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

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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

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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

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Cell body

Nucleus

Large nucleolus

Figure 7.4a

Neuron AnatomyNeuron Anatomy

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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

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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

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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

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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

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Interneurons (association neurons)

Found in neural pathways in the central nervous system

Connect sensory and motor neurons

Functional Classification of Functional Classification of NeuronsNeurons

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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

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Multipolar neurons – many extensions from the cell body

Figure 7.8a

Structural Classification of NeuronsStructural Classification of Neurons

Slide 7.16b

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Bipolar neurons – one axon and one dendrite

Figure 7.8b

Structural Classification of NeuronsStructural Classification of Neurons

Slide 7.16c

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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)

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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

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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

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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

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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

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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

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Figure 7.10

Glia

Neural Support Cells

Nervous Tissue: Support Cells Nervous Tissue: Support Cells (Neuroglia or Glia)(Neuroglia or Glia)

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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

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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

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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

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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