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154 Cards in this Set
- Front
- Back
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What method is used to assess the postsynaptic actions o a transmitter candidates?
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Microionophoresis
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HOw does microionphoresis work?
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Neurotransmitter candidates can be dissolved in solutions that will cause them to acquired a net electrical charge. The tip of the pipette is carefully positioned next to the postsynaptic membrane of the neuron, and the trnasmitter candidate is ejected in very small amounts. A microelectrode in the postysnaptic neuron can be used to measure the effect of the transmitter candidate on the membrane potential. If ionophoretic applicaiton of hte molecule causes electrophysiologicl ahchanges that mimic the effect of transmitter released at the synapse, and if criteria of localizations, synthesis, and release have been met, molecule considered same as neurotransmitter
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Can two neurotransmitters bind tot he same receptor?
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No
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Can one neurotransmitter bind to many receptors?
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yes
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What is a receptor subtype and give an example.
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Each of the different receptors a neurotransmitter binds to; for ex., ACh acts on two different cholinergic receptor subtypes. One is present in heart muscles (muscarinic ACh receptors) and the other is in skeletal muscle (nicotinic ACh receptors)
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Which drugs work against the different cholinergic receptor subtypes.
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Nicotine, derived from teh tobacco plant, is a receptor agonist in the skeletal muscle, but has no effect in the heart; muscarnie, on the other hand, derived from a poisonous species of mushroom, has little or no effect othe skeletal muscle but is an agonist at the cholinergic receptor in the heart
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Do nicotinic or muscarinic receptors exist in the brain?
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Yes, both do
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ASide from targeting drugs that agonize receptors, what is another way to distinguish receptor subtypes?
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Using selective antagonists; curare, for example, inhibits the action of ACh at nitotinic receptors, causing paralysis, and atropine, derived from belladonna plants, antagonizes ACh at muscarinic receptrs. The eyedrops an pothalmologist uses to dilate puils are related to atropine
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What are the 3 subtypes of gluatame receptors?
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1. AMPA receptors
2. NMDA receptors 3. Kainate receptors (Each named for different chemical agonists.) |
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What are the two subtypes of NE receptors?
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1. Alpha
2. Beta |
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What are the two subtypes of GABA receptors?
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1. GABAa
2. GABAb |
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What type of receptors bind endorphins?
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Opiatic receptors
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What are enkephalines?
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Two peptides isolated in the brain that eventually proved to be opiate neurotransmitters
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What is a ligand?
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Any chemical compound that binds to a specific site on a receptor
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What is the ligand-binding method?
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The technique of studying receptors using radioactivtly labeled ligands
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Describe where there are at least 151,887 potential subtypes of GABAa receptors.
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GABAa receptor is a transmitter-gated chlrodie channel. Each channel requires five subunits, and there are five major classes of subunit proteins, designated alpa, beta, gamma, tehta, and epsilon. AT least six different polypeptides (designated alpha1-6) can substitue for one another as the alpha subunity; beta1-4 can substitute as a beta subunit, and gamma1-4 can be used as a gamma subunit; the vast majority of hte possible subunits combinations are never manufactured by neurons, and even if they were, they would not work properly
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Are all neurotransmitters made from amino acids?
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Amino acids and amines are derived from amino acids and peptides are constructed from amino acids; ACh, however, is derived from acetyl CoA and choline
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Are amion acid and amine transmitters stored and released by the same neurons?
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They, they area usually stored and released by separate sets of neurons
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What is Dale's principle?
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The idea that a neuron has only one neurotransmitter. Many peptide-containing neurons violate Dale's principle, cause these cells usually release more than one neurotransmitter: an amino acid or amine and a peptide
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What are co-transmitters?
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Two or more transmitters that are released form one nerve terminal (such as amino acid or amine and peptide)
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What type of neurons synthesize ACh?
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Neurons at the neuromuscular junction; synthesized by all motor neurons in the spinal cord and the brain stem
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what enzyme is required for ACh syntehsis?
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Choline acetyltransferase (ChAT)
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Where is ChAT made?
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Manufactured int he soma and transported to the axon terminal
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Why is ChAT a good marker?
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Only cholinergic neurons contain ChAT, so this enzyme is a good marker for cells that use ACha s a neurotrnasmitter
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How does ChAT get ACh into synaptic vesicles?
