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NERVOUS SYSTEM FUNCTION • communication • regulates & coordinates organ systems Endocrine System • communication system • shares commonalities & differences with nervous system • endocrine system – slow to make changes – changes longer lasting – adjusts metabolic operations of other systems – growth, maturation, sexual development, pregnancy & response to chronic environmental stress • nervous system – faster changes – short lived changes – temporary modifications Neural Tissue • 3% of total body weight • most complex organ system • vital to life • needed for awareness & appreciation of life • basic functional unit-neuron – specialized to carry messages • half the volume of nervous tissue is glial cells or neuroglia – supporting cells – without glial cellsneurons would not function Nervous System Composition • Brain • Spinal cord • Receptors of sense organs • Nerves – connects organs & links nervous system with other systems Anatomical Divisions Central Nervous System • brain & spinal cord – center of integration and control Peripheral Nervous System • nervous system outside brain * spinal cord • consists of: – 31 Spinal nerves » Carry info to & from spinal cord – 12 Cranial nerves » Carry info to & from brain Peripheral Nervous System – Sensory Nervous System • Sensory & Motor Neurons – Sensory-conduct impulses from somatic receptors to CNS – Motor-conduct impulses from the CNS to skeletal muscles – Autonomic Nervous System • Sensory Neurons – conducts impulses from autonomic sensory receptors in visceral organs to the CNS, smooth muscles, cardiac muscle and glands Division of Automatic Nervous System • Sympathetic Division • Parasympathetic division • antagonistic actions • Sympathetic – usually speeds processes • Parasympathetic – usually slows processes Neurons • • • • • • • • • • functional unit of nervous system come in many shapes and sizes most common-multipolar type has cell body, perikaryon or soma surrounded by plasma membrane large nucleus containing a nucleolus cytoskeleton comprised of neurofibrils that extend into extensions of cell mitochondria, golgi bodies, free & fixed ribosomes & endoplasmic reticulum (ER) are also present clusters of rough ER & free ribosomes form nissl bodies lack centrioles – cannot divide & cannot be replaced – stem cells exist but are inactive except in the nose & hippocampus Neuron Structure • Dendrites – highly branched processes extending from soma – each branches into fine processes- dendritic spines – receive information • Axons – long processes – take information away or send information to other cells – capable of propagating electrical impulses or action potentials • cytoplasm of axon axoplasmsurrounded by axolemma • base of axon is initial segment (tigger zone) • attached to nerve cell at thickened region-axon hillock Axon • may branch along their length – side branches or collaterals allow neurons to communicate with several cells at same time • Axon & collaterals ends in fine extensions or telodendria or axon terminals which end in synaptic knobs-filled with synaptic vesicles • Axons maybe encased in a myelin sheath • Between each section of myelin isnode of Ranvier • Where myelin is locatedinternodes • Myelinated axons are able to transmit nerve impulses faster than non-myelinated axons Neuron Function Synapse • synaptic knob of neuron that is sending the messagepresynaptic cell makes contact with postsynaptic cell • presynaptic cell is always a neuron – sends messages • postsynaptic cell can be another nerve cell, muscle cell, gland or an adipocyte – receives messages • when neuron meets another neuron, • synapse can be on a dendriteaxodendritic, • cell body-axosomatic • axon-axoaxonic Synapses • small gap-synaptic cleft separates pre & post synaptic membranes • neurons communicate with other cells with neurotransmitters • packaged in synaptic vesicles found in synaptic knob • released when action potential is propagated down axon of presynaptic cell • postsynaptic side of synapse contains receptors Neuron Classification • Can be classified by structure or function • Structure –relationship of dendrites to cell body and axon • Function –what the neuron does Structural Classification Bipolar – two distinct processes-dendrite and axon – cell body lies in between – found-retina & olfactory epithelium Unipolar or pseudounipolar – axon & dendrite are continuous or fused – cell body lies off to one side – initial segment lies where