Exsc 224 Week 2 Notes
Exsc 224 Week 2 Notes Exsc 224
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This 12 page Class Notes was uploaded by Jane Warther on Saturday January 23, 2016. The Class Notes belongs to Exsc 224 at University of South Carolina taught by Dr. Thompson in Spring 2016. Since its upload, it has received 48 views. For similar materials see Anatomy and Physiology 224 in Education and Teacher Studies at University of South Carolina.
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Date Created: 01/23/16
Chapter 11 Lecture 3 1/19/16 Action Potential (AP) 2. Depolarization 3. Repolarization 1. Resting State 4. Hyperpolarization 3 Membrane Potential (mV) 2 1 4 Time (ms) 70mV resting membrane potential AP is caused by permeability changes in the plasma membrane o Talk about voltagegated channels o Either happen or they don’t, depends on whether or not they reach threshold or not o Same size, same duration Phase 1 o Resting state @ 70mV Only leakage channels for Na+ and K+ are open All gated Na+ and K+ channels are closed Properties of gated channels o Each Na+ channel has two voltagesensitive gates Activation gates Closed at rest; open with depolarization Inactivation gates Open at rest; block channel once it is open o Each K+ channel has one voltage sensitive fate o Closed at rest o Opens slowly with depolarization Phase 2 o Depolarization Local currents open voltagegated Na+ channels Na+ influx causes more depolarization At threshold (55 to 50mV) positive feedback leads to opening of all Na+ channels and a reversal of membrane polarity to +30mV (spike of action potential) Phase 3 o Repolarizing phase Na+ channel slow inactivation gates close Membrane permeability to Na+ declines to resting levels Slow voltagesensitive K+ gates open K+ exits the cell and internal negativity is restored Phase 4 o Hyperpolarization Some K+ channels remain open, allowing excessive K+ efflux This causes afterhyperpolarization of the membrane (undershoot) Greater than at resting membrane potential Recap: Graded potentials are associated with receiving info if total sum is great enough an AP forms o Can vary in size and direction o Can cause rise or decrease in membrane potential o If change in membrane potential is not adequate, nothing happens o What is meant by adequate is by the threshold which is at 55mV If graded potential doesn’t reach threshold nothing happens Reach threshold activate sodiumgated channels, have a huge depolarizing increase overshooting 0mV with the inside of the cell becoming positive for a moment When cell becomes positive, sodiumgated channels close Potassiumvoltage gated channels will open and repolarize, so potassium rushes out of the cell, membrane potential decreases so much the cell is temporarily hyperpolarized Once hyperpolarized potassiumvoltage gated channel closes Potassium is now in cytosol with sodium inside cell o Sodiumpotassium pumps ions back across membrane restoring resting membrane potential where it levels off until it receives another stimulus Takes about 4ms, can have about 250 action potentials in one second Role of the SodiumPotassium Pump Repolarization o Restores the resting electrical conditions of the neuron o Does not restore the resting ionic conditions Ionic redistribution back to resting conditions is restored by the thousands of sodiumpotassium pumps Requires energy to restore from an AP Uses ATP Propagation of an Action Potential Na+ influx causes a patch of the axonal membrane to depolarize Local currents occur Na+ channels toward the point of origin are inactivated and not affected by the local currents Local currents affect adjacent areas in the forward direction Depolarization opens voltagegated channels and triggers an AP Repolarization wave follows the depolarization wave Threshold At threshold Action o Membrane is depolarized by 15 to 20 mV Membrane Potential (mV) o Na+ permeability increases o Na influx exceeds K+ efflux o The positive feedback cycle begins o Subthreshold stimulus Nothing happens if you apply it Stimulus Voltage o Threshold stimulus Time (ms) Cause a greater change in voltage, then we have AP o Action potential All or none phenomenon Either have stimulus that is adequate or not Coding for Stimulus Intensity All action potentials o Coded by number of AP generated Strong stimulus a lot APs generated Increased frequency of AP Weak stimulus less APs generated in a certain amount of time o Are alike o Are independent of stimulus intensity o How does the CNS tell the difference between a weak stimulus and a strong one? Strong stimuli can generate action potentials more often than weaker stimuli The CNS determines stimulus intensity by the frequency