Neurons and Glia

Question 1

What are neurons and glia?

Answer 1

The different types of cells in the nervous system; These are broad categories, within which are many types of cells that differ in structure, chemistry, and function. Nonetheless, the distinction between neurons and glia is important. Although there are approximately equal numbers of neurons and glia in the adult human brain (roughly 85 billion of each type), neurons are responsible for most of the unique functions of the brain. It is the neurons that sense changes in the environment, communicate these changes to other neurons, and command the body’s responses to these sensations. Glia, or glial cells, contribute to brain function mainly by insulating, supporting, and nourishing neighbouring neurons.

Question 2

To study the structure of brain cells, scientists have had to overcome several obstacles. Name 3 of these

Answer 2

the small size; Most cells are in the range of 0.01–0.05 mm in diameter ( could not progress before the development of the compound microscope in the late seventeenth century)

To observe brain tissue using a microscope, it was necessary to make very thin slices, ideally not much thicker than the diameter of the cells. However, brain tissue has a con- sistency like a bowl of Jell-O: not firm enough to make thin slices.

Freshly prepared brain tissue has a uniform, cream-colored appearance under the microscope, with no differences in pigmentation to enable histologists to resolve individual cells.

Question 3

How did scientists overcome the thin slice brain tissue problem?

Answer 3

Early in the nineteenth century, scientists discovered how to harden, or “fix,” tissues by immersing them in formalde- hyde, and they developed a special device called a microtome to make very thin slices.

Question 4

What field did these technological advances spawn?

Answer 4

the field of histology, the microscopic study of the structure of tissues.

Question 5

What breakthrough solved the monochrome problem?

Answer 5

The final breakthrough in neurohistology was the introduction of stains that selectively color some, but not all, parts of the cells in brain tissue.

Question 6

Give an example of one such stain still used today

Answer 6

Nissl showed that a class of basic dyes would stain the nuclei of all cells as well as clumps of material surrounding the nuclei of neurons (Figure 2.1). These clumps are called Nissl bodies, and the stain is known as the Nissl stain.

Question 7

Give two reasons why the Nissl stains are useful

Answer 7

The Nissl stain is extremely useful for two reasons: It distinguishes between neurons and glia, and it enables histologists to study the arrangement, or cytoarchitecture, of neurons in different parts of the brain.

Question 8

The Nissl stain, however, could not tell the whole story. A Nissl-stained neuron looks like little more than a lump of protoplasm containing a nucleus. What stain filled in these gaps and how?

Answer 8

Golgi discovered that soaking brain tissue in a silver chromate solution, now called the Golgi stain, makes a small percentage of neurons become darkly colored in their entirety. This revealed that the neuronal cell body, the region of the neuron around the nucleus that is shown with the Nissl stain, is actually only a small fraction of the total structure of the neuron.

Question 9

What names are given to the larger, swollen part of the cell?

Answer 9

cell body, soma (plural: somata), and perikaryon (plural: perikarya).

Question 10

What are axons and dendrites types of?

Answer 10

The thin tubes that radiate from the soma are called neurites

Question 11

Golgi invented the stain, but a Spanish contemporary (Santiago Ramón y Cajal) used it to greatest effect. How did he do this?

Answer 11

Cajal used the Golgi stain to work out the circuitry of many regions of the brain.

Question 12

Curiously, Golgi and Cajal drew completely opposite conclusions about neurons. How so?

Answer 12

Golgi championed the view that the neurites of different cells are fused together to form a continuous reticulum, or network, similar to the arteries and veins of the circulatory system. According to this reticular theory, the brain is an exception to the cell theory, which states that the individual cell is the elementary functional unit of all animal tissues.

Cajal, on the other hand, argued forcefully that the neurites of different neurons are not continuous with each other and communicate by contact, not continuity.

Question 13

What is this idea that cell theory also applies to neurons known as? Is it or Golgi’s theory better supported?

Answer 13

Neuron doctrine.

With the increased resolving power of the electron microscope, it was finally possible to show that the neurites of different neurons are not continuous with one another. (so yes lol)

Question 14

Explain why the light microscope was required to study neurons and what limits it had

Answer 14

The human eye can distinguish two points only if they are separated by more than about one-tenth of a millimeter (100 um). Thus, we can say that 100 um is near the limit of resolution for the unaided eye. Neurons have a diameter of about 20 um, and neurites can be as small as a fraction of a micrometer. However with the standard light microscope, the limit of resolution is about 0.1 um. Because the space between neurons is only 0.02 um (20 nm)

Question 15

What equipment corrects for these shortcomings?

