Pratique du Shadowing: Neurotransmitters - Apprendre l'anglais à l'oral avec la vidéo
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Neurotransmitters are chemicals that neurons use to communicate with one another, primarily at specialized areas called synapses.
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Most synapses consist of a neuron that is sending a signal, which is called the presynaptic neuron, and a neuron that is receiving the signal,
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or the postsynaptic neuron, as well as a small space between the two called the synaptic cleft.
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Neurotransmitters are typically stored in the presynaptic neuron in small sacs called vesicles
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and are released into the synaptic cleft when an electrical impulse called an action potential occurs in the presynaptic neuron.
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The neurotransmitters attach, or bind, to proteins called receptors, mostly on the postsynaptic neuron.
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There are two general types of neurotransmitter receptors.
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One is called an ionotropic receptor because when a neurotransmitter binds to one of these receptors,
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it causes the opening of an ion channel that lets ions flow across the membrane of the neuron, potentially changing the activity of the cell.
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A general term for a molecule that binds to a receptor is a ligand,
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and because ionotropic receptors consist of an ion channel that opens in response to a ligand binding, they are also called ligand -gated ion channels.
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The other major type of receptor is called a metabotropic receptor, also known as a G -protein -coupled receptor.
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Neurotransmitters also bind to these receptors, but instead of simply opening an ion channel, they cause the activation of a protein called a G -protein.
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The G -protein itself can influence the opening of ion channels, but it can also have other effects,
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such as activating enzymes and initiating signaling cascades within the cell.
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Thus, when neurotransmitters bind to receptors, they cause some action in the postsynaptic neuron.
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In general, that action might be excitatory, meaning it increases the likelihood the postsynaptic neuron will fire an action potential,
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or inhibitory, meaning it decreases the likelihood the postsynaptic neuron will fire an action potential.
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Or it might be a modulatory effect, where it influences the functions of a neuron in more nuanced ways.
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After neurotransmitters have communicated with the postsynaptic neuron,
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they must be removed from the synaptic cleft to keep them from repeatedly binding to and potentially overstimulating receptors.
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A fraction of neurotransmitter molecules will diffuse out of the synaptic cleft, but it is typically a small amount.
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Some synapses, such as those for the neurotransmitter acetylcholine, rely primarily on the use of enzymes to degrade neurotransmitters to remove them from the synaptic cleft.
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Other synapses rely primarily on proteins called transport proteins, which remove neurotransmitters from the synaptic cleft
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and typically shuttle them back into the neuron that released them in a process called reuptake.
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There are over 100 different neurotransmitters, and probably many more yet to be discovered.
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They can be generally grouped into categories such as amine neurotransmitters, amino acids, neuropeptides, and even gas neurotransmitters.
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The rest of this video will explore some examples of our most studied neurotransmitters,
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although it's important to note that while these are some of our best understood neurotransmitters, they are only a small selection of neurotransmitters overall.
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Acetylcholine was the first neurotransmitter discovered, and is named for the two substances used to synthesize it, the nutrient choline and the enzyme acetylcoenzyme A.
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Neurons that contain acetylcholine are called cholinergic.
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There are several clusters of cholinergic neurons throughout the brain.
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Some are found in the basal forebrain.
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They include the medial septal nucleus, the nucleus of the diagonal band, and the nucleus basalis others are found in the brainstem,
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including the pedunculopontine nucleus and lateral dorsal tegmental nucleus.
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Acetylcholine acts on two families of receptors, and each receptor family has several subtypes.
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One family is ionotropic.
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They are called nicotinic acetylcholine receptors because nicotine also binds to and activates the receptors.
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Their activation generally results in excitation of the neuron.
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Another family is metabotropic.
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These are called muscarinic acetylcholine receptors because a substance called muscarin binds to them.
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Their effects depend on the subtype of the receptor.
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The action of acetylcholine in the synapse is terminated by an enzyme called acetylcholinesterase, which breaks acetylcholine down into acetate and choline.
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The choline is then transported back into neurons to synthesize more acetylcholine.
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Acetylcholine has a variety of functions in the nervous system.
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It is the main neurotransmitter used at neuromuscular junctions and is responsible for muscle contraction.
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It is also widely used in the autonomic nervous system.
