brain). The Midbrain which is the smallest part of the brainstem. The diencephalon which is the transition between the brainstem and the cerebral cortex. This is the area where the flexure between the brainstem and the cerebral cortex referred to preciously occurs. Finally, the largest part of the brain is the telencehalon or forebrain. It consists of the massive cerebral cortex and several nuclei that are buried deep within the telencephalon. These are referred to as subcortical nuclei

Question 24

TELENCEPHALON

Answer 24

FOREBRAIN

Cerebral cortex and subcortical nuclei (e.g., Basal Ganglia).

The largest part of the brain. It consists of the cerebral cortex and several nuclei that are buried deep within the cortical mantle. These are referred to as subcortical nuclei.

This is what most people think about when you say “the brain”. Two prominent fissures are evident. The Central Sulcus extends from dorsal to ventral, beginning at about the midpoint of the cerebral cortex. The Lateral Sulcus extends from rostral to caudal. These deep sulci help divide the cortex into lobes: **the Frontal lobe, ** the Parietal lobe, ** The temporal lobe, and the **Occipital lobe.

Question 25

DIENCEPHALON

Answer 25

Encephalon is Greek for “in the head/brain” Diencephalon means “in-between brain,” signifying this part of the CNS lies between the cerebral hemispheres and the brainstem.

Composed of the Thalamus & Hypothalamus

the transition between the brainstem and the cerebral cortex

Most of the diencephalon is covered by the telencephalon or cerebral cortex. It extends from the mammillary bodies to the optic chiasm.

Only a small portion of the diencephalon is evident from the ventral view. Structures that can be seen include the **mammillary bodies which form the posterior border of the diencephalon, and *** the optic nerve. At this point, some of the axons traveling in the optic nerves arising from the right and the left eye cross the midline and continue to visual centers in the cerebral cortex as the optic tract. The point of crossing is called the optic chiasm.

Question 26

Medulla

Answer 26

Direct Rostral extension of the Spinal cord

Question 27

Pons

Answer 27

Latin for Bridge, as it has the appearance of one

Question 28

Midbrain

Answer 28

the smallest part of the brainstem

Question 29

Order of these buggers

Answer 29

spine &raquo_space; mudulla&raquo_space; pons &raquo_space; midbrain &raquo_space; diencephalon&raquo_space; telencephalon

Question 30

Bumps on the brain

Answer 30

(hillocks, tubercles, colliculi, gyri, peduncles, brachium, eminence)

generally indicate the location of underlying nuclei or fiber tracts

Question 31

Grooves

Answer 31

(fissures, sulci, raphe)

divide brain into different regions or represent a midline division of the brain.

Question 32

Cut off nerves are central portions of ______ that emerge from the medulla, pons, midbrain and diencephalon

Answer 32

Cut off nerves are central portions of cranial nerves that emerge from the medulla, pons, midbrain and diencephalon

Question 33

VENTRAL MEDULLA

Answer 33

Cylindrical in shape and continuous with the spinal cord

Is located ventral to the cerebellum.

Numerous cranial nerves arise from the lateral aspect of the medulla bilaterally.

Question 34

the optic nerve

Answer 34

Cranial nerve II

Question 35

central sulcus

Answer 35

extends from dorsal to ventral, beginning at about the midpoint of the cerebral cortex.

Question 36

Lateral Sulcus extends from rostral to caudal.

Answer 36

extends from rostral to caudal.

Question 37

Sulci/ Fissures

Answer 37

depressions between gyri

Question 38

Gyri

Answer 38

elevated ridges of tissue

Question 39

2 prominent fiber tracts are evident:

Answer 39

corpus callosum and fornix

Question 40

Neuron

Answer 40

the Functional Unit of the Nervous System
the Functional Unit of the Nervous System

Information-processing and signaling cell.

Neurons are anatomically and functionally polarized, with electrical signals traveling in only one direction under ordinary physiological circumstances.

Question 41

Neuron Anatomy

Answer 41

Cell Body (Soma, Perikaryon) 
Fills the metabolic needs.

Dendrites (Spines)
Receive info from other neurons via the synapse.