Answer 15

The electron microscope uses an electron beam instead of light to form images, dramatically increasing the resolving power. The limit of resolution for an electron microscope is about 0.1 nm—a million times better than the unaided eye and a thousand times better than a light microscope.

Question 16

Name and describe the fluid inside a neuron

Answer 16

The watery fluid inside the cell, called the cytosol, is a salty, potassium- rich solution that is separated from the outside by the neuronal membrane.

Question 17

Name the most important organelles in a cell

Answer 17

The cell body of the neuron contains the same organelles found in all animal cells. The most important ones are the nucleus, the rough endo- plasmic reticulum, the smooth endoplasmic reticulum, the Golgi apparatus, and the mitochondria.

Question 18

What is the cytoplasm?

Answer 18

Everything contained within the confines of the cell membrane, including the organelles but excluding the nucleus, is referred to collectively as the cytoplasm.

Question 19

What is the nucleus contained in?

Answer 19

It is contained within a double membrane called the nuclear envelope. The nuclear envelope is perforated by pores about 0.1 􏰆m across.

Question 20

What is within the nucleus?

Answer 20

Within the nucleus are chromosomes which contain the genetic material DNA (deoxyribonucleic acid).

Question 21

The DNA in each of your neurons is the same, and it is the same as the DNA in the cells of your liver and kidney and other organs. What distinguished a neuron from a liver cell?

Answer 21

Specific parts of the DNA that are used to assemble the cell. These segments of DNA are called genes.

Question 22

What is meant by gene expression and what does it lead to?

Answer 22

The “reading” of the DNA is known as gene expression. The final product of gene expression is the synthesis of molecules called proteins, which exist in a wide variety of shapes and sizes, perform many different functions, and bestow upon neurons virtually all of their unique characteristics.

Question 23

Where does protein synthesis occur?

Answer 23

Protein synthesis, the assembly of protein molecules, occurs in the cytoplasm.

Question 24

Describe the first step in protein synthesis

Answer 24

Because the DNA never leaves the nucleus, an intermediary must carry the genetic message to the sites of protein synthesis in the cytoplasm. This function is performed by another long molecule called messenger ribonucleic acid, or mRNA. mRNA consists of four different nucleic acids strung together in various sequences to form a chain. The detailed sequence of the nucleic acids in the chain represents the information in the gene, just as the sequence of letters gives meaning to a written word. The process of assembling a piece of mRNA that contains the information of a gene is called transcription, and the resulting mRNA is called the transcript

Question 25

Interspersed between protein-coding genes are long stretches of DNA whose functions remain poorly understood. Some of these regions, however, are known to be important for a particular function in protein synthesis. Describe this and give examples

Answer 25

Regulating transcription: At one end of the gene is the promoter, the region where the RNA-synthesizing enzyme, RNA polymerase, binds to initiate transcription. The binding of the polymerase to the promoter is tightly regulated by other proteins called transcription factors. At the other end is a sequence of DNA called the terminator, or stop sequence, that the RNA polymerase recognizes as the end point for transcription.

Question 26

In addition to the non-coding regions of DNA that flank the genes, there are often additional stretches of DNA within the gene itself that cannot be used to code for protein. What are these regions and their coding sequences called? Are they also transcribed?

Answer 26

These interspersed regions are called introns, and the coding sequences are called exons. Initial transcripts contain both introns and exons, but then, by a process called RNA splicing, the introns are removed and the remaining exons are fused together.

Question 27

Can one gene give rise to different protein products?

Explain how this can happen or why it cannot happen.

Answer 27

In some cases, specific exons are also removed with the introns, leaving an “alternatively spliced” mRNA that actually encodes a different protein. Thus, transcription of a single gene can ultimately give rise to several different mRNAs and protein products.

Question 28

How does the mRNA produce proteins?

Answer 28

mRNA transcripts emerge from the nucleus via pores in the nuclear envelope and travel to the sites of protein synthesis elsewhere in the neuron. At these sites, a protein molecule is assembled much as the mRNA molecule was: by linking together many small molecules into a chain. In the case of protein, the building blocks are amino acids, of which there are 20 different kinds. This assembling of proteins from amino acids under the direction of the mRNA is called translation.

Question 29

What is meant by the genome?

