When people consider the cells in the brain they are often too quick to overlook the most abundant type of brain cell. Scaffolding cells, otherwise known as glial cells, outnumber other types of neurons in the brain. Oligodendrocytes alone outnumber neurons by an estimated ratio of 3:1. There are four different types of glial cells – oligodendrocytes, astrocytes, microglia, and Schwann Cells – each with its own specific attributes, functions, and characteristics.
Oligodendrocytes
Oligodendrocytes are what many people consider the classic glial cell. This is the cell that forms a myelin sheath around axons. The myelin formed by these sheaths (or laminae) allows an action potential to propagate faster than if the axon were unmyelinated because of the time it takes for sodium channels to open. It takes longer for the channels to open than it does for the action potential to propagate, so a myelinated axon will transmit a signal at about 1.7 times the speed of an unmyelinated axon.
Astrocytes
Astrocytes have many functions, some of which are unknown. Astrocytes have long processes, at the ends of which are footpods (or feet). These form tight junctions with one another and tend to surround blood vessels, forming the blood-brain barrier. Astrocytes also surround synaptic clefts, scavenging glutamate or other neurotransmitters that spill out of the synapse. Some people hypothesize that astrocytes even form their own network, feeding off the synapses of neurons like pyramidal cells.
Microglia
Microglia clear away dead or dying cells. These small glial cells can become activated and perform the function of a rudimentary immune system in the brain. Microglia are activated by trauma and help destroy infectious cells.
Schwann Cells
Schwann cells form myelin sheaths in the peripheral nervous system (more commonly know as the spinal cord). Unlike oligodendrocytes, each of which can myelinate more than one axons (in some cases up to 30 different axons), each Schwann cell only myelinates one axons.
Glial cells play many roles in the brain from scavenging debris and providing scaffolding, support, and protection for neurons, to putatively performing information integration functions. While many researchers devote their studies to both the structure and function of glial cells, a better understanding of the functions of glial cells will further efforts in fighting brain-related diseases. Multiple sclerosis, which causes a breakdown of myelin sheaths surrounding axons, and brain trauma damaging nerve cells are just a couple of the many brain-related problems, which a better understanding of glial function will help resolve.
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