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The moving animal - Gateway > Action potentials > Flashcards

Action potentials Flashcards

1
Q

What input do skeletal muscles require to produce muscle contractions?

A

Neurogenic - require synaptic input from neurones to produce muscle contractions

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2
Q

What are neurones?

A

Designed to respond to stimuli and transmit information over long distances

  1. Receive information
  2. Process information
  3. Propagate signal
  4. Transmit signal
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3
Q

What are neurones made up of?

A

Dendrites: points of contact with with other neurones
Axon: permits long distance communication

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4
Q

How does the transmission of information occur in neurones?

A

Electric and chemical signals

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5
Q

What are excitable cells?

A

Neurones, muscle cells and endocrine cells

- excitability is a property of cell membrane

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6
Q

What do excitable cells do?

A

Generate changes in their membrane potential, producing an action potential
Electric current in the neurone is used to rapidly transmit signals through the animal (initiation of contraction in muscles)
- implies rapid and reversible reversal of electrical potential difference across the plasma membrane

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7
Q

What is the significance of the cell membrane in the action potential?

A

Lipid bilayer: resistant to movement of electrically charged ions
- channels through which ions can pass through the membrane

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8
Q

What is the transmembrane or resting potential?

A
  • Living cells have an electric potential across their membranes
  • Inside of the cell more negatively charged, difference in charges = membrane potential (usually between -50 _-100mV) - basis for the resting membrane potential
    Differences in intra and extracellular ionic composition
    Selective permeability of the plasmic membrane
  • Na/K pump and potassium channels are largely responsible for the transmembrane potential
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9
Q

Describe the ionic distribution of intra and extracellular fluid?

A 
Maintenance of electro-neutrality and osmotic pressure 
Extracellular medium 
- Na+: 140mM
- K+: 4mM
- Ca2+: 2mM 
- Cl-: 147mM
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10
Q

How is current flow controlled by ionic channels in membrane? (Resting channels)

A
  • normally open
  • not influenced by potential across the membrane
  • maintain the resting potential in the absence of signalling
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11
Q

How is current flow controlled by ionic channels in membrane? (Gated channels)

A
  • closed when membrane is at rest
  • recognise and select specific ions
  • open in response to specific signals
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12
Q

What are the types of gated channels?

A 
Voltage gated
Ligand gated (ligand fits to receptor site)
Mechanically gated (cytoskeleton stretches channel)
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13
Q

How are ion channels activated?

A

Changes the resting transmembrane potential results in either:

  • local (graded) potentials: changes in transmembrane potential that do not spread far along the membrane from site of initiation
  • action potentials: rapid changes in transmembrane potential; means by which electric signals are propagated along axons
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14
Q

Where are the gated channels located along an axon?

A

Dendrite = ligand-gated channels
Axon/myelin sheath/node of ranvier = voltage-gated Na and K channels
Axon terminal = Voltage gated Ca2+. Na+, K+ channels - mechanically activated

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15
Q

How does depolarisation of a membrane occur?

A

Increased concentration of Na+ on one side of the membrane

- depolarisation occurs as Na+ moves through Na+ channels

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16
Q

What is hyperpolarisation?

A

Potassium channels

  • if a ligand causes an opening of potassium channels: cell becomes hyperpolarised
  • can also be chloride channels
17
Q

Describe the movement of an action potential

A
  1. Momentarily depolarisation of the cell membrane
  2. Membrane potential reverses from -70mV to +40mV and then repolarises, travels along the axon without a drop in amplitude
  3. Produced when the membrane potential is depolarised above a critical value (threshold, around 55mV)
  4. Membrane potential returns to normal rapidly
  5. Movement of very few sodium ions, these ions moving produce infinitiesimal changes in intracellular ion concentrations
18
Q

Describe the positive feedback loop of the depolarisation of the axon?

A

Local depolarisation → increase in Na conductance
↑ ↓
↑ influx of +ve charges
more depolarisation ↓
↑ depolarisation
more influx of +ve charges ↓
↑ more increase in Na conductance

19
Q

What is initiation?

A
  • Stimulation of sensory receptors, initial depolarisation occurs at nerve endings due to changes in the environment
  • synaptic activity, depolarisation to the threshold could be the result of synaptic activity and effect neurotransmitters on the membrane, usually a series of potentials occur in response to a stimulation
  • spontaneous activity, same nerve and muscle cells show spontaneous changes in their membrane potential
20
Q

How does excitablilty vary between regions of axons and among neurons?

A

Due to set of ion channels that are expressed. Voltage sensitive Na+ channels are concentrated at the initial segment of axon (axon hillock)

  • sensory neurons: myelinated axon’s first node of ranvier
  • interneurons and motor neurons: axon hillock
21
Q

What events occur in the depolarising/repolarising phases?

A

Depolarisation
1. Electric event triggers voltage gated channels to open
2. Na+ v-gated channels open immediately
3. Na+ ions flow into cells
Re-polarisation
4. K+ V-gated channels open more slowly
5. When K+ channels do open, Most Na+ channels have already closed
6. K+ outflow returns membrane potential to -70mV

22
Q

What is the refractory period?

A

Period of time in which neuron cannot produce another action potential

  • change in state of Na+ channels that is responsible for the refractory period
  • limits the rate of the AP generation
23
Q

What is the absolute refractory period?

A

For 1-2ms after the spike, neuron unable to produce an AP

24
Q

What is the relative refractory period?

A

During the next several ms, threshold falls back to its normal resting value. Becomes progressively easier to produce an AP

25
Q

Why is an action potential all or none?

A

If graded potential changes the resting potential to threshold potential, an action potential is always produced

  • either a given stimulus produces an action potential, or it does not
  • strong stimuli produce no stronger AP than weak ones
  • strength of the stimulus is encoded in the frequency of the Ap it generates
26
Q

What is electrophysiology?

A

Study of action potentials

  • isolating signals from single axons
  • recording voltage from a single cell
27
Q

What is action potential propagation?

A

Action potential propagates along membrane of axon

  • AP conduction: unmyelinated axons
  • saltatory conduction: myelinated axons
28
Q

What is the speed of propagation in an unmyelinated neurone?

A

1m/sec

29
Q

What is the speed of propagation in a myelinated neurone?

A

Action potential regenerated at nodes of ranvier

- propagation = 25-100m/sec

30
Q

What is acute myelination?

A

Damages the myelin insulation covering Na+ channel

- poor parts of the axon membrane and produces axonal conduction blockage

31
Q

What factors determine conduction velocity?

A

Diameter and myelination

32
Q

What is the direction of propagation?

A

Direction of information is always from the dendrites (synaptic knobs)

  • due to membrane becoming refractory
  • synapses permit conduction in one direction
33
Q

How is an action potential terminated?

A

Chemical synapses
Electrical synapses
Neuromuscular junctions

The moving animal - Gateway (19 decks)

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