Neural Conduction and Synaptic Transmission
Neural Conduction and Synaptic Transmission PS 235
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This 13 page Class Notes was uploaded by Marissa Tawadros on Sunday September 18, 2016. The Class Notes belongs to PS 235 at Butler University taught by Dr. Jennifer Berry in Fall 2016. Since its upload, it has received 2 views. For similar materials see Biological Bases of Behavior in Psychology (PSYC) at Butler University.
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Date Created: 09/18/16
Chapter 4 Resting Membrane Potential Membrane Potential o The difference in electrical charge between the inside and the outside of a cell o Key to understanding how neurons work and how they malfunction Recording the Membrane Potential To record the membrane potential, it is necessary to position the tip of another electrode inside the neuron in the extracellular fluid o Size of the extracellular fluid is not critical, but it is paramount that the tip of the intracellular electrode be fine enough to pierce the neural membrane without severely damaging it. o Microelectrodes The intracellular electrodes with tips that are less than one-thousandth of a mm in diameter When both electrodes are in the extracellular fluid the voltage difference between them is 0 When the tip of the intracellular electrode is inserted into a neuron, a steady potential of about -70mV is recorded o The potential inside the resting neuron is about 70mV less than that outside the neuron o Resting Potential The steady membrane potential of about -70mV In its resting state with the -70mV charge built up, a neuron is said to be polarized Ionic Basis of the Resting Potential Ions o The salts in neural tissue that separate into positively and negatively charged particles Na+ K+ The + indicated that each ion carries a single positive charge o In resting neurons there are more Na+ ions outside the cell than inside, and more K+ ions Ion Channels The specialized pores in the neural membrane that maintain the unequal distributions of Na+ and K+ Each type of ion channel is specialized for the passage of particular ions o Some Na+ ions do manage to enter the resting potential despite the closed sodium channels and some K+ do exit; then why does the resting membrane potential stays fixed? At the same rate that Na+ ions were leaked into the resting neurons, other Na+ ions were actively transported out; at the same time at the same rate that K+ ions leaked out of the resting neurons, other K+ ions were actively transported in Sodium-Potassium Pumps o Transporters in the cell membrane that continually exchange three Na+ ions inside the neuron for two K+ ions outside Transpoter o Mechanisms in the membrane of a cell that actively transport ions or molecules across the membrane Generation and Conduction of Postsynaptic Potential When neurons fire they release from their terminal buttons chemicals called neurotransmitters o Diffuse across the synaptic clefts and interact with specialized receptor molecules on the receptive membranes of the next neurons in the circuit o When neurotransmitters bind molecules bind to postsynaptic receptors they have one of two effects depending on the neurotransmitter, receptor, and receptive membrane Depolarize Decrease the resting membrane potential from -70mV to -67mV Excitatory Postsynaptic Potentials (ESPS) o Increase the likelihood that the neuron will fire Hyperpolarize Increase the resting membrane potential from -70mV to -72mV Inhibitory Postsynaptic Potentials (ISPS) o Decrease the likelihood that a neuron will fire Graded Response Responses whose magnitude is indicative of the magnitude of the stimuli that induce them Weak signals elicit small postsynaptic potentials Strong signals elicit large postsynaptic potentials ESPS and ISPS have two important characteristics It is fast Transmission is decremental Integration of Postsynaptic Potentials and Generation of Action Potentials The post synaptic potentials created at a single synapse typically have little effect on the firing of the postsynaptic neuron o The receptive areas of most neurons are covered in with thousands of synapses and whether or not a neuron fires is determined by the net effect of their activity More specifically, whether or not a neuron fires depends on the balance between the excitatory and inhibitory signals reaching its axon Axon Hillock Conical structure at the junction between the cell body and the axon Where it is believed that action potentials were generated Axon Initial Segment Adjacent to the axon hillock Where the axon potentials are actually generated Threshold of Excitation The level of the sum of the depolarization and hyperpolarization’s reaching the axon initial segment at anytime that is sufficient to depolarize the membrane -65mV Action potential is generated Action