Axon: neurons usually have only one.
Hillock – origin of axon from soma
Initial Segment – segment of axon just before first myelin wrap.
Terminals – ending of axon which usually divides into multiple branches.

Question 42

synapse

Answer 42

Connections between neurons

May be excitatory or inhibitory

Question 43

GLIAL CELL TYPES

Answer 43

Oligodendrocytes

Schwann cells

Microglia

Glial stem cells

Question 44

OLIGODENDROCYTE

Answer 44

Myelinates axons within the CNS.

Each oligodendrocyte may wrap more than 1 axon.

Question 45

Nodes of Ranvier

Answer 45

Gap between myelin wraps

Question 46

SCHWANN CELL

Answer 46

Myelinates axons in the periphery.

Myelin sheaths, sheaths of unmyelinated axons, satellite cells

1 Schwann cell forms a single wrap on one axon.

Most PNS glial cells are variants of the Schwann cell. Some Schwann cells are flattened out as satellite cells that surround the neuronal cell bodies in PNS ganglia. Most, however, envelop axons as they travel through peripheral nerves.

Schwann cells have been implicated in several other functions, including facilitating the regrowth of axons after peripheral nerve injury, helping to regulate extracellular ionic concentrations around neurons and their processes, and collaborating with neurons in some developmental and metabolic processes.

Question 47

Anterograde Transport

Answer 47

– Amino acids.

Inject radioactive amino acid into a region and neurons take it up and transport it to their axon terminals where it can be visualized.

Question 48

Retrograde Transport

Answer 48

– Horseradish peroxidase, microspheres

Inject tracer into area of axon terminals. Terminals take it up and transport it retrogradely to their cells of origin where it can be visualized.

Question 49

Cellular And Molecular Approaches For Studying Connectivity And Molecular Identity Of Nerve Cells:
Antibodies

Answer 49

Antibodies to specific proteins/neurotransmitters can be used to identify populations of neurons.

Question 50

Cellular And Molecular Approaches For Studying Connectivity And Molecular Identity Of Nerve Cells:
Genetic Probes

Answer 50

Genetic Probes can be used to identify specific populations of neurons. Example: a section of the cerebellum can be labeled with a probe for a specific gene that is only expressed in one type of neuron in the cerebellum.

Question 51

Physiological Recording

Answer 51

Stimulate in one area and record in another.

Example: skin is touched and recording is made from primary sensory cortex to determine if the region in which the recording electrode is placed receives input from the stimulated part of the forearm.

Question 52

Physiological Techniques for Studying Connectivity Between Nerve Cells

Answer 52

Reporter gene that codes for visualizable substance (e.g., GFP) is inserted in a genome and is under the control of a cell type-specific promoter (DNA sequence that turns gene “on”). Reporter is only activated in specific cell types that express the gene. Here Dorsal Root Ganglion cells are activated and the distribution of their processes in the periphery and in the CNS are apparent.

Question 53

Cerebrum

Answer 53

Composed of the 2 cerebral hemispheres and the diencephalon.

Question 54

Longitudinal Fissure

Answer 54

Divides the 2 hemispheres of the Cerebrum.

Question 55

Does the brainstem include the cerebellum?

Answer 55

NO!

Question 56

Multipolar

Answer 56

Describes a neuron having more than 2 dendrites and/or axons

Question 57

Bipolar

Answer 57

Describes a neuron having just 2 dendrites and/or axons

Question 58

Dendrites

Answer 58

The tapered extensions of the cell body that collect info from other neurons.

Major organelles: cytoskeleton, mitochondria

provide a great increase in the surface area available for synaptic inputs

The principal but not the only site of synaptic inputs. The dendrites of many neurons have their own extensions, dendritic spines, that may be favored sites for modifying the strength of synapses.

Question 59

Soma

Answer 59

Cell body.
Synthesizes macromolecules and integrates electrical signals.

Major organelles: Nucleus, Golgi apparatus, Nissle substance, cytoskeleton, mitochondria

Question 60

Axon

Answer 60

Conducts info to other neurons.
Single, cylindrical.
May be many centimeters long.
May be myelinated or unmylinated.