dendrites converge and the rest of the process carries an action potential and is therefore considered the axon – Dorsal root ganglion cells & sensory neurons of PNS Multipolar – most abundant & most common – possess two or more dendrites and one axon Functional Classification • Sensory or Afferent • Motor or Efferent • Interneuron Sensory or Afferent Neurons • cell bodies found in peripheral sensory ganglia-collection of nerve cell bodies in PNS • deliver information from sense receptors to CNS • often these are unipolar neurons • there are about 10 X 106 sensory neurons in the body Motor or Efferent Neurons • cell bodies in CNS • axons travel away from CNS to peripheral effectors 6 • there are 0.5 X10 • mainly mulitpolar neurons Interneuron • most abundant type of neurons by function-most are multipolar • 20 billion • often termed association neurons • most found in brain & spinal cord • distribute sensory information & coordinate motor activity • one or more can be found between a sensory & a motor neuron allowing for reflexes REFLEX Neuroglia-Glial Cells • comprise one half of nervous tissue • needed for proper functioning of nerve cells • different types are found in CNS & ANS Ependymal Cells line spinal cord & ventricles form an ependyma or an epithelia cell layer produce CSF patches of cilia on apical surface help circulate CSF Astrocytes largest & most numerous glial cell in CNS create a 3-dimensional supportive framework for CNS secrete nerve growth factors to promote growth & synapse formation maintain composition of tissue fluid cells have extensions or perivascular feet contact blood capillariesstimulate them to form a tight seal called blood-brain barrier • serves to control exchange of blood products with the brain – neural tissue must be physically & biochemically isolated from general circulation Oligodendrocytes & Microglia Oligodendrocytes – have slender cytoplasmic extensions that wrap around other nerve fibers forming myelin sheath – speeds action potential propagation – presence of myelin makes axons appear whitetermed white matter Microglia – least numerous & smallest neuroglila cell of CNS – migrate through tissues engulfing cellular debris, waste products & pathogens Schwann Cells • form sheaths around peripheral axons of neurons in PNS • cell spirals outward as it wraps nerve fiberending in thick outer coilneuilemma • nerve fiber is much longer than one cell can reachtherefore-many Schwann cells are needed to complete myelin sheath • gaps between cells arenodes of Ranvier • myelin covered segments are-internodes Satellite Cells • surround neuron cell bodies in ganglia of PNS • regulate exchange of materials between nerve cell bodies & interstitial fluid Neurophysiology • neural activity begins as a change in resting membrane potential (RMP) • refers to difference in electrical charge between inside & outside of the cell membrane Forces Maintaining Resting Membrane Potential • Cl- & Na+ are high outside • K+ & negatively charged proteins are high inside • inside of cell is negative with respect to outside • there are more positive charged ions outside & a slight excess of negatively charged ions inside Resting Membrane Potential • positive & negative charges attract • held apart by permeability of membrane & by active transport mechanisms • these conditions set up a potential difference – stored energy – stretched spring • RMP = potential difference of an undisturbed neuron • measured in millivolts • -70 mV or -0.07V CELL MEMBRANE PROPERTIES • concentrations differences due to differences in permeability of cell membrane to ions & to active transport mechanisms – If membrane was freely permeable, distribution of chemicals would become even – cell membranes are selectively permeable – Ions enter & leave only through ion channels TRANSPORT MECHANISMS • • • • • • ion movement occurs by leak channels easier for K to diffuse out of a cell than for Na to enter always open allows for slow diffusion of K+ out of the cell & Na into the cell – to maintain resting potential, cell must kick Na out & bring K in – requires energy or ATP – Na/K exchange pump uses carrier protein, Na-K ATPase to push 2 K+ in for every 3 Na+ pumped out Na/K ATPase provides energy to pump ions by splitting a phosphate group from ATPADP Sodium is ejected as quickly as it enters – keeps resting potential Sodium-Potassium Pump Electrochemical Gradient • • • • • • • • • sum of electrical & chemical forces acting across a cell membrane For Na+ – chemical gradient pushes sodium into cell – electrical gradient pulls sodium into cell For K+ – chemical gradient drives K out of cell – electrical gradient opposes K leaving cell • K is attracted to negative charges on inside of cell membrane and repelled by positive charges outside membrane electrochemical gradients for K & Na are primary factors affecting resting potential refers to stored or potential energy like water behind a dam can think of a cell membrane as a dam – even a small opening will release water under tremendous pressure any stimulus increasing membrane permeability to Na or K will produce sudden & dramatic ion movement Stimulusopens Na channelsNa rushes in stimulus opens doorelectrochemical gradient does the rest Ion Channels • any stimulus increasing membrane permeability to Na or K will produce sudden & dramatic ion movement • changes in RMP occur when channels open to allow these ions into or out of the neuron • Leak • Ligand-gated • Mechanically gated • Voltage gated Leak Channels –always open –depend on electrochemical gradient Gated Channels • • • Ligand-Gated – open or close when specific chemicals bind – most abundant – found on dendrites & cell bodies of neurons Voltage Gated – open or close in response to changes in membrane potential – restricted to axons of excitable membranes – most important type – each has two independently functioning gates: an activation gate-closed in resting membrane & opens with proper chemical stimulationNa can enter – inactivation gate-when closedNa stops coming in Mechanically Gated – open or close in response to physical distortion of membrane surface – found on dendrites of sensory neurons especially receptors for touch, pressure, vibration GATED CHANNEL Local Potentials • any stimulus that opens a gated channel disrupts resting potential produces a temporary, localized change in resting membrane potentialgives rise to a graded potential • • • • • • • • Local Potentials when a gated Na channel opens Na ions enter cell due to attraction to negative charges inside cell & because of the higher concentration of Na outside the cell entrance of Na shifts membrane potential to 0 mV any shift of resting potential toward 0 (or becoming more positive) is called depolarization degree of depolarization decreases with distance from opened channel local currents produce changes that cannot spread far from area of stimulation cytosol resists ion movement & some entering Na can move back across through Na channels at a distance from Na entry pointeffect on membrane potential is undetectable –decremental conduction Local Potentials • local potentials are graded • spread of current down axon depends on stimulus • stronger stimulusfurther local potential can be propagated • maximum change in membrane potential is proportional to size of initial stimulus – determined by number of open Na channels more open Na channelsmore Na entersgreater area affected greater degree of depolarization • can be excitatory or inhibitory Opening K Channels • opening gated K channel has opposite effect on membrane potential • as rate of K outflow increases & interior of cell loses positive ionscell is said to be hyperpolarized – membrane potential becomes more negative • Repolarization – when stimulus is removed – cell returns to normal resting potential Action Potential Generation • neurons receive information as graded potentials at dendrites, cell bodies & synaptic terminals • if graded potentials are large enough action potential begins electrical impulse is propagated across surface of membrane and then down axon to synapse Action Potential Generation • when sodium ions enter cell membrane depolarizes local potential begins • local potential must rise to a value termed threshold (about -55mV) for anything to happen • Threshold-minimum voltage needed to open voltage-regulated gates • once threshold is reached neuron fires or produces an action potential All-or-None Principle • Once stimulus depolarizes neuron to thresholdneuron fires at maximum voltage • if threshold is not reachedneuron does not fire • above threshold values do not produce stronger action potentials • Subthreshold values do not produce an action potential Action Potential Steps • Step 1: Resting State & Depolarization to threshold • Step 2: Activation of Na channels & Rapid Depolarization • Step 3: Inactivation of Na channels & Activation of K channels-Repolarizaton • Step 4: Hyperpolarization & return to normal permeability • • • • • Action