Classification of Neurons

Nature of the Nervous System n Camillo Golgi n The Synctium l continuous network l no gaps ~

Nature of the Nervous System n Santiago Ramon y Cajal n The Neuron Doctrine l discrete cells l communication across gaps l synapses ~

Nature of the Nervous System n Golgi won l Proved himself wrong l Golgi Stain - silver chromate Supported Neuron Doctrine Golgi & Cajal shared Nobel Prize (1906) ~

Classification of Neurons l Structural – based on the number of cytoplasmic processes Multipolar neurons Bipolar neurons Unipolar neurons l Functional – based on the direction of impulse transmission Sensory neurons Motor neurons Interneurons (association)

Histology of Nervous Tissue l 2 types of cells Neurons –Structural & functional part of nervous system –Specialized functions Neuroglia (glial cells) –Support & protection of nervous system

NEURONS n Basic functional unit of N.S. n Specialized cell l All cells have same basic properties n information processing Transmits Integrates Stores n Regulation of behavior ~

Neurons l Function Conduct electrical impulses l Structure Cell body –Nucleus with nucleolus –Cytoplasm (perikaryon) Cytoplasmic processes –Dendrites –Axon

Review of Neuron Structure

Nervous Tissue: Neurons Slide 7.8 Copyright © 2003 Pearson Education, Inc. publishing as Benjamin Cummings  Neurons = nerve cells  Cells specialized to transmit messages  Major regions of neurons  Cell body – nucleus and metabolic center of the cell  Processes – fibers that extend from the cell body (dendrites and axons)

Neuron Cell Body Location Slide 7.13 Copyright © 2003 Pearson Education, Inc. publishing as Benjamin Cummings  Most are found in the central nervous system  Gray matter – cell bodies and unmylenated fibers  Nuclei – clusters of cell bodies within the white matter of the central nervous system  Ganglia – collections of cell bodies outside the central nervous system

Neuron Anatomy Slide 7.9b Copyright © 2003 Pearson Education, Inc. publishing as Benjamin Cummings  Cell body  Nucleus  Large nucleolus Figure 7.4a

Neuron Anatomy Slide 7.10 Copyright © 2003 Pearson Education, Inc. publishing as Benjamin Cummings  Extensions outside the cell body  Dendrites – conduct impulses toward the cell body  Axons – conduct impulses away from the cell body (only 1!) Figure 7.4a

Axon Structure l Long, specialized Collaterals = branches Telodendria = termination of axons & collaterals l Cytoplasm = axoplasm l Plasma membrane = axolemma

Anatomy of a Neuron

Axons and Nerve Impulses Slide 7.11 Copyright © 2003 Pearson Education, Inc. publishing as Benjamin Cummings  Axons end in axonal terminals  Axonal terminals contain vesicles with neurotransmitters  Axonal terminals are separated from the next neuron by a gap  Synaptic cleft – gap between adjacent neurons  Synapse – junction between nerves

Nerve Fiber Coverings Slide 7.12 Copyright © 2003 Pearson Education, Inc. publishing as Benjamin Cummings  Schwann cells – produce myelin sheaths in jelly-roll like fashion  Nodes of Ranvier – gaps in myelin sheath along the axon Figure 7.5

Application n In Multiple Scleroses the myelin sheath is destroyed. n The myelin sheath hardens to a tissue called the scleroses. n This is considered an autoimmune disease. n Why does MS appear to affect the muscles?

