The Neuron and Neural Firing
Introduced| English | Meaning |
|---|---|
| all-or-nothing principle/ɔːl ɔː ˈnʌθɪŋ ˈprɪnsɪpl/ | all-or-nothing principle |
| Dendrites/ˈdendraɪts/ | Dendrites |
| cell body/sel ˈbɒdi/ | The part of a neuron containing its nucleus and much of the machinery needed to maintain the cell. |
| axon/ˈæksɒn/ | A long projection of a neuron that carries electrical impulses away from the cell body. |
| myelin sheath/ˈmaɪɪlɪn ʃiːθ/ | An insulating layer around many axons that increases the speed of nerve impulse transmission. |
| Glial cells/ˈɡlaɪəl selz/ | Nervous-system cells that support, protect and regulate the environment around neurons, with some forming myelin. |
| synapse/ˈsɪnæps/ | A junction through which a neuron communicates with another cell, usually using a chemical transmitter. |
| neurotransmitters/ˈnjuːrətrænsmɪtəz/ | Chemical messengers released by neurons to transmit signals across synapses. |
| reuptake/riːˈʌpteɪk/ | The return of released neurotransmitter from the synaptic space into a cell through transporter proteins. |
| Hormones/ˈhɔːməʊnz/ | Chemical messengers released by cells or glands that act on target cells with suitable receptors. |
| Psychoactive drugs/ˌsaɪkəʊˈæktɪv drʌɡz/ | Substances that act on the nervous system and alter mental processes such as mood, perception or attention. |
| tolerance/ˈtɒlərəns/ | A reduced response to a substance after repeated exposure, so the same amount produces less effect. |
| addiction/əˈdɪkʃn/ | A pattern of compulsive substance use or behaviour with impaired control despite harmful consequences. |
| withdrawal/wɪθˈdrɔːl/ | Physical or psychological effects that occur when use of a substance is reduced or stopped after dependence has developed. |
| reflex arc/ˈriːfleks ɑːk/ | The neural pathway connecting a receptor to an effector in a reflex response. |
A signal that is all or nothing
- A neuron does not fire a little bit harder for a stronger stimulus.
- It either fires completely or not at all — the all-or-nothing principle 全或无原则.
- Strength is coded by how often neurons fire, and how many fire, not how hard.
The all-or-nothing principle means that a neuron...
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Strength is coded by the rate of firing and the number of neurons, not the force of one firing.
The parts of a neuron
- Dendrites 树突 receive signals from other neurons.
- The cell body 细胞体 sums them and decides whether to fire.
- The axon 轴突 carries the signal away; a myelin sheath 髓鞘 around it speeds transmission up.
- Glial cells 神经胶质细胞 support and insulate neurons rather than transmitting themselves.
A neuron receives at the dendrites, fires along the axon, and passes the message across the synapse 突触
An action potential: resting, threshold, depolarisation and refractory period
Fire a neuron, step by step
All-or-none: below threshold nothing happens, and above it the impulse is always the same size.
Glial cells transmit signals in the same way neurons do.
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Glial cells support and insulate neurons; the neurons do the transmitting.
Crossing the gap
- Neurons do not touch. The gap between them is the synapse.
- The signal crosses chemically: neurotransmitters 神经递质 are released and bind on the far side.
- Leftover neurotransmitter is taken back in — reuptake 再摄取.
The gap between two neurons, crossed chemically, is the ____.
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Neurons do not touch; neurotransmitters carry the signal across the synapse.
"All-or-nothing" is about one neuron's firing, not about how strongly you feel something. A light touch and a hard press both make neurons fire identically — what differs is the rate of firing and the number of neurons involved.
Select all that the CED lists as consequences of repeated psychoactive drug use.
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Tolerance, addiction and withdrawal are the named consequences; faster reflexes are not.
Neurotransmitters, hormones and drugs
- Each neurotransmitter has specific functions, which can depend on where in the body it acts.
- Hormones 激素 do a similar job outside the nervous system, travelling in the blood — slower, but longer-lasting.
- Psychoactive drugs 精神药物 work by changing neurotransmitter function somewhere in this chain.
- Repeated use can produce tolerance 耐受性 and addiction 成瘾, with withdrawal 戒断反应 on stopping.
Put the steps of a reflex arc in order.
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The reflex is completed in the spinal cord; the brain finds out afterwards.
The reflex arc
- In the spinal cord, the reflex arc 反射弧 shows central and peripheral neurons working together.
- A sensory neuron carries the signal in, an interneuron passes it across, a motor neuron carries the response out.
- The brain is bypassed, which is why your hand moves before you feel the pain.
Many antidepressants block reuptake. The neurotransmitter that would have been reabsorbed stays in the synapse longer, so it keeps acting on the receiving neuron — the drug changed one step in the chain, not the neuron itself.
A neuron fires all-or-nothing. Signals arrive at dendrites, are summed in the cell body, travel down the axon, and cross the synapse as neurotransmitters. Hormones do a slower version in the blood, and psychoactive drugs act by changing one step of this chain.
Write from a threshold of 5 inputs
- A neuron needs 5 equal inputs to reach threshold. Only 4 have arrived.
- Say whether it fires a weak signal.
A neuron needs 5 equal inputs to reach threshold, and only 4 have arrived. Does it fire a smaller signal?
Threshold is a gate. Below it the axon does not send a graded impulse.
What happens when only 4 of the 5 inputs have arrived?
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All-or-nothing means 4 inputs give no impulse, not a weaker one.