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ChAT synthesizes ACh in the cytosol of the axon terminal and the neurotransmitter is transported to synaptic vesicles by ACh transporters
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What is the mechanism whereby ChAT makes Ach?
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ChAT transfers an acetyl group from acetyl CoA to choline. The source of choline is the extracellular fluid. Choline is take up by the cholinergic axon terminal. Choline is combined with acetyl CoA
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What is the rate-determining step in ACh synthesis?
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The transport of choline into the neuron, because the availability of choline limits how much ACh can be synthesized in the axon terminal
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What is AChE?
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Acetylcholinesterase; ACh degradative enzyme that degrades ACh into cholien and acetic acid
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IS AChE a good marker of cholinergic neurons?
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No, because it is also manufactured by some noncolinergic neurons
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Is AChe a fast or slow enzyme?
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AChE has one of the fastest catalytic rates among all known enzymes
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What is typically a result of respiratory paralysis?
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Irreversible inhibition of ACE
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What is tyrosine the precursos of?
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Precursor for 3 different amine neurotransmitters that contain a chemical structure called a catechol, caollectively called catecholamines; dopamine (DA), norepinephrine (NE), and epinephrine/adrenaline
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What are the 3 catecholamines?
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1. dopamine (DA)
2. norpinephrine (NE) 3. epinephrine/adrenaline |
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Where are catecholamines typically found?
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Foudn in regions of hte nervous system involved in the regulatin of movement, mood, attention, and vsiceral function
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Describe the mechanism for catecholamine synthesis.
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1. Conversion of tyrosine to dopa by adding a hydroxyl to the meta position by tyrosine hydroxylase (TH)
2. Dopa is converted into dopamine by dopa decarboxylase 3. Dopamine beta-hydroxylase DBH) converts dopamine to norepinephrine 4. Phentolamine N-methyltransferase (PNMT) converts NE to epinephrine |
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What is the rate-limiting step in catecholamine sythesis?
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The activity of TH
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Describe how TH uses end-product inhibition.
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Decreased catecholamine release by the axon terminal causes the catecholamine concentration in the cytosol to rise, thereby inhibiting TH. (On the other hand, the elevation of Ca2+ that accompanies neurotransmitter release triggers an increase in the activity of TH, so transmitter supply keep s up with demand. Prolonged periods of stimulation cause the synthesis of more mRNA that codes for the enzyme
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What is Parkinson's disease caused by?
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The slow degeneration and death of dopaminergic neurons
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Where is DBH found?
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Within the synaptic vesicles, so DA is transported from the cytosol to the synaptic vesicles, where it can be made into NE
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Where is PNMT found?
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In the cytosol of adrenergic axon termianls ~ NE must first be sytnehsized int eh vesicles, released into the cytosol for conversion to epinephrine, and the epinephrine must again be trasnported into vesicles for release
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What is another name for serotonin?
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5-hydroxytryptamine (5-HT)
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From what is 5-HT derived?
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The amino acid tryptophan
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What do serotonergic neruons regulate?
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Mood, emotional behavior, and sleep
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How abundant are serotonergic neurons?
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No, they are relatively few in number
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Describe the two-step formation of serotonin
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1. tryptophan is conveted into an intermediate called 5-HTP by the enzyme tryptophan hdyroxylase
2. 5-HTP is converted to 5-HT by the enzyme 5-HTP decarboxylase |
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What is the source of brain tryptophan?
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The blood and the source of blood tryptophan is the diet (grains,meat, and dairy products)
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What are the neurotransmittesr at most CNS synapses?
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Glutamate, glycine, and gamma-aminobutyric acid (BAB)
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What are glutamate and glycine syntehsized from?
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Synthesized from glucose and other precursos by teh action of enzymes that exist in all cells
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What concentration of glutamate do glutamate transporters strive to reach?
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50 mM in the synaptic vesicles
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What is GABA formed from?
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GABA is formed from glutamate via glutamic acid decarboxylase (GAD).
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What is a good marker for GABAergic neurons?
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GAD
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What is the major source of synaptic inhibition in the nervous system?
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GABAergic neurons
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What is GABA metabolized by?
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GABA transaminase
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Can ATP be a neurotransmitter?
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Yes; released into the cleft by presynaptic spikes in a CA2+-dependent manner; often packaged in vesicles along with another classic transmitter
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What type of receptors bind ATP?