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Its functions in the brain are still not fully understood, but it does appear to play important roles in memory, arousal, and attention.
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You Dopamine is a monoamine neurotransmitter,
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a term that refers to its chemical structure and the fact that it is derived from an amino acid.
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Dopamine is also a catecholamine, a term that also refers to its chemical structure and the fact that it contains a catechol nucleus.
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To synthesize dopamine, the amino acid tyrosine is converted to L -dopa.
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Then L -dopa is decarboxylated to form dopamine.
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There are several areas of the brain where dopamine neurons are concentrated.
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The largest are the substantia nigra and the ventral tegmental area in the midbrain.
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Other areas include the hypothalamus, olfactory bulb, and retina.
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There are several major dopamine pathways that carry dopamine from these areas of concentration to other parts of the brain.
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Some of the largest are the mesostriatal or nigrostriatal pathway, which stretches from the substantia nigra to the striatum,
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the mesolimbic pathway, which stretches from the ventral tegmental area to the nucleus accumbens and other limbic structures,
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and the mesocortical pathway, which stretches from the ventral tegmental area throughout the cerebral cortex.
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Dopamine acts at G -protein coupled receptors, and there are at least five subtypes of the dopamine receptor.
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Dopamine is removed from the synaptic cleft by a transporter protein called the dopamine transporter.
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Like any neurotransmitter, the functions of dopamine are complex and can't be fully explained with just a short summary.
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Dopamine is linked to movement due to disorders like Parkinson's disease that involve dopamine deficiencies.
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It is also often associated with the processing of rewarding experiences.
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However, dopamine also plays a role in many other functions.
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Norepinephrine, also known as noradrenaline,
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is a monoamine neurotransmitter, a term that refers to its chemical structure and the fact that it's derived from an amino acid.
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It is also a catecholamine, a term that again refers to its chemical structure and the fact that it contains a catechol nucleus.
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Norepinephrine also functions as a hormone.
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It is synthesized from dopamine in a reaction catalyzed by the enzyme dopamine beta -hydroxylase.
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Norepinephrine -producing neurons in the central nervous system are primarily concentrated in the pons and medulla.
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The most prominent of these groups of neurons is a nucleus called the locus coeruleus,
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which is the main site of norepinephrine production for the central nervous system.
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Norepinephrine is also the primary neurotransmitter used by the sympathetic nervous system,
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and is found in clusters of sympathetic neurons located near the spinal cord known as sympathetic ganglia.
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It is also released from the adrenal glands as a hormone.
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Norepinephrine acts on G -protein -coupled receptors referred to as adrenergic receptors or adrenoceptors.
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There are thought to be at least three main types of adrenergic receptors, alpha -1, alpha -2, and beta -adrenergic receptors,
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each of which has multiple subtypes.
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Norepinephrine is removed from the synaptic cleft by a transport protein called the norepinephrine transporter.
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Like any neurotransmitter, the actions of norepinephrine depend on the type of receptor it activates and where that receptor is located.
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Thus, although norepinephrine in the central nervous system is frequently associated with arousal, alertness, and attention, the full extent of its actions are more complex.
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Its release in the sympathetic nervous system is typically associated with responses linked to increased activity, like elevated heart rate and blood pressure.
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Serotonin is a monoamine neurotransmitter, a term that refers to its chemical structure and the fact that it is derived from an amino acid.
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To synthesize serotonin, the amino acid tryptophan is converted to 5 -hydroxytryptophan,
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or 5 -HTP, and 5 -HTP is converted to serotonin, or 5 -HT.
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Serotonin neurons are primarily found in the brainstem, in clusters of neurons called the raphe nuclei.
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Serotonin neurons from the raphe nuclei project throughout the brainstem and brain, and provide serotonin to the rest of the central nervous system.
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Researchers have identified seven different families of serotonin receptors, which differ from one another in distribution, the substances that bind to them,
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and the effects they mediate.
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All but one of these families of receptors consists of G -protein -coupled receptors.
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The other receptor family consists of ligand -gated ion channels.
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Within these seven families of receptors, 14 receptor subtypes have been identified as well.
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Serotonin is removed from the synaptic cleft by a transport protein called the serotonin transporter, or CERT.
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Start.
