Neuron action potential - physiology

00:10:24
https://www.youtube.com/watch?v=BbUcWbtVjT4

概要

TLDRThe video provides an in-depth explanation of neurons and their role in the nervous system. Neurons are the fundamental cells composed of dendrites, the soma, and an axon. Dendrites receive chemical signals from other neurons through neurotransmitters, opening ion channels and creating an electrical signal. This signal, known as an action potential, travels rapidly down the axon, triggering the release of neurotransmitters at the synapse. The electric charge of the neuron is due to differing ion concentrations inside and outside the cell, with a resting potential of about -65mV. When enough excitatory signals change the membrane potential, voltage-gated sodium channels at the axon hillock open, allowing sodium influx and rapid depolarization, leading to action potential propagation. Myelinated axons allow for faster signal transmission as the charge jumps between nodes of Ranvier, a process called saltatory conduction. This intricate signaling enables efficient nerve communication, vital for various physiological responses.

収穫

  • 🧠 Neurons have dendrites, soma, and axons for signal transmission.
  • 🔌 Action potentials are critical for neuron communication.
  • 📉 Resting potential is typically -65 mV due to ion concentrations.
  • 🧪 Neurotransmitters convert chemical to electrical signals.
  • ⚡ Voltage-gated sodium channels trigger depolarization.
  • 🔁 Absolute refractory period ensures signal timing and direction.
  • ⚡ Saltatory conduction speeds up signal transmission along axons.
  • 🧪 EPSPs and IPSPs influence neuron firing likelihood.
  • 🧫 Nodes of Ranvier facilitate fast electrical signal jumps.
  • 🔧 Sodium-potassium pump helps restore resting potential.

タイムライン

  • 00:00:00 - 00:10:24

    Neurons are fundamental units of the nervous system, composed of dendrites, a soma, and an axon. Dendrites receive signals via neurotransmitters, converting these into electrical signals. When signals reach a certain threshold, they trigger an action potential that travels down the axon, facilitating neurotransmitter release and further signal propagation. Neurons utilize an action potential to effectively transmit signals internally despite their length. The resting membrane potential arises from the distribution of ions, with neurotransmitters affecting ligand-gated ion channels altering this potential. This process can lead to excitatory or inhibitory postsynaptic potentials depending on the ion flow, potentially triggering an action potential to further carry the signal along the neuron.

マインドマップ

ビデオQ&A

  • What are the three main parts of a neuron?

    The three main parts of a neuron are dendrites, the soma (cell body), and the axon.

  • What role do neurotransmitters play in neuron communication?

    Neurotransmitters bind to receptors on dendrites to convert chemical signals into electrical signals.

  • What is an action potential?

    An action potential is an electrical signal that travels down the axon, triggering the release of neurotransmitters.

  • Why does the cell have an electric charge?

    The cell has an electric charge due to the different concentrations of ions inside and outside the cell.

  • What is the resting membrane potential of a neuron?

    The resting membrane potential of a neuron is typically around -65 millivolts.

  • What happens during depolarization?

    During depolarization, positive charge enters the cell, making it less negative and less polar.

  • What is saltatory conduction?

    Saltatory conduction is the fast transmission of electrical signals where the charge jumps between nodes of Ranvier on myelinated axons.

  • What is the role of myelin in nerve signal transmission?

    Myelin speeds up nerve signal transmission by insulating the axon and causing the charge to jump between nodes of Ranvier.

  • What is the difference between EPSP and IPSP?

    EPSP involves a net influx of positive charge, making the cell more likely to fire, while IPSP involves a net influx of negative charge, making it less likely.

  • What prevents action potentials from happening too close together?

    The absolute refractory period, during which sodium channels are inactivated, prevents action potentials from occurring too closely together.

