Chapter 12.4: Characteristic and Types of Glial Cells, Myelination

Glial cells (neuroglia)

• nonexecitable, support cells in CNS and PNS

• same number of glial as there are neurons

• account for half volume of NS

Glial characteristics

• mitosis

• protect and nourish neurons

• physical scaffolding for nervous tissue

• guide migrating neurons during development

• critical normal function at neural synapses

Glial cells within CNS: Astrocytes

• star-shaped

• processes end in perivascular feet

• most abundant glial in CNS

• form blood-brain barrier by wrapping feet around brain capillaries

• regulates fluid composition

  • regulates potassium concetration

• form structural support for nearby neurons

• assist neuronal decelopment

• alter synaptic activity (add, eliminate, influence)

• occupy space of dying neurons

Glial cells within CNS: Ependymal

• line cavities in brain and spinal cord

• part of choroid plexus which produces cerebrospinal fluid

Glial cells within CNS: Microglia

• small cells wander CNS and replicate infection

• phagocytic cells of immune system

Glial cells within CNS: Oligodendrocytes

• large cells with slender extensions

• extensions wrap around axons of neurons forming myelin sheath

Glial cells within PNS: Satellite

• around neuronal bodies in ganglion

• electrically insulate and regulate exchange of nutrients and waste

Glial cells within CNS: Schwann (neurolemmocytes)

• elongated, flat cells

• surround PNS axons with myelin

• allow faster potential propagation

Myelination

process wrapping axon with myelin

Myelin

• several layers of membrane of glial cells

• high lipid content gives glossy-white appearance and insulates axon

• glia are Schwann cells in PNS; oligodendrocytes in CNS

• schwann cells encircle neuron axon, wrap it forming myeling sheath

• schwann cells cytoplasm and nucleus pushed to peripher forming neurilemma

• schwann myelinate only 1 mm of axon, several are needed

• gaps are nodes of ranvier

Myelin in PNS

• schwann cells encircle neuron axon, wrap it forming myeling sheath

• schwann cells cytoplasm and nucleus pushed to peripher forming neurilemma

• schwann myelinate only 1 mm of axon, several are needed

• gaps are nodes of ranvier

Myelin in CNS

• oligodendrocytes myelinate 1 mm of multiple axons, each at multiple spots

• no neurilemma formed

• neurofibril nodes between adjacent wrapped segments