Major organelles: mitochondria, transport vesicles, cytoskeleton,

The single, cylindrical appendage used by most neurons to conduct action potentials away from the soma and toward axon terminals that make synaptic endings on other neurons.

Question 61

Axon Terminals

a.k.a., synaptic endings

Answer 61

Vesicle-filled. Close to the parts of other neurons.

Most are axodendritic or axosomatic.

Major organelles: mitochondria, synaptic vesicles

Question 62

Sensory Neurons

Answer 62

Are directly sensitive to various stimuli (like touch or temperature change), or receive direct connections from nonneuronal receptor cells.

unipolar

Sensory axons are located in the dorsal root ganglia of spinal nerves, and enter the spinal cord and divide into a large number of branches, most of which terminate on neuronal processes in the spinal gray matter.

Question 63

Motor Neuron

Answer 63

End directly on muscles, glands, or other neurons in PNS ganglia.

multipolar

Motor axons are located in the spinal gray matter, leave the spinal cord and enter spinal nerves.

Question 64

Interneuron

Answer 64

In the broadest sense of the term, neurons that are located entirely within the CNS and interconnect other neurons. However, the term is usually used to refer to small local-circuit neurons.

Question 65

projection neuron

Answer 65

Have long axons that connect different areas (e.g., corticospinal neurons).

Question 66

Golgi Stain

Answer 66

The Golgi stain reveals the shapes of the arborizations of cortical neurons by completely staining a small percentage of them.

For many years, the major technique available for studying the shapes & sizes of neurons was Golgi staining, a method that infiltrates all the processes of a small percentage of neurons with heavy metals, causing them to stand out from an unstained or counterstained background.

One drawback of Golgi staining is that it stains a subset of neurons indiscriminately, revealing relatively little about the function of an individual cell.

Question 67

Astrocytes

Answer 67

Greek for “star cell”

2nd major category of CNS glial cells.

Adult astrocytes fall into 2 broad classes—protoplasmic astrocytes, found in gray matter, and fibrous astrocytes, found in white matter. (A third type of astrocyte, called radial glia, is present during development and forms a scaffolding that helps guide growing axons.) Despite their somewhat different appearances, protoplasmic and fibrous astrocytes have basically similar characteristics. Astrocytes have a well-developed cytoskeleton that is dominated by intermediate filaments but also includes microtubules and actin filaments, consistent with a role as structural support elements in the CNS. In addition, some astrocyte processes have enlarged end-feet that are applied either to the surface of CNS capillaries or to the surface of the CNS itself; other processes abut neurons, dendrites, synaptic endings, and nodes of Ranvier. This carpeting of otherwise exposed surfaces with astrocyte processes underlies the role of these cells in the regulation of extracellular ionic concentrations and in the transfer of metabolites to and from neurons. Finally, astrocytes are a major part of the limited armamentarium available to the CNS in responding to injury; they hypertrophy, increase their production of intermediate filaments, and form dense, gliotic scars.

Question 68

Gray Matter

Answer 68

Area that’s dense with cell bodies and dendrites.

Is actually pinkish-gray because of the rich blood supply.

Gray matter contains motor neurons, the endings of incoming sensory axons and long descending tracts, local interneurons, and projection neurons whose axons enter long ascending tracts.

• Nuclei are specialized areas of gray matter.
• A cortex is made of gray matter (e.g., cerebral and cerebellar cortices)

Question 69

cortex

Answer 69

A cortex is made of gray matter (e.g., cerebral and cerebellar cortices).

It’s an area where gray matter forms a layered surface that covers some part of the CNS.

Question 70

Names for white matter

Answer 70

tract, fasciculus, funiculus, lemniscus, peduncle.

These are mostly descriptive terms that also make sense. Fasciculus and funiculus mean “little bundle” and “string,” respectively. Lemniscus means “ribbon” and is used for tracts that are flattened out in cross section. Peduncle means “little foot” and is used for a site where axons funnel down into a compact bundle.

Question 71

White matter

Answer 71

Refers to areas where there is a predominance of axons.

