Morphology and Cytology of Neurons and Glia

I.          Neurons

            A.            similar to other body cells with a few exceptions

1.            generally do not divide after a certain point (although current research suggests some poorly understood exceptions)

                        2.            have unique membranes specialized for communication

            B.            Some of the principle components

                        1.            membrane

                        2.            cytosol (+organelles=cytoplasm)

                        3.            nucleus

                        4.            mitochondria

                        5.            endoplasmic reticulum

                                        a.    rough

                                        b.    smooth

                        6.            Golgi apparatus

                        7.            cytoskeleton

                                    a.            microfilaments (~3-5 nM, actin)

                                    b.            neurofilaments (~10 nM, involved in Alzheimer’s D.)

c.            microtubules (~20-30 nM, transport substances via ortho- and retrograde transport)

            C.            Common Neuronal Structures

                        1.            soma (cell body)

                        2.            dendrites (may be none, may be thousands)

                        3.            axon (generally one, may or may not be myelinated)

                        4.            axon terminals

                                    a.            button (buton) (e.g., cholinergic neurons)

                                    b.            varicosities (e.g., adrenergic neurons)

                                    c.            terminal arbor (may be low-high degree of arborization)

            D.            General Types of Neurons

                        1.            multipolar (e.g., motor)

                        2.            bipolar (e.g., sensory)

                        3.            unipolar

a.            true unipolar (has one process that serves as both dendrite and axon)

b.            pseudounipolar (has one stalk that divides into a dendritic and an axonal process)

            E.            Functional Categories

                        1.            Sensory (afferent)

                        2.            Motor (efferent)

                        3.            interneurons

                                    a.            local circuit

                                    b.            projection

II.         Supporting Cells (Glia)

            A.            Macroglia

                        1.            astrocytes

                        2.            radial glia (become astrocytes after completion of development)

3.            oligodendrocytes (serve as myelin in the CNS, do NOT promote axonal regeneration)

4.            Schwann cells (serve as myelin in the PNS, promote axonal regeneration following injury)

5.            ependymal cells (line the fluid-filled cavities, create arachnoid membrane, help with fluid transport within the vesicles and central canal, form a passive brain-CSF barrier, are multipotent)

            B.            Microglia

III.       From Nerve Fiber to Nerve

            A.            cell bodies

                        1.            nuclei (CNS)

2.            ganglia in (PNS)

            B.            axons

                        1.            tracts (CNS)

                        2.            nerves (PNS)

            C.            layers of the nerve sheath (from outside to inside)

                        1.            epineurium

                                    a.            connective tissue (collagen)

                                    b.            also contain blood and lymph vessels

                        2.            perineurium

                                    a.            surrounds fascicles (bundles of axons)

                                    b.            allows for separate tracts

                        3.            endoneurium

a.            surrounds individual axons, whether they are myelinated or not

IV.        Blood-Brain Barrier (and Brain-Blood Barrier)

            A.            Formed of endothelial cells

                        1.            endothelial cells form capillary wall and are tightly joined

                        2.            astrocytes lie on top of endothelial cells to further create a barrier

            B.            weaker at certain areas--circumventricular organs

                        1.            e.g., area postrema

                        2.            allows low levels of toxins to penetrate and trigger vomiting

                        3.            very important protective mechanism