Ch. 12Nervous Tissue

Objectives

• Understand how the nervous system is divided and the types of cells that are found in nervous tissue

• Know the anatomy of a neuron and the structural and functional types of neurons

• Understand what a potential is and how this can transmit an impulse

• Understand what occurs at the synapse

The Nervous System

• Maintains internal coordination– Sensory information– Processing– Response

• Two major subdivisions– Central (CNS)

• Brain and spinal cord– Peripheral (PNS)

• Nerves and ganglia

Divisions of Nervous System

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Neurons• Communication cells of the nervous system

• Properties that allow communication– Excitability– Conductivity– Secretion

• Three functional classes– Afferent (sensory) neurons– Interneurons (association neurons)– Efferent (motor) neurons

Neuron Structure

• Soma – control center

• Dendrites

• Axon Hillock

• Axon

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Structural Classification

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Axonal Transport• Axonal transport – two way transport of materials to and from the

soma

• Anterograde – movement away from soma down axon– Kinesin motor protein used

• Retrograde – movement up axon toward soma– Dynein motor protein used

• Two types of transport– Fast axonal transport

• Rate of 10 – 400 mm/day• Anterograde or retrograde

– Slow axonal transport• Rate of .5 – 10 mm/day• Only anterograde

Glial Cells• Four types of glial cells

– Astrocytes• Spatial orientation and support• Synapse formation

– Thrombospondin• Repair and barrier formation• Nourish• Degradation of neurotransmitters• K+ regulation

– Oligodendrocytes• myelination

– Microglia• Immune protection• Nerve growth factor

– Ependymal cells• Internal lining of CNS• Production of CSF• Neural stem cells

• Two types of glial cells found only in PNS– Schwann cells

• Myelination

– Satellite cells• Provide electrical insulation around

soma• Chemical regulation

Myelination

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Neural Communication

• Neurons are excitable cells because they produce electric signals when excited

• Terms to know– Polarization• Due to electric potential

– Depolarization– Repolarization– Hyperpolarization

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Electrical Signals

• Produced by changes in ion movement across the plasma membrane– Leak or gated channels

• Voltage gated channels – Membrane permeability changes due to triggering

events

• Two types of signals– Local potentials – Action potentials

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Local Potentials

• Short range changes in voltage

• Distinguished from action potential due to:– Graded– Decremental• Weaken from point of origin

– Reversible– Excitatory or inhibitory

Action Potentials• Transient, large changes in membrane potential– Potential will typically reverse within the cell

• Inside becomes positive

• Occur when a graded potential reaches a threshold potential (-50mV in neuron)

• Caused by the opening of voltage-gated Na+ and K+ channels– Open only if threshold is reached– Ions move down their gradients– Depolarization caused by Na+ entering cell– Repolarization caused by K+ leaving cell

Action Potential• Contiguous conduction

– Action potential spreads down the membrane of the axon

• Refractory period– Ensure the one way transmission

of the action potential• Absolute • Relative

• All-or-none law– Responds to a triggering event

with maximal potential or not

• Frequency of action potential determines strength

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Action Potential Velocity

• Myelination increases speed of conduction– Voltage gated channels only found at nodes– Saltatory conduction– Schwann cells and oligodendrocytes

• Fiber diameter– The larger the diameter the faster the actin

potential is propagated

Signal Transduction• Unmyelinated axons– Action potential excites adjacent voltage gated

channels (opens them) allowing more Na+ in• Continues down the length of axon

• Myelinated axons– Saltatory conduction

• Na+ diffuses towards next node and reaches threshold

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Refractory Period

• Period of resistance to restimulation

• Absolute refractory period– No stimulus of any strength will stimulate a new

action potential

• Relative refractory period– New action potential may be triggered, but

requires unusually strong stimulus

Synapses and Neural Integration

• How do neurons communicate with other cells?– Can terminate at a muscle, gland, or neuron

• Synapse– Two types

• Electrical and chemical– Pre-synaptic and post-synaptic neurons

• Axodendritic, axosomatic, axoaxonic synapses– Neurotransmitter

• Release promoted by Ca2+• Can excite or inhibit• Quickly removed from synaptic cleft

Synapse

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Synaptic Transmission• Excitatory cholinergic synapse

– Ach released and binds with receptors on target cell– Receptors are ligand regulated ion channels– Channels open, Na+ in and K+ out

• Inhibitory GABA-ergic synapse– γ – aminobutyric acid – GABA binds to ligand regulated channels– Channels open, Cl- in

• Excitatory adrenergic synapse– Norepinephrine binds to receptor protein– Activates secondary messenger system– Leads to the opening of ion channels or to enzyme activation

Neural Integration

• Ability of neurons to process, store, and recall information– Occurs at synapse

• Neural integration is based on postsynaptic potentials– EPSP– IPSP– Summation, facilitation, inhibition

Grand Postsynaptic Potential

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Making Sense of it All

• Neural coding– Converting information into a meaningful pattern

of action potentials

• Labeled line code– Fibers leading to the brain recognize specific

stimulus type