Many axons have a myelin sheath that is mostly lipid and has a fatty, white appearance.

White matter contains long descending tracts (from the brainstem and cerebrum), long ascending tracts (to the brainstem, cerebellum, and cerebrum), and local axons interconnecting different spinal levels.

Question 72

Tract Naming

Answer 72

Many tracts have two-part names that provide information about the nature of the tract.

The first part of the name refers to the location of the neuronal cell bodies from which these axons originate, and the second part refers to the site where they terminate.

A spinocerebellar tract is a collection of axons with cell bodies in the spinal cord and synaptic endings in the cerebellum.

Question 73

Ribosomes

Answer 73

stud the surface of the rough endoplasmic reticulum and float freely in the cytoplasm between the cisternae.

are stained intensely by basic dyes, appearing by light microscopy as clumps called Nissl bodies or Nissl substance

Question 74

Nissl bodies

Answer 74

Clumps of rough endoplasmic reticulum that are prominent in the cell bodies and proximal dendrites of neurons.

Question 75

long descending tracts

Answer 75

from the brainstem and cerebrum

Question 76

long ascending tracts

Answer 76

to the brainstem, cerebellum, and cerebrum

Question 77

cytoskeleton

Answer 77

Organelles are embedded in a network of filamentous protein polymers that extend throughout the neuron and its processes, collectively constituting the neuronal cytoskeleton

Question 78

Microtubules

Answer 78

Cylindrical assembly of 13 strands of tubulin polymers. Microtubules are major cytoskeletal components, and serve as the “railroad tracks” for fast axonal transport.

Cylindrical assemblies, about 25 nm in diameter, of 13 strands (protofilaments) of protein arranged around a hollow core.

Each protofilament is a polymer of the protein tubulin; an assortment of additional proteins associated with the microtubules links them to one another, to other cytoskeletal elements, and to various organelles as they travel toward or away from the cell body.

Question 79

Neurofilaments

Answer 79

A ropelike assembly of protein polymers; one of three major cytoskeletal components. Neurofilaments aggregate into neurofibrils after some preparation methods, and can be stained and seen microscopically.

the neuron’s version of the intermediate filaments found in most cells, are multiply twisted, assemblies of strands of polymers involving at least 3 different proteins from the cytokeratin family. Neurofilaments are about 10 nm in diameter, too small to be seen under the light microscope, but they aggregate in response to certain chemical fixatives. When silver stains are applied, such aggregates can be visualized as neurofibrils

Question 80

microfilaments

Answer 80

A twisted pair of actin filaments, forming the thinnest of the 3 major types of cytoskeletal elements.

microfilaments, (7 nm), are twisted pairs of actin filaments. All 3 kinds of cytoskeletal elements contribute to maintaining the shape of the neuron. Microtubules also serve as the substrate along which organelles are transported through neuronal processes. Microfilaments are important for anchoring membrane molecules in place (e.g., receptor molecules at synapses), for shuttling things to and from the cell membrane, and for movement of the advancing tip of growing axons.

Question 81

dendritic tree

Answer 81

the total array of dendrites,

it can have an elaborate structure

Question 82

axon hillock

Answer 82

The axon arises abruptly from an axon hillock on one side of the neuronal cell body or one of its proximal dendrites. Bundles of microtubules, accompanied by neurofilaments and mitochondria, funnel through the axon hillock into the initial segment of the axon

Question 83

initial segment

Answer 83

typically the most electrically excitable part of the neuron

part of the axon

Question 84

axonal transport

Answer 84

active process needed for normal function.

A variety of substances ranging from “used” organelles to intracellular chemical messengers need to be transported from synaptic endings back to the soma.

Microtubules serve as the “railroad tracks” for fast transport.

Some things move preferentially in the anterograde direction, others in the retrograde direction. This is made possible by the longitudinal polarity of microtubules: tubulin is a structurally polarized molecule and can be added only in one orientation to one end (called the plus end) of an existing microtubule. Axonal microtubules are oriented with their plus ends pointing away from the soma. 2 ATPases associated with microtubules serve as the motors for fast transport. Kinesin & Dynein moves some components in the retrograde direction (Fig. 1-17).

