The Human Brain : From Neurone to Nervous System

GLIAL CELLS


  1. Glial cells have been described as the cells that support neurones in the CNS. They occur in much the same numbers as neurones, and have many functions that are relevant to neurones.

  2. There are four main types of glia, with separate functions:
    • Astrocytes link cerebral blood vessels and neurones, and supply nutrients to neurones; they also participate in the removal and metabolism of neurotransmitters.

    • Oligodendrocytes provide the myelin wrapping of large axons in the CNS, and are therefore responsible for maintaining a high speed of conduction in central nervous pathways.

    • Microglia destroy pathogens and remove dead neurones. They are the dormant phagocytes of the central nervous system, and can be activated to become phagocytes in these tissues.

    • Ependymal cells are ciliated cells that line the ventricular system, and form a cellular barrier between the cerebrospinal fluid and the nervous system.







MicroGlia

Microglia: the dormant phagocytes of the CNS.

Microglia exist in several forms: they are normally present in an inactive form, but can be stimulated or primed to become active.

Active microglia are phagocytic and remove cell debris such as dead or damaged, degenerating neurones, bacteria and viruses.


The factors that trigger activation of Microglia include:

  • viruses, bacteria and some CNS toxins.
  • dead cells or cellular debris
  • damaged neurones.
  • other activated microglia.





Ependymal cells are cells within the membrane surrounding the ventricles and central canal of the spinal cord. They have cilia in contact with the cerebrospinal fluid and processes that extend into the neuropil, where they make contact with other glia or neurones.

Ependymal Cells

Ependymal cells line the cavities of the CNS and make up the walls of the ventricles.

Ependymal cells have cilia that beat and help circulate the CSF and make up the Blood-CSF barrier.


Tanycytes

Tanycytes are specialised ependymal cells found in the floor of the third ventricle; these cells have processes that extend deep into the hypothalamus and it is thought that they may convey signals from the CSF to the nuclei of the rostral ventral hypothalamus.

Tanycytes possibly have a role in the release of gonadotrophic hormone releasing hormone.




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