Answer 29

the entire length of DNA that comprises the genetic information in our chromosomes

Question 30

Give two reasons why a new understanding of genes is now possible

Answer 30

because the human genome has been sequenced. We now know the 25,000 “words” that comprise our genome, and we know where these genes can be found on each chromosome.

Furthermore, we are learning which genes are expressed uniquely in neurons. This knowledge has paved the way to understanding the genetic basis of many diseases of the nervous system.

Question 31

How may the genetic basis of diseases work in terms of the genomes? (2)

Answer 31

In some diseases, long stretches of DNA that contain several genes are missing; in others, genes are duplicated, leading to overexpression of specific proteins

Question 32

What are these abnormalities called and when do they usually occur?

Answer 32

These sorts of mishaps, called gene copy number variations, often occur at the moment of conception when paternal and maternal DNA mix to create the genome of the offspring.

Some instances of serious psychiatric disorders, including autism and schizophrenia, were recently shown to be caused by gene copy number variations in the affected children

Question 33

Other nervous system disorders are caused by mutations, how does this happen?

Answer 33

“typographical errors”—in a gene or in the flanking regions of DNA that regulate the gene’s expression. In some cases, a single protein may be grossly abnormal or missing entirely, disrupting neuronal function.

An example is fragile X syndrome, a disorder that manifests as intellectual disability and autism and is caused by disruption of a single gene

Question 34

How problematic are most mutations?

Answer 34

Many of our genes carry small mutations, called single nucleotide polymorphisms, which are analogous to a minor misspelling caused by a change in a single letter. These are usually benign, like the difference between “color” and “colour”—different spelling, same meaning. However, sometimes the mutations can affect protein function (consider the difference between “bear” and “bare”—same letters, different meaning). Such single nucleotide polymorphisms, alone or together with others, can affect neuronal function.

Question 35

Why are genes relevant to neuroscience and name an breakthrough for studying this field

Answer 35

Genes make the brain, and understanding how they contribute to neuronal function in both healthy and diseased organisms is a major goal of neuroscience. An important breakthrough was the development of tools for genetic engineering—ways to change organisms by design with gene mutations or insertions.

Question 36

Why has genetic engineering been used most in mice?

Answer 36

Because they are rapidly reproducing mammals with a central nervous system similar to our own.

Question 37

What are knockout mice?

Answer 37

One gene has been deleted (or “knocked out”). Such mice can be used to study the progression of a disease, like fragile X, with the goal of correcting it

Question 38

Name and describe two other approaches in research with mice to investigate the expression of certain genes

Answer 38

Another approach has been to generate transgenic mice, in which genes have been introduced and overexpressed; these new genes are called transgenes.

Knock-in mice have also been created in which the native gene is replaced with a modified transgene.

Question 39

How may neuroscientist use the information present by the human genome project to further explore long-standing questions about the biological basis of neurological and psychiatric disorders as well as to probe deeper into the origins of individuality?

Answer 39

The brain is a product of the genes expressed in it. Differences in gene expression between a normal brain and a diseased brain, or a brain of unusual ability, can be used to identify the molecular basis of the observed symptoms or traits.

Question 40

How is the level of gene expression usually defined? What implication does this have for the methodology?

Answer 40

The level of gene expression is usually defined as the number of mRNA transcripts synthesized by different cells and tissues to direct the synthesis of specific proteins. Thus, the analysis of gene expression requires comparing the relative abundance of various mRNAs in the brains of two groups of humans or animals.

Question 41

How may such a comparison between two groups be carried out? Describe one method

Answer 41

One way to perform such a comparison is to use DNA microarrays, which are created by robotic machines that arrange thousands of small spots of synthetic DNA on a microscope slide. Each spot contains a unique DNA sequence that will recognize and stick to a different specific mRNA sequence.

To compare the gene expression in two brains, one begins by collecting a sample of mRNAs from each brain. The mRNA of one brain is labeled with a chemical tag that fluoresces green, and the mRNA of the other brain is labeled with a tag that fluoresces red. These samples are then applied to the microarray. Highly expressed genes will produce brightly fluorescent spots, and differences in the relative gene expression between the brains will be revealed by differences in the color of the fluorescence

Question 42

To achieve high efficiency of expression of the exogenous DNA in a recipient cell, what did Mario Capecchi have to do?

Answer 42

He had to attach small fragments of viral DNA, which we now understand to contain enhancers that are critical in eukaryotic gene expression.