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Purinergic receptors, some of which are transmitter-gated ion channels; there is also a large class of G-protein -coupled purinergic receptors
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Describe retrograde signaling using endocannabinoids.
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1. Vigorous firing of neurons in the postsynaptic neuron causes Ca2+ channels to open ~ Intracellular CA2+ rises
2. Elevated CA2+ stimulate sthe syntehsis of endocannabinoic molecules 3. Endocanabinoids bind to CB1 receptors, reducing the opening of presynaptic calcium channels 4. CBI receptors are G-protei-coupled receptors and their main effect i often to reduc ethe opening of presynaptic calcium channels ~ ability to release neurotransmitter (usually GABA or gluatamate is impaired) |
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Are endocannabinoids packaged inv esicles?
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NO, they are manufactured rapidly and on-demand
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Are endocannabinoids membrane permeable?
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Yes
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Besides endocannabinoids, what else is a retrograde messenger?
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Nitric oxide (NO)
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What is nitric oxide (NO) syntehsized from?
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Arginine
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GIve 4 examples of transmitter-related chemicals that are used elsewhere in the nervous system.
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1. Highest serotonin levels are in the blood platlets
2. ATP is the energy source for all cells 3. Nitric oxide is released from endothelial cells and causes smooth muscles of blood vessels to relax 4. Highest levels of ACh are in the cornea of the eye |
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Describe the structure of the nicotinic (ACh) receptor.
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Made of two alpha subunits, and a beta, gamma, and delta subunit; Eac subunit has four separate segments (M1-M4) that will coil into alpha helices that span the membrane; amino acid residues of these segments are hydrophobic; ACh bind to the alpha subunits
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Describe the structure of glutamate receptors
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Likely have four subunits that comprise a functional channel; M2 region of the glutamate subunit does not span the membrane; rather, it forms a hairpin that both enters and exits from the inside of the membrane; resembles a potassium channel so thought that glutamate receptors and potassium channels evolved from a common ancestral ion channel
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What is the pharmacology of a binding site?
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Describes which trasmitters affect them and how drugs interact with them
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What 4 properties of neurotransmitter channels distinguish them from one another?
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1. Pharmacology of their binding sites
2. Kinetics of the transmitter binding process 3. Selectivity of the ion channels 4. Conductance of open channels |
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What are the three glutamate receptor subtypes?
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AMPA, NMDA, and kaintate
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Describe the difference in function/location between the three glutamate receptor subtypes.
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AMPA and NMDA-gated channels mediate the bulk of fast excitatory synaptic transmission in the brain. AMPA -gated channels are permeable to NA+ and K+; net effect of activating htem at normal, negative membrane potentials is to admit Na+ ions into the cell, causing a rapid and large depolarization. AMPA receptors mediate excitatory transmission of the CNS synapses much in the same way nicotinic receptors mediate synaptic excitation at neuromuscular junctions. NMDA receptors cause excitation by admitting NA+ (they are additionally permeable to Ca2+, however, and they are voltage-dependent); Ca2+ triggers presynaptic neurotransmitter release, causing so NMDA can cause changes that lead to long-term memory. are Kainate receptors also exist throughout the brain, but their functions are not clearly understood.
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Describe why NMDA channels are voltage-dependent.
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At normal negative resting membrane poteitals, teh channel becomes clooged by Mg2+ ions, and the "magnsium block" prevenst other ions from passing freely through the NMDA channel. Mg2+ pops out of hte pore only when the membrane is depolarized, which usually follows the activation of AMPA channel at the same and neighboring synapses. Both glutamate and depolarization must coincide before the channel will pass current.
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Describe what types of neurotransmitters regulate inhibition in the CNS vs. the rest of hte nervous system
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GABA mediates most synaptic inhibition in the CNS; glycine mediates most of the rest. Both gate chloide channels
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What do too much and too little inhibition respectively lead to?
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Too much inhibitoin causes a loss of consciousness and coma; too little leads to a seizure
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What two drugs bind to their own distinctive sites on the outside face of the GABAa channel?
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Benzodioazepines (such as Valium) and barbiturates (such as pheobarbital)
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When coupled with the presence of GABA, what do benzodioazepines and barbiturates respectively do?
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Benzodiazepines increase the frequency of channel openings
Barbiturates increase the duration of channel openings. The results in each case is more inhibitory Cl- current, stronger IPSPs, and the behavioral consequences of enhanced inhibition. |
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Besides behzodioazepines and barbiturates, what are two other modulators of GABAa receptors?
