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CH12IntrotoNervSyst

TermDefinition
The nervous system, together with the endocrine system, governs homeostasis by sensing and responding to changes in normal physiological body set points
The nervous system controls and integrates all body activities
Cellular organization of nervous system ▪ Neurons ▪ Neuroglia
NERVOUS SYSTEM FUNCTIONS ▪ Sensory function ▪ Detecting internal and external stimuli ▪ Integrative function ▪ Processing of sensory information ▪ Motor function ▪ Eliciting a response by activating effectors, such as muscles and glands
ORGANIZATION OF NERVOUS SYSTEM -Central Nervous System (CNS) -Peripheral Nervous System (PNS)
Central Nervous System (CNS) ▪ Consists of brain and spinal cord ▪ Responsible for integration of incoming sensory information, generation of motor commands, and thoughts and memories
Peripheral Nervous System (PNS) ▪ Consists of all nervous tissue outside CNS ▪ Cranial nerves - arise from brain ▪ Spinal nerves - arise from spinal cord ▪ Ganglia - clusters of nervous tissue ▪ Enteric plexuses - networks in GI tract organ walls ▪ Sensory receptors - detect intern
PERIPHERAL SUBDIVISIONS -SNS -ANS -ENS
Somatic (SNS) ▪ Sensory neurons: from somatic and special sensory receptors to CNS ▪ Motor neurons: to skeletal muscles under voluntary control from CNS
Autonomic (ANS) ▪ Sensory neurons: from visceral organs to CNS ▪ Motor neurons: to smooth muscle, cardiac muscle and glands under involuntary control from CNS
Enteric (ENS) ▪ Function somewhat independently of ANS and CNS ▪ Monitors GI tract and controls operation involuntarily
NEURONS ▪ Responsible for most of the unique functions of the nervous system ▪ Possess electrical excitability, like muscle cells ▪ Convert stimulus to an action potential (nerve impulse) ▪ Most are amitotic
Neurons-Three major parts of each cell ▪ Cell body ▪ Dendrites ▪ Axon
NEURON CELL BODY-Nucleus surrounded by cytoplasm and many typical cellular organelles ▪ Mitochondria ▪ Golgi complex ▪ Lysosomes and other vesicles
NEURON CELL BODY-Some specialized organelles ▪ Nissl bodies ▪ Neurofibrils
Nissl bodies prominent clusters of rough endoplasmic reticulum for protein synthesis
Neurofibrils cytoskeleton bundles of intermediate fibers and microtubules for movement of material within cytoplasm, between cell body and axon
NEURON CELL BODY PROCESSES -dendrite -axon
Dendrite ▪ Usually multiple, short, tapering and highly branched ▪ Receive incoming impulses
Axon ▪ Single, long, thin process from neuron cell body with axoplasm and axolemma ▪ Conduct impulse from cell body to another neuron, muscle fiber, or gland cell ▪ Axon hillock ▪ Axon collaterals ▪ Axon terminals
Axon hillock cone-shaped junction with cell body; typically acts as trigger zone for impulse
Axon collaterals side branches along length
Axon terminals often swollen into synaptic end bulb at synapse for communication with other cells
NEURON CELL PRODUCTS ▪ Produced in neuron cell body ▪ Synthesize new products ▪ Recycles old ones
NEURON CELL PRODUCTS ARE CONDUCTED IN axoplasm between cell body and axon terminals ▪ Microtubule transport of cell products ▪ Anterograde transport - from cell body toward axon terminals ▪ Retrograde transport - from axon terminal back to
Axon damage can alter neuron chemical and electrical signals
STRUCTURAL CLASSIFICATION OF NEURONS -multipolar -bipolar -unipolar
Multipolar ▪ Several dendrites and one axon ▪ Most neurons in the brain and spinal cord
Bipolar ▪ One dendrite and one axon ▪ Found in retina of eye, inner ear, olfactory area
Unipolar ▪ One process arising from the cell body that branches into two axon-like processes ▪ Most sensory neurons, with cell bodies in ganglia of spinal and cranial nerves
FUNCTIONAL CLASSIFICATION OF NEURONS -sensory(afferent) -motor(efferent) interneuron
Sensory (afferent) Carry sensory information into the CNS
Motor (efferent) Carry information out of the CNS to effectors (muscles and glands)
Interneuron Located within the CNS and integrate (process) incoming sensory information from sensory neurons and then elicit a motor response by activating motor neurons
NEUROGLIA (GLIA) ▪ Smaller and more numerous than neurons ▪ Can divide readily by mitosis ▪ Do not generate or conduct impulses (action potentials)
