BIO 311D 1st Edition Lecture 26Outline of Last Lecture I. Action PotentialII. Hyperpolarization and DepolarizationIII. Graded Potentials and Action PotentialsOutline of Current Lecture I. Neurons communicate with other cells at synapsesII. Generation of postsynaptic potentialsIII. Summation of postsynaptic potentialsIV. Generation of postsynaptic potentialsV. Modulating signals at synapsesVI. NeurotransmittersCurrent LectureNeurons communicate with other cells at synapses• At electrical synapses, the electrical current flows from one neuron to another• At chemical synapses, a chemical neurotransmitter carries information across the gap junction• Most synapses are chemical synapses• The presynaptic neuron synthesizes and packages the neurotransmitter in synaptic vesicles located in the synaptic terminal• The action potential causes the release of the neurotransmitter • The neurotransmitter diffuses across the synaptic cleft and is received by the postsynaptic cellInjecting ethylene glycol tetraacetic acid, a chelating agent that prevents calcium ions from moving across membranes to a synaptic region would likely:A. Increase the release of neurotransmitters by the presynaptic neuronB. Decrease the release of neurotransmitters by the presynaptic neuronC. Result in neurotransmitters bing released but could not bind to its receptors on the post synaptic neuronGeneration of Postsynaptic Potentials• Direct synaptic transmission involves binding of neurotransmitters to ligand-gated ion channels in the postsynaptic cell• Neurotransmitter binding causes ion channels to open, generating a postsynaptic potential• Postsynaptic potentials fall into two categories• Excitatory postsynaptic potentials (EPSPs) are depolarizations that bring the membrane potential toward threshold• Inhibitory postsynaptic potentials (IPSPs) are hyperpolarizations that move the membrane potential farther from threshold• After release, the neurotransmitter• May diffuse out of the synaptic cleft• May be taken up by surrounding cells • May be degraded by enzymesSummation of Postsynaptic Potentials• Most neurons have many synapses on their dendrites and cell body• A single EPSP is usually too small to trigger an action potential in a postsynaptic neuronGeneration of Postsynaptic Potentials• If two EPSPs are produced in rapid succession, an effect called temporal summation occurs• In spatial summation, EPSPs produced nearly simultaneously by different synapses on the same postsynaptic neuron add together • The combination of EPSPs through spatial and temporal summation can trigger an actionpotential• Through summation, an IPSP can counter the effect of an EPSP• The summed effect of EPSPs and IPSPs determines whether an axon hillock will reach threshold and generate an action potentialModulated Signaling at Synapses• In some synapses, a neurotransmitter binds to a receptor that is metabotropic• In this case, movement of ions through a channel depends on one or more metabolic steps• Binding of a neurotransmitter to a metabotropic receptor activates a signal transduction pathway in the postsynaptic cell involving a second messenger• Compared to ligand-gated channels, the effects of second-messenger systems have a slower onset but last longerNeurotransmitters• There are more than 100 neurotransmitters, belonging to five groups: acetylcholine, biogenic amines, amino acids, neuropeptides, and gases• A single neurotransmitter may have more than a dozen different receptorsAcetylcholine• Acetylcholine is a common neurotransmitter in vertebrates and invertebrates• It is involved in muscle stimulation, memory formation, and learning• Vertebrates have two major classes of acetylcholine receptor, one that is ligand gated and one that is metabotropicThe use of organophosphate pesticides that inhibit acetylcholinesterase, an enzyme that breaks down acetylcholine, could cause skeletal muscles to A. Undergo more graded depolarization’s because acetylcholine would remain in the synaptic cleftB. undergo more graded hyperpolarizations because acetylcholine would remain in the synaptic cleft longerC. Undergo more graded depolarizations because acetylcholine would prevent ligand gatedion channels from openingAmino Acids• Amino acid neurotransmitters are active in the CNS and PNS• Known to function in the CNS are– Glutamate– Gamma-aminobutyric acid (GABA)– GlycineBiogenic Amines• Biogenic amines include – Epinephrine– Norepinephrine– Dopamine– Serotonin• They are active in the CNS and PNSNeuropeptides• Several neuropeptides, relatively short chains of amino acids, also function as neurotransmitters• Neuropeptides include substance P and endorphins, which both affect our perception ofpain• Opiates bind to the same receptors as endorphins and can be used as painkillersGases• Gases such as nitric oxide and carbon monoxide are local regulators in the
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