Chapter 2 - Neurons and Glia

Question 1

How many neurons and glia are in the adult human brain?

Answer 1

Roughly 85 billion of each type.

Question 2

What are some of the major functions of the NEURONS?

Answer 2

Sensing changes in the environment, communicate these changes to other neurons, and command the body’s responses to these sensations.

Question 3

What are some of the major functions of the GLIA?

Answer 3

They contribute to the brain function mainly by insulating, supporting, and nourishing neighboring neurons.

Question 4

What is the average size of most brain cells?

Answer 4

0.01 - 0.05 mm in diameter.

Question 5

What is one method to harden, or ‘fix’ tissues? How to slice this?

Answer 5

Immerse them in formaldehyde. With a microtome, you can make very thin silces of it.

Question 6

What is the field of histology?

Answer 6

The microscopic study of the structure of tissues.

Question 7

What was the major finding by Franz Nissl regarding neurohistology?

Answer 7

A class of basic dyes would stain the nuclei of all cells as well as clumps of material surrounding the nuclei of neurons. These clumps are Nissl bodies, and the stain is known as the Nissl stain.

Question 8

Why is the Nissl stain useful?

Answer 8

1) It distinguishes between neurons and glia, and 2) it enables histologists to study the arrangement, or CYTOARCHITECTURE, of neurons in different parts of the brain. Cytoarchitecture led us to realize that the brain consists of regions.

Question 9

What was the major finding by Camillo Golgi regarding neurohistology?

Answer 9

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. Thanks to this, we realize that the region around the nucleus is only a small fraction of the neuron.

Question 10

What are the names used for the swollen region containing the cell nucleus?

Answer 10

Cell body, soma (pl: somata), perikaryon (pl: perikarya).

Question 11

What are the thin tubes that radiate from the soma?

Answer 11

They are neurites. Neurites are further divided into dendrites and axons.

Question 12

What is the size ratio of axons and dendrites?

Answer 12

Dendrites are rarely longer than 2mm. However, axons can extend to more than 1 meter.

Question 13

Describe the properties of the axon.

Answer 13

There is usually a single axon from the neuron. This has a uniform diameter throughout its length, and any branches generally extend at right angles. They act as “wires” that carry the output of the neurons.

Question 14

Describe the properties of the dendrite.

Answer 14

They are rarely longer than 2mm, and many of them extend from the cell body. They generally taper to a fine point (become thinner towards the end). Dendrites act as the antennae of the neuron and receive incoming signals.

Question 15

What is the contribution of Cajal to histology?

Answer 15

He used the Golgi stain to work out the circuitry of many regions of the brain. He argued that neurons communicate by contact, not continuity. This idea that cell theory applies to neurons as well came to be known as the neuron doctrine.

Question 16

Describe the contents of the soma.

Answer 16

The cell body is about 20 um in diameter. The watery fluid inside the cell, the cytosol, is a salty, potassium-rich solution that is separated from the outside by the neuronal membrane. Within the soma, there are membrane-enclosed organelles, same ones you would find in all animal cells.

Question 17

What is cytoplasm?

Answer 17

Everything within the membrane except the nucleus.

Question 18

Describe the nucleus.

Answer 18

It is spherical, centrally located, and about 5-10 um across. It is contained within a double membrane, the nuclear envelope, which is perforated by pores about 0.1 um across. Within the nucleus are chromosomes that contain the DNA. The DNA contains the genes, that are parts of the DNA used to assemble the cell.

Question 19

Describe genes and gene expression.

Answer 19

Gene expression is the reading of the DNA. The final product of gene expression is the synthesis of proteins. This protein synthesis occurs in the cytoplasm.

Question 20

What is transcription?

Answer 20

The process of assembling a piece of mRNA that contains the information of a gene. The mRNA consists of four different nucleic acids strung together in various sequences to form a chain.

Question 21

What are the promoters and terminators in a gene?

Answer 21

The PROMOTER is the region where the RNA-synthesizing enzyme, RNA polymerase, binds to initiate transcription. The TERMINATOR, also called the stop sequence, is a sequence of DNA that the RNA polymerase recognizes as the end point for transcription.

Question 22

What are EXONS and INTRONS?

