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Nervous Tissue Ch. 17, p257

Nervous Tissue • Neurons • Neuroglia – CNS: • • • •

Astrocytes Oligodendrocytes Microglia Ependymal Cells

– PNS: • Schwann Cells • Satellite Cells

Neuron Anatomy • Cell body (aka: perikaryon= cyton =soma) – single, central nucleus with large nucleolus, cell organelles • Dendrites – for receiving signals, conduct impulses toward the cell body • Axon – for sending signals, conduct impulses away from the cell body to effectors – Covered by lipid myelin

Myelin

Myelin Sheath Formation

• Myelination begins during fetal development, but proceeds most rapidly in infancy.

Myelin Sheath • Insulating layer around a nerve fiber • 20% protein & 80 % lipid (looks white) • In PNS, hundreds of layers wrap axon – the outermost coil is schwann cell (neurilemma)

• In CNS, no neurilemma • Nodes of Ranvier – Gaps between myelin segments

• Initial segment (area before 1st schwann cell) & axon hillock form trigger zone where signals begin

Axon Nerve Impulse Conduction Speed

Myelinated - 300MPH

Unmyleinated - 2 MPH

Neuron Classification • Unipolar Neuron – Single process from cell body that is divided into axon and dendrite – Conduct impulses toward CNS, sensory neurons

• Bipolar Neuron – One dendrite and one axon – Occurs in retina of eye

• Multipolar Neuron – Multiple dendrites, one axon – Carry impulses away from CNS, Motor Neurons

Neuron Model

Neuron Model

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Anatomy of the Brain and Cranial Nerves Ch. 19, pg. 279