Question 43

What did he observe when many copies of a gene were injected into a cell nucleus? What does this demonstrate?

Answer 43

All of the molecules ended up in an ordered head-to-tail arrangement, called a concatemer (Figure B). This was astonishing and could not have occurred as a random event. He went on to unequivocally prove that homologous recombination, the process by which chromosomes share genetic information during cell division, was responsible for the incorporation of the foreign DNA

These experiments demonstrated that all mammalian somatic cells contain a very efficient machinery for swapping segments of DNA that have similar sequences of nucleotides. Injection of a thousand copies of a gene sequence into the nucleus of a cell resulted in chromosomal insertion of a concatemer containing a thousand copies of that sequence, all oriented in the same direction.

Question 44

Give steps of achievement between these experiments involving injecting a DNA sequence into the nucleus of an egg and

Answer 44

1. Carried out research attempting to directly alter gene DNA sequences in mammalian cultured cells by homologous recombination.
2. Obtained results supporting the ability to do gene targeting in cultured mammalian cells(Despite arguments that the probability of the exogenously added DNA sequence ever finding the DNA sequence similar enough to enable homologous recombination in living mammalian cells was vanishingly small.)
3. Because the frequency of gene targeting was low, if we were to be successful in transferring our technology to mice, we needed mouse embryonic stem cells capable of contributing to the formation of the germ line—the sperm and eggs—in mature animals.
4. Martin Evans from Cambridge started isolating EK cells (rather than embryonal carcinoma
   (EC)), that resembled EC cells but were derived from a normal mouse embryo rather than from tumors.
5. Both learned how to maintain these marvelous cells and use them to generate mice capable of germ line transmission.

Question 45

What are microglia and what happens when you mutate them?

Answer 45

Cells that migrate into the brain after being generated in the bone marrow along with immune and blood cells. Mutating these cells in mice results in a pathology remarkably similar to the human condition called trichotillomania, a type of obsessive- compulsive disorder characterized by strong urges to pull out one’s hair.

Amazingly, transplanting normal bone marrow into mutant mice permanently cures them of this pathological behavior. Now, we are deeply immersed in trying to understand the mechanism of how microglia control neural circuit output and, more importantly, exploring the intimate relationship between the immune system (in this case microglia) and neuropsychiatric disorders such as depression, autism, schizophrenia, and
Alzheimer’s disease.

Question 46

Where exactly does protein synthesis occur and what role does this area play in this process?

Answer 46

Protein synthesis occurs at dense globular structures in the cytoplasm called ribosomes. mRNA transcripts bind to the ribosomes, and the ribosomes translate the instructions contained in the mRNA to assemble a protein molecule. In other words, ribosomes use the blueprint provided by the mRNA to manufacture proteins from raw material in the form of amino acids.

Question 47

In neurons, many ribosomes are attached to stacks of membrane. What are these called and what function do they carry out?

Answer 47

In neurons, many ribosomes are attached to stacks of membrane called rough endoplasmic reticulum, or rough ER (seen as Nissl bodies). Rough ER abounds in neurons, far more than in glia or most other non-neuronal cells. Rough ER is a major site of protein synthesis in neurons. As the protein is being assembled, it is threaded back and forth through the membrane of the rough ER, where it is trapped (see notes for diagram)

Question 48

But not all ribosomes are attached to rough ER. What are these called and what is the difference between proteins synthesized on the rough ER and those synthesized on the unattached ribosomes?

Answer 48

Many are freely floating and are called free ribosomes. Several free ribosomes may appear to be attached by a thread; these are called polyribosomes. The thread is a single strand of mRNA, and the associated ribosomes are working on it to make multiple copies of the same protein.

The difference between proteins synthesized on the rough ER and those synthesized on the free ribosomes depends on the intended fate of the protein molecule. If it is destined to reside within the cytosol of the neuron, then the protein’s mRNA transcript shuns the ribosomes of the rough ER and gravitates toward the free ribosomes. However, if the protein is destined to be inserted into the membrane of the cell or an organelle, then it is synthesized on the rough ER.

Question 49

Why does the smooth ER have its name and what function is it thought to carry out?

Answer 49

The remainder of the cytosol of the soma is crowded with stacks of membranous organelles that look a lot like rough ER without the ribosomes, so much so that one type is called smooth endoplasmic reticulum, or smooth ER.

