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Transcript
Nervous System: Nervous Tissue! (Chapter 12)! Neural Tissue! -3% of body mass! -cellular, ~20% extracellular space! -two categories of cells:! 1. Neurons: conduct nervous impulses! 2. Neuroglia / glial cells: “nerve glue”, ! ! supporting cells! Lecture Materials! for! Amy Warenda Czura, Ph.D.! Suffolk County Community College! Organization of Nervous System! Eastern Campus! Primary Sources for figures and content:! Marieb, E. N. Human Anatomy & Physiology 6th ed. San Francisco: Pearson Benjamin Cummings, 2004.! Martini, F. H. Fundamentals of Anatomy & Physiology 6th ed. San Francisco: Pearson ! Benjamin Cummings, 2004. 1. Central Nervous System (CNS)! -spinal cord, brain! -function: integrate, process, coordinate ! ! sensory input and motor output! 2. Peripheral Nervous System (PNS)! -all neural tissue outside CNS! -function: carry info to/from CNS via nerves! Nerve = bundle of axons (nerve fibers) with ! ! blood vessels and CT! -cranial nerves " brain! -spinal nerves " spinal cord! 2. Motor/Efferent Division! ! -CNS ! effectors! A. Somatic Nervous System! ! -“voluntary nervous system”! ! -to skeletal muscles! B. Autonomic Nervous System (ANS)! ! -“involuntary nervous system”! ! -to smooth & cardiac muscle, glands! ! 1. Sympathetic Division ! ! !- “fight or flight”! ! 2. Parasympathetic Division! ! !- “rest and digest”! ! (tend to be antagonistic to each other)! Histology of Nervous System! Neuron! -function:conduct nervous impulses (message)! -characteristics:! 1. Extreme longevity! 2. Amitotic (exceptions: hippocampus, ! ! olfactory receptors)! 3. High metabolic rate: need O2 and glucose! Divisions of PNS:! 1. Sensory/Afferent Division! ! -sensory receptors ! CNS! A. Somatic afferent division! ! -from skin, skeletal muscles, joints! B. Visceral afferent division! ! -from internal organs! Amy Warenda Czura, Ph.D. 1 SCCC BIO130 Chapter 12 Lecture Notes Structure:! ! ! ! ! ! -large soma / perikaryon! -large nucleus, large nucleolus (rRNA)! -many mitochondria, ribosomes, RER, Golgi: ! ! (#ATP, #protein synthesis to produce ! ! !neurotransmitters)! -Nissl bodies: visible RER & ribosomes, gray! -neurofilaments = neurofibrils, neurotubules ! ! (internal structure)! -no centrioles! -2 types of ! ! processes:! (cell extensions)! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! 2. Axon: ! ! -single, long ! ! -carry an action potential away from soma! ! -release neurotransmitters at end to signal ! !next cell! ! -long ones = “nerve fibers”! -contains:! !-neurofibrils & neurotubules (abundant)! !-vesicles of neurotransmitter! !-lysosomes, mitochondria, enzymes! !-no Nissl bodies, no Golgi (no protein ! ! !synthesis in axon)! -connects to soma at axon hillock! -covered in axolemma (membrane)! -may branch: axon collaterals! -end in synaptic terminals or knobs! -may have myelin sheath: protein+lipid! !-protection! !-insulation! !-increase speed of impulse! CNS: myelin from oligodendrocytes! PNS: myelin from Schwann cells/! ! !neurilemma cells! Amy Warenda Czura, Ph.D. 1. Dendrites: ! -receive info ! -carry a graded potential toward soma! -contain same organelles as soma! -short, branched! -end in dendritic spines! Axoplasmic transport! -move materials between soma and terminal! -along neurotubules on kinesins! -Anterograde transport = soma ! terminal! ! (neurotransmitters from soma)! -Retrograde transport = terminal ! soma! ! (recycle breakdown products from used ! ! !neurotransmitters)! ! Some viruses use retrograde transport to ! ! !gain access to CNS (Polio, Herpes, ! ! !Rabies)! 2 SCCC BIO130 Chapter 12 Lecture Notes Synapse! -site where neuron ! communicates with ! another cell: ! neuron or effector! Structural classification of neurons:! 1. Anaxonic neurons: ! -dendrites and axon look same ! -brain and special sense organs! 