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CH12IntrotoNervSyst
| Term | Definition |
|---|---|
| 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 |
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