Nervous Tissue Ectodermal in origin; Neurulation Formation of neural tube and neural crest. Cell Types: a) Neurons - CNS and PNS b) Ependyma - CNS c) Neuroglia - CNS d) Schwann Cells - PNS
Nerve Cell Origins A) Neural Tube: 1) Matrix Layer - matrix cells become ependymal -> neural tube lining cells 2) Mantle Layer – migrate from matrix a) Glioblasts Astroblasts -> Astrocytes Oligodendroblasts -> Oligodendrocytes b) Neuroblasts -> neurons 3) Marginal Layer - mantle neuron cell bodies form gray matter while their axons migrate out into an outer Marginal layer forming white matter of CNS.
Nerve Cell Origins B) CNS CT forms Microglia cells. • Neural Crest: - PNS and other structures 1) Chromaffin Cells 2) Schwann Cells 3) Melanocytes 4) Odontoblasts
Neurons Nerve cells are capable of depolarization (Excitable) Function in impulse formation, transfer, interpretation. Neurons form functional links (circuits) a) afferent = sensory neuron that respond to changes in environment (stimuli) b) interneurons = connector neurons c) efferent = motor neuron (response)
Neuron Cell Body with nucleus = Perikaryon Cell Body - cytoplasm has prominent numbers of basophilic granules (ribosomes and ER) = Nissl's Bodies Usually two kinds of fibrous processes - dendrite and axon. Dendrite a) non-myelinated, b) brings impulse to NCB, c) contains microtubules. Axon a) myelinated, b) takes impulse from NCB, c) contains microfilaments.
Neuron Morphology Several levels of neuron organization: a) apolar - modified neurons with no fibrous process, unique to Pineal gland and Adrenal medulla. b) unipolar (pseudounipolar) - one axon, no dendrite, PNS afferent. c) bipolar - one axon and one dendrite; special neuron types; retina of eye; olfactory neuron. d) multipolar; one axon and many dendrites; CNS efferent and afferent; PNS Efferents.
Fibrous Processes Axons Cylinder of cytoplasm = axoplasm; Up to 1 meter (40 inches) Plasma membrane = axolemma; Axons are encased in a sheath of cells; Schwann in the PNS, Oligodendrocytes in CNS.
Myelin Encasing cells have myelin (lipid) in membranes - produce a white appearance in living state. When the cell wraps many times = myelinated or medullated neuron with white outer cover; Serves to speed up impulse transfer Ends of Schwann Cells produce segmented nodes = Nodes of Ranvier
RF = reticular fibers P = perineurium (nucleus) S = Schwann Cell (nucleus)
Unmyelinated Neurons Present in CNS and PNS In PNS, axon in simple cleft in Schwann Cell, no wrapping, no Nodes of Ranvier. In CNS, totally unsheathed, many in number (3-4 x as many as myelinated.)
Ultrastructural features of myelinated (A) and unmyelinated (B) nerve fibers. • Nucleus and cytoplasm of a Schwann cell; • axon; • microtubule; • neurofilament; • myelin sheath; • mesaxon; • node of Ranvier; • interdigitating processes of Schwann cells at the node of Ranvier; • side view of an unmyelinated axon; • basal lamina.
Synaptic Communication Gap between adjacent neuron ends = synapse. Chemical Synapse: In the synapse, the axon terminal secretes chemical neurotransmitter (e.g. acetylcholine) which transfers impulse from one neuron to the next via a specific receptor on the post-synaptic cell. Common neurotransmitters include: Acetylcholine, Norepinephrine, Dopamine, Serotonin and Glutamate, and GABA Electrical Synapse: Gap junctions - cytoplasmic connections between adjacent cells.