Cells of the nervous system

Question 1

What is a neurone?

Answer 1

Main informational processing unit of the nervous system.

It is responsible for the generation and conduction of electrical signals. They are used to communicate with one another via chemicals that are released at the synapse.

Neurones are supported by neuroglia, comprising of different cell types.

Question 2

What characteristics differ between neurones?

Answer 2

Differences in number (abundance) and the shape of their processes (location as well)

It is this diversity that makes them less vulnerable to degredation in CNS conditions like Alzheimer’s, Parkinsons, MS and ALS (amyotrophic lateral sclerosis)

Question 3

What are the characteristics of neurones?

Answer 3

• Large nucleus
• Prominent nucleolus
• Abundant RER and free ribosomes
• Well developed golgi- high levels of protein trafficking via the secretory pathway
• Abundant mitochondria
• Highly organised cytoskeleton
• In general, it is a highly organised metabolically active cell

Question 4

What are the dendrites?

Answer 4

Dendrites are input to neurones from other neurones. Spread from cell body and branch frequently have tree structure to increase SA of neurone.

Thin dendrite protrusions (an actual lump sticking out from the main bit) called dendritic spines recieve the majority of the synapses. Dendritic spines can have multiple synapses.

Large pyramidal neurones can have 30,000-40,000 spines.

Question 5

Describe a pyramidal cell

Answer 5

The centre is like a triangle (pyramid).

Primary dendrites come off the vertices (corners) of the pyramid.

There are secondary dendrites branching off the primary

Tertiary dendrites branch off the secondary

THERE IS ONLY 1 AXON

Dendrites have over 80,000 spines per cell

Question 6

Which type of neurones are present in the cerebellum?

Answer 6

The purkinje neurones

Dendrite have an enormous number of spines (80,000) and the human cerebellum has approx 15 mill purkinje cells.

Question 7

What are the axons? - what is their function?

Answer 7

They conduct impulses away from cell body- they emerge at the axon hillock. There is usually only ONE axon per cell.

They may branch a little after leaving the cell body and at the target- form axon collaterals.

Contains abundant intermediate filaments and microtubules- the intermediate filaments are needed for tensile strength as the cells are very long.

Axons can be myelinated or unmyelinated- exposed bits of myelinated axons are called nodes of ranvier.

Question 8

Why is it important that the axon remains the same diameter all the way down?

Answer 8

In order to maintain the same speed of transmission of AP.

Question 9

Describe the segregation of axons into different bits

Answer 9

Paranode- the area where the ends of the myelin are bound to the axon to form tight junctions- this prevents leakage of current under the myelin sheath.

Juxtaparanode- an area adjacent to the paranode where you find VGKC and VGCC.

NB- about the fluorescent labelling- a fluorescent antibody has been used against a protein involved in binding ends of the myelin to the axon to form tight junctions.

Question 10

What are the 2 types of axon terminals and describe them

Answer 10

Axons will often branch extensively close to the target (terminal arbor) and they form synaptic terminals with the target.

They can either be:

• Boutons- the classic one on one pre-synaptic to post-synaptic
• Varicosities- this is when the synapse with many smooth muscle cells as it passes- there are multiple swellings containing neurotransmitter.

Question 11

Characteristics of synapses

Answer 11

• Abundant mitochondria- 45% of tot energy consumption is needed for ion pumping and synaptic transmission- its is a process that is sensitive to oxygen deprivation.
• Synaptic vesicles are packaged in the golgi and shipped by fast anterograde transport.
• Specialised mechanisms for association of synaptic vesicles with the plasma membrane
• Competing inputs are integrated in the post-synaptic neurone- neuronal integration

Question 12

Describe this with respect to the densities

Answer 12

Under the microscope, you have synaptic vesicles (neurotransmitter) and mitochondria. Synaptic density (increased number of protein), important for control of the mechanism of the synapse

Question 13

What is synaptic organisation?

Answer 13

Neurones recieve input from multiple places.

Each synapse uses a diversity of chemical transmitters, excitatory and inhibitory.

Question 14

How are synapses organised?

Answer 14

Synapses can form by attaching to different areas.

They are named based on which parts of the 2 neurones synapses:

• (a) axo-dendritic= often excitatory
• (b) axo-somatic = often inhibitory
• (c) axo-axonic= often modulatory

Question 15

What is the neuronal cytoskeleton?

Answer 15

Axons can range in length- from micrometers to meters.

Neuronal filaments play a critical role in determining axon caliber.

