L2. Histology of the CNS

Question 1

What is the key cell type of the CNS?

Answer 1

The neuron

Question 2

What are glial cells? Give some examples

Answer 2

These are the supporting cells (the glue) of the CNS

Eg. Astrocytes, oligodendrocytes, schwann cells, ependymal cells and satellite cells

Question 3

What are the immune cells of the CNS?

Answer 3

Microglia (resident immune cells)

Question 4

The CNS contains grey and white matter containing neurons and glia.

Describe the cortex (outer cm of grey matter)

Answer 4

Is divided into 6 distinct and functional layers of neurons containing different types of neurons.
(Layer IV is very important)

The cortex is responsible for complex thoughts, sensations and motor functions

Question 5

Describe the ventricular system of the CNS

Answer 5

a series of hollow regions/compartments in the brain for the passage of CSF

Question 6

What is the choroid plexus?

Answer 6

A vascular structure arising from the walls of each ventricle. It is the system that produces the CSF.

On the inside surface of the ventricles, COLUMNAR CELLS called EPENDYMAL CELLS that are lining the ventricle

Question 7

Describe ependymal cells

Answer 7

Low columnar or cuboidal cells lining the central canal of the spinal cord and the ventricles in the brain.

Some places have CILIATED ependymal cells to aid CSF flow

They have NO BASEMENT MEMBRANE (have interdigitations projecting into the spinal cord to anchor themselves) - they are not epithelial cells.

Question 8

How do neurons differ from other cells?

[5 main characteristics]

Answer 8

They are specialised for signalling (have transduction methods)

Morphologically distinct (different functions are represented by the different appearances)

Rapidly communicate

Are electrically active cells

Span and communicate over long distances

Question 9

Generally, neurons have three characteristics in terms of metabolic activity and differentiation. What are these?

Answer 9

1. High level of protein synthesis
2. Metabolically limited
3. Terminally differentiated (no cell division)

Question 10

Dendrites have a definite shape but there are some subtle dynamic changes occurring in real time. Describe when and how this would occur

Answer 10

There are fine processes on dendrites that change rapidly in response to neural stimulation (due to dynamic processes in the cell).

It occurs by the action of actin, intermediate filaments and microtubules (the cytoskeleton) in assembly and disassembly.

Question 11

Describe how actin, intermediate filaments and the microtubules act to maintain and change the shape of cells in the CNS

Answer 11

Actin - dynamic assembly/ disassembly allowing for shape change and movement

Intermediate filaments - are permanent structures

Microtubules - are dynamic and composed of tubulin. Very important for axonal transport.

Question 12

What is meant by neurons are highly polarised?

Answer 12

Their apical and basal aspects are different and specialised. They have an axon on one end and dendrites on the other (separated by the cell body or soma)

Question 13

What is the difference between the dendrites and the axon?

Answer 13

Dendrites are generally small processes that RECEIVE information from other neurons

Axons are often very long processes and are the main CONDUCTING unit for carrying signals to other neurons

Question 14

Describe the volume distribution of neurons

What does this mean?

Answer 14

A high proportion of the total neuronal cell volume is in the axons and the dendrites (not in the soma)

This means that the transport through the axons is critical and needs to be efficient.

It also means that damage to neurons mainly occurs in the axons and the soma is relatively spared.

Question 15

Neurons are electrically active conducting electrical impulses from one place to another. How is this performed?

Answer 15

The membranes of the neurons carry GRADED POTENTIALS (unequal distribution of charged ions on either side of the membrane)

AND

A way to maintain these gradients (active pumps and ion channels)

Question 16

Describe the difference between the electrical signalling between the inputs of signals vs. the outputs of cells

Answer 16

Inputs to the cell through the dendrites are PASSIVE electrotonic spread (local)

Outputs from the cell through axons are via an active ACTION POTENTIAL

Question 17

What is the output signal of the cell?

Answer 17

Secretion of a chemical message at a synapse by release of synaptic vesicles.

Question 18

Neurons are highly synthetic cells with high levels of protein production. Why is this so?

Answer 18

Protein is required for ion channels, receptors, cytoskeleton

Question 19

What are Nissl Bodies?

