Neurons and Glia

Question 1

Describe the cell body of the neuron.

Answer 1

contains protein-making apparatus (e.g. nucleus, endoplasmic reticulum, etc.)

*largest cell body is ~40um in scale

Question 2

What are Dendrites?

Answer 2

long thin extensions from the cell body in order to create surface area necessary for hundreds/thousands of synaptic inputs

*largest dendritic tree is ~400um in scale

Question 3

What is the Axon?

Answer 3

single long fibre extending from the cell body and terminates in boutons (synaptic terminals), which are the chemical communication points between different nerve cells across a synapse

*longest axon is ~1.5m in scale

Question 4

How can we look at a nerve cell?

Answer 4

Under high powered microscope:

1) Saline solution injected to get rid of blood, followed by formaldehyde fix, to harden the brain up
2) Brain cut precisely into even sections to be put under a smooth cover slip and a high-powered microscope objective close enough to see it
3) Sectioning the specimen, either by:

> microtome technique: embedding specimen in wax to hold it together allowing it to come off the microtome in a nice strip

> cryostat technique: freezing the specimen, surround it with sugar solution that is easy to wash out, allowing you to take off nice thin sections of tissue

4) Tissue treated with solvents which get rid of all lipids an fat in brain tissue (especially myelin) to make it transparent.
5) Nissl Stain which stains nerve cell bodies to be able to visualise them under microscope

Question 5

Why do we have to get rid of red blood cells to examine a nerve cell under a microscope?

Answer 5

because red blood cells are dark and will obscure details therefore they need to be removed

Question 6

What is the Nissl stain?

Answer 6

A stain comprised of a basic dye - stains the Nissl body of neurons

Question 7

What is the Importance of the Nissl Stain in further discoveries?

Answer 7

Nissl stain allowed Brodmann to divide brain into what he thought were different functional areas because they had different stripe patterns within the cortex

Question 8

What is the Disadvantage of the Nissl stain?

Answer 8

nerve cells were just seen as little coloured blobs

Question 9

What is the Golgi’s method?

Answer 9

staining technique which allowed you to see the cell body, dendritic tree and, to some extent, the beginning of the axons

silver chromate creates a dense black stain, which labelled a small% of the cells present in their entirety

Question 10

What is the Disadvantage of the Golgi Stain?

Answer 10

much of the axon could not be seen

Question 11

Describe the Nerve cell classifications.

Answer 11

Multipolar

-many dendrites coming off from cell body

Bipolar

-two extensions from cell body (one axon and one dendrite)

Pseudounipolar

-single axon splits into two branches; one branch runs to peripheral tissues and other to CNS

Question 12

What are cells labelled with to be able to see the axon?

Answer 12

To see the axon, cells are labelled with a small, soluble molecule biocytin which is made visible via a chemical reaction:

• exposing the sections to antibodies with biocytin binding sites
• then antibodies raised against those antibodies
• then a substance is attached to that outer layer of antibodies, either:

> a fluorescent protein-visible under appropriate illumination

> enzyme (e.g. horseradish peroxidase HRP) which catalyses the conversion of soluble biocytin into a dark, dense insoluble molecule

> both allow for the visualisation of the local axonal tree and its detail

Question 13

What is the Disadvantage of biocytin injection?

Answer 13

only gives you local axonal tree and it doesn’t give you long range connections

Question 14

How are long range axonal connections visualised?

Answer 14

by injecting an extracellular label which will be taken up by nerve cells and axonal terminals

nerve cells will transport it down to the synaptic terminals (anterograde tracing) to visualise the termination points of axons

axonal terminals will transport it back to the cell bodies (retrograde tracing), allowing us to see the origin of the axon, and shape and dendritic patterns of those nerve cells

Question 15

How are intracellular structures visualised?

Answer 15

Intracellular structures are invisible to light microscopy. To view intracellular structures of nerve cell, use an electron microscopy:

• uses a beam of electrons and a camera in place of light rays and the observer’s eye
• ultrathin sections, typically 3-60nm
• magnification>100,000x
• resolution to <0.5nm

Question 16

What structures does the electron microscopy capture?

Answer 16

Mitochondria
-nerves very metabolically active

Axons/Dendrites (neurites) packed together

Synapses
-confirmed by presence of vesicles full of neurotransmitter

Cytoskeleton

Question 17

What is the Difference in nerve cell membrane thickness at terminal?

Answer 17

thicker pre-synaptic membrane because there is a SNARE protein complex which,
pulls the vesicles into the membrane when an action potential arrive and calcium enters the pre-synaptic terminal, becoming part of the membrane and releasing its neurotransmitter

thicker post-synaptic membrane because it is full of protein receptor complexes and other molecules associated with receiving the neurotransmitter and converting it into an electrical potential

Question 18

What is the Dendritic spine?