of impulses – rate coding Absolute Refractory Period When cell depolarizes Time from the opening of the Na+ channels until the resetting of the channels Ensures that each AP is an all or none event Enforces oneway transmission of nerve impulses Period from when cell reaches threshold and depolarizes until the cell depolarizes and reaches threshold again from threshold to threshold During this period, the cell can’t depolarize a second time Relative Refractory Period Begins when membrane potential falls below threshold and lasts through hyperpolarization period until resting membrane potential returns to normal Follows the absolute refractory period o Most Na+ channels have returned to their resting state o Some K+ channels are still open o Repolarization is occurring Threshold for AP generation is elevated Exceptionally strong stimulus may generate an AP Possible for neuron to depolarize again but it takes a larger stimulus than normal to do it Conduct Velocity Rate at which the AP propagates down the length of the axon Conduction velocities of neurons vary widely 2 things that effect it: o 1. Effect of axon diameter Large diameter fibers have less resistance to local current flow and have faster impulse conduction o 2. Effect of myelination Continuous conduction in unmyelinated axons is slower than salutatory conduction in myelinated axons In a bare plasma membrane o Receive stimulus at dendrite will cause local change in membrane potential o Change in membrane potential doesn’t travel very far from site of stimulus, it decays rapidly because there aren’t voltagegated channels in dendrites o It takes a lot of stimulus to create an adequate stimulus to create AP on the dendrite In an unmyelinated axon o Do have voltagegated channels(talking about sodium in picture) o If there is an adequate stimulus will cause sodium voltage gated channels to open, sodium rushed into cell, cell depolarizes o Change in membrane potential travels to the next sodium gated channel, sodium rushes in , cell depolarizes o And it keeps repeating o The signal propagates down length of axon In a myelinated axon o Myelin decreases the AP that occur along length of axon o There are few sodiumgated channels in myelinated axon because myelin wraps around axon with no sodiumgated channels where myelin is o Acts as insulator , allows change in membrane potential to last at a longer distance where it will come upon a node of Ranvier which will trigger sodium gated channels, depolarize membrane and the cycle repeats o Takes less time for AP to propagate down the membrane o Salutatory conduction activation of sodiumgated channels at Node of Ranvier Nerve Fiber Classification Nerve fibers are classified according to: o Diameter o Degree of myelination o Speed of conduction Group A fibers o Largest diameter neurons o Heavily myelinated o Fastest conduction o Examples Somatic sensory (sensory input) Motor fibers Group B fibers o Intermediate diameter o Lightly myelinated o Moderately fast o Examples ANS fibers (involuntary motor) Group C fibers o Smallest diameter o Unmyelinated o Slowest o Example ANS fibers (involuntary motor) Certain type of sensory The Synapse A junction that mediates information transfer from one neuron: o To another neuron or o To an effector cell Axodendritic synapse – most common Axosomatic synapse most common Axoaxonic synapse Presynaptic neuron conducts impulses toward the synapse o Has AP, will release neurotransmitter which will causes graded potential on post synaptic neuron Postsynaptic neuron transmit impulses away from the synapse Electrical vs. Chemical Synapses Electrical o Very rapid o Have gap junctions o Unidirectional or bidirectional o Are important in : Embryonic nervous tissue Some brain regions Heart Chemical o Specialized for the release and reception of neurotransmitters o Typically composed of two parts Axon terminal or buton presynaptic neuron Doesn’t physically touch postsynaptic region space is known as synaptic cleft o Extracellular fluid passes through this region Receptor region postsynaptic neuron (dendrites or soma with ligandgated channels) How does one neuron communicate with another neuron? Via neurotransmitters Information Transfers how one neuron communicates with another neuron 1. Action potential arrives at axon terminal or button o Its goal to release neurotransmitter into synaptic cleft 2. Voltage gated Ca2+ channels open and Ca2+ enter the axon terminal 3. Ca2+ entry causes neurotransmitter containing synaptic vesicles to release their contents by exocytosis 4.Neurotransmitter diffuses across the synaptic cleft and binds to specific receptors on the postsynaptic membrane 5. Binding of neurotransmitter opens ion channels, resulting in graded potentials 6. Neurotransmitter effects are terminated by : o Reuptake o Enzymatic degradation o Diffusion Quiz 1 1. Integration occurs in the PNS false it occurs in CNS 2. Afferent fibers are sensory input true 3. what specific cell tyes are responsible for the production of myelin in the CNS and PNS CNS oligodendrocytes PNS schwann cells 4. The Na+/K+ pumps move sodium into extracellular fluid and potassium into the cytosol. 