Potential Steps Step 1: Depolarization to threshold Step 2: Activation of Na channels & Rapid Depolarization – at threshold voltage-regulated Na gates open quickly sodium rushes into the cellrapid depolarization – membrane potential changes from -70mV to more positive value Step 3: Inactivation of Na channels & activation of K channels – as membrane potential passes 0 mV, sodium gates are inactivatedbegin to close – by the time they all close and Na inflow ceases voltage peaks at about +35mV Na channels close & voltage regulated K channels open – both electrical & chemical gradients favor movement of K into cell – sudden loss of positive changes (K+) shifts membrane potential back toward resting levelrepolarization begins K gates remain open longer than Na gatesK leaves the cell than Na enters causes membrane potential to be more negative than original RMP-membrane hyperpolarized Step 4: Return to normal permeability Refractory Periods • from time action potential begins until normal resting potential is reestablished, membrane cannot respond to stimuli • Refractory Period • if second stimulus is applied <0.001 second after the first, will not trigger an impulse • membrane cannot respond • all voltage regulated Na channels are open or inactivated • Absolute Refractory period • Relative refractory period • time when another action potential can occur if membrane is sufficiently depolarized – requires a larger than normal stimulus Action Potential Propagation • once threshold has been reachedchange in electrical potential passively spreads along axon to adjacent regions of membrane from area to area in series of steps • at each stepmessage is repeated • because same events occur over & over this is called propagation Propagation Types • Continuous –Unmyelinated fibers • Saltatory Conduction or Leaping –Myelinated fibers Continuous Propagation • • • • • • unmyelinated fibers begins at initial segment of axon Step 1: membrane potential becomes positive briefly Step 2: local current develops & spreads in all directions depolarizing adjacent parts of membrane – continues in a chain reaction Steps 3-4: more distant parts of membrane are affected – action potential moves forward – cannot reverse – because previous segment is still in absolute refractory period speed of propagation is about 2mph which can be increased to 300mph with myelination-300mph Saltatory Conduction • • • myelinated axon axolemma is wrapped in myelin sheath makes continuous conduction impossible – myelin increases resistance to ion flow – ions cross best at nodes • Depolarization occurs only at nodes – action potential begins at initial segment which produces a local current that skips internode & depolarizes closest node – action potential jumps from node to nodesalatory propagation – moves message faster – uses less energy Propagation Review Nerve Fiber Types • • • • • Type A fibers – largest & myelinated – send impulses at 300mph – carry sensory information to CNS about body position, balance, touch and pressure from skin – motor neurons to skeletal muscles Type B fibers – smaller & myelinated – send impulses at 40mph Type C fibers – Unmyelinated – <2um in diameter – carry impulses at 2mph Type B & C carry information to CNS about temperature, pain, gentle touch & pressure carry information to smooth muscle, cardiac muscle, glands & other peripheral effectors Synaptic Activity • messages are conveyed from neuron to neuron via synapses • presynaptic cell converges on a postsynaptic cell • where pre & post synaptic cells meet is synapse • there are electrical & chemical synapses Electrical Synapses • found in both CNS & PNS • pre & post synaptic membranes are locked at gap junctions • integral membrane proteins or connexions possess pores allowing for ion passage • changes in membrane potential of one cell produces a local current in the other since cells share common membrane Chemical Synapses • neurons are not directly connected – there is a gap-synaptic cleft • action potential is transferred with small chemicalsneurotransmitters • In electrical synapses action potential is always propagated – need not be so in chemical synapses • post synaptic cell can be adjusted to respond more or less Excitatory & Inhibitory Action Potentials • some neurotransmitters are excitatory – depolarize postsynaptic cell promoting an action potential • other neurotransmitters are inhibitory – hyperpolarize postsynaptic membranesuppress action potential • effect depends on post