Answer 18

small protrusion from the dendrites of some cells that receives synaptic input

Question 19

Describe the Development of dendritic spines.

Answer 19

Early in development, dendritic spines put out long, thin projections searching for axons to connect with. Where they find a potential axon connection, they make a test connection. If that test connection is good then they will strengthen that connection and keep it. As the connection strengthens the spine shortens and develops a club head giving a big surface area for a big powerful synapse.

Question 20

Under the electron microscope, why does the axon have a dotted look?

Answer 20

the dots are the cut ends of the cytoskeleton

Question 21

What are the Components of the cytoskeleton?

Answer 21

Microtubules
Microfilaments
Neurofilaments

Question 22

What are Microtubules?

Answer 22

• tubes of tubulin which run throughout the cell
• provide support for axons and dendrites
• motor molecules such as kinesin will travel anterogradely and retrogradely along microtubules carrying substances from one of the cell the other:

> from cell body to axon (anterograde): structural proteins, neurotransmitter-associated proteins, organelles (e.g. mitochondria)

> from axon to cell body (retrograde): signalling proteins, debris and used materials to be recycled

Question 23

What are Microfilaments (actin filaments)?

Answer 23

• provide support, helping to maintain shape of cell body and neurites
• play vital role in neural embryonic growth, helping to shape axons and dendrites and make sure they go to the right place and adopt the right shape by producing long thin projections that stick out of the growth cone of axons and dendrites in all directions looking for guidance signals in their environment
• change the shape of dendritic spines and hence the strength of synapses during memory formation

Question 24

What are Neurofilaments?

Answer 24

variety of different proteins associated with neurofilaments (e.g. Tau protein)

Tau protein binds together with cytoskeleton elements

Question 25

What is the Effect of abnormal Tau protein?

Answer 25

forms dense intracellular tangles of cytoskeleton which can be seen on pathological specimens, associated with Alzheimer’s disease

Question 26

Describe the process of Genetic Manipulation to look at neural circuits?

Answer 26

• fluorescent protein gene from jellyfish is inserted into an animal via viral vector
• with modern techniques, genes can be instructed to only produce that protein under very specific circumstances

we can engineer mice to express tracer protein genes only in specific cells:

• cells using a specific neurotransmitter
• cells of a specific type
• cells that make a specific type of connection

beneficial as you don’t have to:

• inject a tracer in
• do a histochemical technique
• cut the brain into sections

Question 27

What are the Advantages of using genetic manipulation to look at neural circuits?

Answer 27

If the brain doesn’t have to be cut into sections and the brain can be made transparent, you can end up with a block of tissue perfectly transparent and in it, all of the cells of a particular type are glowing.

Combining the ability to force a single type of brain cell to make fluorescent proteins with the “clarity” technique lets you see entire neural sub-systems in un-sectioned brain.

Question 28

What is a Glial cell?

Answer 28

cells in the nervous system that support, nourish, and protect neurones

Question 29

Name the different Types of glial cells.

Answer 29

3 glial cells:

• astrocytes
• oligodendrocytes
• neurolemmocytes (Schwann cells)

Question 30

What are all the glial cells made from?

Answer 30

neural tube

-by the same stem cells that make nerve cells, but later in development they switch over from nerve cells to glial cells

Question 31

What is the function of astrocytes?

Answer 31

Astrocytes are the most common type of glia in the brain.
They regulate the extracellular environment in the brain by enclosing the synaptic junctions and actively removing the neurotransmitters from the synaptic cleft

Question 32

What is the function of microglia?

Answer 32

· microglia are immune cells that migrate into the CNS very early in development:
>help direct the development of neurones, and constantly monitor their health thereafter (and that other cells)
>become amoeboid and travel to areas of injury/infection
>engulf and eliminate microbes, damaged cells and other particulate matter
>secrete factors essential for recovery and repair of nervous system (e.g. after infection has been cleared)

Question 33

What is the function of Myelinating Glia (neurolemmocytes and oligdendrocytes)?

Answer 33

· neurolemmocytes (Schwann cells) each myelinate a single axon running down a peripheral nerve
· oligodendrocytes myelinate multiple axons within the central nervous system (e.g. white matter in brain)
· in both cases, the cell wraps around its own extracellular membrane around the nerve cell, leaving layers of phospholipid membrane; underneath this membrane the actual nerve cell itself does not introduce channel proteins into its own extracellular membrane underneath the myelin, ensuring that the axon is well insulated and that it does not leak ions, helping to speed action potentials
· some axons are unmyelinated, but are still protected by oligodendrocytes and neurolemmocytes (they do not lie there in the tissue completely exposed

Question 34

Why are microglial cells not considered glial cells?

Answer 34

because they arise from the mesoderm and not from the neural tube

Question 35

What is the Difference between microglial cells and other immune cells?

Answer 35

other immune cells can’t get into the nervous system due to the blood brain barrier