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  • 00:00:04
    Neurons are the cells that make up our nervous system, and they’re made up of three main
  • 00:00:08
    parts.
  • 00:00:09
    The dendrites, which are little branches off of the neuron that receive signals from other
  • 00:00:13
    neurons, the soma, or cell body, which has all of the neuron’s main organelles like
  • 00:00:18
    the nucleus, and the axon which is intermittently wrapped in fatty myelin.
  • 00:00:25
    Those dendrites receive signals from other neurons via neurotransmitters, which when
  • 00:00:29
    they bind to receptors on the dendrite act as a chemical signal.
  • 00:00:34
    That binding opens ion channels that allow charged ions to flow in and out of the cell,
  • 00:00:39
    converting the chemical signal into an electrical signal.
  • 00:00:42
    Since a single neuron can have a ton of dendrites receiving input, if the combined effect of
  • 00:00:47
    multiple dendrites changes the overall charge of the cell enough, then it triggers an action
  • 00:00:52
    potential- which is an electrical signal that races down the axon up to 100 meters per second,
  • 00:00:58
    triggering the release of neurotransmitter on the other end and further relaying the
  • 00:01:02
    signal.
  • 00:01:04
    So neurons use neurotransmitters as a signal to communicate with each other, but they use
  • 00:01:09
    the action potential to propagate that signal within the cell.
  • 00:01:13
    Some of these neurons can be very long, especially ones that go from the spinal cord to the toes,
  • 00:01:18
    so the movement of this electrical signal is super important!
  • 00:01:22
    But why does the cell have an electric charge in the first place?
  • 00:01:27
    Well, it’s based on the different concentrations of ions on the inside versus outside of the
  • 00:01:31
    cell.
  • 00:01:32
    Generally speaking, there are more Na+ or sodium ions, Cl- or chloride ions, and Ca2+
  • 00:01:39
    or calcium ions on the outside, and more K+ or potassium ion and A- which we just use
  • 00:01:46
    for negatively charged anions, on the inside.
  • 00:01:50
    Overall, the distribution of these ions gives the cell a net negative charge of close to
  • 00:01:56
    -65 millivolts relative to the outside environment - this is called the neuron’s resting membrane
  • 00:02:01
    potential.
  • 00:02:05
    When a neurotransmitter binds to a receptor on the dendrite, a ligand-gated ion channel
  • 00:02:10
    opens up to allow certain ions to flow in, depending on the channel.
  • 00:02:14
    Ligand-gated literally means that the gate responds to a ligand, which in this case is
  • 00:02:19
    a neurotransmitter.
  • 00:02:21
    So if we take the example of a ligand-gated Na+ ion channel, which, when it opens, lets
  • 00:02:26
    Na+ flow into the cell.
  • 00:02:28
    The extra positive charge that flows in makes the cell less negative (since remember it’s
  • 00:02:33
    usually -65mV), and therefore less “polar” - so that’s why gaining positive charge
  • 00:02:39
    is called depolarization.
  • 00:02:43
    Neurotransmitters typically open various ligand-gated ion channels all at once, so ions like sodium
  • 00:02:48
    and calcium, may flow in, while other ions like potassium, may flow out, which would
  • 00:02:54
    actually mean some positive charge leaves the cell.
  • 00:02:58
    In the end though - when it’s all added up - if there is a net influx of positive
  • 00:03:02
    charge, then it’s called an excitatory postsynaptic potential (EPSP).
  • 00:03:09
    In contrast, the opening of only ligand-gated Cl- ion channels would cause a net influx
  • 00:03:14
    of negative charge, creating an inhibitory postsynaptic potential (IPSP), making the
  • 00:03:20
    cell potential more negative or repolarizing it.
  • 00:03:24
    Now, a single EPSP or IPSP causes only a small change on the resting membrane potential,
  • 00:03:31
    but, if there are enough EPSPs across multiple sites on the dendrites then collectively they
  • 00:03:36
    can push the membrane potential to a specific threshold value- typically about -55mV, although
  • 00:03:43
    this can vary by tissue.
  • 00:03:46
    When this occurs, it triggers the opening of voltage-gated Na+ channels at the start
  • 00:03:51
    of the axon - the axon hillock, voltage-gated channels open in response to a change in voltage,
  • 00:03:59
    and when these open sodium to rush into the cell.
  • 00:04:02
    The influx of sodium ions and the resulting change in membrane potential causes nearby
  • 00:04:07
    voltage-gated sodium channels to open up as well - setting off a chain reaction that continues
  • 00:04:12
    down the entire length of the axon—which is our action potential, and when this happens,
  • 00:04:17
    we say that the neuron has ‘fired.’
  • 00:04:20
    Once a lot of sodium has rushed across the neuronal membrane, the call actually becomes
  • 00:04:25
    positively charged relative to the external environment - up to about +40mV.
  • 00:04:31
    The depolarization process ends when the sodium channel stops allowing sodium to flow into
  • 00:04:36
    the cells- a process known as inactivation.
  • 00:04:41
    But this state is different than when the channel’s closed, or open for that matter,
  • 00:04:46
    which is what most of the other channels have.
  • 00:04:49
    The voltage-gated sodium channel, though, is unique in that it has what’s known as
  • 00:04:53
    the inactivation gate, which blocks sodium influx shortly after depolarization, until
  • 00:04:59
    the cell repolarizes and the channel enters the closed state again and the inactivation