Answer 19

Large granular bodies made up of rough ER and free ribosomes which are the site of protein synthesis

(The nucleus of neurons appear large and pale)

Question 20

What is more numerous? Astrocytes or Neurons?

Answer 20

Astrocytes

Question 21

What are astrocytes?

Answer 21

Star shaped neuroglial cells

Question 22

What are the passive functions of the astrocytes? [5]

Answer 22

Neurotransmitter uptake and metabolism (stop the neural signal)

K+ homeostasis (remove K from the synapse following cell depolarisation)

Maintains neuronal energy supply (glucose transport from the blood to the neurons)

Maintenance of the Blood Brain Barrier

Injury response and recovery

Question 23

What are the (newly discovered) functions of astrocytes?

Answer 23

They are able to modulate neural function

Modulation of blood flow (in response to neuronal needs)

Are excitable and can communicate with one another

Question 24

Glial cells express glutamate transporters that suck up glutamate and transporters for GABA.

What are these substances?

Answer 24

Glutamate (excitatory) and GABA (inhibitory) are amino acids acting as neurotransmitters.

Thus the glial cells are able to be involved in Neurotransmitter uptake and metabolism (stopping the neural signal)

Question 25

How do glial cells communicate with one another?

Answer 25

Through modulations in intracellular calcium (calcium waves)

The glial cells have SYNAPTIC VESICLES containing glial transmitters as a result of the calcium wave

These transmitters then impact surrounding glial cells and/or surrounding neurons

Question 26

What triggers/initiates glial communication/calcium waves?

Answer 26

Neurotransmitters like ATP, glutamate, trauma and touch, spontaneously or by inflammatory mediators.

Question 27

How are neurons affected by the calcium wave of glial cells?

Answer 27

Neurons are INHIBITED by the calcium wave (hyperpolarisation) in a mechanism that involves release of ATP from glia

Question 28

Describe how glial cells are related to blood vessels

Answer 28

Astrocytes very intimately surround blood vessels and thus regulate VASCULAR TONE.

The calcium wave causes either a vasodilation or vasoconstriction of vessels

Question 29

What is the predominant glial cell of white matter?

Answer 29

The Oligodendrocyte in the CNS

schwann cells in the PNS

Question 30

What is the function of the oligodendrocyte?

Answer 30

Myelinating axons in the CNS producing the myelin sheath that provides the electrical insulation for neurons (speeds up the conduction of the action potential)

Each oligodendrocyte extends processes to wrap around parts of several different axons (schwann cells wrap around a single axon)

Question 31

Does the myelin sheath wrap around the whole axon?

Answer 31

No

Small gaps called the NODES OF RANVIER where current ‘jumps’ across using a number of ion channels from one node to another - action potentials are not a free flowing current of electrical energy

Question 32

What is meant by the statement: “The CNS is Immune Privileged”?

Answer 32

The presentation of an antigen to the brain and then subsequent presentation of the same antigen elsewhere in the body will NOT lead to a typical immune response

There are no B and T cells and normal immunological responses occurring in the brain.

Question 33

Describe microglial cells

Answer 33

The resident, specialised macrophages of the CNS: are haemopoietic (derived from bone marrow), are phagocytic

They constantly survey the CNS (dynamically change all the time)

They are also involved in pruning the synapses (developmental processes)

Question 34

Describe how microglial cells survey their surroundings

Answer 34

They send processes out to the CNS environment to survey the area and then retract them continuously.

They are abel to change very rapidly in response to inflammation or injury by up-regulating cytokines and growth factors.

Question 35

Describe the organisation of peripheral nerve fibres

Answer 35

Axons are surrounded by schwann cells and supported by ENDONEURIUM (loose vascular support tissue) to make up nerve fibres.

Nerve fibres are arranged in nerve bundles or FASCICLES.
Each fascicle is surrounded by collagenous tissue called the PERINEURIUM.

Multiple fascicles can be grouped together to form a nerve in a layer of collagenous tissue called the EPINEURIUM.

Question 36

What are ganglia?

What is the support cell of these ganglia?

Answer 36

Ganglia are aggregations of cell bodies of neurons OUTSIDE the CNS

There are 2 types: sensory ganglia (DRG) and autonomic ganglia (post-ganglionic neuron cell bodies)

These are supported by satellite cells