Smooth ER is heterogeneous and performs different functions in different locations. Some smooth ER is continuous with rough ER and is believed to be a site where the proteins that jut out from the membrane are carefully folded, giving them their three-dimensional structure. Other types of smooth ER play no direct role in the processing of protein molecules but instead regulate the internal concentrations of substances such as calcium.

Question 50

What is the golgi apparatus? Name a possible function of it

Answer 50

The stack of membrane-enclosed disks in the soma that lies farthest from the nucleus is the Golgi apparatus. This is a site of extensive “post-translational” chemical processing of proteins. One important function of the Golgi apparatus is believed to be the sorting of certain proteins that are destined for delivery to different parts of the neuron, such as the axon and the dendrites.

Question 51

Describe the structure of mitochondria in neurons

Answer 51

Within the enclosure of their outer membrane are multiple folds of inner membrane called cristae (singular: crista). Between the cristae is an inner space called matrix

Question 52

What primary function do the mitochondria have? Describe this process

Answer 52

Mitochondria are the site of cellular respiration. When a mitochondrion “inhales,” it pulls inside pyruvic acid (derived from sugars and digested proteins and fats) and oxygen, both of which are floating in the cytosol. Within the inner compartment of the mitochondrion, pyruvic acid enters into a complex series of biochemical reactions called the Krebs cycle.

Question 53

What is the product of the krebs cycle and does function does this product have?

Answer 53

The biochemical products of the Krebs cycle provide energy that, in another series of reactions within the cristae (called the electron-transport chain), results in the addition of phosphate to adenosine diphosphate (ADP), yielding adenosine triphosphate (ATP), the cell’s energy source. When the mitochondrion “exhales,” 17 ATP molecules are released for every molecule of pyruvic acid that had been taken in. ATP is the energy currency of the cell

Question 54

What function does the neuronal membrane serve? Describe its structure

Answer 54

The neuronal membrane serves as a barrier to enclose the cytoplasm inside the neuron and to exclude certain substances that float in the fluid that bathes the neuron. The membrane is about 5 nm thick and is studded with proteins. Some of the membrane-associated proteins pump substances from the inside to the outside. Others form pores that regulate which substances can gain access to the inside of the neuron.

Question 55

What is the function of the cytoskeleton and name its important parts

Answer 55

The cytoskeleton gives the neuron its characteristic shape. The “bones” of the cytoskeleton are the microtubules, microfilaments, and neurofilaments.

The cytoskeleton is not static. Elements of the cytoskeleton are dynamically regulated and are in continual motion. Your neurons are probably squirming around in your head even as you read this sentence.

Question 56

Contrast the structure of the “bones” of the cytoskeleton

Answer 56

Measuring 20 nm in diameter, microtubules are relatively large. A microtubule appears as a straight, thick-walled hollow pipe. The wall of the pipe is composed of smaller strands that are braided like rope around the hollow core.

Measuring only 5 nm in diameter, microfilaments are about the same thickness as the cell membrane. Microfilaments are braids of two thin strands that are polymers of the protein actin. Actin is one of the most abundant proteins in cells of all types, including neurons, and is believed to play a role in changing cell shape.

With a diameter of 10 nm, neurofilaments are inter- mediate in size between microtubules and microfilaments. A neurofilament consists of multiple subunits (building blocks) that are wound together into a ropelike structure. Each strand of the rope consists of individual long protein molecules, making neurofilaments mechanically very strong.

Question 57

Where are each of the “bones” of the cytoskeleton located in the cell?

Answer 57

microtubules run longitudinally down neurites.

Microfilaments are found throughout the neuron, they are particularly numerous in the neurites. In addition to running longitudinally down the core of the neurites like microtubules, microfilaments are also closely associated with the membrane. They are anchored to the membrane by attachments with a meshwork of fibrous proteins that line the inside of the membrane like a spider web.

Neurofilaments exist in all cells of the body as intermediate filaments; only in neurons are they called neurofilaments. The difference in name reflects differences in structure among different tissues. For example, a different intermediate filament, keratin, composes hair when bundled together.

Question 58

What process is involved in the construction of these “bones” of the cytoskeleton? Describe it

Answer 58

In microtubules each of the smaller strands consists of the protein tubulin. A single tubulin molecule is small and globular; the strand consists of tubulins stuck together like pearls on a string. The process of joining small proteins to form a long strand is called polymerization; the resulting strand is called a polymer. Polymerization and depolymerization of microtubules and, therefore, of neuronal shape can be regulated by various signals within the neuron.

