Summary
Highlights
The neuron is the star, but it relies heavily on a 'supporting cast' of cells, primarily glia cells. The central nervous system (CNS) has fluid-filled ventricles lined by ependymal cells that maintain fluid balance. Capillaries are also essential for supplying oxygen and nutrients to aerobic cells and removing metabolic end products.
Glia, meaning 'glue' in Greek, refers to neuron-supporting cells. Their exact number compared to neurons varies in estimates but is generally considered to be at least as numerous, if not 2-10 times more. Glia cells are not electrically excitable but use other cellular processes like exocytosis and endocytosis. They serve as stem cells, offering some limited regenerative capacity to the nervous system, especially evident in recovery from mild brain injuries.
Astrocytes are star-shaped glia cells that provide an 'insulation effect' by separating neurons, contributing to the health of synapses. They regulate extracellular potassium levels, which is crucial for neuronal activity. Astrocytes also improve signaling efficiency by cleaning out neurotransmitters after communication and stimulating the growth of new synapses through growth factors.
Microglia cells are derived from bone marrow and act as the resident immune cells of the central nervous system. They are responsible for cleaning up normal metabolic end products and protecting against harmful invaders like bacteria and viruses, as well as injured cells. They secrete signaling molecules (cytokines) to modulate inflammatory responses, being careful not to damage healthy tissue, as seen in autoimmune disorders.
Schwann cells are glia cells found in the peripheral nervous system (PNS) that produce myelin, the insulation around axons. Myelination is not a continuous sheath but has interruptions called nodes of Ranvier. Myelin acts as electrical insulation, preventing the length-dependent decay of action potentials and significantly increasing the speed and efficiency of communication (e.g., >100 meters per second for myelinated fibers).
Oligodendrocytes are the glia cells responsible for myelination in the central nervous system (CNS). Unlike Schwann cells, which myelinate one axon, a single oligodendrocyte can myelinate up to 30 different axons, making them more efficient. They are attracted to active axons by metabolic end products like ATP, adenosine, and lactic acid, which signal the need for myelination to support communication.