Different cell types in nervous system divisions-CNS ▪ Astrocytes ▪ Oligodendrocytes ▪ Microglia ▪ Ependymal cells
Different cell types in nervous system divisions-PNS ❑ Schwann cells ❑ Satellite cells
Astrocytes ▪ Star shaped with many processes ▪ Provide nutrients to neurons and maintain proper chemical environment
Oligodendrocytes ▪ Smaller and fewer processes than astrocytes ▪ Form and maintain myelin sheath around CNS axons
Microglia Small phagocytotic cells
Ependymal cells ▪ Cuboidal cells forming layer with cilia and microvilli ▪ Line fluid-filled spaces of CNS, circulate and control chemical exchange with cerebrospinal fluid
Schwann cells ▪ Form and maintain myelin sheath around single PNS axon ▪ Support multiple unmyelinated PNS axons ▪ Participate in axon regeneration in PNS
Satellite cells ▪ Flat cells surrounding cell bodies of neurons in PNS ganglia ▪ Support and regulate exchange of materials with interstitial fluid
Myelin sheath multilayered lipid and protein produced in neuroglia that wrap extensions of plasmalemma around axons
MYELINATION ▪ Oligodendrocytes in CNS - each around part of several different neuron axons ▪ Schwann cells in PNS - around single neuron axon ▪ Neurolemma ▪ Myelin sheath ▪ Nodes of Ranvier ▪ Electrically insulate and increase speed of nerve impulse conducti
Neurolemma outer cytoplasmic layer with nucleus; aids in axon regeneration after damage
Myelin sheath inner layers wrapped around axon
Nodes of Ranvier gaps in myelin sheath between cells
White matter in CNS ▪ Presence of myelinated axons from oligodendrocytes ▪ Myelin sheath lacks neurolemma, so CNS axons show little regeneration after damage ▪ Deep in brain; superficial in spinal cord
Gray matter in CNS ▪ Unmyelinated axons ▪ Neuron cell bodies ▪ Neuroglia ▪ Superficial in brain; deep in spinal cord
Neurons create two types of electrical signals -graded potentials -action potentials
Graded potentials short-distance communication
Action potentials longer distance communication
Action potential in neuron is a nerve impulse which? travels along axon
Neurotransmitter release at synapse is triggered by? action potential arriving at axon terminal
Neurotransmitter can stimulate graded potential in? next cell ▪ Sequence: sensory, integration, motor
Graded and action potentials depend upon two features of neuron plasma membrane ▪ Resting membrane potential ▪ Ion channels (membrane proteins)
Resting membrane potential ▪ An electrical potential difference across the plasma membrane ▪ Voltage difference in an excitable neuron ▪ Current is created by flow of ions across membrane down their electrochemical gradient
Ion channels (membrane proteins) ▪ Gated ion channels open or close in response to specific stimuli ▪ When open, ion movement changes membrane potential
TYPES OF ION CHANNELS ▪ Leak channels ▪ Ligand-gated channels ▪ Mechanically gated channels ▪ Voltage-gated channels
Leak channels Randomly open and close
Ligand-gated channels Specific chemical (ligand) binding to receptor opens or closes channel
Mechanically gated channels Mechanical stimulation distorts position to open or close channel
Voltage-gated channels ▪ Change in membrane potential opens channel ▪ Participate in generation and conduction of action potentials
RESTING MEMBRANE POTENTIAL ▪ More negative ions along inside of cell membrane and more positive ions along outside ▪ Separation of charges forms potential energy ▪ Can be measured with microelectrodes and voltmeter ▪ Neurons typically “polarized” with –70 mV
FACTORS CONTRIBUTING TO POLARIZATION ▪ Unequal distribution of ions across plasma membrane ▪ Inability of most anions to leave the cell ▪ Electrogenic nature of the sodium-potassium pump
Unequal distribution of ions across plasma membrane ▪ Extracellular fluid rich in Na+ and Cl– ▪ Cytosol full of K+ organic phosphate & amino acids ▪ More K+ than Na+ leak channels - greater permeability to K+ increases negative potential inside
Inability of most anions to leave the cell Most attached to non-diffusible molecules
Electrogenic nature of the sodium-potassium pump ▪ Maintain resting membrane potential ▪ Pump Na+ out as fast as it leaks in ▪ Return K+ to interior to leak out again
GRADED POTENTIALS Small deviations from resting membrane potential
GRADED POTENTIALS result from opening or closing of ligand-gated and mechanically gated channels in response to stimulus
