Содержание
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The Nervous System
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Functions of the Nervous System
Sensory input—gathering information To monitor changes occurring inside and outside the body Changes = stimuli Integration To process and interpret sensory input and decide if action is needed
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Motor output A response to integrated stimuli The response activates muscles or glands
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Figure 7.1
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Structural Classification of the Nervous System
Central nervous system (CNS) Brain Spinal cord Peripheral nervous system (PNS) Nerves outside the brain and spinal cord Spinal nerves Cranial nerves
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Functional Classification of the Peripheral Nervous System
Sensory (afferent) division Nerve fibers that carry information to the central nervous system Motor (efferent) division Nerve fibers that carry impulses away from the central nervous system
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Organization of the Nervous System
Figure 7.2
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Functional Classification ofthe Peripheral Nervous System
Motor (efferent) division (continued) Two subdivisions Somatic nervous system = voluntary Autonomic nervous system = involuntary
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Nervous Tissue: Support Cells
Support cells in the CNS are grouped together as “neuroglia” Function: to support, insulate, and protect neurons
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Astrocytes Abundant, star-shaped cells Brace neurons Form barrier between capillaries and neurons Control the chemical environment of the brain
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Figure 7.3a
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Microglia Spiderlike phagocytes Dispose of debris
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Figure 7.3b
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Ependymal cells Line cavities of the brain and spinal cord Circulate cerebrospinal fluid
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Figure 7.3c
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Oligodendrocytes Wrap around nerve fibers in the central nervous system Produce myelin sheaths
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Figure 7.3d
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Satellite cells Protect neuron cell bodies Schwann cells Form myelin sheath in the peripheral nervous system
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Figure 7.3e
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Nervous Tissue: Neurons
Neurons = nerve cells Cells specialized to transmit messages Major regions of neurons Cell body—nucleus and metabolic center of the cell Processes—fibers that extend from the cell body
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Cell body Nissl substance Specialized rough endoplasmic reticulum Neurofibrils Intermediate cytoskeleton Maintains cell shape
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Figure 7.4
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Cell body Nucleus Large nucleolus Processes outside the cell body Dendrites—conduct impulses toward the cell body Axons—conduct impulses away from the cell body
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Figure 7.4
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Axons end in axonal terminals Axonal terminals contain vesicles with neurotransmitters Axonal terminals are separated from the next neuron by a gap Synaptic cleft—gap between adjacent neurons Synapse—junction between nerves
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Myelin sheath—whitish, fatty material covering axons Schwann cells—produce myelin sheaths in jelly roll–like fashion Nodes of Ranvier—gaps in myelin sheath along the axon
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Figure 7.5
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Neuron Cell Body Location
Most neuron cell bodies are found in the central nervous system Gray matter—cell bodies and unmyelinated fibers Nuclei—clusters of cell bodies within the white matter of the central nervous system Ganglia—collections of cell bodies outside the central nervous system
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Functional Classification of Neurons
Sensory (afferent) neurons Carry impulses from the sensory receptors to the CNS Cutaneous sense organs Proprioceptors—detect stretch or tension Motor (efferent) neurons Carry impulses from the central nervous system to viscera, muscles, or glands
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Figure 7.7
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Interneurons (association neurons) Found in neural pathways in the central nervous system Connect sensory and motor neurons
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Neuron Classification
Figure 7.6
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Structural Classification of Neurons