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Ethanol and neurosteroids
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What are the 3 steps involved in G-proteins-coupled receptor transmission?
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1. Binding of the neurotransmitter to the receptor protein
2. Activation of G-proteins 3. Activation of effector systems |
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Describe the structure of most G-protein-coupled receptors.
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Consist of a single polypeptide containing seven membrane-spanning alpha helices. Two of the extracellular loops of the polypeptide form the transmitter binding sites. Structural variations in this region determine which neurotransmitters, agonists, and antagonists bind to the receptor.
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What is G-protein short for?
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Guanosine triphosphate (GTP) binding protein
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Describe the mechanism of G-protein receptors.
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GdP-bound G-protein bumps into the proper type of receptor and if that receptor has a transmitter molecule bound to it, then the G-protein receptor releases GDP and exchanges it for a GTP that it pick sup from the cytosol. The activated GTP-bound G-protein splits into two parts (the Galpha subunitplus GTP and the Gbeta complex). Both can then move on to influence various effector proteins. The Galpha subunit it itself an exzyme that eventually breaks down GTP i into GDP . Afterwards, Galpha and G beta subunits come back together, allowing the cycle to begin again.
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What are the two ways Gproteins can exert their effects on effector proteins?
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Can either effect G-protein-gated ion channels or G-protein-activated enzymes.
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What is the shortcut pathway called?
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G-proteins that exert their effect on G-protein -gated ion channels
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Give an example of a short-cut pathway.
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Muscarinic receptors in the heart. These ACh receptors are coupled via G-proteins to potassium channels, explaining why ACh slows the heart rate.
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Describe the speed of shortcut pathways.
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Althought not quite as fast as transmitter-gated channels, which use no intermediary between receptor and channel, it is faster than the second messenger cascade
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Describe the difference in binding of NE to beta G-protein receptors vs. alpha G-protein receptors.
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Binding of NE to beta receptors activates Gs which activates adenylyl cyclase to generate cAMP to activate protein kinase A. Binding of NE to alpha receptors activates Gi which inhibit rs adenylylcyclase
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Describe the second messenger pathway that involves PLC.
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Activated G-proteins stimulate enzyme PLC which splits PIP2 into DAG and IP3. DAG stimulates downstream PKC. IP3 stimulates release of CA2+ from intracellular stores. This can trigger widespread and long-lasting effect, such as CaMK which is implicated in, among other things, the molecular mechanisms of memory
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What does activating the beta type of NE receptors on cardiac muscle cells do?>
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Causes rise in cAMP, which activates PKA, which phosphorylates the cell's voltage-gated calcium cahnnels, adn this enhances their activity. More Ca2+ flows, and the heart beats more strongly.
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What are protein phosphatases?
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They act rapidly to remove phosphate groups
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What are the most common excitatory and inhibitory neurotransmitters, respectively, in the brain?
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Glutamate is the most common excitatory neurotransmitter; GABA is the pervasive inhibitory neurotransmitter
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What is divergence?
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The ability of one transmitter to activate more than one subtype of receptor, causing more than one type of postsynaptic response.
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What is convergence?
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Multiple transmitters, each activating their own receptor type, can converge to affect the same effector systems
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What are 5 advantages of invertebrate nervous systems?
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1. Small nervous systems
2. Large neurons 3. Identifiable neurons 4. Identifiable circuits 5. Simple genetics |
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Which motor neuron in aplysia receives direct monosynaptic sensory input from the siphon?
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L7, which innervates the muscles that produce gill withdrawal.
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How do we know that the underlying changes resulting in inhibition do not occur at the sensory nerve endings of the skin or at the muscle?
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Making microelectrode recordings from the sensory neuron showed that the sensory neuron continued to fire action potentials in response to sin stimulation, even as the motor response decreased; electrically stimulating the motor neuron always evokes the same amount of muscle contractions.
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Habituation of gill-withdrawal reflex is associated with ______.
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presynaptic modification; a critical step is the entry of Ca2+ into the terminal
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How does sensitization occur?
