top of page
2.5. Glial Cells
Overview of Glial Cells
Glial cells are support cells in the nervous system. Glial cells are different from neurons because glial cells do not conduct action potentials and they do not synapse with other nearby cells. Instead, they provide structural and protective support for neurons. There are many more glial cells than neurons. In fact, it is estimated that for every 1 neuron there are 10 glial cells. There are five types of glial cell:
- Astrocytes
- Oligodendrocytes
- Ependymal cells
- Microglia
- Radial glia
Astrocytes and Oligodendrocytes
Astrocytes
These are the most numerous glial cells in the brain. They have lots of functions, so only their main ones will be described here. It was previously thought that astrocytes contributed to the formation and structural stability of the blood-brain barrier (BBB), by extending ‘astrocytic feet’ around the brain capillaries and thereby preventing toxins and unwanted chemicals from entering the delicate brain matter. However, more recent evidence suggests that astrocytes don't make a large contribution to the BBB. Instead they help to control the diameter of blood vessels by allowing them to dilate - this means they help control blood flow to particularly active brain regions. Astrocytes also maintain the health of synapses and regulate ion and neurotransmitter levels (specifically, astrocytes remove excess K+ ions and excess glutamate neurotransmitter).
Oligodendrocytes
These provide myelination around neurons in the central nervous system, as shown in Figure 2.5.2. Myelination speeds up the transmission of action potentials by making them ‘jump’ along the neuron at specific myelin-free points called nodes of Ranvier. Oligodendrocytes are very similar to Schwann cells in the peripheral nervous system, but the difference is oligodendrocytes myelinate between five and eight neurons, whereas one Schwann cell myelinates only one neuron.
These are the most numerous glial cells in the brain. They have lots of functions, so only their main ones will be described here. It was previously thought that astrocytes contributed to the formation and structural stability of the blood-brain barrier (BBB), by extending ‘astrocytic feet’ around the brain capillaries and thereby preventing toxins and unwanted chemicals from entering the delicate brain matter. However, more recent evidence suggests that astrocytes don't make a large contribution to the BBB. Instead they help to control the diameter of blood vessels by allowing them to dilate - this means they help control blood flow to particularly active brain regions. Astrocytes also maintain the health of synapses and regulate ion and neurotransmitter levels (specifically, astrocytes remove excess K+ ions and excess glutamate neurotransmitter).
Oligodendrocytes
These provide myelination around neurons in the central nervous system, as shown in Figure 2.5.2. Myelination speeds up the transmission of action potentials by making them ‘jump’ along the neuron at specific myelin-free points called nodes of Ranvier. Oligodendrocytes are very similar to Schwann cells in the peripheral nervous system, but the difference is oligodendrocytes myelinate between five and eight neurons, whereas one Schwann cell myelinates only one neuron.
Ependymal Cells, Microglia and Radial Glia
Ependymal Cells
These line the ventricles of the brain and have cilia that help to waft CSF out of the ventricles and around the ventricular system (see Section 2.6.2. CSF and the Ventricles). They also actively secrete Na+ ions which contributes to the formation of CSF.
Microglia
These are the immune cells of the brain. They are essentially macrophages which ingest and destroy foreign particles and remove parts of damaged neurons. In many situations, they also take part in gliosis and thus migrate to the site of injury in the brain.
Radial Glia
Radial cells are used during development of the central nervous system. They act as scaffolds for newborn neurons to migrate from the ventricular surface of the brain to the pial surface of the brain, where their final locations will be.
These line the ventricles of the brain and have cilia that help to waft CSF out of the ventricles and around the ventricular system (see Section 2.6.2. CSF and the Ventricles). They also actively secrete Na+ ions which contributes to the formation of CSF.
Microglia
These are the immune cells of the brain. They are essentially macrophages which ingest and destroy foreign particles and remove parts of damaged neurons. In many situations, they also take part in gliosis and thus migrate to the site of injury in the brain.
Radial Glia
Radial cells are used during development of the central nervous system. They act as scaffolds for newborn neurons to migrate from the ventricular surface of the brain to the pial surface of the brain, where their final locations will be.
Clinical Top Tip:
Astrocytoma
An astrocytoma is a type of glioma – essentially a cancerous brain tumour consisting of mutated glial cells. Astrocytomas can grow throughout the brain and are graded based on their severity, with grade I being the least aggressive and grade IV being the most aggressive and likely to spread to other parts of the brain. In the UK, about 1/3 of all brain tumours and brain cancers are astrocytomas. Read more by visiting The Brain Tumour Charity website using the link below.
Related Video
Related Video
bottom of page