Potential o A massive but momentary (lasting for 1 mm) reversal of the membrane potential from about -70mV to +50mV o Not graded responses o All-or- None Reponses Action potentials occur to their full extent or do not occur at all Integration o Adding or combining a number of individual signals into one overall signal Spatial Summation Shows how local ESPSs that are produced simultaneously on different parts of the receptive membrane sum to form a greater ESPS Shows how simultaneous ISPSs sum to form a greater ISPS Shows how simultaneous ESPSs and ISPSs sum to cancel each other out Temporal Summation Shows how postsynaptic potentials in rapid succession at the same synapse sum to form a greater signal Conduction of Action Potentials Voltage-Activated Ion Channels o Ion channels that open or close in response to changes in the level of the membrane potential o Where action potentials are produced and how they are conducted along the axon Iconic Basis of Action Potentials The membrane potential of a neuron at rest is relatively constant despite the high pressure acting to drive Na+ ions into the cell o Because the resting membrane is relatively impermeable to Na+ ions and because few that do pass in are pumped out o Change when the membrane potential of the axon is depolarized to the threshold of excitation of the ESPS Voltage-activated sodium channels in the axon membrane open wide, and Na+ ions rush in, driving the membrane potential from about -70mV to +50mV. The change associated with the influx of Na+ triggers the opening of voltage-activated potassium channels o K+ ions near the membrane are driven out of the cell through these channels o First by their relatively high internal concentration then when the action potential is near its peak o After 1 millisecond the sodium channels close End of the rising phase and beginning of the repolarization Potassium channels gradually close once repolarization has been achieved o Too many K+ ions flow out of the neuron because of this o Left hyperpolarized Refractory Periods Absolute Refractory Period o A brief period of 1-2 milliseconds after the initiation of an action potential during which it is impossible to elicit another one Relative Refractory Period o The period which it is possible to fire the neuron again but only by applying higher-than-normal levels of stimulation o The end is the point at which the amount of stimulation necessary to fire a neuron returns to baseline o Follows the absolute refractory period Responsible for the fact that action potentials normally travel along axons in only one direction Responsible for the fact that the rate of neural firing is related to the intensity of the stimulation Higher levels of stimulation= more firing Axonal Conduction of Action Potentials The conduction of action potentials along an axon differs for the conduction of ESPSs and ISPSs in two ways: o The conduction of action potentials along an axon is nondecremental Do not grow weaker as they travel o Axon potentials are conducted more slowly than postsynaptic potentials Axonal conduction is largely active o Once generated, it travels passively along the axonal membrane to the adjacent voltage-activated sodium channels, which have not yet opened o The arrival of the electrical signal opens these channels, thereby allowing Na+ in into the neuron and generate a full blown action potential on this portion of the membrane This signal is then conducted passively to the next sodium channels, where another action potential is activated Repeated until all action potentials are activated in the terminal buttons Since there are so many ion channels and they are so close together, usually it is thought that an axonal conduction is a single wave of excitation spreading actively at a constant speed along the axon Always spreads passively through the cell body and dendrites of the neuron Antidromic Conduction o Axonal conduction opposite to the normal direction; conduction from axon terminal buttons back toward the cell body Orthodromic Conduction o Axonal conduction in the normal direction-from the cell body toward the terminal buttons Conduction in Myelinated Axons In myelinated axons, ions can pass through the axonal membrane only at the nodes of Ranvier When action potential are generated in a myelinated axon, the signal is conducted passively, along the first segment of myelin to the next node of Ranvier o Although the signal is somewhat diminished by the time it reaches that node, it is still strong enough to open the voltage-activated sodium channel at the node and to generate an action potential Myelination increases the speed of axonal conduction o Conduction along the myelinated segments of the axon is passive, occurs instantly, and thus the signal “jumps” along the axon from node to node o Much faster than unmyelinated axons o Salatory Conduction