Answer 22

EXONS are the parts of a gene that are coded into mRNA. INTRONS are stretches of DNA within the gene that cannot be used to code for protein. During RNA SPLICING the introns are removed from the transcription after the initial transcript.

Question 23

Why do neurons differ from other cells in the body?

Answer 23

Because of the specific genes they express as proteins.

Question 24

What are some causes of psychic disorders?

Answer 24

In some diseases, GENE COPY NUMBER VARIATIONS cause loss of long stretches of DNA that contain several genes. In other diseases, MUTATIONS appear in a gene or in the flanking regions of DNA that regulate gene expression.

Question 25

What are KNOCKOUT MICE, TRANSGENIC MICE and KNOCK-IN MICE?

Answer 25

Knockout mice are mice that have had one gene deleted. They can be used to study the progression of a disease.
Transgenic mice are mice in which genes have been introduced and overexpressed.
Knock-in mice are mice where the native gene is replaced with a modified transgene.

Question 26

Describe the function of the ROUGH ENDOPLASMIC RETICULUM in neurons.

Answer 26

Many ribosomes (but not all), where protein synthesis occurs through translation, are attached to stacks of membrane (rough ER). Rough ER is very prominent in neurons, far more than in glia or non-neuronal cells.

Question 27

Describe the function of the SMOOTH ENDOPLASMIC RETICULUM in neurons.

Answer 27

Stacks of membraneous organelles that look a lot like rough ER without the ribosomes. Also called smooth ER. It is heterogenous 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 are folded. Other types of smooth ER play no direct role but instead regulate the internal concentrations of substances such as calcium.

Question 28

Describe the function of the GOLGI APPARATUS.

Answer 28

The Golgi apparatus 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 29

Describe the function of the MITOCHONDRION.

Answer 29

It is a very abundant organelle in the soma, and within the enclosure of their outer membrane are multiple folds of inner membrane called CRISTAE (sing: crista). Between the cristae is an inner space called matrix. They are the site of CELLULAR RESPIRATION.

The Mitochondrion pulls inside pyruvic acid and oxygen, and releases 17 ATP molecules for every molecule of pyruvic acid that was taken in.

The Mitochondrion is the place of the Krebs cycle that produces ATP, which is the cell’s energy source.

Question 30

Describe the function of the NEURONAL MEMBRANE.

Answer 30

The neuronal membrane is a barrier that encloses the cytoplasm inside the neuron and excludes certain substances that float in the fluid that bathes the neuron. Some membrane-associated proteins pump substances from the inside to outside, and others form pores that regulate which substances can gain access to the inside of the neuron.

Question 31

Describe the function of the CYTOSKELETON.

Answer 31

The cytoskeleton is a scaffolding that gives the neuron its characteristic shape. The “bones” are the microtubules, microfilaments, and neurofilaments. The cytoskeleton is not static, as its elements are dynamically regulated and are in continual motion.

Question 32

What are MICROTUBULES?

Answer 32

Measuring 20 nm in diameter, microtubules are relatively large and run longitudinally down neurites. They are the building blocks of the cytoskeleton. Other components of the cytoskeleton are neurofilaments (10 nm) and microfilaments (5 nm).

Question 33

What are MICROFILAMENTS?

Answer 33

Microfilaments are particularly numerous in the neurites. They are braids of two thin strands that are polymers of the protein actin. They are constantly undergoing assembly and disassembly, like microtubules. They are also anchored to the membrane by attachments with a meshwork of fibrous proteins that line the inside of the membrane like a spider web.

Question 34

What are NEUROFILAMENTS?

Answer 34

They exist in all cells of the body as intermediate filaments, only in neurons they are called neurofilaments. These resemble the bones and ligaments of the skeleton most closely. They consist of multiple subunits, building blocks, that are wound together like a rope-like structure. They are very strong mechanically.

Question 35

Describe the function of the AXON.

Answer 35

-A structure only found in neurons.
-Begins with the AXON HILLOCK, which tapers away from the soma.
-No rough ER extends into the axon. Few, if any free ribosomes in mature axons.
The protein composition of the axon membrane is fundamentally different from that of the soma membrane.
-No protein synthesis (due to no ribosomes)
-May extend from less than a millimeter to over a meter long.
-The diameter of axons is variable, ranging from 1-25 um in humans and to as large as 1mm in squid. This affects the speed of the nerve impulse -> thicker axons cause faster travel.

