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 Pre

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