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9
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Nervous TissueNervous Tissue
Chapter 9
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Structures of Nervous Structures of Nervous SystemSystem
• Brain- Neurons enclosed in skull• Spinal cord– connects to brain & enclosed in
spinal cavity • Nerves- bundles of neuronal axons
Cranial emerge from brain; spinal nerves- emerge from spinal
cord• Ganglia- groups of cell bodies outside brain
& spinal cord• Enteric plexuses- networks in digestive
tract• Sensory receptors- monitor changes in
internal or external environments
Figure 9.1
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FunctionFunction
• Sensory Receptors & afferent nervesCarry information into brain & spinal cord
• Integration- information processingPerception = awareness of sensory input Carry by short interneurons
• Motor activity- efferent nervesSignals to glands and muscles (effectors)
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OrganizationOrganization
• Central Nervous System (CNS)• Peripheral Nervous System (PNS)• Subdivided: Somatic (SNS) &
Autonomic (ANS) nervous systems•Also
• INPUT-Afferent or Sensory division• OUTPUT- Efferent or Motor division
Figure 9.2
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Nervous SystemNervous System
• Neuron= nerve cell Specialized for signal carrying & information
processing• Neuroglia cells-support, nourish &
protect neuronsNeuroglia critical for homeostasis of
interstitial fluid around neurons
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Neuronal StructureNeuronal Structure• Cell body- nucleus, cytoplasm with
typical organelles• Dendrites- highly branched input
structures emerging from cell body• Axon- conducts away from cell body
toward another neuron or effectorEmerges at cone-shaped axon hillock
• Axon terminals -at end of axon with synaptic bulbs
Figure 9.3
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MyelinationMyelination
• Axons covered with a myelin sheath Many layered lipid & protein creating insulationsIncreases speed of nerve conduction.
• Nodes of Ravier= gaps in the myelinNodes are important for signal conduction
• Some diseases destroy myelin- E.g. multiple sclerosis & Tay-Sachs
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Gray and White MatterGray and White Matter
•White matter- primarily myelinated axons
•Gray matter- cell bodies, dendrites, unmyelinated axons, axon terminals & neuroglia
• spinal cord gray matter is centrally located
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Other termsOther terms
• Gray matter in brain covers surface of cerebrum & cerebellum – cortex
• deep cluster of neuronal cell bodies = nucleus
• Bundle of white matter in CNS= Tract
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NeurogliaNeuroglia
• ~ half the volume of CNS• Cells smaller than neurons• Can multiply and divide and fill
in brain areas• Do not conduct nerve impulses
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Neuroglia-supportNeuroglia-support• Astrocytes- blood brain barrier• Oligodendrocytes- myelin in CNS• microglia - defense• Ependymal cells- CSF production• Schwann- PNS cell support•Satellite cells- in PNS ganglia
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Action PotentialsAction Potentials
• Action potentials = nerve impulses• Require a membrane potential
electrical charge difference across cell membrane – like a Battery
• Ion Channels- allow ions to move by diffusion = current
• If no action potential then resting cell has resting membrane potential
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Ion ChannelsIon Channels• Allow specific ions to diffuse across
membraneMove from high concentration to low or toward area of opposite charge
• Leakage channels• Gated channels- require trigger to
open•Voltage- Gated channels respond
to a change in membrane potential
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Resting Membrane Resting Membrane PotentialPotential
•Leakage channels• Cytosol high in K+ & interstitial fluid
high in Na+ (sodium –potassium pumps)• Leakage lets K+ through easily and
Na+ poorly• inside is negative relative to outside• actual value depends on the relative
leakage channel numbers
Figure 9.4
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Action Potential (AP)Action Potential (AP)
• Series of active events• Channels actively open & close• Some initial event is required to
reach a voltage threshold (~ = - 55 mv)
•Stimulus = any event bringing membrane to threshold
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Action PotentialAction Potential
•Then•Depolarizing phase-
membrane potential rises and becomes positive
•Repolarizing phase- potential restored to resting valueMay overshoot =hyperpolarizing phaseThen recovery to rest.
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Active EventsActive Events
• Stimulus to reach threshold• Na+ channel opens=>• Na+ ions enter=> • positive potential=>• Causes K+ channel opening =>• repolarization
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All- or -NoneAll- or -None
• This sequence is always the same• If threshold then the same size of
changes occur no larger or smaller APs
• Stimulus must reach threshold to start• After one AP there is a short period
before next can be triggered= refractory period
Figure 9.5
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Conduction of Nerve Conduction of Nerve ImpulsesImpulses
• Each section triggers next locally• Refractory period keeps it going
the right direction• unmyelinated fiber- continuous
conduction• With myelin- saltatory conduction
Can only be triggered at Nodes of Ranvier• Myelinated fibers faster & larger
neurons faster
Figure 9.6a
Figure 9.6b
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Synaptic TransmissionSynaptic Transmission• Sequence of events at synapse • Triggered by voltage change of
the Action Potential• Sending neuron = presynaptic• Receiving neuron = postsynaptic• Space between = synaptic cleft•Neurotransmitter carries signal
across cleft
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Events at SynapseEvents at Synapse• AP arrives at presynaptic end bulb=>• Opens voltage gated Ca2+ channels=>
Ca2+ flows into cell• increased Ca2+ concentration =>• exocytosis of synaptic vesicles=>• Neurotransmitter released into cleft• Diffuse across and bind to receptors
in postsynaptic cell membrane
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Synaptic TransmissionSynaptic Transmission• Binding at receptors • Chemical trigger of ion channels• May depolarize or hyperpolarize
postsynaptic cell membrane• If threshold reached at axon
hillock then postsynaptic cell action potential results
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Synaptic TransmissionSynaptic Transmission• Finally the neurotransmitter must
be removed from the cleft-• Diffusion away• Destroyed by enzymes in cleft• Transport back into presynaptic
cell• Neuroglia destruction
Figure 9.7
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NeurotransmittersNeurotransmitters
• AcetylCholine (Ach)- common in PNSMay be stimulatory or inhibitory
• Amino Acids- Glutamate, Aspartate, gamma aminobutyric acid
(GABA), glycine• Modified amino acids-
Norepinephrine (NE), Dopamine (DA), serotonin• Neuropeptides – endorphins• Nitric oxide (NO)