Question 36

Describe AXON BRANCHES.

Answer 36

Axons branch often. These branches are called AXON COLLATERALS. They can travel long distances to communicate with different parts of the nervous system. Sometimes an axon collateral returns to communicate with the same cell that gave rise to the axon. These axon branches are called RECURRENT COLLATERALS.

Question 37

Describe the function of the AXON TERMINAL.

Answer 37

All axons have a beginning (hillock) middle (axon proper) and end (axon terminal, or terminal bouton). The terminal is the site where the axon comes in contact with other neurons or other cells, and passes information on to them. This point of contact is the SYNAPSE.

Question 38

Describe the function of the SYNAPSE.

Answer 38

The point of contact where axons come into contact with other neurons. Sometimes axons have many short branches at their ends, and each branch forms a synapse on dendrites or cell bodies in the same region. These branches are called the TERMINAL ARBOR.

Question 39

Describe the function of the BOUTONS EN PASSANT.

Answer 39

Axons sometimes form synapses at swollen regions along their length, and then continue to terminate elsewhere. These are boutons en passant.

Question 40

How does the cytoplasm of the axon terminal differ from that of the axon?

Answer 40

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 41

What is INNERVATION?

Answer 41

When a neuron makes synaptic contact with another cell, it is said to innervate that cell, or to provide innervation.

Question 42

Describe the process of information transfer in the synapse.

Answer 42

The synapse has two sides: presynaptic and postsynaptic. The presynaptic side generally consists of an axon terminal, and the postsynaptic cleft can be a dendrite or the soma of another neuron. The space between these membranes is the SYNAPTIC CLEFT. The transfer of information at the synapse from one neuron to another is SYNAPTIC TRANSMISSION.

Question 43

How does information travel at the synapse?

Answer 43

At most synapses, information in the form of electrical impulses is converted in the terminal into a chemical signal that crosses the cleft. On the postsynaptic membrane, this chemical signal is converted again into an electrical one.

Question 44

What is AXOPLASMIC TRANSPORT?

Answer 44

Ribosomes are absent in axon terminals. They create the proteins of the cell. Thus, proteins need to be passed down the axon. Movement of material down the axon is called AXOPLASMIC TRANSPORT. The degeneration of axons that occurs when they are cut is called Wallerian degeneration. There is slow and fast axoplasmic transport.

Question 45

What is ANTEROGRADE and RETROGRADE transport?

Answer 45

Anterograde transport is movement of materials via KINESON down the axon. Retrograde transport is transport of materials from the axon terminal to soma. In retrograde transport, the “legs” are DYNEIN.

Question 46

What are DENDRITES?

Answer 46

Neurites that resemble the branches of a tree. They form dendritic branches, and are covered with thousands of synapses. Dendrites of some neurons are covered with specialized structures called DENDRITIC SPINES that receive some types of synaptic input.

Question 47

How can neurons be classified by NEURONAL STRUCTURE?

Answer 47

1. Number of neurites: Unipolar, bipolar, and multipolar.
2. Dendrites: The shape of the neuron with dendrites (e.g., stellate, pyramidal cells), or the spines of the dendrites (spiny or aspinous dendrites).
3. Connections: Primary sensory neurons (neurites in the sensory surfaces of the body), motor neurons (axons that form synapses with muscles and command movements), interneurons (only connect with other neurons).
4. Axon length: Golgi type I neurons / projection neurons (extend from one part of the brain to another) and Golgi type II neurons / local circuit neurons (short axons that do not extend beyond the vicinity of the cell body).

Question 48

How can neurons be classified by GENE EXPRESSION?

Answer 48

1. Differences in gene expression cause pyramidal cells and stellate cells to develop different shapes.
2. Neurons use different neurotransmitters. Motor neurons that command voluntary movements release acetylcholine, and are cholinergic.

Question 49

What is GREEN FLUORESCENT PROTEIN?

