Post on 20-Jan-2016
Classification of Neurons
Nature of the Nervous System
Camillo Golgi The Synctium
continuous network
no gaps ~
Nature of the Nervous System
Santiago Ramon y Cajal The Neuron Doctrine
discrete cells communication across
gaps synapses ~
Nature of the Nervous System
Golgi won Proved himself wrong Golgi Stain -
silver chromate
Supported Neuron DoctrineGolgi & Cajal shared Nobel Prize (1906) ~
Classification of Neurons Structural – based on the number of
cytoplasmic processesMultipolar neuronsBipolar neuronsUnipolar neurons
Functional – based on the direction of impulse transmission
Sensory neuronsMotor neuronsInterneurons (association)
Histology of Nervous Tissue
2 types of cellsNeurons
– Structural & functional part of nervous system– Specialized functions
Neuroglia (glial cells)– Support & protection of nervous system
NEURONS
Basic functional unit of N.S. Specialized cell
All cells have same basic properties information processing
Transmits
Integrates
Stores Regulation of behavior ~
Neurons Function
Conduct electrical impulses Structure
Cell body – Nucleus with nucleolus – Cytoplasm (perikaryon)
Cytoplasmic processes– Dendrites– Axon
Review of Neuron Structure
Nervous Tissue: NeuronsNervous Tissue: Neurons
Slide 7.8Copyright © 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 LocationNeuron Cell Body Location
Slide 7.13Copyright © 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 AnatomyNeuron Anatomy
Slide 7.9bCopyright © 2003 Pearson Education, Inc. publishing as Benjamin Cummings
Cell body
Nucleus
Large nucleolus
Figure 7.4a
Neuron AnatomyNeuron Anatomy
Slide 7.10Copyright © 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 Long, specialized
Collaterals = branches
Telodendria = termination of axons & collaterals
Cytoplasm = axoplasm Plasma membrane = axolemma
Anatomy of a Neuron
Anatomy of a Neuron
Axons and Nerve ImpulsesAxons and Nerve Impulses
Slide 7.11Copyright © 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 CoveringsNerve Fiber Coverings
Slide 7.12Copyright © 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
In Multiple Scleroses the myelin sheath is destroyed.
The myelin sheath hardens to a tissue called the scleroses.
This is considered an autoimmune disease.
Why does MS appear to affect the muscles?
Neuron ClassificationNeuron Classification
Slide 7.15Copyright © 2003 Pearson Education, Inc. publishing as Benjamin Cummings
Figure 7.6
Types of Neurons
Functional classificationSensory or afferent: Action potentials toward CNSMotor or efferent: Action potentials away from CNSInterneurons or association neurons (CNS)Principle neurons / projection neurons (CNS)
Structural classificationMultipolar, bipolar, unipolar
Shorthand
Neuron Classification
Categories overlap ~
Functional Classification of Functional Classification of NeuronsNeurons
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 Functional Classification of NeuronsNeurons
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
Sensory Motor
UnipolarBipolarMultipolar
By morphology (# of neurites)
Structural Classification of NeuronsStructural 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 NeuronsStructural 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 NeuronsStructural 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 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
Neuroglia of PNSSchwann cells
– Form myelin sheaths of PNS
Satellite cells
Classification of Glial Cells
Neuroglia of CNS
Nerve Fibers of the PNS An axon and its sheaths
Myelinated axon – Axon is surrounded by a myelin sheath
Unmyelinated axon– Axon has no myelin sheath
Myelin White matter of nerves, brain, spinal cord Composed primarily of phospholipids Production
Developing Schwann cells wind around axon
FunctionIncreases speed of impulse conduction
Insulation and maintenance of axon
Schwann Cells and Peripheral Axons
Myelin Neurilemma
Peripheral cytoplasmic layer of the Schwann cell enclosing the myelin sheath
Nodes of RanvierUnmyelinated gaps between
segments of myelin
Impulses “jump” from node to node
A Myelinated Axon
Nerve Fibers of the CNS Umyelinated 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
Spiny or Aspinous ~
Other Classification Schemes
Axon length Golgi Type I - long Golgi Type II - short (local
signalling) Chemistry
neurotransmitter released ~
How Neurons Function How Neurons Function (Physiology)(Physiology)
Slide 7.17Copyright © 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 ImpulseStarting a Nerve Impulse
Slide 7.18Copyright © 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 PotentialThe Action Potential
Slide 7.19Copyright © 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 PropagationNerve Impulse Propagation
Slide 7.20Copyright © 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
Receives information Integrates information~
Axon carries information away from soma Electrical signal Axon terminal releases chemical message~
Continuation of the Nerve Impulse Continuation of the Nerve Impulse between Neuronsbetween Neurons
Slide 7.21Copyright © 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:
Within a neuron, electrical signals called
action potentials travel along the axon.
Communication between neurons is
mediated via synaptic transmission (e.g. by
means of neurotransmitters such as
glutamate, GABA, acetylcholine).
Transfer of information from one part of
body to another.
How Neurons Communicate at How Neurons Communicate at SynapsesSynapses
Slide 7.22Copyright © 2003 Pearson Education, Inc. publishing as Benjamin Cummings
Figure 7.10
Glia
Neural Support Cells
Nervous Tissue: Support Cells Nervous Tissue: Support Cells (Neuroglia or Glia)(Neuroglia or Glia)
Slide 7.5Copyright © 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 CellsNervous Tissue: Support Cells
Slide 7.6Copyright © 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 CellsNervous Tissue: Support Cells
Slide 7.7aCopyright © 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
Neuroglia divide. Neurons do not. Most brain tumors are “gliomas.” Most brain tumors involve the
neuroglia cells, not the neurons. Consider the role of cell division in
cancer!
Support Cells of the PNSSupport Cells of the PNS
Slide 7.7bCopyright © 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
Provide physical support Regulating chemical
content of extracellular fluid localizes
neurotransmitters K+ concentration
Blood-brain Barrier ~
BrainCapillary
Blood-Brain Barrier
TypicalCapillary
BBB: Function
Maintains stable brain environment
large fluctuations in periphery Barrier to poisons Retains neurotransmtters & other
chemicals Regulates nutrient supplies
glucose levels
active transport ~
Myelin
Wrap around axon Saltatory Conduction
faster transmission CNS: oligodendroglia
or oligodendrocytes
PNS: Schwann cells ~
Myelinated and Unmyelinated Axons
Myelinated axonsMyelin protects and insulates axons from one anotherNot continuous
Nodes of Ranvier Unmyelinated
axons
Saltatory Conduction
Other non-neuronal cells
Microglia phagocytosis immune-like function
Ependymal cells line walls of ventricles role in cell migration during
development ~
Chemical Synapses
Neuronal Pathways and Circuits
Organization of neurons in CNS variesConvergent pathways Divergent pathwaysOscillating circuits