5. where do you find ligand gated channels on neurons? Dendrites or soma (cell body) Chapter 11 Lecture 4 Postsynaptic potentials Graded potentials –they are changes in membrane potentials o Vary in size unlike AP o Sometimes called receptor potentials Strength determined by o Amount of neurotransmitter released o Time the neurotransmitter is in the area Types of postsynaptic potentials o 1. EPSP excitatory postsynaptic potentials Also known as depolarizing graded potentials membrane moves toward 0 or threshold o 2. IPSP inhibitory postsynaptic potentials These are hyperpolarizing membrane potential moves away from zero, becomes more negative o EPSP or IPSP only tell the direction not if they are adequate Excitatory Synapses and EPSPs Neurotransmitter binds to and opens chemically gated channels that allow simultaneous flow of Na+ and K+ in opposite directions Na+ influx is greater that K+ efflux, causing a net depolarization Threshold EPSP helps trigger AP at axon hillock if EPSP is of threshold strength and opens the voltagegated channels Causes depolarization Inhibitory Synapses and IPSPs Stimulus Neurotransmitter binds to and opens channels Time (ms) for K+ or Cl Causes a hyperpolarization (the inner surface of membrane Membrane potential (mV) becomes more negative) Threshold Reduces the postsynaptic neuron’s ability to produce an AP Stimulus Integration: Summation ime (ms) Membrane potential (mV) Presynaptic (E1) fires one time, get a graded potential (EPSP), then it decays, fires another time , get a graded potential EPSP) and then it decays again o Action potential was inadequate so it didn’t reach threshold o If time between the two 2 of E1 firing is small , one can cause a change in membrane potential, and the firing of a second upon the first causes a greater change in membrane potential o The adding of the two is called summation o Does not take 2 EPSPs to reach threshold o No summation is when two signals are fired at different times and do not overlap You need 2 that are close together that fall on top of each other to combine and reach threshold o When one neuron fires multiple times causing a large change in membrane potential it is called temporal summation Two presynaptic neurons fire at the same time , they can cause a larger effect than if one fired o This is called spatial summation Signal from one postsynaptic neuron (EPSP )and a presynaptic neurons(IPSP) cancels each other out Synapses can’t be inhibitory and excitatory Neurotransmitters Acetylcholine o At nicotinic ACh receptors(on skeletal muscles, autonomic ganglia, and in the CNS) Functional classes: excitatory and direct action Will cause EPSP Sites where secreted CNS: widespread throughout cerebral cortex, hippocampus and brain stem o At muscarinic ACh receptors( on visceral effectors and in the CNS) Functional classes: excitatory or inhibitory depending on subtype of muscarinic receptor and indirect action via second messengers Multiple kinds of muscarinic receptors which is why it is either inhibitory or excitatory M2 receptor receptor for ACh in heart o When activated causes heart rate to decrease o Inhibitory receptor Sites where secreted PNS: all neuromuscular junctions with skeletal muscle; some autonomic motor endings (all preganglionic and parasympathetic postganglionic fibers) o One of the most abundant in the body o Get names of receptors whether they are stimulated by nicotine or muscarinic o Depends on the receptor ACh binds to whether it is excitatory or inhibitory Biogenic Amines o Amino acids that are highly modified o Norepinephrine ( noradrenaline) Functional classes: Excitatory or inhibitory depending on receptor type it binds to Indirect action via second messengers Sites where secreted: CNS: brain stem, particularly in the locus coeruleus of the midbrain; limbic system; some areas of cerebral cortex o Dopamine Synthesized in same pathway as norepinephrine “feel good” neurotransmitter Functional classes Excitatory or inhibitory depending on the receptor type bound Indirect action via second messengers Sites where secreted: CNS: substantia nigra of midbrain; hypothalamus; is the principal neurotransmitter of extrapyramidal system PNS: some sympathetic ganglia o Serotonin Too