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Nervous Nervous SystemSystem
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
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
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
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
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
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
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
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
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
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
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
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
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
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
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
SynapseSynapse – place where signals are transmitted from one neuron, called the prepre--synapticsynaptic neuronneuron, to another neuron, called thepostpost--synapticsynaptic neuronneuron
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
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
•ExcitationExcitation – occurs when a stimulus triggers the opening of stimulus-gated Na+ channels
• InhibitionInhibition – stimulus triggers the opening of stimulus-gated K+ channels
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
Absolute refractory Absolute refractory periodperiod – very brief period when a local area of an axon’s membrane resists restimulation
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
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
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