Question 85

anterograde

Answer 85

Transport away from the soma

Question 86

retrograde

Answer 86

transport toward the soma

Question 87

Slow axonal transport

Answer 87

moves soluble proteins—such as cytoskeletal proteins and cytoplasmic enzymes—in the anterograde direction at rates of a few millimeters a day; the mechanism of this movement is still not understood.

Question 88

Fast axonal transport

Answer 88

moves membrane-associated substances—mitochondria, lysosomes, vesicles of neurotransmitter precursors, and membrane components—at rates up to 400 mm a day.

Microtubules serve as the “railroad tracks” for fast transport.

Question 89

Kinesin

Answer 89

One of 2 ATPases associated with microtubules that serves as the motors for fast transport.

Kinesin bridges between microtubules and some membrane-associated cell components and moves them toward the plus end of the microtubule (i.e., in the anterograde direction).

Question 90

Dynein

Answer 90

One of 2 ATPases associated with microtubules that serves as the motors for fast transport.

Dynein moves some components in the retrograde direction

Question 91

tracer use

Answer 91

Neuroanatomical techniques take advantage of axonal transport to map out the connections between neurons .

Appropriate tracer substances injected into/near known neuronal cell bodies are transported anterogradely, revealing the locations of the neurons’ synaptic terminals.

Conversely, tracers injected near synaptic endings are taken up by the endings and transported retrogradely to the cell bodies (a method used covertly by some viruses, such as herpes, to gain access to the nervous system).

Question 92

Degeneration techniques

Answer 92

introduced in the 1800’s and are based on the reactions of neurons to injury. If an axon is severed, its formerly attached cell body undergoes a characteristic series of cytological changes (chromatolysis). Therefore, examining brain sections for chromatolytic cells can reveal the locations of the cell bodies of origin of the severed axons. While the cell body undergoes chromatolysis, the portion of the axon distal to the cut degenerates (Wallerian degeneration). The same distal changes occur if the damage is inflicted at the ultimate proximal location (i.e., if the cell body is destroyed). Special staining methods can be used to selectively stain degenerating axons or their synaptic terminals. Therefore, if a particular nucleus is destroyed, the path of axons originating there and the sites of their termination can be determined.

Although a great deal of information was gained over the years with the aid of degeneration techniques, their use is not without pitfalls. It is technically difficult, and sometimes impossible, to completely destroy a particular structure without also damaging nearby structures. In addition, because the segregation of gray and white matter is not absolute, axons passing through a given nucleus can be destroyed along with the cell bodies forming the nucleus. For these and other reasons, techniques that take advantage of axonal transport proved to be a great advance. Early methods of this type used radioactive substances (usually tritiated amino acids) that were introduced into an area of gray matter, taken up by the resident neurons, incorporated into macromolecules, and transported down the axons of these neurons. Eventually the synaptic terminals of these axons become radioactive. Subsequent methods have used the introduction of a marker substance (often a protein) into selected areas of gray matter, where it encounters synaptic terminals. The terminals take up the protein and transport it back to the parent neurons. A protein commonly used in such experiments is an enzyme called horseradish peroxidase, which can be detected with great sensitivity and resolution by appropriate histochemical procedures. Although this technique is typically used for retrograde transport studies, it can be used simultaneously for anterograde transport studies, labeling not only the neurons that project to a given area of gray matter but also the targets of axons that leave it. Certain fluorescent dyes can also be used in retrograde transport studies. By injecting two different dyes at two different sites in the nervous system, it is possible to determine whether any neurons have branching axons that project to both sites.

Question 93

Wallerian degeneration

Answer 93

Degeneration and removal of an axon and any associated myelin distal to a point of transection.

Question 94

chromatolysis

Answer 94

Swelling and apparent loss of Nissl substance of a neuronal cell body in response to axonal damage.

