nervous system
DESCRIPTION
Biology 120 Presentation - Chapter 12 - Nervous systemTRANSCRIPT
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Nervous Nervous SystemSystem
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The nervous systemnervous system and the endocrineendocrine system together integrate oneone communication function of the human body
The nervous system is subdivided in a variety of ways according to its structure, the direction of information flow, and the control of effectors
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Central nervous systemCentral nervous system
• Structural and functional center of the entire nervous system
• Consists of the brainbrain and spinal cordspinal cord• Integrates incoming informationincoming information from the senses,
evaluates the informationevaluates the information, and initiates an initiates an outgoing responseoutgoing response
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Peripheral nervous systemPeripheral nervous system
• Consists of the nerve tissues that lie on the periphery or regions outside the CNS
• Cranial nervesCranial nerves – nerves that originate from the brain (or through the skull)
• Spinal nervesSpinal nerves – nerves that originate from the spinal cord
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A-A-fferentfferent division division – consists of all incoming sensorysensory or afferent pathways
E-E-fferentfferent division division – consists of all outgoing motormotor or efferent pathways
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Somatic nervous systemSomatic nervous system – carry information to the somatic effectors (skeletal muscles)
• Somatic motor divisionSomatic motor division – efferent pathways • Somatic sensory divisionSomatic sensory division – afferent
pathways• Integrating centersIntegrating centers – receive the
sensory information and generate the efferent response signal
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Autonomic nervous systemAutonomic nervous system – carry information to the autonomic (visceral) effectors• Sympathetic divisionSympathetic division – pathways that exit
from the middle portions of the spinal cord; involved in preparing the body to deal with immediate threats: the fight-or-flight responsefight-or-flight response
• ParaPara--sympathetic divisionsympathetic division – exit from the brain or lower portions of the spinal cord and coordinate the body’s normal resting activities; “rest-and-repair” division
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NeuronsNeurons consist of a cell body, one or more dendrites, and one axon
• Distal ends of dendritesDistal ends of dendrites of sensory neurons are called receptors because they receive the stimuli that initiate nerve signals• Axon hillockAxon hillock – tapered portion of the cell body;
“decides” whether to send the impulse any farther in the neuron• AxonsAxons with larger diameters conduct nervous impulses
faster than those with smaller diameters • Synaptic knobsSynaptic knobs – release neuroneuro--transmitterstransmitters
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Classification of neuronsClassification of neurons – three distinct structural types of neurons
• MultiMulti--polarpolar neurons neurons – have only one axon but several
dendrites
• BiBi--polarpolar neurons neurons – have only one axon and also only one highly branched dendrite
• UniUni--polarpolar neurons neurons – sensory neurons with a single process extending from the cell body
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Classification of neuronsClassification of neurons – according to the direction in which they conduct impulses
• AA-fferent -fferent (sensory) neurons(sensory) neurons – transmit nerve impulses to the spinal cord or brainto the spinal cord or brain
• EE-fferent -fferent (motor) neurons(motor) neurons – transmit nerve impulses away from the brain or spinal away from the brain or spinal cordcord to or toward muscles or glands
• InterInter-neurons-neurons – conduct impulses from afferent neurons to or toward motor neurons
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Reflex arcReflex arc – automatic signal conduction route to and from the CNS• Most common form of reflex arc is the
three-neuron arc; consists of an: Afferent neuronInterneuronEfferent neuron
SynapseSynapse – junction between the synaptic knobs of one neuron and the dendrites (or cell body) of another neuron
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Neur/o-Neur/o-gli/ogli/o - al- al (NeuroglialNeuroglial)– pertaining to the support cells, glial cells, of nerves.