Neuron Classification Slide 7.15 Copyright © 2003 Pearson Education, Inc. publishing as Benjamin Cummings Figure 7.6

Types of Neurons n Functional classification l Sensory or afferent: Action potentials toward CNS l Motor or efferent: Action potentials away from CNS l Interneurons or association neurons (CNS) l Principle neurons / projection neurons (CNS) n Structural classification l Multipolar, bipolar, unipolar

Shorthand Neuron Classification Categories overlap ~

Functional Classification of Neurons Slide 7.14b Copyright © 2003 Pearson Education, Inc. publishing as Benjamin Cummings  Interneurons (association neurons)  Found in neural pathways in the central nervous system  Connect sensory and motor neurons

Functional Classification of Neurons Slide 7.14a Copyright © 2003 Pearson Education, Inc. publishing as Benjamin Cummings  Sensory (afferent) neurons  Carry impulses from the sensory receptors  Cutaneous sense organs  Proprioceptors – detect stretch or tension  Motor (efferent) neurons  Carry impulses from the central nervous system

By function (connections) Interneuron SensoryMotor

Unipolar BipolarMultipolar By morphology (# of neurites)

Structural Classification of Neurons Slide 7.16a Copyright © 2003 Pearson Education, Inc. publishing as Benjamin Cummings  Multipolar neurons – many extensions from the cell body Figure 7.8a

Structural Classification of Neurons Slide 7.16b Copyright © 2003 Pearson Education, Inc. publishing as Benjamin Cummings  Bipolar neurons – one axon and one dendrite Figure 7.8b

Structural Classification of Neurons Slide 7.16c Copyright © 2003 Pearson Education, Inc. publishing as Benjamin Cummings  Unipolar neurons – have a short single process leaving the cell body Figure 7.8c

Neuroglia l Neuroglia of CNS Astrocytes –Form the blood-brain barrier –Form a structural framework for the CNS –Repair damaged neural tissue –Control the interstitial environment of the CNS Oligodendrocytes –Form myelin sheaths CNS Microglia –Phagocytose foreign microbes, etc. Ependymal –Line ventricles of the brain, secrete cerebrospinal fluid l Neuroglia of PNS Schwann cells –Form myelin sheaths of PNS Satellite cells

Classification of Glial Cells

Neuroglia of CNS

Nerve Fibers of the PNS l An axon and its sheaths Myelinated axon –Axon is surrounded by a myelin sheath Unmyelinated axon –Axon has no myelin sheath

Myelin l White matter of nerves, brain, spinal cord l Composed primarily of phospholipids l Production Developing Schwann cells wind around axon l Function Increases speed of impulse conduction Insulation and maintenance of axon

Schwann Cells and Peripheral Axons

Myelin l Neurilemma Peripheral cytoplasmic layer of the Schwann cell enclosing the myelin sheath l Nodes of Ranvier Unmyelinated gaps between segments of myelin Impulses “jump” from node to node

A Myelinated Axon

Nerve Fibers of the CNS l Umyelinated l Myelinated Production of myelin is from oligodendrocytes Nodes of Ranvier are less numerous

Nerve Fibers of the CNS

Structural Classification of Neurons

Functional Classification of Neurons

By Dendrite Structure Pyramidal vs.Stellate Dendritic Spines n Spiny or Aspinous ~

Other Classification Schemes n Axon length l Golgi Type I - long l Golgi Type II - short (local signalling) n Chemistry l neurotransmitter released ~

How Neurons Function (Physiology) Slide 7.17 Copyright © 2003 Pearson Education, Inc. publishing as Benjamin Cummings  Irritability – ability to respond to stimuli  Conductivity – ability to transmit an impulse  The plasma membrane at rest is polarized  Fewer positive ions are inside the cell than outside the cell

Starting a Nerve Impulse Slide 7.18 Copyright © 2003 Pearson Education, Inc. publishing as Benjamin Cummings  Depolarization – a stimulus depolarizes the neuron’s membrane  A deploarized membrane allows sodium (Na + ) to flow inside the membrane  The exchange of ions initiates an action potential in the neuron Figure 7.9a–c

The Action Potential Slide 7.19 Copyright © 2003 Pearson Education, Inc. publishing as Benjamin Cummings  If the action potential (nerve impulse) starts, it is propagated over the entire axon  Potassium ions rush out of the neuron after sodium ions rush in, which repolarizes the membrane  The sodium-potassium pump restores the original configuration  This action requires ATP