Transmission of action potentials in myelinated axons The Velocity of Axonal Conduction At what speed are action potentials conducted along an axon? o Conduction is faster in larger diameter axons o Faster in myelinated axons Conductions in Neurons without Axons Action potentials are the means by which axons conduct all-or- none signals nondecrementally over relatively long distances o Many neurons in mammalian brains either do not have axons or have very short ones and many of these neurons do not normally display action potentials Conduction in these interneurons is typically passive and decremental Synaptic Transmission: Chemical Transmission of Signals among Neurons Structure of Synapses Axodendritic synapses o Synapses of axon terminal buttons on dendrites o Dendritic Spines Nodules of various shapes that are located on the surfaces of many dendrites Axosomatic synapses o Synapses of axon terminal buttons on somas (cell bodies) Dendrodendritic synapses o Capable of transmission in either direction Axonaxonic sysnapses o They can mediate presynaptic facilitation and inhibition Axoaxonic synapse on or near a terminal button can selectively facilitate or inhibit the effects of that button on the postsynaptic neuron Advantage: they can selectively influence one particular synapse rather than the entire presynaptic neuron Directed Synapses o Synapses at which the site of the neurotransmitter release and the site of the neurotransmitter reception are in close proximity Nondirected Synapses o Synapses at which the site of release is at some distance from the site of reception Synthesis, Packaging, and Transport of Neurotransmitter Molecules Two basic categories of neurotransmitter molecules: o Small and large o Neuropeptides Short amino acid chains comprising between 3 and 36 amino acids; short proteins Large neurotransmitters Assembled in the cytoplasm of the cell body on ribosomes Packaged in vesicles by the cell body’s Golgi complex and transported by microtubules to the terminal buttons o Vesicles are larger and do not congregate as closely to the presynaptic membrane o Small-molecule neurotransmitters are typically synthesized in the cytoplasm of the terminal button and packaged in synaptic vesicles by the button’s Golgi complex Synaptic Vesicles Small spherical membranes that store neurotransmitter molecules and release them into the synaptic cleft Golgi Complex Structures in the cell bodies and terminal buttons of neurons that package neurotransmitters and other molecules in vesicles o Once filled with neurotransmitter, the vesicles are stored in clusters next to the presynaptic membrane Release of Neurotransmitter Molecules Exocytosis o The process of neurotransmitter release When a neuron is at rest, synaptic vesicles that contain small-molecule neurotransmitters tend to congregate near sections of the presynaptic membrane are rich in voltage-activated calcium channels When stimulated by action potentials these 2+¿ channels open, Ca¿ ions enter the button The entry of the 2+¿¿ ions causes synaptic Ca vesicles to fuse with the presynaptic membrane and empty their contents into the synaptic cleft o At many synapses, one action potential causes the release of neurotransmitter molecules from one vesicle o Neuropeptides are typically released gradually in response 2+¿ to general increases in the level of intracellular ¿ ions Ca Activation of Receptors by Neurotransmitter Molecules Once released, neurotransmitter molecules produce signals on postsynaptic neurons by binding to receptors in the postsynaptic membrane o Receptor A protein that contains binding sites for only particular neurons A neurotransmitter can influence only those cells that have receptors for it o Ligand Any molecule that binds to another o Receptor Subtypes The different types of receptors The binding of a neurotransmitter to one of its receptor subtypes can influence a postsynaptic neuron in one of two fundamentally different ways, depending on the receptor Ionotropic Receptors o Associated with ligand-activated ion channels Metabotropic Receptors o Associated with single proteins and G proteins o More prevalent o Effects are slower to develop, longer lasting, more diffuse, and more varied o Different kinds but each is attached to a serpentine signal protein that winds its way back and forth through the cell membrane 7 times The m receptor is attached to a proportion of the signal protein outside the neuron The G protein is attached to a proportion of the signal protein inside the neuron When a neurotransmitter binds to a m receptor, a subnit of the associated G protein breaks away The sunbit may move along the inside surface of the membrane and bind to a nearby ion channel, introducing ESPS or ISPS Trigger the synthesis of a chemical called second messenger Second Messenger o A chemical synthesized in a neuron in response