Chapter 12: Glial Cells

Neurons: Excitable, present in CNS and PNS, larger and less numerous than glial cells, and incapable of mitosis in adults.
Glial cells (neuroglia): Non-excitable, present in CNS and PNS, smaller and outnumber neurons, capable of mitosis.
General functions: Protect and nourish neurons, supply physical scaffolding, guide migrating neurons during development, and support neural synapse function.

Astrocytes: Most abundant CNS glia; star-shaped with perivascular feet; maintain blood-brain barrier, regulate interstitial fluid composition, offer structural support, assist development, and replicate to occupy space left by dying neurons.
Ependymal cells: Ciliated simple cuboidal or simple columnar cells lining brain ventricles and spinal cord central canal; contribute to the choroid plexus to produce and circulate cerebrospinal fluid (CSF).
Microglia: Small, phagocytic immune cells that wander the CNS, replicate during infection, engulf pathogens, and clear cellular debris.
Oligodendrocytes: Large cells with extensions that wrap around CNS axons to form myelin sheaths without forming a neurilemma; a single cell can myelinate $1\,mm$ of multiple axons.

Neurolemmocytes (Schwann cells): Encircle PNS axons to form myelin sheaths and a peripheral neurilemma (containing cytoplasm and nucleus); consecutive cells leave gaps called neurofibril nodes (nodes of Ranvier).
Satellite cells: Positioned around neuronal cell bodies in ganglia to provide electrical insulation and regulate nutrient and waste exchange.

Perivascular feet of astrocytes wrap around brain capillaries to protect delicate neurons from potential blood toxins, wastes, and abnormal solute concentrations.
Brain capillaries contain specialized endothelial cells with tight junctions and a thickened basement membrane; lipophilic substances such as alcohol easily cross.
Absent in the Choroid Plexus, Hypothalamus, and Pineal Gland.

Clear, colorless liquid formed by the choroid plexus (layer of ependymal cells and capillaries within the pia) inside brain ventricles.
Functions: Provides buoyancy (reduces brain's apparent weight by $95\%$ ), cushions against physical trauma, and stabilizes the chemical environment.
Composition: Contains higher concentrations of $Na^+$ and $Cl^-$ and lower concentrations of $K^+$ , $Ca^{2+}$ , and glucose relative to blood plasma.
Flow Route: Choroid Plexus \rightarrow Lateral Ventricles \rightarrow Interventricular foramen \rightarrow Third Ventricle \rightarrow Cerebral Aqueduct \rightarrow Fourth Ventricle \rightarrow Medial and lateral apertures \rightarrow Subarachnoid Space \rightarrow Arachnoid Villi \rightarrow Dural Venous Sinuses.

Process of wrapping an axon with lipid-rich, glossy-white myelin membrane to provide electrical insulation and enable saltatory conduction, increasing propagation speed.
Unmyelinated axons propagate signals more slowly; they associate with Schwann cells (without forming a myelin sheath) in the PNS and are not associated with oligodendrocytes in the CNS.