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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)