BIO 311D 1st Edition Lecture 26 Outline of Last Lecture I Action Potential II Hyperpolarization and Depolarization III Graded Potentials and Action Potentials Outline of Current Lecture I Neurons communicate with other cells at synapses II Generation of postsynaptic potentials III Summation of postsynaptic potentials IV Generation of postsynaptic potentials V Modulating signals at synapses VI Neurotransmitters Current Lecture Neurons 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 cell Injecting 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 neuron B Decrease the release of neurotransmitters by the presynaptic neuron C Result in neurotransmitters bing released but could not bind to its receptors on the post synaptic neuron Generation 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 enzymes Summation 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 neuron Generation 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 action potential 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 potential Modulated 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 longer Neurotransmitters 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 receptors Acetylcholine 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 metabotropic The 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 cleft B undergo more graded hyperpolarizations because acetylcholine would remain in the synaptic cleft longer C Undergo more graded depolarizations because acetylcholine would prevent ligand gated ion channels from opening Amino Acids Amino acid neurotransmitters are active in the CNS and PNS Known to function in the CNS are Glutamate Gamma aminobutyric acid GABA Glycine Biogenic Amines Biogenic amines include Epinephrine Norepinephrine Dopamine Serotonin They are active in the CNS and PNS Neuropeptides Several neuropeptides relatively short chains of amino acids also function as neurotransmitters Neuropeptides include substance P and endorphins which both affect our perception of pain Opiates bind to the same receptors as endorphins and can be used as painkillers Gases Gases such as nitric oxide and carbon monoxide are local regulators in the PNS
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