Nerve Tissue & The Nervous System 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 Neurulation Nerve Cell Origins
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Ectodermal in origin; Neurulation
Formation of neural tube and neural crest.
a) Neurons - CNS and PNS
b) Ependyma - CNS
c) Neuroglia - CNS
d) Schwann Cells - PNS
A) Neural Tube:
1) Matrix Layer - matrix cells become ependymal -> neural tube lining cells
2) Mantle Layer – migrate from matrix
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.
B) CNS CT forms Microglia cells.
1) Chromaffin Cells
2) Schwann Cells
Nerve cells are capable of depolarization
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)
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.
b) brings impulse to NCB,
c) contains microtubules.
b) takes impulse from NCB,
c) contains microfilaments.
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;
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.
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
P = perineurium (nucleus)
S = Schwann Cell (nucleus)
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.)
Gap between adjacent neuron ends = 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
Gap junctions - cytoplasmic connections between adjacent cells.