ch. 12 nervous tissue. objectives understand how the nervous system is divided and the types of...
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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