much of it you go to sleep Not enough of it you are depressed Functional classes: Mainly inhibitory Indirect action via second messengers; direct action at 5HT3 receptors Sites where secreted: CNS: brain stem, especially midbrain; hypothalamus; limbic system; cerebellum, pineal gland; spinal cord Amino Acids can be neurotransmitters too o GABA(aminobutyric acid) Functional Classes: Generally inhibitory o Glutamate Functional classes: Generally excitatory Peptides o Endorphins (e.g., dynnorphin, enkephalins) o “feel good” neurotransmitter operating via opiate receptors Functional classes: General inhibitory Indirect action via second messengers Sites where secreted : CNS: widely distributed in brain; hypothalamus; limbic system; pituitary ;spinal cord Purines o ATP Functional classes: Excitatory or inhibitory depending on receptor type it is bound to o Often associated with signaling pain Direct and indirect actions via second messengers Sites where secreted: CNS: basal nuclei, induces Ca2+ wave propagation in astrocytes PNS: dorsal root ganglion neurons o Adenosine Causes you to be mentally unalert Caffeine blocks adenosine from binding to its receptor Functional Classes: Generally inhibitory o Signaling molecule Indirect action via second messengers Sites where secreted: Throughout CNS Gases and Lipids o All are neuromodulators o Nitric oxide (NO) Functional classes: Excitatory o Carbon monoxide (CO) Functional classes: Excitatory Endocannabinoids (e.g., 2arachidonoylglycerol, anaddamide) Functional classes: Inhibitory Neurotransmitter Receptors Two types o 1. Channellinked receptors (direct) Ex. ACh and amino acids o 2. G proteinlinked receptors (Indirect) Ex. Biogenic amines, neuropeptides, and dissolved gases If ions move across the membrane it is not a direct affect to activate the membrane ChannelLinked (Ionotropic) Receptors Ligandgated ion channels o Ligand binds to closed receptor, causes it to open and then the ions flow through the channel Action is immediate and brief Excitatory (EPSP) o Receptors are channels o Na+ influx = depolarization Inhibitory (IPSP) o Receptors allow: Cl influx or K+ efflux Hyperpolarization occurs for either of them G ProteinLinked Receptors they cause formation of an intracellular second messenger (cyclic AMP ) that bring about the cell’s response don’t directly open channels, a domino effect opens them effect exerted through second messenger slower to respond and fall off and decay than ligandgated channels signal lasts a little longer so usually is a little more potent Second Messenger Systems o 1. Neurotransmitter (1 messenger) binds and activates receptor o 2. Receptor activates G protein Used GTP as energy source o 3. G protein activates adenylate cyclase (enzyme) Catalyzes a reaction o 4. Adenylate cyclase converts ATP to cAMP(2 messenger) o 5a. cAMP changes membrane permeability by opening or closing ion channels o 5b. cAMP activates enzymes o 5c. cAMP activates specific genes Neural Integration: Neuronal Pools Functional groups of neurons that: o Integrate incoming information o Forward the processed information to other destinations Simple Neuronal Pool o Single presynaptic(input) fiber branches and synapses with several postsynaptic neurons in the pool o Amplifying circuit Types of Circuits in Neuronal Pools Diverging circuit o One incoming fiber stimulates an everincreasing number of fibers, often amplifying circuits o May affect a single pathway or several o Common in both sensory and motor systems Converging circuit o Opposite of diverging circuits , resulting in either strong stimulation or inhibition o Also common in sensory and motor systems o Multiple inputs all converging into one Reverberating (oscillating) circuit o Chain of neurons containing collateral synapses with previous neurons in the chain o Keeps things active –how you stay alert o Ex; alertness Parallel afterdischarge circuit o Incoming fiber stimulates several neurons in parallel arrays to stimulate a common output cell Patterns of Neural Processing Serial Processing o Ex: reflexes rapid, automatic responses to stimuli that always cause the same response o Normally doesn’t happen in isolation o Usually accompanied by parallel processing o Reflex arcs (pathways) have five essential components Stimulus ↓ 1. Receptor 2. Sensory neuron 3. Integration center 4. Motor neuron 5. Effector ↓ Response Parallel processing o Input travels along several pathways o One stimulus promotes numerous responses o Travels to two different destinations simultaneously o Important for higherlevel mental functioning o Ex: a smell may remind one of the odor and associated experiences
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