Microfilaments are braids of two thin strands that are polymers of the protein actin. Like microtubules, actin microfilaments are constantly undergoing assembly and disassembly, and this process is regulated by signals in the neuron.

Question 59

Describe one class of proteins that participate in the regulation of microtubule assembly and function. What dysfunction are changes to these proteins associated with?

Answer 59

Microtubule-associated proteins, or MAPs: Among other functions (many of which are unknown), MAPs anchor the microtubules to one another and to other parts of the neuron. Pathological changes in an axonal MAP, called tau, have been implicated in the dementia that accompanies Alzheimer’s disease

Question 60

How does the axon differ to the soma, organelles, membrane, and cytoskeleton in its relationship to the neuron?

Answer 60

These structures are not unique to neurons but are found in all the cells in our body. The axon is a structure found only in neurons and highly specialised for the transfer of information over distances in the nervous system.

Question 61

What marks the beginning of the axon? What two other noteworthy features also distinguish the axon from the soma?

Answer 61

The axon begins with a region called the axon hillock, which tapers away from the soma to form the initial segment of the axon proper

1. No rough ER extends into the axon, and there are few, if any, free ribosomes in mature axons.
2. The protein composition of the axon membrane is fundamentally different from that of the soma membrane.

Because there are no ribosomes, there is no protein synthesis in the axon. This means that all proteins in the axon must originate in the soma.

Question 62

What are axon collaterals? Name and describe a specific type of axon collateral

Answer 62

Axons often branch, and these branches, called axon collaterals, can travel long distances to communicate with different parts of the nervous system.

Occasionally, an axon collateral returns to communicate with the same cell that gave rise to the axon or with the dendrites of neighbouring cells. These axon branches are called recurrent collaterals.

Question 63

The diameter of an axon is variable, ranging from less than 1 nm to about 25 nm in humans and to as large as 1 mm in squid.

Why is this variation in axon size important?

Answer 63

the speed of the electrical signal that sweeps down the axon—the nerve impulse— depends on the axonal diameter. The thicker the axon, the faster the impulse travels.

Question 64

What names are given to the start, middle and end of the axon?

Answer 64

All axons have a beginning (the axon hillock), a mid- dle (the axon proper), and an end. The end is called the axon terminal or terminal bouton (French for “button”), reflecting the fact that it usually appears as a swollen disk.

Question 65

Sometimes axons have many short branches at their ends, and each branch forms a synapse on dendrites or cell bodies in the same region.

What are these branches collectively called?

Answer 65

These branches are collectively called the terminal arbor

Question 66

What are boutons en passent?

Answer 66

Sometimes axons form synapses at swollen regions along their length and then continue on to terminate elsewhere. Such swellings are called boutons en passant

Question 67

In either case, when a neuron makes synaptic contact with another cell what is this called?

Answer 67

It is said to innervate that cell, or to provide innervation.

Question 68

Name 4 ways in which the cytoplasm of the axon terminal differs from that of the axon

Answer 68

1. Microtubules do not extend into the terminal.
2. The terminal contains numerous small bubbles of membrane, called
   synaptic vesicles, that measure about 50 nm in diameter.
3. The inside surface of the membrane that faces the synapse has a particularly dense covering of proteins.
4. The axon terminal cytoplasm has numerous mitochondria, indicating a high energy demand.

Question 69

What is meant by synaptic transmission?

Answer 69

The presynaptic side generally consists of an axon terminal, whereas the postsynaptic side may be a dendrite or the soma of another neuron. The space between the presynaptic and postsynaptic membranes is called the synaptic cleft. The transfer of information at the synapse from one neuron to another is called synaptic transmission.

Question 70

Describe the process of synaptic transmission

Answer 70

At most synapses, information in the form of electrical impulses traveling down the axon is converted in the terminal into a chemical signal that crosses the synaptic cleft. On the postsynaptic membrane, this chemical signal is converted again into an electrical one. The chemical signal, called a neurotransmitter, is stored in and released from the synaptic vesicles within the terminal. As we will see, different neurotransmitters are used by different types of neurons.

Question 71

What is meant by Wallerian degeneration? Why is this useful?

Answer 71

Because ribosomes are the protein factories of the cell, their absence means that the proteins of the axon must be synthesised in the soma and then shipped down the axon. Indeed, in the mid-nineteenth century, English physiologist Augustus Waller showed that axons cannot be sustained when separated from their parent cell body. The degeneration of axons that occurs when they are cut is now called Wallerian degeneration. Because it can be detected with certain staining methods, Wallerian degeneration is one way to trace axonal connections in the brain.