Five basic types:• AstroAstro--cytescytes – largest and most numerous type of glia; help form the form the
blood-brain barrierblood-brain barrier (BBB)
• Microglia – serve a protective function when the brain is under attack by microorganisms
• Ependymal cellsEpendymal cells – produce the fluidfluid that fills the cavities in the brain and spinal cord
• Oligo-dendro-cytes – help hold nerve fibers together and also produce the vitally important myelin sheath in CNS
• Schwann cellsSchwann cells – support nerve fibers in the PNS and sometimes form a myelin sheath around them
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Neur/o – al – (Neur/o – al – (NeuralNeural)) – Pertaining to nerves.NervesNerves - bundles of peripheral nerve fibers (axons) held together by several layers of connective tissues
• EndoEndo-neurium-neurium – delicate layer of fibrous connective tissue surrounding each nerve fiber
• PeriPeri-neurium-neurium – connective tissue layer surrounding each bundle of nerve fibers (fascicles)
• EpiEpi-neurium-neurium – fibrous coat surrounding numerous fascicles
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TractsTracts – bundles of nerve fibers within the CNS
White matterWhite matter – bundles of myelinated fibersmyelinated fibers
Gray matterGray matter – cell bodiescell bodies and unmyelinated fibers
NucleusNucleus – distinct regions of gray matter within the CNS
GangliaGanglia – distinct regions of gray matter within the PNS
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Mixed nervesMixed nerves – carry both sensorysensory (afferent) and motormotor (efferent) fibers
Sensory nervesSensory nerves – contain mostly afferent fibers
Motor nervesMotor nerves – contain mostly efferent fibers
Nerve fibers can sometimes be repaired if the damage is not extensive
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Nerve impulseNerve impulse – wave of electrical energy that travels along the plasma membrane of the nerve
Cell membrane potentialCell membrane potential – difference in electrical charge across their plasma membranes• Resting membrane potential (RMP)Resting membrane potential (RMP) – mechanism that
maintains the potential voltage; when the neuron is not in an excited state, we say that the neuron is “at rest”; typically –70 mV
• The slight excess of positive ions on the outer surface is produced by ion transport mechanisms and the membrane’s permeability characteristics
• The membrane’s selective permeability characteristics help maintain a slight excess of positive ions on the outer surface of the membrane
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SynapseSynapse – place where signals are transmitted from one neuron, called the prepre--synapticsynaptic neuronneuron, to another neuron, called thepostpost--synapticsynaptic neuronneuron
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Types of synapsesTypes of synapses
• Electrical synapseElectrical synapse – where two cells are joined end-to-end by gap junctions; as a result, an action potential simply continues along the postsynaptic plasma membrane as if it belonged to the same cell
• Chemical synapsesChemical synapses – use a chemical neuro-transmitter to send the message to the postsynaptic cell:
• Synaptic knobSynaptic knob – contains many small sacs (vesicles) filled with neurotransmitter molecules
• Synaptic cleftSynaptic cleft – fluid-filled space (about one millionth of an inch in width) between a synaptic knob and the plasma membrane of a postsynaptic neuron
• Post-synaptic neuronPost-synaptic neuron – has protein molecules embedded in it, each facing toward the synaptic knob and its vesicles
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NeuroNeuro--transmitterstransmitters – means by which neurons “talk” to one another; more than 50 compounds are now known to be neurotransmitters
NeuroNeuro--transmitterstransmitters are commonly classified by by their functiontheir function or by their chemical structure
• ExcitatoryExcitatory neurotransmitters • InhibitoryInhibitory neurotransmitters
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•ExcitationExcitation – occurs when a stimulus triggers the opening of stimulus-gated Na+ channels
• InhibitionInhibition – stimulus triggers the opening of stimulus-gated K+ channels
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Action potentialAction potential – an electrical signal that travels along the surface of a neuron’s plasma membrane• When an adequate stimulus is applied to a neuron, the
Na+ channels open at the point of stimulation. Na+ diffuses rapidly into the cell at the site of this local depolarization
• If the magnitude of the local depolarization exceeds a limit called the threshold potential (about –59 mV), then additional Na+ channels are opened
• As more Na+ rushes into the cell, the membrane moves rapidly toward 0 mV and then continues in a positive direction to a peak of +30 mV
• The action potential is an all-or-none response• Once the peak of the action potential is reached, the
membrane potential begins to move back toward the resting potential of –70 mV in a process called repolarization
• Because the K+ channels often remain open as the membrane reaches its resting potential, too much K+ may rush out of the cell; hyperpolarization
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Absolute refractory Absolute refractory periodperiod – very brief period when a local area of an axon’s membrane resists restimulation
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Conduction of the action potentialConduction of the action potential
• The action potential never moves never moves backwardbackward, restimulating the region from which it just came
• In myelinated fibers, the insulating properties of the thick myelin sheath resist ion myelin sheath resist ion movementmovement and the resulting flow of current
• The rate at which a nerve fiber conducts an impulse depends on its diameterdiameter and also on the presence or absence of a myelin presence or absence of a myelin sheathsheath
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Severe psychic depressionpsychic depression occurs when a deficit of norepinephrinenorepinephrine, dopaminedopamine, serotoninserotonin, and other amines exists in certain brain synapses
• Anti-depressant drugsAnti-depressant drugs – some of these inactivate dopamine and serotonin; others called SSRIs (serotonin-specific reuptake inhibitors) produce antidepressant effects by inhibiting the uptake of serotonin
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CocaineCocaine – produces a temporary feeling of well-being in cocaine abusers by blockingblocking the uptake of dopaminedopamine
AnestheticsAnesthetics – produce their effects by inhibitinginhibiting the opening of sodium channelssodium channels in the nerve cell membrane, thus blocking the initiation and conduction of nerve impulses
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