2. Bipolar neurons: ! -1 dendrite, 1 axon ! -soma in middle! -rare: special sense organs, ! ! relay from receptor to neuron! -presynaptic cell sends message along axon to ! ! axon terminal! -postsynaptic cell receives message as ! ! neurotransmitter! Neurotransmitter = chemical, transmits signal ! ! from pre- to post- synaptic cell across ! ! synaptic cleft ! Synaptic knob = small, round, when ! ! postsynaptic cell is neuron, synapse on ! ! dendrite or soma! Synaptic terminal = complex structure, at ! ! neuromuscular or neuroglandular junction! 3. Unipolar neurons:! -1 long axon, dendrites at one ! ! end, soma off side (T shape)! -most sensory neurons! 4. Multipolar neurons:! -2 or more dendrites! -1 long axon! -99% all neurons! -most CNS! Functional Classification of Neurons:! 1. Sensory/Afferent neurons! -transmit info from sensory receptors to CNS! 3. Interneurons / Association neurons! -distribute sensory info and coordinate motor ! ! activity! -between sensory and motor neurons! -in brain, spinal cord, autonomic ganglia! -most are multipolar! -most unipolar! -soma in peripheral sensory ganglia! Ganglia = collection of cell bodies in PNS! A. Somatic sensory neurons! ! -receptors monitor outside conditions! B. Visceral sensory neurons! ! -receptors monitor internal conditions! 2. Motor/Efferent neurons! -transmit commands from CNS to effectors! -most multipolar! A. Somatic motor neurons! ! -innervate skeletal muscle! ! -conscious control or reflexes! B. Visceral/Autonomic motor neurons! ! -innervate effectors on smooth muscle, ! ! cardiac muscle, glands, adipose! Amy Warenda Czura, Ph.D. Neuroglia =supporting cells! Neuroglia in CNS! -outnumber neurons 10:1! -half mass of brain! 3 SCCC BIO130 Chapter 12 Lecture Notes 1. Ependymal cells! -line central canal of spinal ! cord and ventricles of brain! -secrete cerebrospinal fluid ! (CSF)! -have cilia to circulate CSF! -CSF: cushion brain, nutrient & gas exchange! 3. Oligodendrocytes! -wide flat processes wrap ! local axons = myelin ! sheath! -1 cell contributes myelin to many ! neighboring axons! -lipid in membrane insulates axon for faster ! action potential conductance! -gaps on axon between processes/myelin = ! Nodes (of Ranvier), necessary to conduct ! impulse! -white, myelinated axons = “white matter”! 2. Astrocytes! -most abundant CNS ! neuroglia! -varying functions:! a. blood brain barrier: ! ! processes wrap capillaries, control ! ! chemical exchange between blood and ! interstitial fluid of brain! b. framework of CNS! c. repair damaged neural tissue! d. guide neuron development in embryo! e. control interstitial environment: regulate conc. ions, gasses, nutrients, neurotransmitters! 4. Microglia! -phagocytic! -wander CNS! -engulf debris, pathogens! -important CNS defense ! (no immune cells or antibodies)! Neuroglia in PNS! 1. Satellite cells! -surround somas in ganglia! -isolate PNS cells! -regulate interstitial environment of ganglia! Cells in the CNS! 2. Schwann cells /! Neurilemma cells! -myelinate axons in PNS! -whole cells wraps axon, ! !many layers, organelles compressed in ! superficial layer (neurilemma)! -Nodes (of Ranvier) between cells! Amy Warenda Czura, Ph.D. 4 SCCC BIO130 Chapter 12 Lecture Notes -vital to repair of peripheral never fibers after ! ! injury: guide growth to original synapse! Neurophysiology! Neurons: conduct electrical impulse! -requires transmembrane potential = electrical ! ! difference across cell membrane! -cells: positive charge outside (pump cations ! out) and negative charge inside (proteins)! +! +! +! +! +! +! +! +! +! +!+! +! +! +! +! +! +! +! +! +! +! +! +! +! +! +! +! -!-!-!-!-!-!-!-! -! -! -! -! -!-! -!-!-!-! -! -!