Question 72

Wallerian degeneration occurs because the normal flow of materials from the soma to the axon terminal is interrupted. What is this movement of material called?

Answer 72

Axoplasmic transport

Question 73

Describe how axoplasmic transport was first discovored and why it doesn’t tell the whole story

Answer 73

Researchers found that if they tied a thread around an axon, material accumulated on the side of the knot closest to the soma. When the knot was untied, the accumulated material continued down the axon at a rate of 1–10 mm per day. However if all material moved down the axon by this transport mechanism alone, it would not reach the ends of the longest axons for at least half a year.

New methods involved injecting the somata of neurons with radioactive amino acids. Recall that amino acids are the building blocks of proteins. The “hot” amino acids were assembled into proteins, and the arrival of radioactive proteins in the axon terminal was timed to calculate the rate of transport. This fast axoplasmic transport (so named to distinguish it from slow axoplasmic transport described by Weiss) occurred at a rate as high as 1,000 mm per day.

Question 74

Explain how fast axoplasmic transport works

Answer 74

Material is enclosed within vesicles, which then “walk down” the microtubules of the axon. The “legs” are provided by a protein called kinesin, and the process is fueled by ATP. Kinesin moves material only from the soma to the terminal. All movement of material in this direction is called anterograde transport.

In addition to anterograde transport, there is a mechanism for the movement of material up the axon from the terminal to the soma. This process is believed to provide signals to the soma about changes in the metabolic needs of the axon terminal. Movement in this direction, from terminal to soma, is called retrograde transport. The molecular mechanism is similar to anterograde transport, except the “legs” for retrograde transport are provided by a different protein, dynein.

Question 75

What is meant by a dendritic tree?

Answer 75

The dendrites of a single neuron are collectively called a dendritic tree; each branch of the tree is called a dendritic branch.

Question 76

What are dendritic spines and what is their purpose believed to be?

Answer 76

The dendrites of some neurons are covered with specialized structures called dendritic spines that receive some types of synaptic input. Spines look like little punching bags that hang off the dendrite. They are believed to isolate various chemical reactions that are triggered by some types of synaptic activation. Spine structure is sensitive to the type and amount of synaptic activity. Unusual changes in spines have been shown to occur in the brains of individuals with cognitive impairments

Question 77

How does the cytoplasm of dendrites compare to that of axons?

Answer 77

For the most part, the cytoplasm of dendrites resembles that of axons. It is filled with cytoskeletal elements and mitochondria. One interesting difference is that polyribosomes can be observed in dendrites, often right under spines

Question 78

How may neurons be classified by their neuronal structure? (4)

Answer 78

Number of Neurites: total number of neurites (axons and dendrites) that extend from the soma

Dendrites: Dendritic trees can vary widely from one type of neuron to another. Some have inspired names with flourish, like “double bouquet cells” or “chandelier cells.” Others have more utilitarian names, such as “alpha cells.” Classification is often unique to a particular part of the brain.

Connections

Axon length

Question 79

Name 3 different types of neurons under the number of neurites classification

Answer 79

A neuron with a single neurite is said to be unipolar. If there are two neurites, the cell is bipolar, and if there are three or more, the cell is multipolar. Most neurons in the brain are multipolar.

Question 80

Name 2 broad classifications of neurons according to the dendrite classification in the cerebral cortex

Answer 80

stellate cells (star shaped) and pyramidal cells (pyramid shaped)

Question 81

How else may neurons be classified under the dendrite classification? How do they interact with stellate cells and pyramidal cells classification?

Answer 81

whether their dendrites have spines. Those that do are called spiny, and those that do not are called aspinous. These dendritic classification schemes can overlap. For example, in the cerebral cortex, all pyramidal cells are spiny. Stellate cells, on the other hand, can be either spiny or aspinous.

Question 82

Name three types of neuron according to the connections classification

Answer 82

Information is delivered to the nervous system by neurons that have neurites in the sensory surfaces of the body, such as the skin and the retina of the eye. Cells with these connections are called primary sensory neurons.

Other neurons have axons that form synapses with the muscles and command movements; these are called motor neurons.

But most neurons in the nervous system form connections only with other neurons. In this classification scheme, these cells are called interneurons.