  • 00:05:04
    gate stops blocking influx, although even though the inactivation gate’s not blocking,
  • 00:05:09
    the channel’s still closed so no sodium enters the cell.
  • 00:05:13
    This middle open state therefore is the only state where sodium gets let into the cell
  • 00:05:18
    through the channel, and this is a very short window of time.
  • 00:05:24
    Now in addition to these sodium voltage-gated channels, we’ve also got potassium voltage-gated
  • 00:05:29
    channels, which are slow to respond and don’t open until the sodium channels have already
  • 00:05:33
    opened and become inactivated.
  • 00:05:35
    The result is that after the initial sodium rush into the cell, potassium flows out of
  • 00:05:41
    the cell down its own electrochemical gradient- removing some positive charge and blunting
  • 00:05:46
    the effect of the sodium depolarization.
  • 00:05:48
    The potassium channels, do not have a separate inactivation gate and therefore remain open
  • 00:05:54
    for slightly longer, which means that there is a period of time when there is a net movement
  • 00:05:58
    of positive ions out of the cell, causing the membrane potential to become more negative,
  • 00:06:03
    or repolarize.
  • 00:06:05
    During this repolarization phase, the cell also relies on the sodium-potassium pump,
  • 00:06:10
    an active transporter that moves three sodiums out of the cell and two potassiums into it.
  • 00:06:17
    It’s during this repolarization phase that the cell’s in its absolute refractory period,
  • 00:06:23
    since the sodium channels are inactivated and won’t respond to any amount of stimuli.
  • 00:06:29
    This absolute refractory period keeps the action potentials from happening too close
  • 00:06:32
    together in time, but also keeps the action potential moving in one direction.
  • 00:06:39
    The combined efforts of this pump and the extended opening of the potassium channels
  • 00:06:44
    results in a small period of overcorrection where the neuron becomes hyperpolarized relative
  • 00:06:48
    to the resting potential, and at this point the sodium channels go back to their initial
  • 00:06:53
    closed state, and for a short period the potassium channels stay open.
  • 00:06:57
    Now we’re in the relative refractory period since the sodium channels are closed but can
  • 00:07:03
    be activated, but because the potassium channels are still open and we’re in a hyperpolarized
  • 00:07:08
    state, so it’s takes a strong stimulus to do so.
  • 00:07:12
    Finally, as the potassium channels close, the neuron returns to it’s resting membrane
  • 00:07:17
    potential.
  • 00:07:18
    Alright, as a quick graphical recap, with membrane potential on the y and time on the
  • 00:07:25
    x.
  • 00:07:26
    First we start at resting potential of around -65 mV and voltage-gated sodium and potassium
  • 00:07:32
    channels are closed, we receive EPSPs enough to hit threshold at about -55 mV, voltage-gated
  • 00:07:41
    sodium channels open and we reach a peak of about +40 mV, at which point the sodium channels
  • 00:07:47
    become inactivated and we’re in the absolute refractory period.
  • 00:07:53
    Voltage-gated potassium channels open, and along with the sodium-potassium pump, start
  • 00:07:57
    to repolarize the cell, so much so that it overshoots and hyperpolarizes the cell.
  • 00:08:04
    Next the sodium channels enter their closed resting state as potassium channels start
  • 00:08:09
    to close we’re in the relative refractory period, until finally they all close and we
  • 00:08:15
    reach our resting membrane potential.
  • 00:08:18
    Alright, so this process of positive sodium ions moving in and depolarizing the cell transmits
  • 00:08:25
    the electrical signal down the length of the axon.
  • 00:08:28
    Great.
  • 00:08:29
    But really, this process isn’t that fast.
  • 00:08:32
    So that’s where the fatty myelin comes in, which comes from glial cells like Schwann
  • 00:08:36
    cells or oligodendrocytes.
  • 00:08:39
    These myelinated areas don’t have voltage-gated ion channels spanning the membrane, so ions
  • 00:08:44
    can’t simply flow into the cell, that only happens in the spots between the myelin, called
  • 00:08:49
    nodes of Ranvier.
  • 00:08:51
    So instead of propagating via channels, the charge essentially jumps from node to node.
  • 00:08:58
    That said though, these ions aren’t just diffusing down the length of the myelin to
  • 00:09:03
    the other side...that’d be way to slow.
  • 00:09:06
    What actually happens is more like the sodium ions rushing in bumps other positive sodium
  • 00:09:11
    ions already inside the cell, which bumps another one, and so on until it reaches the
  • 00:09:18
    next node.
  • 00:09:20
    The charge moving in this way with the myelinated areas moves really fast, and is called saltatory
  • 00:09:26
    conduction, which makes it look like the action potential “jumps” from one one node to
  • 00:09:30
    the next.
  • 00:09:32
    Okay extremely quick recap - neuron action potentials happen when dendrites receive enough
  • 00:09:38
    EPSPs to open voltage-gated sodium channels, which cause rapid depolarization of the neuronal
  • 00:09:43
    membrane and propagation of an electrical charge from node to node down the length of
  • 00:09:48
    the axon.
  • 00:09:51
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  • 00:09:59
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タグ
  • neurons
  • action potential
  • neurotransmitters
  • depolarization
  • signal transmission
  • myelin
  • resting potential
  • EPSP
  • IPSP
  • saltatory conduction