Answer 49

Green fluorescent protein, GFP, is encoded by a gene discovered in jellyfish. It is used in neuroscience research. When illuminated with the appropriate wavelength of light, the GFP fluoresces bright green, allowing visualization of the neuron in which it is expressed.

Question 50

What are GLIA divided into?

Answer 50

The glia are divided into ASTROCYTES and MYELNIATING GLIA.

Question 51

What are ASTROCYTES?

Answer 51

The cells that fill most of the spaces between neurons. They likely influence whether a neurite can grow or retract. They also regulate the chemical content of the extracellular space. The primary function is not clear.

Question 52

What are MYELNIATING GLIA?

Answer 52

These are OLIGODENDROGLIAL and SCHWANN cells, located in different locations. They create myelin sheaths that spiral around axons. The myelin sheath is interrupted periodically, leaving nodes of Ranvier where the axonal membrane is exposed. Myelin also speeds the propagation of nerve impulses down the axon.

Question 53

What are the differences between OLIGODENDROGLIAL and SCHWANN cells?

Answer 53

Oligodendroglia are found only in the central nervous system and contribute myelin to several axons, and Schwann cells are only found in the peripheral nervous system and each Schwann cell myelinates only a single axon.

Question 54

What NON-NEURONAL cells are in the brain?

Answer 54

1. EPENDYMAL CELLS: They line fluid-filled ventricles within the brain and play a role in directing cell migration during brain development.
2. MICROGLIA: They function as phagocytes to remove debris left by dead or degenerating neurons and glia. They also appear to be involved in remodeling synaptic connections by gobbling them up.
3. VASCULATURE: Arteries, veins and capillaries that deliver essential nutrients and oxygen to neurons via blood.

Question 55

What is the neuron doctrine, in a single sentence? To whom is this credited?

Answer 55

The idea that cell theory also applies to neurons. Credited to Santiago Cajal.

Question 56

Which parts of a neuron are shown by a Golgi stain that are not shown by a Nissl stain?

Answer 56

The axons and the dendrites. The Nissl stain distinguishes between neurons and glia, and enables histologists to study the CYTOARCHITECTURE (arrangement of neurons in different parts of the brain)

Question 57

What are three physical characteristics that distinguish axons from dendrites?

Answer 57

1. Dendrites can appear in trees, whereas the axon is usually singular and branches only at the end.
2. Dendrites are on average much shorter than axons.
3. Some dendrites have dendritic spines.
4. Axons arise from an axon hillock, dendrites directly from the receiving surface of the soma.
5. Axons have neurotransmitter containing vesicles.

Question 58

Of the following structures, state which ones are unique to neurons and which are not: nucleus, mitochondria, rough ER, synaptic vesicle, Golgi apparatus.

Answer 58

Unique: Synaptic vesicle.

Non-unique: Nucleus, mitochondria, rough ER, Golgi apparatus.

Question 59

What are the steps by which the information in the DNA of the nucleus directs the synthesis of a membrane-associated protein molecule?

Answer 59

Transcription, RNA splicing, and translation. The process is called gene expression.

Question 60

Colchicine is a drug that causes microtubules to break apart (depolymerize). What effect would this drug have on anterograde transport? What would happen in the axon terminal?

Answer 60

Anterograde transport is done on the microtubules, so breaking them apart would disrupt this transport. The axon terminal would no longer receive proteins from the soma.

Question 61

Classify the cortical pyramidal cell based on (1) the number of neurites, (2) the presence or absence of dendritic spines, (3) connections, and (4) axon length.

Answer 61

1. Multipolar.
2. Spinal.
3. Interneurons.
4. Tens of centimeters, from one part of brain to another.

Question 62

Knowledge of genes uniquely expressed in a particular category of neurons can be used to understand how those neurons function. Give one example of how you could use genetic information to study a category of a neuron.

Answer 62

By using DNA microarrays that are filled with spots of synthetic DNA with gene specific sequences. Mix mRNA from neurons of two brains, and you can see how the gene expression differs from brain to brain.

Question 63

What is myelin? What does it do? Which cells provide it in the central nervous system?

Answer 63

Myelin is the wrapping that insulates axons. It is provided by the oligodendroglial cells in the central nervous system. (And Schwann cells in the peripheral nervous system)