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In terms of function, serotonin is often linked to mood, in part due to the understanding that many antidepressants cause serotonin levels to rise.
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However, an attempt to define any neurotransmitter by one function is inevitably an oversimplification.
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In truth, serotonin's role in mood is very complex, and depression is not likely to be due to a simple serotonin deficiency.
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Additionally, serotonin is involved in a long list of functions other than mood.
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In most cases, its actual role in these functions is still not completely understood.
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Glutamate is an amino acid that also functions as a neurotransmitter.
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Although glutamate is obtained through the diet, it cannot pass the blood -brain barrier, and thus must be synthesized in the brain.
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It can be synthesized from alpha -ketoglutarate, an intermediate product in the citric acid cycle.
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Glutamate generally has excitatory actions, meaning that when it interacts with the receptors of a neuron,
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it makes that neuron more likely to fire at an action potential.
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It is, in fact, used at the vast majority of excitatory connections in the brain, and at more than half of all synapses in the brain.
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Glutamate interacts with several different types of receptors.
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There are three identified ionotropic glutamate receptors, named for substances that activate them—NMDA, AMPA, and kinate receptors.
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When activated, all three allow positively charged sodium ions to flow into a postsynaptic neuron,
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depolarizing the neuron and making it more likely to fire an action potential.
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NMDA receptors have unique characteristics that make them well -suited to be involved in synaptic plasticity, or synaptic changes that occur in response to experience,
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which are an important component of learning and memory.
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There are also three identified types of metabotropic glutamate receptors.
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These receptors have more varied effects than ionotropic glutamate receptors and may be involved with excitatory or inhibitory actions.
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Glutamate is removed from the synaptic cleft by a class of transporter proteins called the excitatory amino acid transporters, or EAATs.
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EAATs carry glutamate into neurons and glial cells.
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Glutamate taken into glial cells is converted to the amino acid glutamine by the enzyme glutamine synthetase.
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Glutamine is then transported back into neurons where it is converted back to glutamate.
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This process is referred to as the glutamate -glutamine cycle.
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Although GABA's primary functions are as a neurotransmitter, it has the structure of an amino acid and thus is referred to as an amino acid neurotransmitter.
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It is synthesized from another amino acid neurotransmitter, glutamate, in a reaction catalyzed by the enzyme glutamic acid decarboxylase.
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The function of GABA changes over the course of neural development, but in the mature brain it acts primarily as an inhibitory neurotransmitter.
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In other words, when GABA interacts with the receptors of a neuron,
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it generally makes the neuron less likely to fire an action potential or release neurotransmitters.
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There are two types of receptors GABA interacts with, GABA -A and GABA -B receptors.
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GABA -A receptors are ionotropic receptors.
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When GABA binds to the GABA -A receptor, it causes the opening of an associated ion channel that is permeable to the negatively charged ion chloride.
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When negative chloride ions flow into the neuron,
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they hyperpolarize the membrane potential of the neuron and make it less likely the neuron will fire an action potential.
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GABA -B receptors are metabotropic, or G -protein coupled receptors.
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When activated, they frequently cause the opening of potassium channels.
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These channels allow positively charged potassium ions to flow out of the neuron, again making the neuron hyperpolarized and less likely to fire an action potential.
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The actions of GABA are terminated by proteins called GABA transporters, which transport GABA from the synaptic cleft into neurons or glial cells,
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where it is degraded primarily by mitochondrial enzymes.
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Because GABA can reduce neural transmission, increased GABA activity can have sedative effects.
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Accordingly, a number of drugs that have such effects, like alcohol and benzodiazepines, increase activity at the GABA receptor.
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Vocabulaire et conseils d’expression pour cette leçon
Cette leçon d’expression orale s’appuie sur la vidéo « Neurotransmitters ». Les mots qui reviennent le plus souvent : receptor, neuron, neurotransmitter, dopamine, protein. Cette vidéo contient 137 phrases et 2111 mots à répéter en shadowing. La partie parlée dure 14:01. Le locuteur parle à un rythme naturel d’environ 151 mots par minute, proche d’une conversation courante. Seuls 65 % des mots font partie des 3 000 mots les plus courants en anglais, le vocabulaire est donc exigeant.