Nerve Impulse Propagation Slide 7.20 Copyright © 2003 Pearson Education, Inc. publishing as Benjamin Cummings  The impulse continues to move toward the cell body  Impulses travel faster when fibers have a myelin sheath Figure 7.9c–e

Stimuli n Receives information n Integrates information~

n Axon carries information away from soma n Electrical signal n Axon terminal releases chemical message~

Continuation of the Nerve Impulse between Neurons Slide 7.21 Copyright © 2003 Pearson Education, Inc. publishing as Benjamin Cummings  Impulses are able to cross the synapse to another nerve  Neurotransmitter is released from a nerve’s axon terminal  The dendrite of the next neuron has receptors that are stimulated by the neurotransmitter  An action potential is started in the dendrite

Neuronal Communication: n Within a neuron, electrical signals called action potentials travel along the axon. n Communication between neurons is mediated via synaptic transmission (e.g. by means of neurotransmitters such as glutamate, GABA, acetylcholine). n Transfer of information from one part of body to another.

How Neurons Communicate at Synapses Slide 7.22 Copyright © 2003 Pearson Education, Inc. publishing as Benjamin Cummings Figure 7.10

Glia Neural Support Cells

Nervous Tissue: Support Cells (Neuroglia or Glia) Slide 7.5 Copyright © 2003 Pearson Education, Inc. publishing as Benjamin Cummings  Astrocytes  Abundant, star-shaped cells  Brace neurons  Form barrier between capillaries and neurons  Control the chemical environment of the brain (CNS) Figure 7.3a

Nervous Tissue: Support Cells Slide 7.6 Copyright © 2003 Pearson Education, Inc. publishing as Benjamin Cummings  Microglia (CNS)  Spider-like phagocytes  Dispose of debris  Ependymal cells (CNS)  Line cavities of the brain and spinal cord  Circulate cerebrospinal fluid Figure 7.3b, c

Nervous Tissue: Support Cells Slide 7.7a Copyright © 2003 Pearson Education, Inc. publishing as Benjamin Cummings  Oligodendrocytes (CNS)  Produce myelin sheath around nerve fibers in the central nervous system Figure 7.3d

Neuroglia vs. Neurons n Neuroglia divide. n Neurons do not. n Most brain tumors are “gliomas.” n Most brain tumors involve the neuroglia cells, not the neurons. n Consider the role of cell division in cancer!

Support Cells of the PNS Slide 7.7b Copyright © 2003 Pearson Education, Inc. publishing as Benjamin Cummings  Satellite cells  Protect neuron cell bodies  Schwann cells  Form myelin sheath in the peripheral nervous system Figure 7.3e

Support Cells / Glia PNSCNS

Astrocytes n Provide physical support n Regulating chemical content of extracellular fluid l localizes neurotransmitters l K+ concentration n Blood-brain Barrier ~

Brain Capillary Blood-Brain Barrier Typical Capillary

BBB: Function n Maintains stable brain environment large fluctuations in periphery n Barrier to poisons n Retains neurotransmtters & other chemicals n Regulates nutrient supplies glucose levels active transport ~

Myelin n Wrap around axon n Saltatory Conduction l faster transmission n CNS: oligodendroglia or oligodendrocytes n PNS: Schwann cells ~

Myelinated and Unmyelinated Axons n Myelinated axons l Myelin protects and insulates axons from one another l Not continuous Nodes of Ranvier n Unmyelinated axons

Saltatory Conduction

Other non-neuronal cells n Microglia l phagocytosis l immune-like function n Ependymal cells l line walls of ventricles l role in cell migration during development ~

Chemical Synapses

Neuronal Pathways and Circuits n Organization of neurons in CNS varies l Convergent pathways l Divergent pathways l Oscillating circuits