GRADED POTENTIALS typically occur in sensory receptors, dendrites, and cell bodies
Hyperpolarization membrane has become more negative
Depolarization membrane has become less negative
Size of graded potential varies with the? strength of the stimulus
▪ Generating graded potentials: opening or closing of ion channels cause a localized flow of current along the membrane ▪ Mechanically-gated channels ▪ Ligand-gated channels
Summation process by which graded potentials add together
ACTION POTENTIALS ▪ Rapid electrical events occurring in two phases ▪ Depolarizing phase ▪ Repolarizing phase
ACTION POTENTIALS Follow the ▪ “all-or-none” principle ▪ Once threshold depolarization occurs, voltage-gated channels open ▪ Creates an action potential that is always the same size (amplitude) ▪ Subthreshold stimulus will not create action potential ▪ Suprathreshold stimulus wi
SEQUENCE OF EVENTS: DEPOLARIZING PHASE ▪In response to graded potential creating a threshold stimulus ▪ Voltage-gated Na+ channels quickly open ▪ Na+ ions rush into the cell ▪ Membrane potential becomes positive
SEQUENCE OF EVENTS: REPOLARIZING PHASE ▪ Voltage-gated K+ channels open slowly ▪ Na+ channel inactivation gates close ▪ K+ ions flow out of cell ▪ Membrane potential starts to repolarize
SEQUENCE OF EVENTS: AFTER-HYPERPOLARIZING PHASE ▪ Voltage-gated K+ channels remain open, allowing large outflow of K+ ions ▪ Membrane potential becomes even more negative than resting membrane potential ▪ Voltage-gated K+ channels close ▪ Membrane potential eventually returns to resting level ▪ Cyc
REFRACTORY PERIOD ▪ Follows an action potential ▪ Period during which an excitable cell cannot generate another action potential -Absolute refractory period -Relative refractory period
Absolute refractory period ▪ Even a very strong stimulus cannot generate second action potential ▪ Na+ inactivation channels must return to resting state before they can reopen
Relative refractory period ▪ A very strong stimulus can initiate a second action potential ▪ K+ channels still open after Na+ inactivation channels have returned to resting state
PROPAGATION OF ACTION POTENTIAL From trigger zone to axon terminal
PROPAGATION OF ACTION POTENTIAL ▪ As Na+ flows into open channels in one area of membrane, depolarizing ▪ In adjacent segment of membrane voltage-gated Na+ channels open, regenerating another action potential
PROPAGATION OF ACTION POTENTIAL ▪Propagate in only one direction ▪ Region of axon that has just undergone an action potential is in its refractory period
TYPES OF PROPAGATION OF NERVE IMPULSE ▪ Continuous ▪ Saltatory
Continuous ▪ Occurs in unmyelinated axons ▪ Step-by-step depolarization and repolarization of each adjacent segment of axolemma
Saltatory ▪ Occurs in myelinated axons ▪ More rapid ▪ Voltage-gated channels present primarily at nodes of Ranvier ▪ Action potential appears to “leap” from node to node ▪ Less overall movement of Na+ and K+ ions during propagation, so less ATP energy used by s
FACTORS THAT AFFECT SPEED ▪ Amount of myelination ▪ Propagate more rapidly along myelinated axons ▪ Axon diameter ▪ Large diameter axons propagate faster than smaller ones due to large surface area ▪ Temperature ▪ Cooled axons propagate more slowly
ENCODING OF STIMULUS INTENSITY ▪ All nerve impulses (action potentials) are same amplitude and permit long distance communication ▪ Graded potentials vary in amplitude depending upon stimulus strength and function only for short-distance communication
ENCODING OF STIMULUS INTENSITY ▪ Stimulus intensity is “coded” ▪ Frequency of action potentials generated at trigger zone directly related to intensity of stimulus ▪ Number of sensory neurons activated to threshold at same time by a stimulus related to stimulus intensity
SYNAPSE ▪ Special junction between neurons ▪ Presynaptic neuron - carries impulse toward synapse ▪ Postsynaptic neuron - carries impulse away from synapse
SYNAPSE Location ▪ Axodendritic - axon to dendrite ▪ Axosomatic - axon to cell body ▪ Axoaxonic - axon to axon hillock
TYPES OF SYNAPSES ▪ Chemical ▪ Electrical
Chemical ▪ Conduct impulses through synaptic cleft ▪ Nerve impulse in presynaptic axon opens Ca2+ channels in synaptic end bulb ▪ Ca2+ stimulates release of neurotransmitter ▪ Neurotransmitter crosses synaptic cleft and binds to receptors on postsynaptic neuron