to the binding of a neurotransmitter to a metabotropic receptor in its cell membrane o Once created a second messenger diffuses through the cytoplasm and may influence the activities of the neuron in a variety of ways Autoreceptors Monitor the number of neurotransmitter molecules in the synapse, reduce subsequent release when the levels are high, increase the subsequent release when they are low Metabotropic receptors that have two unconventional characteristics o They bind to their neuron’s own neurotransmitter molecules o They are located on the presynaptic membrane rather than the postsynaptic Reuptake, Enzymatic Degradation, and Recycling If nothing intervened, a neurotransmitter molecule would remain active in the synapse, in the effect clogging that channel of communication Two mechanisms terminate synaptic messages and keep that from happening o Reuptake The drawing back into the terminal button of the neurotransmitter molecules after their release into the synapse More common of the two mechanisms o Enzymatic Degradation The breakdown of chemicals by enzymes Enzymes Proteins that stimulate or inhibit biochemical reactions without being affected by them Acetylcholinesterase The enzyme that breaks down acetylcholine, one of the few neurotransmitters for which enzymatic degradation is the main mechanism of synaptic deactivation Glia, Gap Junctions, and Synaptic Transmission Gap Junction o The narrow spaces between adjacent cells that are bridged by fine, tubular, cytoplasm-filled protein channels called connexins o Connect the cytoplasm of two adjacent cells allowing electrical signals and small molecules to pass from one cell to the next o Transmit signals more rapidly than chemical synapses o Sometimes called electrical synapses Neurotransmitters 3 classes of conventional small-molecule neurotransmitters o Amino acids o Monoamines and acetylcholine o Unconventional neurotransmitters 1 class of large-molecule neurotransmitters o Neuropeptides Amino Acid Neurotransmitters Amino Ac Fast acting, directed synapses in the CNS Molecular building blocks of proteins o Glutamate The brain’s most prevalent excitatory neurotransmitter, whose excessive release causes much of the brain damage resulting from cerebral ischemia o Aspartate Glycine An amino acid neurotransmitter that is a constituent of many of the proteins we eat o GABA The amino acid neurotransmitter that is synthesized from glutamate Most prevalent inhibitory neurotransmitter in the mammalian nervous system Monoamine Neurotransmitters Slightly larger than amino acids and their effects are more diffuse Present in small groups of neurons whose cell bodies are located in the brain stem o Highly branched axons with many varicosities from which monoamine neurotransmitters are diffusely released into the extracellular fluid Catecholamine’s Monoamine neurotransmitters that are synthesized from the amino acid tyrosine o Dopamine o Epinephrine o Norepinephrine Indolamines o The class of monoamine neurotransmitters that are synthesized from tryptophan Serotonin The only member of this class that is found in the mammalian nervous system Acetylcholine A small-molecule neurotransmitter that is in a class by itself Created by adding an acetyl group to a choline molecule Neurotransmitter at the neuromuscular junctions at many of the synapses in the autonomic CNS and at synapses in several parts of the CNS Unconventional Neurotransmitters Soluble-Gas Neurotransmitter o Nitric Oxide o Carbon Monoxide o Produced in the neural cytoplasm and immediately diffuse through the cell membrane into the extracellular fluid and into nearby cells Once inside, they stimulate the production of a second messenger and then deactivated by being converted into other molecules o Retrograde transmission At some synapses they transmit feedback signals from the postsynaptic neuron to the presynaptic neuron Endocannabinoids A class of unconventional neurotransmitters that are similar to THC Synthesized from fatty compounds in the cell membrane Tend to be released from the dendrites and cell body Tend to have most of their effects on presynaptic neurons, inhibiting subsequent synaptic transmission o Anandamide Neuropeptides Neuropeptide transmitters o Peptides that function as a neurotransmitter Pituitary Peptides Contain neuropeptides that were first identified as hormones released by the pituitary Hypothalamic Peptides Contains neuropeptides that were first identified as hormones released by the hypothalamus Brain-gut Peptides Contains neuropeptides that were first discovered in the gut Opioid Peptides Contains neuropeptides that are similar in structure to the active ingredients of opium Miscellaneous Peptides Catch-all category that contains all the neuropeptides that do not fit into one of the other four categories
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