Question 83

Name two types of neuron according to the axon length classification. How do they interact with stellate cells and pyramidal cells classification?

Answer 83

Some neurons have long axons that extend from one part of the brain to the other; these are called Golgi type I neurons, or projection neurons. Other neurons have short axons that do not extend beyond the vicinity of the cell body; these are called Golgi type II neurons, or local circuit neurons.

In the cerebral cortex, for example, pyramidal cells usually have long axons that extend to other parts of the brain and are therefore Golgi type I neurons. In contrast, stellate cells have axons that never extend beyond the cerebral cortex and are therefore Golgi type II neurons.

Question 84

We now understand that most differences between neurons ultimately can be explained at which level? Give an example of this and how it is used in a practical sense

Answer 84

The genetic level. For example, differences in gene expression cause pyramidal cells and stellate cells to develop different shapes.

Once a genetic difference is known, that information can be used to create transgenic mice that allow detailed investigation of neurons in this class

Question 85

Describe a common method of how transgenic mice can be used to investigate neurons

Answer 85

A foreign gene encoding a fluorescent protein can be introduced and placed under the control of a cell type–specific gene promoter. Green fluorescent protein (usually simply abbreviated as GFP), encoded by a gene discovered in jellyfish, is used commonly in neuroscience research.

When illuminated with the appropriate wavelength of light, the GFP fluoresces bright green, allowing visualisation of the neuron in which it is expressed. Genetic engineering methods are now commonly used for measuring and manipulating the functions of neurons in different categories

Question 86

What is referred to as the ‘sleeping giants of neuroscience’ and why?

Answer 86

Some neuroscientists consider glia the “sleeping giants” of neuroscience. Indeed, we continue to learn that glia contribute much more importantly to information processing in the brain than has been historically appreciated.

Glia contribute to brain function mainly by supporting neuronal functions. Although their role may be subordinate, without glia, the brain could not function properly.

Question 87

What is the name and four functions of the most numerous glia in the brain?

Answer 87

The most numerous glia in the brain are called astrocytes. These cells fill most of the spaces between neurons. Consequently, astrocytes probably influence whether a neurite can grow or retract.

An essential role of astrocytes is regulating the chemical content of this extracellular space. For example, astrocytes envelop synaptic junctions in the brain, thereby restricting the spread of neurotransmitter molecules that have been released. Astrocytes also have special proteins in their membranes that actively remove many neurotransmitters from the synaptic cleft.

Astrocytic membranes also possess neurotransmitter receptors that, like the receptors on neurons, can trigger electrical and biochemical events inside the glial cell.

Besides regulating neurotransmitters, astrocytes also tightly control the extracellular concentration of several substances that could interfere with proper neuronal function. For example, astrocytes regulate the concentration of potassium ions in the extracellular fluid.

Question 88

Name and describe the functions of two other glial cells

Answer 88

Unlike astrocytes, the primary function of oligodendroglial and Schwann cells is clear. These glia provide layers of membrane that insulate axons. this wrapping, called myelin, spirals around axons in the brain (myelin sheath)

Question 89

Why are there gaps in the myelin and what are these called?

Answer 89

The sheath is interrupted periodically, leaving a short length where the axonal membrane is exposed. This region is called a node of Ranvier. Myelin serves to speed the propagation of nerve impulses down the axon.

Question 90

how do oligodendroglia and Schwann cells differ?

Answer 90

In their location and some other characteristics. Oligodendroglia are found only in the central nervous system (brain and spinal cord), whereas Schwann cells are found only in the peripheral nervous system (parts outside the skull and vertebral column). Another difference is that one oligodendroglial cell contributes myelin to several axons, whereas each Schwann cell myelinates only a single axon.

Question 91

Describe three other forms of cells which exist in the brain which have not previously been mentioned

Answer 91

First, special cells called ependymal cells line fluid-filled ventricles within the brain and play a role in directing cell migration during brain development.

Second, a class of cells called microglia function as phagocytes to remove debris left by dead or degenerating neurons and glia.

Finally, in addition to glial and ependymal cells, the brain also has vasculature: arteries, veins, and capillaries that deliver via the blood essential nutrients and oxygen to neurons.

Question 92

Why has microglia attracted much attention recently?

Answer 92

As they appear to be involved in remodeling synaptic connections by gobbling them up. Microglia can migrate into the brain from the blood, and disruption of this microglial invasion can interfere with brain functions and behavior.