تدريب Shadowing: Neurotransmitters - تعلم التحدث بالإنجليزية عبر الفيديو
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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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المفردات وملاحظات التحدث لهذا الدرس
هذا الدرس في المحادثة مبني على الفيديو “Neurotransmitters”. أكثر الكلمات تكرارًا في الفيديو: receptor, neuron, neurotransmitter, dopamine, protein. يحتوي هذا الفيديو على 137 جملة و2111 كلمة للتدرب على الترديد. مدة الكلام 14:01. يتكلم المتحدث بسرعة طبيعية، نحو 151 كلمة في الدقيقة، قريبة من المحادثة اليومية. 65% فقط من الكلمات ضمن أكثر 3,000 كلمة شيوعًا في الإنجليزية، فالمفردات صعبة نسبيًا.
أهم المفردات في هذا الفيديو
15 كلمة من الفيديو تستحق التعلّم، مع النطق والمعنى:
| الكلمة | النطق | المعنى |
|---|---|---|
| receptor اسم | /ɹɪˈsɛp.tə/ | مُسْتَقْبِل |
| acid اسم | /ˈæsɪd/ | حَامِض, حَمْض |
| nucleus اسم | /ˈnuː.kli.əs/ | نَوَاة |
| enzyme اسم | /ˈɛn.zaɪm/ | إِنْزِيم |
| activate فعل | /ˈæktɪˌveɪt/ | فَعَّلَ |
| refer فعل | /ɹɪˈfɜː/ | أَشَارَ |
| convert فعل | /kənˈvɝt/ | حَوَّلَ |
| substance اسم | /ˈsʌbstəns/ | مَادَّة |
| ion اسم | /ˈaɪ.ɑn/ | شَارِد, أَيُون |
| mood اسم | /muːd/ | مَزَاج |
| stretch فعل | /stɹɛt͡ʃ/ | تَمَدَّدَ |
| alpha اسم | /ˈæl.fə/ | أَلْفَا |
| cluster اسم | /ˈklʌstɚ/ | عُنْقُود, عُنْقُود نَجْمِيّ |
| derive فعل | /dɪˈɹaɪv/ | استمد |
| gate اسم | /ˈɡeɪ̯t/ | بَوَّابَة |
القواعد في هذا الفيديو
أكثر التراكيب التي يستخدمها المتحدث، مع العبارات نفسها من الفيديو:
| التركيب | في الفيديو |
|---|---|
| المبني للمجهول be + اسم المفعول — التركيز على ما يحدث لا على من يفعله | is called · are typically stored · are released |
| المضارع التام have/has + اسم المفعول — حدث ماضٍ ما زال له أثر الآن | have communicated · have identified · have been identified |
نطق يحتاج إلى انتباه
- أصوات “th”: sympathetic /ˌsɪm.pəˈθɛt.ɪk/, pathway /ˈpæθˌweɪ/, synthesize /ˈsɪnθəsaɪz/
- أصوات “sh” و“zh”: potentially /pəˈtɛnʃ(ə)li/, activation /ˌæktəˈveɪʃən/, deficiency /dɪˈfɪʃənsi/, transmission /tɹænsˈmɪʃən/, concentration /ˌkɑn.sənˈtɹeɪ.ʃən/
- الكلمات الطويلة — انتبه لموضع النبر: primarily /ˈpɹaɪ.mə.ɹə.li/, sympathetic /ˌsɪm.pəˈθɛt.ɪk/, potentially /pəˈtɛnʃ(ə)li/, communicate /kəˈmjuːnɪkeɪt/, positively /ˈpɑ.zɪ.tɪv.li/
أصوات يجدها متحدثو العربية صعبة:
- /p/ — ليس /b/: انطقه مع نفخة هواء: receptor /ɹɪˈsɛp.tə/, primarily /ˈpɹaɪ.mə.ɹə.li/, sympathetic /ˌsɪm.pəˈθɛt.ɪk/, pathway /ˈpæθˌweɪ/, potentially /pəˈtɛnʃ(ə)li/
- /v/ — ليس /f/: مع اهتزاز الأوتار الصوتية: activate /ˈæktɪˌveɪt/, convert /kənˈvɝt/, derive /dɪˈɹaɪv/, positively /ˈpɑ.zɪ.tɪv.li/, activation /ˌæktəˈveɪʃən/
- تتابع الصوامت — لا تضف حركة بينها: bind /ˈbaɪ̯nd/, primarily /ˈpɹaɪ.mə.ɹə.li/, substance /ˈsʌbstəns/, interact /ɪn.təˈɹækt/, consist /kənˈsɪst/
كيف تتدرب بهذا الفيديو
- استمع إلى الفيديو كاملًا مرة واحدة دون أن تتكلم، ودوّن الكلمات التي لا تعرفها.
- ابدأ بسرعة 0.75×، وردّد جملة جملة، ثم عُد إلى السرعة العادية عندما يصبح الأمر سهلًا.
- سجّل صوتك وقارنه بالأصل، مع الانتباه إلى كلمات مثل receptor, acid, nucleus.
ما هي تقنية التظليل الصوتي؟
التظليل الصوتي (Shadowing) تقنية تعلم لغة مدعومة علمياً، طُورت أصلاً لتدريب المترجمين الفوريين المحترفين. الطريقة بسيطة لكنها قوية: تستمع لصوت إنجليزي أصلي وتكرره فوراً بصوت عالٍ — كظل يتبع المتحدث بتأخير 1-2 ثانية. تُظهر الأبحاث تحسناً كبيراً في دقة النطق والتنغيم والإيقاع وربط الأصوات والاستماع والطلاقة.