synaptic cell receptors -not neurotransmitter • ACHdepolarizes most post synaptic membranes but inhibits post synaptic membranes in heart • synapses can be categorized as to type of neurotransmitter that is released at the presynaptic terminal Synapse Types Based on Neurotransmitter • Cholinergic synapses – release ACH – found-all neuromuscular junctions, CNS, at all neuron-neuron synapses in PNS, & at all neuroglandular junctions in the parasympathetic ANS • Adrenergic synapses – release norepinephrine – found in brain & ANS Other Neurotransmitters • Gaba or gamma aminobutyric acid – inhibitory neurotransmitter – synapses termed GABA-ergic synapses • Dopamine – found in CNS – can be inhibitory or excitatory – inadequate amounts are found in those with Parkinson’s disorder • Serotonin – found in CNS – inadequate amounts have been linked to depression Release of Neurotransmitter • Step 1: Action potential arrives at synaptic knobdepolarizes itopens voltage regulated Ca channels • Step 2: extracellular Ca enters synaptic knobtriggers exocytosis of NT into synaptic cleft • Step 3: NTdiffuses across cleft and binds to receptors on post synaptic cell which depolarizes postsynaptic membrane and increases Na permeability • If enough NT is released post synaptic membrane depolarizes or hyperpolarizes Synaptic Delay • there is a delay of about 0.5ms between arrival of action potential at synaptic knob & effect on postsynaptic membrane • corresponds to time needed for Ca influx & neurotransmitter release • fewer number of synapses shorter total synaptic delay faster response • fastest response is a reflex • has just one synapse Removal of Neurotransmitter • • • • • • Diffusion Neurotransmitter diffuses out of the synapse Uptake by Cells Neuron may reabsorb (reuptake) the NT Enzymatic Degradation ACHE-acetylcholinesterase decomposes ACHacetate + choline Neural Integration • Neurons process, store & recall information • more synapses a neuron hasgreater its information-processing ability • neural integration is based on postsynaptic potentials produced by neurotransmitters • there are two types – excitatory post synaptic potentials (EPSP) – inhibitory post synaptic potentials (IPSP) EPSPS & IPSPS • EPSPs – occur due to opening of chemically regulated membrane channels that depolarize cell membrane – graded – only affect area immediately surrounding synapse • IPSPs – hyperpolarize post synaptic membrane – some due to opening of chloride gates – others due to opening of K channels – graded and local Neural Integration • One EPSP or IPSP will not result in an action potential • Summation is responsible for integrating EPSPs & IPSPs in post synaptic neurons • Individual EPSPs & IPSPs sum in several ways Temporal &Spatial Summation • Temporal summation – addition of stimuli in rapid succession – occurs at single synapse – every time action potential arrives vesicles discharge ACH into synaptic cleft – each time an action potential arrivesmore chemically regulated channels open degree of depolarization increases • Spatial summation – simultaneous stimuli at different locations – multiple synapses are active simultaneously – action potentials have cumulative effect on membrane potential Neural Pools • neurons usually work in large groups-neural pools • groups-a few hundred to a few thousand • function depends on how neurons are connected- neural circuit Neural Circuits • • • • • Simple Series Circuit Diverging Circuit Converging Circuit Reverberation Circuit Parallel Circuit Simple Series Circuit • presynaptic neuron stimulates a single post synaptic neuron • which then stimulates another • and so on and so on Diverging Circuit • one nerve fiber branches & synapses with several postsynaptic cells • each postsynaptic cells synapses with several more Converging Circuit • several neurons synapse on same post synaptic neuron Reverberation Circuit • neurons stimulate each other in a liner sequence • one of the neurons sends an axon collateral back to first neuron in circuit – type of positive feedback • each time a neuron fires it stimulates itself to refire • once circuit is activatedit continues to stimulate itself Parallel After Discharge Circuits • Presynaptic neuron stimulates several groups of neurons • each group reconverge on a single postsynaptic neuron • differing number of synapses between first & last neuron causes impulses to have varying synaptic delays • last neuron exhibits multiple EPSPs or IPSPs. • complex mental processing such as math