12.3: Supportive Cells (Neuroglia)

Types of Neuroglia (glia)

50:1 (neuroglia cells: neurons); function is to protect the neurons and aid their function; 6 types

Oligodendrocyte

Type of neuroglia in CNS; bulbous body with numerous arm-like processes (up to 15), each reaches out to a nerve fiber and spirals around it (myelin sheath); function is to insulate the nerve fiber from ECF and speeds up signal conduction in nerve fiber

Astrocytes

Type of neuroglia in CNS that is most abundant; most functionally diverse; covers entire brain and most nonsynaptic regions of the neurons (acts as seal and cover); function: supportive framework of nerve tissue, blood brain barrier, convert blood glucose to lactate (regulates just enough nourishment, not too much), secrete nerve growth factors communicate electrically with neurons, regulate the chemical composition of tissue fluid, and astrocytosis or sclerosis (forms scar tissue (hardened mass) and fill space when neurons are damaged)

Ependymal Cells

Type of neuroglia in CNS; like the cuboidal epithelial cell; cilia are present on apical (one side) surface and help circulate cerebrospinal fluid (CSF); lines internal cavities of the brain; function: produce (CSF) cerebral spinal fluid (clear liquid, that bathes the CNS and fills its cavities)

Microglia

Type of neuroglia in CNS;

small macrophages (developed from monocytes (WBC); function: phagocytize dead nerve tissue, microorganisms and other foreign matter; concentrate in damaged areas

Schwann Cells

Type of neuroglia in PNS;

envelop nerve fibers of PNS (winds around the axon and produces myelin sheath); function: assist in regeneration of damaged fibers

Satellite Cells

Type of neuroglia in PNS; Surround the neuron cell bodies (soma) in ganglia of PNS; provides electrical insulation and regulates chemical environment

Myelin Sheath

An insulating layer around a nerve fiber (axon); formed by oligodendrocytes (CNS) and schwann cells (PNS); each oligodendrocyte reaches out to many nerve fibers, but it takes many oligodendrocytes to cover one nerve fiber; comprise 20% proteinsand 80% lipids (phospholipids, glycolipids, and cholesterol); MOA: reaches out to the nerve fiber and spirals then fiber repeatedly (with little cytoplasm between them) and then basal lamina and then endoneurium (fibrons connective tissue); most of cytoplasm and nucelensis in between the myelin sheath and neurilemma. Spirals around a single nerve fiber (~ 100’s of layers) and outermost layer is specifically named neurilemma

Myelination

Begins around 14th week of fetal development, not a lot though around birth, rapidly increase in infancy, completed around late adolescence (dietary fat improve for development); function is initiating a nerve signal

Unmyelinated Axons

The CNS has no myelin; PNS “unmyelinated” areas still have a single layer of myelin (of Schwann cells and neurilamna) and somewhat overlays itself along the edges; basal lamina on the very outside

Conduction Velocity

Speed at which a nerve signal travels down an axon, depends on 2 factors

Diameter of the fiber (axon)

Is a determinant of conduction speed; conduction occurs along surface of nerve fiber, increase in diameter means more surface area which leads to increase conduction speed

Presence or absence of Myelin

Increases myelin means increase speed of conduction; different speeds of conduction are needed in the body

Nerve Regeneration

Injury which leads to severed distal end of axon and it’s myelin sheath degenerate and macrophages remove debris, axon stump sprouts numerous extensions and one of them makes contact with the regeneration tube (formed by neurilemma and endoneurium), sprout/extension begins to grow rapidly that makes contact and others are reabsorbed (not in use), tube guides the regenerating axon until healed (back to original position); can only occur in PNS, not CNS; soma and some neurolemma has to be present