-! Voltage = measure of potential energy ! generated by separation of opposite charges! Current = flow of electrical charges (ions)! Cell can produce current (nervous impulse) ! ! when ions move to eliminate the potential ! difference (volts) across the membrane! Resistance = restricts ion movement (current)! ! (high resistance = low current); membrane ! has resistance, restricts ion flow/current! Ohm’s Law: current = voltage ÷ resistance! Current highest when voltage high and ! ! resistance low! Cell voltage set at -70mV but membrane! ! resistance can be altered to create current! Membrane resistance depends on permeability! ! to ions: open or close ion channels! Cell must always have some resistance or ions! ! would equalize, voltage = zero, ! ! no current generated = no nervous impulse! Membrane ion channels:! -allow ion movement (alter resistance)! -each channel specific to one ion type! 1. Passive channels (leak channels)! -always open, free flow! -sets resting membrane potential at -70mV! Amy Warenda Czura, Ph.D. 2. Active channels! -open/close in response to signal! A. Chemically regulated/ Ligand-gated! ! -open in response to chemical binding! ! -located on any cell membrane ! ! ! !(dendrites, soma) ! ! ! ! !! ! ! ! ! ! 5 B. Voltage regulated channels! -open/close in response to shift in ! !transmembrane potential! -excitable membrane only: conduct ! !action potentials (axolemma, ! !sarcolemma) ! SCCC BIO130 Chapter 12 Lecture Notes ! ! ! ! C. Mechanically regulated channels! -open in response to membrane ! !distortion! -on dendrites of sensory neurons for ! !touch, pressure, vibration! Sodium-Potassium Pump:! -uses ATP to move 3 Na+ out 2 K+ in ! ! (70% of neuron ATP for this)! -runs anytime cell not conducting impulse! -creates high [K+] inside and high [Na+]outside! When Na+ channel opens:! - Na+ flows into cell:! !1. Favored by diffusion gradient! !2. Favored by electrical gradient! open channel = $resistance = #ion flow/current! When K+ channel opens:! - K+ flows out of cell:! !1. Favored by diffusion gradient only! !2. Electrical gradient repels K+ exit! ! - Thus less current than Na+ ! When channel opens, ions flow along ! ! ! electrochemical gradient:! ! !-diffusion (high conc. to low)! ! !-electrical attraction/repulsion! Channels open = resistance low = ions move ! ! until equilibrium potential: depends on ! ! !-diffusion gradient! ! !-electrical gradient! Equilibrium Potential! ! For K+ = -90mV! ! Na+ channel opens = Na+ flows in, ! ! ! depolarization (cell less negative)! K+ channel opens = K+ flows out, ! ! !hyperpolarization (cell more negative)! Graded potentials:! -occur on any membrane: dendrites and somas! -can be depolarizing or hyperpolarizing! -amount of depolarization or hyperpolarization ! ! depends on intensity of stimulus: ! ! ! # channels open = # voltage change! -passive spread from site of stimulation over ! short distance! -effect on membrane potential decreases with ! ! distance from stimulation site! -repolarization occurs as soon as stimulus is ! ! removed: leak channels & Na+/K+ pump ! ! reset resting potential! For Na+ = +66mV! Open channel ! current ! graded potential! Graded potential = localized shift in ! ! transmembrane potential due to ! ! movement of charges in to /out of cell! Amy Warenda Czura, Ph.D. Graded potential = localized change in ! ! transmembrane potential, not nervous ! ! impulse (message)! 6 SCCC BIO130 Chapter 12 Lecture Notes The Generation of an Action Potential! If big enough depolarization = action potential! ! = nervous impulse = transmission to ! !next cell! Action potentials:! -occur on excitable membranes only ! ! (axolemma, sarcolemma)! -always depolarizing! -must depolarize to threshold (-55mV) before ! ! action potential begins ! ! (voltage gated channels on excitable ! ! !membrane open at threshold to ! ! ! !propagate action potential)! - “all-or-none” : all stimuli that exceed ! ! threshold will produce identical action ! ! potentials! -action potential at one site depolarizes ! ! adjacent sites to threshold! -propagated across entire membrane surface ! ! without decrease in strength! -55 mV! (Handout)! 