Electrical ▪ Conduct impulses though gap junctions ▪ Allow faster communication and synchronization of cell group activity ▪ Common in visceral smooth and cardiac muscle
SEQUENCE AT CHEMICAL SYNAPSE ▪ Nerve impulse arrives at synaptic end bulb ▪ Depolarization opens voltage-gated Ca2+ channels, Ca2+ flows into axon terminal ▪ Triggers exocytosis of synaptic vesicles with neurotransmitter
SEQUENCE AT CHEMICAL SYNAPSE ▪ Neurotransmitter released into synaptic cleft and diffuse ▪ Bind to receptors on postsynaptic neuron and opens ligand-gated channels ▪ Creates postsynaptic potential, which generates action potential if threshold
TYPES OF POSTSYNAPTIC POTENTIALS ▪ Excitatory postsynaptic potential (EPSP) ▪ Inhibitory postsynaptic potential (IPSP)
Excitatory postsynaptic potential (EPSP) ▪ Neurotransmitter creates depolarizing graded potential at the postsynaptic neuron’s membrane ▪ Brings membrane potential closer to threshold than resting membrane potential (less negative) ▪ More likely to respond to next EPSP
Inhibitory postsynaptic potential (IPSP) ▪ Neurotransmitter creates hyperpolarizing graded potential at the postsynaptic neuron’s membrane ▪ Brings membrane potential farther from threshold than resting membrane potential (more negative) ▪ Less likely to generate an action potential
SUMMATION OF POSTSYNAPTIC POTENTIALS ▪ Determines whether the postsynaptic neuron will generate an action potential ▪ Process by which graded potentials from many different presynaptic neurons are added and integrated ▪ EPSP ▪ Action potential ▪ IPSP
EPSP Total excitatory effect is greater than inhibitory, but still subthreshold
Action potential Total excitatory effect is greater than inhibitory and reach threshold
IPSP Total inhibitory effect is greater than excitatory
REMOVAL OF NEUROTRANSMITTER Essential for normal synaptic cleft function ▪ Diffusion ▪ Enzymatic degradation ▪ Uptake by cells
Diffusion Diffuse away from cleft and membrane receptors
Enzymatic degradation ▪ Inactivated by specific enzyme in synaptic cleft ▪ Example: acetylcholinesterase
Uptake by cells ▪ Actively transported back into neuron that released them (reuptake) and recycling into synaptic vesicle ▪ Actively transported into neighboring neuroglia (uptake)
SUMMARY: NEURONAL STRUCTURE -Dendrites ▪ Neuron cell body ▪ Junction of axon hillock and initial segment of axon ▪ Axon ▪ Axon terminals and synaptic end bulbs
Dendrites Receive stimuli through ligand-gated or mechanically gated ion channels to produce EPSPs or IPSPs
Neuron cell body Receives stimuli through ligand-gated ion channels to produce EPSPs or IPSPs
Junction of axon hillock and initial segment of axon Trigger zone in many neurons for summation
Axon Propagates nerve impulse (action potential) without change in amplitude if reach threshold
Axon terminals and synaptic end bulbs Inflow of Ca2+ triggers release of neurotransmitter
REPAIR AND REGENERATION OF NEURONS ▪ Plasticity ▪ Regeneration
Plasticity ▪ Capability of nervous system to change based on experience ▪ Individual neurons can sprout new dendrites, synthesize new proteins, change synaptic contacts
Regeneration ▪ Capability to replace or repair destroyed cells ▪ Very limited in nervous system
DAMAGE AND REPAIR IN CNS ▪ Little to no repair in brain and spinal cord ▪ Injury is usually permanent
DAMAGE AND REPAIR IN CNS ▪ Even if neuron cell body is intact, severed axons unable to repair or regrow ▪ Myelin of oligodendrocytes inhibits regrowth ▪ Astrocytes near damage proliferate and form scar tissue barrier
DAMAGE AND REPAIR IN CNS ▪ New neurons able to arise in hippocampus area only, area of brain crucial for learning ▪ Ongoing research ▪ Stimulate existing axons to bridge injury gap ▪ Stimulate dormant stem cells to replace lost cells
DAMAGE AND REPAIR IN CNS ▪ As long as cell body is intact, and Schwann cells’ neurolemmas are functional, dendrites and axons in PNS may be repaired
DAMAGE AND REPAIR IN CNS Chromatolysis in cell body Nissl bodies break up into fine granular masses
DAMAGE AND REPAIR IN CNS Wallerian degeneration of damaged axon ▪ Distal portion of axon and myelin sheath degenerates ▪ Neurolemma remains
DAMAGE AND REPAIR IN CNS Regeneration tube ▪ Schwann cells multiply by mitosis and form tube ▪ Tube guides axon growth across injury area ▪ In time, Schwann cells reform myelin sheath with nodes of Ranvier
Created by: fieldslady80