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Glial Cells

Notes

Glial Cells

Sections


GLIAL CELLS

Summary

Overview

Glial cells vs neurons

  • Glial cells are smaller than neurons and are closely associate with neurons.

Gray matter

  • Comprises neurons' dendrites and cell bodies

White matter

  • Comprises the neurons' axons

6 TYPES OF GLIAL CELLS

Central Nervous System

1) Astrocytes (Macroglia)

  • Star-shaped
  • Support and brace neurons
  • Anchor neurons to supply lines
  • Help determine capillary permeability (helping to form blood/brain barrier)
  • Guide young neuron migration and synapse formation
  • "Mop up" extracellular potassium and neurotransmitters
  • Two Key Subcategories of Astrocytes:
    • a) Protoplasmic astrocytes: thick, branched processes that attach to neurons and vessels.
    • b) Fibrous astrocyte: long, thin, unbranched processes; found in white matter.

Clinical correlation: Astrocytoma

2) Microglial cells

  • Thorny processes monitor neuronal health
  • Can morph into special type of macrophage to phagocytose pathogens or dead neurons

3) Ependymal cells

  • Form a somewhat permeable barrier between cerebrospinal fluid filled cavities and the tissue of the brain or spinal cord (not to be confused with blood/brain barrier)
  • Cilia can help circulate the cerebrospinal fluid

Clinical correlation: Ependymoma

4) Oligodendrocytes (Macroglia)

  • Similar shape to astrocytes
  • Create myelin sheath within central nervous system
  • A single oligodendrocyte can create multiple myelin segments

Clinical Correlation: Oligodendroglioma, Multiple sclerosis

B) Peripheral Nervous System

5) Satellite cells

  • Small cells surrounding neuronal cell bodies in the peripheral nervous system
  • Thought to have many of the same support functions as astrocytes

6) Schwann cells (neurolemmocytes)

  • Create myelin sheath in the peripheral nervous system
  • Each Schwann cell creates single myelin segment, so there is a bulge in each segment where the nucleus and most of the cytoplasm is located

Clinical Correlation: Guillain-Barre Syndrome

Full-Length Text

  • Here we will explore glial cells (aka neuroglia), which, in the central nervous system, outnumber neuronal cells, although there is significant intertextual variation as to by how much.
  • To begin, start a table to denote some key concepts of glial cells.
  • Denote that they are smaller than neurons and are closely associated with them.
  • Denote that we divide the 6 types of glia into their central nervous system (CNS) (ie, brain and spinal cord) and peripheral nervous system (PNS) (ie, peripheral nerve) types.
  • Denote that the CNS comprises 4 main types: astrocytes, microglial cells, ependymal cells, and oligodendrocytes.
  • Denote that we use the term macroglia to describe both astrocytes and oligodendrocytes.
  • Denote that in the PNS, there are 2 cell types: satellite cells and Schwann cells.

Let's build a diagram that spans from the CNS to the PNS.

  • Subdivide CNS into gray matter and white matter.
  • Within the CNS milieu, draw a couple of neurons and a capillary.
  • Gray matter comprises the neurons' dendrites and cell bodies and the white matter contains the axons.
  • Begin with astrocytes, which are star-shaped (the shape gives the cell type its name).
  • Draw a protoplasmic astrocyte in the gray matter; it has thick, branched processes that attach to neurons and vessels – they grab hold of many cells with their multiple processes.
  • Draw a fibrous astrocyte in the white matter; it has long, thin, unbranched processes.
    • We limit our speciation of astrocytes to these two main categories, although others exist.
  • Denote that astrocytes have many functions:
    • They support and brace neurons.
    • They anchor neurons to the supply lines.
    • Help determine capillary permeability (helping to form the blood/brain barrier).
    • Guide young neuron migration and synapse formation.
    • They "mop up" extracellular potassium and neurotransmitters.
    • They might even be able to influence neuronal functioning.

Now, microglia.

  • Draw a tiny microglial cell near one of the neurons.
  • Denote that the thorny processes, which touch the neurons allow the microglial cell to monitor neuronal health.
  • Also denote that microglial cells can morph into a special type of macrophage capable of phagocytosing pathogens or dead neurons.

Next, ependymal cells.

  • Within the central nervous system gray matter, draw a cerebrospinal fluid filled ventricle.
  • Surround it with cuboidal-shaped ependymal cells though these cells can have various shapes.
  • Show that cilia on some of the ependymal cells track into the cerebrospinal fluid – depending on the type and location, not all ependymal cells have cilia.
  • Denote that the ependymal cells form a somewhat permeable barrier between the cerebrospinal fluid filled cavities and the brain and spinal cord tissue.
  • Do not confuse this with the blood/brain barrier, which provides a unique barrier between the brain tissue and the blood and requires astrocytes to form.
  • Denote that their cilia help circulate the CSF.

Now, oligodendrocytes.

  • Now, within the white matter, draw the cell body of an oligodendrocyte.
  • Next, draw its processes.
  • Then, show that the processes hold myelin sheaths, which wrap around the axons.
  • Denote that oligodendrocytes have a similar shape to astrocytes but fewer processes.
  • Denote that oligodendrocytes create the myelin sheath within the central nervous system.
    • A single oligodendrocyte is capable of not only creating multiple myelin segments on a single axon, but also providing myelin to multiple axons.
  • Label the space between myelin segments as a Node of Ranvier.
  • Myelin is fundamental to the speed of axonal transmission, which we discuss further elsewhere.

Now let's switch gears and look at the glial cells found in the peripheral nervous system, the satellite cells and the Schwann cells.

  • Draw a pseudounipolar neuron as follows:
  • Draw the neuronal cell body.
  • Then, a short axon.
  • Then, show that it connects to an axon that projects centrally and peripherally.
  • Draw satellite cells along the surface of the cell body.
  • Denote that satellite cells are thought to have many of the same support functions as astrocytes.
  • Draw Schwann cells (which are sometimes called neurolemmocytes) surrounding the axon forming the myelin sheath.
  • Denote that these cells create the myelin sheath in the peripheral nervous system.
  • Label a Node of Ranvier.
  • Because each Schwann cell creates a single myelin segment, there is a bulge in each segment containing the nucleus and most of the cytoplasm.

This concludes our tutorial on glial cells.

UNIT CITATIONS:

  1. Marieb, E. N. & Hoehn, K. Human Anatomy & Physiology, 10th ed. (Pearson, 2016).
  1. Campbell, N. A. & Reece, J. B. Biology, 7th ed. (Pearson Benjamin Cummings, 2005).
  1. Alberts, B., Johnson, A., Lewis, J., Raff, M., Roberts, K. & Walter, P. Molecular Biology of the Cell, 5th ed. (Garland Science, 2008).
  1. Alberts, B., Bray, D., Hopkin, K., Johnson, A., Lewis, J., Raff, M., Roberts, K. & Walter, P. Essential Cell Biology, 3rd ed. (Garland Science, 2010).
  1. Bear, M. F., Connors, B. W. & Paradiso, M. A. Neuroscience: Exploring the Brain, 4th ed. (Wolters Kluwer, 2016).
  1. Haines, D. E. (edited by). Fundamental Neuroscience for Basic and Clinical Applications, 3rd ed. (Churchill Livingstone Elsevier, 2006).
  1. Hammond, C. Cellular and Molecular Neurophysiology, 4th ed. (Academic Press, 2014).
  1. Siegel, A. & Sapru, H. N. Essential Neuroscience. (Lippincott Williams & Wilkins, 2010).