1. Depolarization to threshold:! - a graded potential depolarizes local ! membrane and flows toward the axon! - if threshold is met (-55mV) at the hillock, an ! action potential will be triggered! 2. Activation of sodium channels and rapid ! depolarization:! - at threshold (-55mV), voltage-regulated ! sodium channels on the excitable axolemma! membrane open! - Na+ flows into the cell depolarizing it! - the transmembrane potential rapidly changes ! from -55mV to +30mV! 3. Inactivation of sodium channels and ! activation of potassium channels:! - at +30mV Na+ channels close and K+ ! channels open! - K+ flows out of the cell repolarizing it! 4. Return to normal permeability:! - at -70mV K+ channels begin to close! - the cell hyperpolarizes to -90mV until all ! channels finish closing! - leak channels restore the resting membrane ! potential to -70mV! Restimulation only when Na+ channels closed:! influx of Na+ necessary for action potential! Cell has ions for thousands of action potentials! Eventually must run Sodium-Potassium pump ! ! (burn ATP) to reset high [K+] inside and ! ! high [Na+] outside! ! (Death = no ATP, but stored ions can ! ! !generate action potentials for awhile)! Propagation of Action Potentials! -once generated must be transmitted length of ! ! axon: hillock to terminal! -speed depends on: ! ! 1. Degree of myelination (yes or no)! ! 2. Axon diameter! 1. Myelination! A. Continuous Propagation: ! ! -unmyelinated axons! ! -whole membrane depolarizes and ! ! repolarizes sequentially hillock to ! ! terminal! ! -only forward movement; membrane ! ! behind always in absolute refractory ! period! Absolute Refractory Period = -55mV ! ! (threshold) to +30mV, Na+ channels open, ! membrane cannot respond to additional ! ! stimulus! Relative Refractory Period = +30mV to ! ! -70mV (return to resting potential), Na+ ! ! channels closed, membrane capable of ! ! second action potential but requires ! ! larger/longer stimulus (threshold elevated)! Amy Warenda Czura, Ph.D. 7 SCCC BIO130 Chapter 12 Lecture Notes Saltatory Propagation! Continuous Propagation! ! ! ! ! ! ! ! ! B. Saltatory propagation! -myelinated axons! -depolarization only on exposed ! !membrane at nodes! -myelin insulates covered membrane ! !from ion flow! -action potential jumps from node to ! !node: faster and requires less ! !energy to reset! Myelination: ! -requires space, metabolically expensive! -only important fibers large and myelinated! -occurs in early childhood! -results in improved coordination! Multiple Sclerosis = genetic disorder, myelin ! ! on neurons in PNS destroyed ! ! ! numbness, paralysis! 2. Axon diameter! -larger axon ! less resistance ! easier ion ! !flow ! faster action potential! A. Type A Fibers/Axon! ! - 4-20µm diameter ! ! - myelinated (saltatory propagation)! ! - action potentials 140m/sec! ! - carry somatic motor and somatic ! ! !sensory info! B. Type B Fibers/Axon! ! - 2-4µm diameter! ! - myelinated (saltatory propagation)! ! - action potentials 18m/sec! ! - carry autonomic motor and visceral ! ! !sensory info! C. Type C Fibers/Axon! ! - < 2µm diameter! ! - unmyelinated (continuous propagation)! ! - action potentials 1m/sec! ! - carry autonomic motor and visceral ! ! !sensory info! Amy Warenda Czura, Ph.D. Synapse = junction between transmitting ! ! neuron (presynaptic cell) and receiving ! ! cell (postsynaptic cell), where nerve ! ! impulse moves from one cell to next! Two types:! 1. Electrical Synapse! -direct contact via gap junctions! -ions flow directly from pre to post cell! -less common synapse! -in brain (conscious perception)! 2. Chemical synapse! -most common! 