Glial cells Flashcards
What is the main cell type present within the CNS?
- Neuron
-Glial cell, which can be further subdivided into four different cell types
What are the four glial cell types?
- Astrocytes
- Oligodendrocytes
- Microglia
- Ependymal
What is the most abundant type of glial cells?
Astrocytes which are actually more abundant than neurons in the brain
Describe the main functions of astrocytes.
Glial cells were long since thought of the ‘glue’ only providing structural support within the brain. However the star shaped astrocytes have a dynamic role in communicating with both each other and neurons via both gap junctions and by releasing neurotransmitters such as glutamate and ATP.
They do also have a functional role in structural support in their ability to position neurons as well as scaffolding in guiding developing neurons.
Astrocytes form part of the blood brain barrier with their foot processors forming contact with blood vessels within the CNS.
Astrocytes, if damage occurs in part of the brain has the ability to form scar tissue leading to the development of the ‘glial scar’
Astrocytes ensure the correct functioning of neurons which require specific extracellular environments to be able to function. Astrocytes contain transporters within their membrane which are able to uptake neurotransmitters within the synaptic cleft back into the synaptic terminal in which they are then metabolised/degraded by intercellular enzymes. The precursors are then stored by the astrocytes before being returned to the neurons.
Astrocytes are also important for maintaining extracellular potassium concentrations. Potassium efflux occurs after an action potential. Extracellular potassium must remain low as otherwise neurons will depolarise, preventing neuronal function.
Astrocytes are involved in the formation of new synapses and their maintenance.
Astrocytes can also modulate neurotransmitter release at the synapse.
State the overall functions of the astrocytes.
- Dynamic role in communication with other astrocytes and neurons
- Structural support
- Scaffolding for developing axons
- Formation of the BBB, foot processors contact blood vessels
- Maintenance of extracellular environment with neurotransmitters and potassium levels
- Modulate synapse function with formation of new synapses
- Controls release of neurotransmission at the synapse
Provide an example of a transporters present on astrocytes to facilitate neurotransmitter reuptake.
Astrocytes play a crucial role in the reuptake in the primary excitatory neurotransmitter Glutamate, to prevent accumulation of Glutamate within the synapse and hence excitotoxicity
The astrocytic glutamate transporters include:
- Glutamate-aspartate transporter (GLAST) - Glutamate transporter-1 (GLT-1) and their human homologs excitatory amino acid transporter 1 (EAAT1) and 2 (EAAT2)
In which conditions is excitotoxicity associated with?
Growing evidence has shown that excitotoxicity is associated with various neurological disorders, including:
- Amyotrophic lateral sclerosis (ALS)
- Alzheimer’s disease (AD)
- Parkinson’s disease (PD)
-Manganism
- Ischemia
- Schizophrenia
- Epilepsy
- Autism.
This may be due to dysregulation of GLAST/GLT-1, ongoing research is being conducted
At which level may GLAST/GLT-1 become dysregulated?
The expression and function of GLAST/GLT-1 may become dysregulated at:
Genetic
Epigenetic
Translational
Transcriptional
Leading to high levels of extracellular glutamate and excitotoxicity
What is the potential role of astrocyte dysfunction in the physiology of epilepsy?
In patients with pharmacoresistant temporal lobe epilepsy and epilepsy models it was revealed there was alterations in expression, localization, and function of astroglial K+ and water channels. In addition, malfunction of glutamate transporters and the astrocytic glutamate-converting enzyme, glutamine synthetase, has been observed in epileptic tissue.
What is the overriding function of microglia?
They are the immune cells of the CNS and have similar roles to that of macrophages within the periphery, and so overall mediates neuroinflammation and response to infection.
This includes:
- Primary source of proinflammatory cytokines to mediate and co-ordinate immune response
- Responsible for the elimination of elimination of microbes, dead cells, redundant synapses, protein aggregates, and other particulate and soluble antigens that may endanger the CNS
- Pivotal role in brain development and equally implicated in neurodegeneration
What are the differences between resting and activated microglia?
These two forms have very different both functions and structures.
Resting microglia are highly ramified (highly spread/branched out) and are involved in homeostatic functions, controlling the environment within the CNS tissue
Activated microglia begin to retract their processors and become less ramified. Eventually will become amemoid (crumpled together in a ball). They become more motile in this state, can move to areas of tissue damage. They can then release pro-inflammatory cytokines.
What do oligodendrocytes do?
They form the myelin sheath around the neuronal axons and will cause myelination of axons around the vicinity (white matter).
Where are ependymal cells located?
These epithelial cells are found lining the fluid filled cavities (ventricles) within the CNS.
What is the function of the ependymal cells?
These cells are responsible for the secretion of cerebrospinal fluid. These cells also contain cilia and these protrusion facilitates the movement of this secreted fluid within the ventricles. These cells also line the BBB.