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Third cell (L29) is activated by the head shock and makes a synapse on the axon terminal of the sensory neuron. The neurotransmitter released by this third neuron (5-HT) sets in motion a molecular cascade that sensitizes the sensory axon terminal so that it lets in more Ca2+ per action potential. The serotonin receptor on the sensory axon terminal and is a G-protein coupled metabotropic receptor. Stimulatino fo this receptor leads to the production of intracellular second messengers. This second smessegner in cAMP produced from ATP by adenylyl cyclase. It activates protein kinase A. In the sensory nerve terminal, one of htese preotiens is a potassium channel, and phosphorylation of this channel causes it to close. The closure of potassium channels in the axon terminal leads to a prolongation of the presynaptic action potential. This results in more Ca2+ entry through voltage-gated calcium channels during the action potential, and therefore, more quanta of neurotransmitter are released.
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What happens if stimulation of the tail was paired with stimulation of the siphon?
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The subsequent response to siphon stimulation alone was much greater than what could be accounted for by sensitization.
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How does adenylyl cyclase act in the presence of elevated Ca2+?
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Adenylyl cyclase churns out more cAMP. More cAmP means more activation of portein kinase A, more phosphorylation of potassium channels, and therefore the release of more transmitter molecules.
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When does learning occur?
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Occurs when a presynaptic Ca2+ pulse coincides with the G-protein coupled activation of adenyly cyclase, which stimulates the production of a lot of cAMP.
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When does memory occur?
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Occurs when potassium channels are phosphorylated and neurotransmitter release is enhanced.
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What is the fluid behind the cornea called?
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The aqueous humor
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What is the transparent surface behind the iris called?
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The lens
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What is the difference between the aqueous humor and the vitreous humor?
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The aqueous humor lies between the cornea and the iris, whereas the vitreous humor lies in the middle of the eye; vitreous humor serves as pressure to keep the eyeball spherical
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What is the focal distance?
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The distance from the refractive surface to the point where parallel light rays converge; depends on the curvature of the cornea; the tighter the curve, the shorter the focal distance
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What is the refractive power of the cornea?
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42 diopters; (prescription eyeglasses have a power of only a few diopters)
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What is accommodation?
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The process whereby the shape of the lens changes to allow for greater refractive power, which is required to focus the diverging rays of an approaching object
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What is the pupillary light reflex?
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The continuous adjustment of the pupil to acommodate different ambient light levels; involves connections between the retina and neurons in the brain stem that control the muscles that constrict the pupils; constricting the pupil increases the depth of focus (so when squint you can see farther)
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Where is the left visual field imaged?
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On the right side of the retina
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The distance across the retina can be described in terms of what?
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Degrees of visual angle
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What is the most direct pathway of neurons for visual information to exit the eye?
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Photoreceptors --- (horizontal cells) --- bipolar cell ---- (amarcrine cells) ---- ganglion cells
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What do horizontal cells do?
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Receive input frmo the photoreceptors and project neurites laterally to influence surrounding bipolar cells and photoreceptors.
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What do amarcrine cells do?
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Amarcrine cells receives input from bipolar cells and project laterally to influence surrounding ganglion cells, bipolar cells, and other amarcrine cells
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What are the only sources of neural neural output form the retina?
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Ganglion cells
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What is one reason the inside-out arrangement is advantageous?
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The pigmented epithelium that lies below the photoreceptors plays a critical role in the maintenance of the photoreceptors and photopigments; absorbs any light that passes entirely through the retina, minimizing the reflection of the light within the eyes that would blur the image
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What is the inner nuclear layer?
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Contains the cell bodies of the bipolar cells, the horizontal, and amarcrine cells.
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What is the outer nuclear layer?
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Contains the cell bodies of the photoreceptors.
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What is the inner plexiform layer?
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Contains connections between inner nuclear layer and ganglion cell.
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What is the outer plexiform layer?
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Contains connections between outer nuclear layer and the inner nuclear layer
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How many photoreceptors are at the back fo the retina?
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125 million
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Describe the structure of the photoreceptors.
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Every photoreceptor has four regions: an outer segment (containing a stack of membranous disks which contain the phootpigments taht absorb light), an inner segment, a cell body, and a synaptic terminal.
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Which have more disks, rods or cones?
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Rods ~ 1,000 times more sensitive to light than cones
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Does the peripheral retina have more rods or cones?
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Rods
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Does the peripheral retina have more photoreceptors or ganglion cells?
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Photoreceptors
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Why are the peripheral retina more sensitive to light?
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1. Rods are specialized for low light
2. There are more photoreceptors feeding information to each ganglion cell |
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What happens to the appearance of a faint star when its position in your visual field changes?