8 SCCC BIO130 Chapter 12 Lecture Notes Events at a Synapse:! ! e.g.Cholinergic Synapse! (Acetylcholine as neurotransmitter)! (Handout)! -pre and post cells separated by synaptic cleft! -presynaptic neuron releases neurotransmitter ! ! to trigger effect on post synaptic cell! -dynamic: facilitate or inhibit transmission, ! ! depends on neurotransmitter:! ! 1. Excitatory Neurotransmitters = ! ! !-depolarization! ! !-propagate action potential! ! 2. Inhibitory Neurotransmitters = ! ! !-hyperpolarization! ! !-suppress action potential! Propagation across chemical synapse always ! ! slow but allows variability! Neurotransmitter Mechanism of Action! Post synaptic potential = graded potential ! ! caused by a neurotransmitter due to ! ! opening or closing of ion channels on ! ! post synaptic cell membrane! Two types:! 1. Excitatory Post Synaptic Potential (EPSP)! ! -causes depolarization! 2. Inhibitory Post Synaptic Potential (IPSP)! ! -causes hyperpolarization! ! -inhibits postsynaptic cell (need larger ! ! !stimulus to reach threshold)! Multiple EPSPs needed to trigger action ! ! potential in post cell axon! EPSP summation:! 1. Temporal summation! -single synapse fires repeatedly: string of ! ! !EPSPs in one spot! -each EPSP depolarizes more until ! ! !threshold reached at hillock! 1. Direct effect on membrane potential! (Handout)! 2. Indirect effect on membrane potential! (Handout)! Amy Warenda Czura, Ph.D. 9 SCCC BIO130 Chapter 12 Lecture Notes Most nervous system activity results from ! ! interplay of EPSPs and IPSPs to ! ! promote differing degrees of facilitation! ! or inhibition to allow constant fine ! ! tuning of response! Neuromodulators = chemicals that influence ! ! synthesis, release, or degradation of ! ! neurotransmitters thus altering normal ! ! response of the synapse! Common Neurotransmitters:! 1. Acetycholine- cholinergic synapses! -excitatory! -direct effect! -skeletal neuromuscular junctions, many ! !CNS synapses, all neuron to neuron ! ! PNS, all parasympathetic ANS! 2. Norepinephrine- adrenergic synapses! -excitatory! -second messengers! -many brain synapses, all sympathetic ANS ! ! effector junctions! 2. Spatial summation! -multiple synapses fire simultaneously! -collective depolarization reaches threshold! Facilitated = depolarized; brought closer to ! ! threshold by some sort of stimulus, less ! ! stimulus now required to reach threshold! ! (e.g. caffeine)! Post Synaptic Potentiation:! -repeat stimulation of same synapse ! ! ! conditions synapse, pre cell more easily ! ! stimulates post cell to threshold (repetition)! 3. Dopamine! -excitatory or inhibitory ! -second messengers! -many brain synapses, many functions! ! -responsible for reward feeling! ! !-cocaine: inhibits removal = “high”! ! -Parkinson’s disease: damage neurons =! ! !ticks, jitters! 4. Serotonin! -inhibitory! -direct or second messenger! -brain stem for emotion! ! -anti-depression/ anti-anxiety drugs ! ! !block uptake! 5. Gamma aminobutyric acid (GABA)! -inhibitory! -direct effect! -brain: anxiety control, motor coordination! ! -alcohol: augments effects = loss of ! ! !coordination! Amy Warenda Czura, Ph.D. Factors that disrupt neural function:! 1. pH: normal = 7.4! @ pH 7.8 ! spontaneous action potentials ! ! != convulsions! @ pH 7.0 ! no action potentials! ! ! = unresponsive! 2. Ion concentrations! high extracellular [K+] ! depolarize ! ! membranes = death, cardiac arrest! 3. Temperature: normal = 37°C! -higher: neurons more excitable! ! (fever = hallucinations)! -lower: neurons non-responsive! ! (hypothermia = lethargy, confusion)! 4. Nutrients! -neurons: no reserves, use a lot of ATP! -require constant and abundant glucose! -glucose only! 5. Oxygen! -aerobic respiration only for ATP! -no ATP = neuron damage/death! 10 SCCC BIO130 Chapter 12 Lecture Notes
