Download Ch. 12 Nervous Tissue

Ch. 12
Nervous 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
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•
Spatial orientation and support
Synapse formation
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Repair and barrier formation
Nourish
Degradation of neurotransmitters
K+ regulation
myelination
– Microglia
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Immune protection
Nerve growth factor
– Ependymal cells
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Two types of glial cells found only in
PNS
– Schwann cells
•
Myelination
Thrombospondin
– Oligodendrocytes
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Internal lining of CNS
Production of CSF
Neural stem cells
– Satellite cells
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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
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–
–
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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