Figure 7.8a Multipolar neurons—many extensions from the cell body
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Bipolar neurons—one axon and one dendrite Figure 7.8b
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Unipolar neurons—have a short single process leaving the cell body Figure 7.8c
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Functional Properties of Neurons
Irritability Ability to respond to stimuli Conductivity Ability to transmit an impulse
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Nerve Impulses
Resting neuron The plasma membrane at rest is polarized Fewer positive ions are inside the cell than outside the cell Depolarization A stimulus depolarizes the neuron’s membrane A depolarized membrane allows sodium (Na+) to flow inside the membrane The exchange of ions initiates an action potential in the neuron
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Figure 7.9a–b
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Action potential If the action potential (nerve impulse) starts, it is propagated over the entire axon Impulses travel faster when fibers have a myelin sheath
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Figure 7.9c–d
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Repolarization Potassium ions rush out of the neuron after sodium ions rush in, which repolarizes the membrane The sodium-potassium pump, using ATP, restores the original configuration
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Figure 7.9e–f
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Transmission of a Signal at Synapses
Impulses are able to cross the synapse to another nerve Neurotransmitter is released from a nerve’s axon terminal The dendrite of the next neuron has receptors that are stimulated by the neurotransmitter An action potential is started in the dendrite
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Figure 7.10 Axonterminal Vesicles Synapticcleft Actionpotentialarrives Synapse Axon oftransmittingneuron Receivingneuron Neurotrans-mitter is re-leased intosynaptic cleft Neurotrans-mitter bindsto receptoron receivingneuron’smembrane Vesiclefuses withplasmamembrane Synaptic cleft Neurotransmittermolecules Ion channels Receiving neuron Transmitting neuron Receptor Neurotransmitter Na+ Na+ Neurotransmitterbroken downand released Ion channel opens Ion channel closes
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Figure 7.10, step 1 Axonterminal Vesicles Synapticcleft Actionpotentialarrives Synapse Axon oftransmittingneuron Receivingneuron
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Figure 7.10, step 2 Axonterminal Vesicles Synapticcleft Actionpotentialarrives Synapse Axon oftransmittingneuron Receivingneuron Vesiclefuses withplasmamembrane Synaptic cleft Ion channels Receiving neuron Transmitting neuron
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Figure 7.10, step 3 Axonterminal Vesicles Synapticcleft Actionpotentialarrives Synapse Axon oftransmittingneuron Receivingneuron Neurotrans-mitter is re-leased intosynaptic cleft Vesiclefuses withplasmamembrane Synaptic cleft Neurotransmittermolecules Ion channels Receiving neuron Transmitting neuron
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Figure 7.10, step 4 Axonterminal Vesicles Synapticcleft Actionpotentialarrives Synapse Axon oftransmittingneuron Receivingneuron Neurotrans-mitter is re-leased intosynaptic cleft Neurotrans-mitter bindsto receptoron receivingneuron’smembrane Vesiclefuses withplasmamembrane Synaptic cleft Neurotransmittermolecules Ion channels Receiving neuron Transmitting neuron
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Figure 7.10, step 5 Axonterminal Vesicles Synapticcleft Actionpotentialarrives Synapse Axon oftransmittingneuron Receivingneuron Neurotrans-mitter is re-leased intosynaptic cleft Neurotrans-mitter bindsto receptoron receivingneuron’smembrane Vesiclefuses withplasmamembrane Synaptic cleft Neurotransmittermolecules Ion channels Receiving neuron Transmitting neuron Receptor Neurotransmitter Na+ Ion channel opens
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Figure 7.10, step 6 Axonterminal Vesicles Synapticcleft Actionpotentialarrives Synapse Axon oftransmittingneuron Receivingneuron Neurotrans-mitter is re-leased intosynaptic cleft Neurotrans-mitter bindsto receptoron receivingneuron’smembrane Vesiclefuses withplasmamembrane Synaptic cleft Neurotransmittermolecules Ion channels Receiving neuron Transmitting neuron Receptor Neurotransmitter Na+ Na+ Neurotransmitterbroken downand released Ion channel opens Ion channel closes
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Figure 7.10, step 7 Axonterminal Vesicles Synapticcleft Actionpotentialarrives Synapse Axon oftransmittingneuron Receivingneuron Neurotrans-mitter is re-leased intosynaptic cleft Neurotrans-mitter bindsto receptoron receivingneuron’smembrane Vesiclefuses withplasmamembrane Synaptic cleft Neurotransmittermolecules Ion channels Receiving neuron Transmitting neuron Receptor Neurotransmitter Na+ Na+ Neurotransmitterbroken downand released Ion channel opens Ion channel closes
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