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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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nervous tissue
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
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 origins6
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
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
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
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
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
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

slide17

RF = reticular fibers

P = perineurium (nucleus)

S = Schwann Cell (nucleus)

unmyelinated neurons
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.)

slide20
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
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.