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You can see it when it's imaged on the peripheral retina, but disappears when imaged on the central retina.
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When is the fovea?
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the region of retina most highly specialized for high-resolution vision; a thinning of the retina at the center of the macula
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Why does the fovea have greater visual acuity?
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Other cells above the photoreceptors are pushed aside (resulting in the pit-like appearance)
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How does light stimulation change the membrane potential?
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Light stimulation of the photopigment activates G-proteins (transducin), which in turn activate an effector enzyme (phosphodiesterase (PDE)) that breaks down cGMP to GMP. This change causes the Na+ channel to close and the membrane potential is thereby altered. (Light reduces cGMP cause the Na+ channel to close, so photoreceptors) hyperpolarize)
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What is the membrane potential of the rod outer segment in complete darkness?
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About -30 mV
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What is the dark current?
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The movement of positive charge across the membrane, which occurs in the dark.
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What is the rhodopsin?
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Pigment in the membrane of the stacked disks in the rod outer segments that absorb electromagnetic light
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What is opsin?
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The receptor protein for light
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What is retinal?
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The prebound agonist in opsin that responds to light; derivative of Vitamin A; activation in light cuases a change in conformaiton that activates the opsin.
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What is bleaching?
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The process whereby the absorption of light cuases a change in the conformation of retinal.
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How is the transmission of the light signal an example of amplification?
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Many G-proteins are activated by each photopigment molecule, and each PDE enzyme breaks down more than one cGMP molecule.
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What is the major difference in the process of phototransduction in cones vs. rods?
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Use different types of opsin
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What wavelength of light maximally activates "blue" cones?
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430 nm
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What wavelength of light maximally activates "green" cones?
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530 nm
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What wavelength of light maximally activates "red" cones?
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560 nm
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What is the theory that brain assigns colors based on a comparison of the readout of the three cone types?
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young-Helmholtz trichromacy theory
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What color is perceived when all types of cones are equally active?
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White
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To what wavelength are rods sensitive?
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500 nm (perceived as blue-green)
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Why are dashbord lights blue-green? What is the alternate view?
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Take advantage of the spectral sensitivity of the rods. The alternative view is that the lights should be bright red because this wavelength affects mainly cones, leaving the rods unsaturated
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How long does dark adaptation take?
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20-25 minutes
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By what factor does sensitivity to light increase during dark adaptation?
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A millionfold
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By what factor can the size of the pupil change?
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16
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What happens during dark adaptation?
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Unbleached rhodopsin is regenerated; adjustment of the functional circuitry of the retina so that the information from more rids is available to each ganglion cell
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How long does light adaptation take?
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Light adaptation
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What happens regarding the polarization of the cones when step into bright light?
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Initially cones are very hyperpolarized when step into bright light; gradual depolarization to about -35 mv occurs
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Describe the calcium-based mechanisms that ensure the photoreceptors are always able to register relative changes in light level
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cGMP-gated sodium channels admit calcium. In the dark, Ca2+ enters the cones ~ inhibitory effect on enzymes synthesizing cGMP (cGMP is not formed in the dark); when these channels close in light, Ca2+ is curtailed, so more cGMP is synthesized, allowing the cGMP-gated channels to open again.
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What transmitter is released by photoreceptors?
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Glutamate
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Are photoreceptors depolarized or hyperpolarized in the dark and light, respectively?
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Photoreceptors depolarize in the dark and hyperpolarize in the light
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What are OFF bipolar cells?
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Glutamate-gated cation channels that mediate a classical depolarizing EPSP from the influx of Na+ in darkness
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What are ON bipolar cells?
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Have G-protein-coupled receptors that respond to glutamate by hyperpolarizing the cell in the presence of light
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What is the receptive field?
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The area of retian that, wehn stimulated with light, changes the cell's membrane potential. The receptive field of a bipolar cell consists of two parts: a circular area of retina providing direct photoreceptor input, called the receptive field center, and a surrounding area of retina providing input via horizontal cells, called the receptive field surround
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What are the two parts of the receptive field of a bipolar cell?
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1. Receptive field center: circular area of retina providing direct photoreceptor input
2. Receptive field surrounds: a surrounding area of retina providing input via horizontal cells |
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One millimeter on the retina corresponds to a visual angle of about _____?
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3.5 degrees
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