Vocabulaire clé de cette vidéo
15 mots de la vidéo à retenir, avec leur prononciation et leur sens :
| Mot | Prononciation | Sens |
|---|---|---|
| receptor nom | /ɹɪˈsɛp.tə/ | récepteur |
| acid nom | /ˈæsɪd/ | acide |
| nucleus nom | /ˈnuː.kli.əs/ | noyau |
| enzyme nom | /ˈɛn.zaɪm/ | enzyme |
| primarily adverbe | /ˈpɹaɪ.mə.ɹə.li/ | principalement, essentiellement |
| activate verbe | /ˈæktɪˌveɪt/ | actionner, activer |
| refer verbe | /ɹɪˈfɜː/ | référer |
| convert verbe | /kənˈvɝt/ | convertir |
| substance nom | /ˈsʌbstəns/ | substance |
| ion nom | /ˈaɪ.ɑn/ | ion |
| interact verbe | /ɪn.təˈɹækt/ | interagir |
| consist nom | /kənˈsɪst/ | rame |
| sympathetic adjectif | /ˌsɪm.pəˈθɛt.ɪk/ | sympathique |
| pathway nom | /ˈpæθˌweɪ/ | voie |
| mood nom | /muːd/ | humeur, mauvaise humeur |
La grammaire de cette vidéo
Les structures que le locuteur utilise le plus, avec les mots exacts de la vidéo :
| Structure | Dans la vidéo |
|---|---|
| Voix passive be + participe passé — l’accent est mis sur ce qui arrive, pas sur qui le fait | is called · are typically stored · are released |
| Present perfect have/has + participe passé — une action passée qui compte encore maintenant | have communicated · have identified · have been identified |
Prononciation à surveiller
- Les sons « th »: sympathetic /ˌsɪm.pəˈθɛt.ɪk/, pathway /ˈpæθˌweɪ/, synthesize /ˈsɪnθəsaɪz/
- Les sons « sh » et « zh »: potentially /pəˈtɛnʃ(ə)li/, activation /ˌæktəˈveɪʃən/, deficiency /dɪˈfɪʃənsi/, transmission /tɹænsˈmɪʃən/, concentration /ˌkɑn.sənˈtɹeɪ.ʃən/
- Mots longs — placez bien l’accent: primarily /ˈpɹaɪ.mə.ɹə.li/, sympathetic /ˌsɪm.pəˈθɛt.ɪk/, potentially /pəˈtɛnʃ(ə)li/, communicate /kəˈmjuːnɪkeɪt/, positively /ˈpɑ.zɪ.tɪv.li/
Les sons difficiles pour les francophones :
- /h/ — il se souffle, il n’est pas muet: likelihood /ˈlaɪklihʊd/, hormone /ˈhoɹmoʊn/
- /tʃ/ et /dʒ/ — à ne pas adoucir en « ch » et « j »: stretch /stɹɛt͡ʃ/, mature /məˈt͡ʃʊ(ə)ɹ/, junction /ˈd͡ʒʌŋkʃən/, researcher /ˈɹiˌsɝ.t͡ʃɚ/, attach /əˈtæt͡ʃ/
- /r/ anglais — langue recourbée, sans frotter la gorge: receptor /ɹɪˈsɛp.tə/, primarily /ˈpɹaɪ.mə.ɹə.li/, refer /ɹɪˈfɜː/, interact /ɪn.təˈɹækt/, stretch /stɹɛt͡ʃ/
Comment s’entraîner avec cette vidéo
- Écoutez la vidéo en entier une fois sans parler et notez les mots que vous ne connaissez pas.
- Commencez à la vitesse 0,75×, répétez phrase par phrase, puis revenez à la vitesse normale quand cela devient facile.
- Enregistrez-vous et comparez avec l’original, en faisant attention à des mots comme receptor, acid, nucleus.
Qu'est-ce que la technique du Shadowing ?
Le Shadowing est une technique d'apprentissage des langues fondée sur la science, développée à l'origine pour la formation des interprètes professionnels. Le principe est simple mais puissant : vous écoutez de l'anglais natif et le répétez immédiatement à voix haute — comme une ombre suivant le locuteur avec un décalage de 1 à 2 secondes. Les recherches montrent une amélioration significative de la précision de la prononciation, de l'intonation, du rythme, des liaisons, de la compréhension orale et de la fluidité.