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Nervous Tissue Graduate Microanatomy Spring 1999 Sandra H. Bolanos Central Nervous System • Brain • Spinal Cord Brain • Cranium (brain case) • Cortex Gray matter Nerve cell bodies • Interior Portion White matter Axons Ventricles CSF Spinal Cord • Occupies two-thirds of the vertebral canal • Enveloped by meninges • Cervical and lumbar enlargements • Gray matter centrally located Meninges • 3 layers of connective tissue membranes • Surround and protect brain and spinal cord Pia Mater Arachnoid Dura Mater Peripheral Nervous System • Nerves that emerge from central nervous system: 12 pairs of cranial nerves 31 pairs of spinal nerves Autonomic Nervous System Sympathetic Parasympathetic Cellular Composition of CNS • Neurons and their processes • Support cells: Astrocytes Oligodendrocytes Ependymal cells Microglia Astrocytes • In developing NS, form structural framework to guide migration of developing neurons • In developed NS, form structural scaffolding for more specialized neural elements • Clear ECM of by-products of neural activity and contain glycogen reserves • Extend foot processes around blood capillaries Blood Brain Barrier • Capillaries of the CNS not fenestrated and have intercellular tight junctions between endothelial cells • Highly resistant to passage of ions or small molecules • Do not exhibit transendothelial transport in small vesicles • Astrocytes may influence capillaries to express these unique properties Oligodendrocytes • Produce myelin within CNS • Each cell sends out several processes and myelinates several nearby axons Ependymal Cells • Epithelial-like • Ciliated • Line cavities of brain ventricles and central canal of spinal cord forming sheets of cuboidal cells in contact with CSF Microglia • Specialized macrophages • In areas of injury, proliferate and become actively phagocytic in clearing cellular debris and ingesting damaged myelin Composition of PNS • Ganglia - a peripheral collection of nerve cell bodies together with efferent and afferent axons and support cells • Nerves - a collection of axons linked together by support tissue into an anatomically defined trunk Ganglia • • • • Sensory (spinal sensory ganglia) Non-sensory (sympathetic or parasympathetic) Dorsal root ganglia, cranial ganglia, autonomic ganglia Neuron cell bodies, support cells, loose fibrocollagenous tissue Nerves • Motor Innervate skeletal muscle (CNS) Innervate smooth muscle (PNS) • Sensory • Myelinated or Non-myelinated • Axons, Schwann cells, fibroblast cells, blood vessels - 3 types of support tissue in a nerve trunk: • Endoneurium Surrounds individual axons, their associated Schwann cells, and capillary blood vessels • Perineurium Surrounds groups of axons and endoneurium to form fascicles • Epineurium binds individual nerve fascicles into a nerve trunk • Endoneurium Collagen fibers that are longitudinally oriented ECM rich in GAGs and sparse fibroblasts • Perineurium 7-8 concentric layers of epithelium-like flattened cells separated by layers of collagen and joined by junctional complexes Each layer of cells surrounded by an external lamina • Epineurium Outer sheath of loose fibrocollagenous tissue May also include adipose tissue and muscular artery that supplies the nerve trunk Peri Epi Fascicle Endo Schwann Cells • PNS • Myelinate only one axon Neurons • • • • Gather information from sensory receptors Process information and provide a memory Generate appropriate signals to effector cells Cell body, dendrites, axon, and synaptic bouton Cell Body • • • • • • • • • • Pale-staining Conspicuous nucleolus Little heterochromatin Nissl bodies Golgi complex Mitochondria Lysosomes Neurofilaments Microfilaments Microtubules Dendrites • • • • Radiating processes of the cell body Receive signals (synapses) from other neurons Broader than axons Extensive branching increases cell surface available for receiving signals from other neurons • Organelles similar to those of the perikaryon (no Golgi bodies) • As distance from cell body increases, smooth ER and NFs reduced but MTs and mitochondria still prominent Axons • A single long process capable of generating a nerve impulse • More slender and usually longer than dendrites • Branch at right angles • Can be quite long (spinal motor neurons that supply foot muscles 40 inches in length) • Axon Hillock is conical extension of cell body from which axon arises • Axoplasm lacks Nissl bodies • • • • Branches as it approaches its end forming small expansions Terminal boutons contact other cells to form a synapse At synapse, chemicals or electrical signals pass from one neuron to another cell known as the effector cell Neurotransmitters act rapidly and locally to activate their target cells and neuromodulators that regulate these events Neuron Shapes Neuron Types Myelin • Insulation minimizes leakage of current from membrane speeds up conduction along axons • Reduction of electrical capacitance wide axons lower capacitance than narrow ones increases diameter of axons increases speed of nerve conduction • Myelin-producing cells Oligodendrocytes (CNS) Schwann cells (PNS) • Nodes of Ranvier space between each unit of myelin increase efficiency of nerve conduction Myelin Synapses • Specialized region of contact where NT released from axon to stimulate another cell Axonal Transport • • • • • • Anterograde or retrograde Fast or slow Microtubules Motor proteins Ca2+ ATP Minus end Kinesin Plus end Dynein