Microtubules are abundant in the nervous system

Question 16

What is involved in fast axonal transport?

Answer 16

Vesicles associated with motors are moved down the axon at 100-400mm per day.

Proteins are packaged into a vesicle and then the vesicle is targeted down towards the presynaptic membrane. The microtubules are polarised so the vesicles can only move in one direction.

Retrograde transport- moving vesicles back to the cell body. The transport travels in 2 directions but down two different microtubules.

Question 17

What is the axonal damage in MS?

Answer 17

If there is a restriction to the axon (e.g. traumatic injury or inflammatory disease) you begin to see SWELLINGS

SWELLINGS - vesicles of neurotransmitter keep being transported down the axon and they accumulate because they have no where to go.

Question 18

What are the different morphological subtypes of neurones?

Answer 18

1. PSEUDOUNIPOLAR- sensory neurones have 2 fused process which are axonal in structure (one axon splits into 2). The signal recieved passes into the axonal terminal without going to the soma.
2. BIPOLAR- (like retinal bipolar cells) involved in white matter of the cerebral cortex.
3. Golgi type 1 multipolar
4. Golgi type 2 multipolar

Question 19

Describe golgi type 1 multipolar

Answer 19

• Highly branched dendritic trees
• axons extend long distances

Examples:

• Pyramidal cells of the cerebral cortex
• Purkinje cells of the cerebellum
• Anterior horn cell of the spinal cord
• Retinal ganglion cells

Question 20

Golgi type 2 multipolar cells

Answer 20

Highly branched dendritic trees, short axons.

The axons terminate quite close to the cell body of origin

E.g. stellate cells of the cerebral cortex and cerebellum.

Question 21

What are the functional classifications of neurones?

Answer 21

1. Sensory neurones
2. Motor neurones
3. Interneurones (responsible for modification, coordination, integration, facilitation and inhibition of sensory input)

Question 22

What are neuroglia?

Answer 22

Neuroglia: glue of the nervous system

They are the support cells with varied functions. They are essential for functioning of neurones.

They include the following:

Astroglia

Oligodendroglia

Microglia

Immature progenitors

Ependymal cells

Schwann cells

Satellite glia

Question 23

describe astroglia

Answer 23

Multiprocessed- star shaped

Most abundant cell type in the brain- numerous intermediate filament bundles in cytoplasm (GFAP)- give axons their tensile strength. They signal to each other by gap junctions. In grey matter, they appear as fuzzy balls- protoplasmic because of overlapping processes.

Question 24

What are the functions of astroglia?

Answer 24

1. Scaffold for neuronal migration and axon growth during development.
2. Formation of blood-brain barrier. They have feet onto the blood vessels. Ordered arrangement of astrocyes with minimal overlap- some have contact with blood vessels, others have contact with neurones.
3. Transport of substances from blood to neurons.
4. Segregation of neuronal processes (synapses). They need to be separated between one another.
5. Removal of neurotransmitters.
6. Synthesis of neurotrophic factors.
7. Neuronal-glial and glial neuronal signalling
8. Potassium ion buffering
9. Glial scar formation

Question 25

What are the oligodendroglia?

Answer 25

They are the myelin forming cells of the CNS. There are 2 types:

1. interfascicular
2. Perineuronal

They have a small spherical nuclei with thin processes. There are prominent ER and golgi and it is metabolically highly active.

They are supposed to make and maintain the myelin sheath- each cell can produce up to 40 sheaths.

Question 26

What is myelin?

Answer 26

It is a lipid rich insulating membrane.

It has up to 50 layers (lamellae)

There are light and dark bands

Question 27

What are the myelin disease states?

Answer 27

Multiple sclerosis- loss of myelin due to autoimmune reaction

Adrenoleukodystrophy- leads to progressive loss of myelin

Question 28

What are the microglia?

Answer 28

They are the immune cells of the CNS

Derived from bone marrow during early development.

They are the macrophage of the CNS. They are APCs and involved in immune surveillance.

they have a role in tissue modelling and synaptic stripping.

Question 29

What is the microglia morphology?

Answer 29

When they respond to change, the cell shape changes and if the stimulus is strong enough it becomes phagocytic

MS- potentially, the microglia could be the ones destroying the myelin.

Question 30

What are schwann cells?

Answer 30

Myelin producing cells of the PNS.

Comes under the peripheral glia.

Each schwann cell can only produce one myelin sheath.

They surround unmyelinated axons and they promote axon regeneration.

They are not found in the CNS as they would take up too much space.