Question 95

Synaptic Contacts Mediate Information Transfer between Neurons

Answer 95

An enlargement (the presynaptic element) of a distal axonal branch abuts part of another neuron (the postsynaptic element), separated from it by a synaptic cleft 10-20 nm across. The presynaptic ending contains membrane-bound packets (synaptic vesicles) of neurotransmitter molecules; some vesicles release their contents into the synaptic cleft in response to electrical activity. The neurotransmitter diffuses across the synaptic cleft, binds to receptor molecules in the postsynaptic membrane, and causes an electrical signal in the postsynaptic neuron.

Most synapses have an axonal ending as the presynaptic element and part of a dendrite as the postsynaptic element, but in fact, any part of a neuron can be presynaptic to any part of another neuron (or sometimes even to itself).

Question 96

internode

Answer 96

The myelin between 2 nodes, formed by a single Schwann cell.

Adjacent internodes form the projections that cover the node between them. Internodes range in length from 0.2 - 2 mm, with larger-diameter axons having longer internodes and thicker myelin sheaths. This arrangement is part of what allows larger axons to conduct electrical signals more rapidly.

Question 97

unmyelinated axons

Answer 97

simply embedded in individual Schwann cells.

Most of the smaller axons in peripheral nerves do not acquire myelin sheaths. This lack of myelin, together with their small diameter, leads to relatively slow conduction of electrical signals by unmyelinated axons.

Question 98

Glia

Answer 98

Greek for “glue”

In contrast to the PNS, there are multiple kinds of glial cells in the CNS.

Question 99

Neurons, dendrites, synapses

Answer 99

Collect, integrate, transmit information; synthesize macromolecules

Located Gray matter

Question 100

Axons

Answer 100

Conduct information

Located in White matter

Question 101

Oligodendrocytes

Answer 101

Form myelin sheaths

Found in White (and gray) matter

Question 102

Protoplasmic astrocytes

Answer 102

Provide mechanical and metabolic support, response to injury

Found in Gray matter

Question 103

Fibrous astrocytes

Answer 103

Provide mechanical and metabolic support, response to injury

Located in White matter

Question 104

Microglia

Answer 104

Phagocytosis, response to injury

Found in Gray (and white) matter

Question 105

Ependymal cells

Answer 105

Line ventricles and choroid plexus, secrete cerebrospinal fluid

Walls of ventricles

Question 106

oligodendrocytes

Answer 106

Many CNS axons are wrapped in myelin sheaths that are fundamentally similar to those of PNS axons, except that in the CNS the sheaths are formed by a different population of glial cells called oligodendrocytes. As in the periphery, larger axons have thicker myelin and longer internodes. However, as the name implies (oligodendro- is Greek for “tree with a few branches”), individual oligodendrocytes produce internodes on multiple axons. A single oligodendrocyte may have dozens of branches, each ending as an internode. Unlike Schwann cells, oligodendrocyte processes do not envelop unmyelinated CNS axons, which can be directly exposed to the extracellular environment. Given their role as myelin-producing cells, oligodendrocytes are most prominent in white matter, but they are also found in gray matter. There they provide myelin sheaths for axons traversing the gray matter and also participate (along with other glial cell types) in surrounding neurons and their processes in a manner analogous to satellite cells in the PNS.

Question 107

radial glia

Answer 107

A third type of astrocyte that is present during development and forms a scaffolding that helps guide growing axons.

Question 108

Ependymal Cells

Answer 108

The CNS develops embryologically from a neuroepithelial tube. The cavity of the tube persists in the adult CNS as a system of ventricles with an epithelial lining of ependymal cells. In some locations the ependymal cells are specialized as a secretory epithelium that produces the cerebrospinal fluid (CSF) that fills the ventricles and bathes the CNS.

Question 109

Microglial Cells

Answer 109

Microglial cells, as their name implies, are smaller than oligodendrocytes and astrocytes (which together are sometimes called macroglia). Microglia seem not to be involved in the minute-to-minute metabolism and electrical signaling of the nervous system. Instead, they play the other major role in the nervous system’s response to injury. In the normal, healthy nervous system they use their numerous processes to sweep through extracellular spaces looking for damage or disease. When they find something, they proliferate, migrate to the affected site, transform into macrophages,* and devour pathogens and neuronal debris.