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LQB388: Medical Physiology 1 > 3 - Excitable Tissues > Flashcards

3 - Excitable Tissues Flashcards

1
Q

What are excitable tissues? And what are the major types?

A
  • Tissues that utilise electrical signals that travel along cells and can be readily transferred from cell to cell
  • Neurones and Muscle (skeletal, cardiac and smooth)
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2
Q

How do you record the membrane potential in a single cell?

A

Use a voltmeter with microelectrodes. Place one extracellularly and one intracellularly, then measure the potential difference

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3
Q

What is resting membrane potential?

A

Membrane potential of an excitable cell at rest. Membrane is polarised

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4
Q

Define polarised

A

2 sides are different charges

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5
Q

What happens during depolarisation

A

Membrane potential decreases in magnitude from RMP (inside of the cell becomes less negative)

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6
Q

What happens during repolarisation

A
  • Restoration of the difference in charge
  • MP increases in magnitude back towards RMP
  • Inside of the cell becomes more negative
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7
Q

What happens during hyperpolarisation?

A
  • MP increases in magnitude from RMP
  • Membrane is more polarised (more negative)
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8
Q

What happens during hyperpolarisation?

A
  • MP increases in magnitude from RMP
  • Membrane is more polarised (more negative)
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9
Q

What are graded potentials? And what are their key characteristics?

A
  • Occur when an excitable tissue is subjected to an excitatory/inhibitory stimulus
  • Small changes in MP (1-30mv)
  • Transient (lasting 10’s of ms)
  • Proportional to size of stimulus
  • Produce local not global effects (only effects part around the stimulus, not the whole cell)
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10
Q

Explain depolarising graded potentials

A
  • Produced by an excitatory stimulus applied to the cell
  • Causes a transient depolarisation of the membrane
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11
Q

Describe hyperpolarising graded potentials

A
  • Inhibitory stimulus applied to the cell
  • Causes a transient increase in the membrane potential
  • More negative than the RMP
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12
Q

What are the general features of an action potential?

A
  • Large, fast complex changes in MP by large excitatory stimulus
  • Affects the whole cell (once initiated, travels over the whole cell)
  • To get an AP, depolarising GP must be large enough to reach threshold (varies from cell to cell)
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13
Q

Describe the phases of action potentials

A
  1. **Depolarising phase: **period between threshold and the peak. Inside of the cell is positive compared to the outside
  2. Repolarising phase: Period between peak and RMP
  3. Hyperpolarising phase: Becomes more negative before returning to RMP
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14
Q

What are the typical membrane values at threshold and peak of action potential?

A
  • Threshold: -65mV
  • Peak: +30mV
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15
Q

Key differences between graded and action potentials

A
  1. AP always the same size, unlike GP (all or none principle. if threshold is reached, whole sequence occurs)
  2. APs are quicker (AP = few ms)
  3. Action potential propagation. Movement of AP. GP is localised.
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16
Q

What is conduction velocity?

A
  • Speed of AP propagation is rapid
  • Vary b/n cells (0.5-130 m/sec)
17
Q

What is frequency encoding?

A
  • APs encode info by frequency of APs
  • Expressed in Hz
  • Freq in excitable tissues = no. AP per second
18
Q

Describe an experiment for frequency coding

A
  • Measuring MP of a neurone in response to skin indentation by blunt probe
  • As the probe is advanced, series of AP in neurone that last as long as skin deformation and have a consistent frequency
  • Increase the force applied to the probe, higher freq of AP happens
  • Therefore, higher stimulus intensity is encoded by a higher freq of AP
19
Q

What are the normal intracellular and extracellular ion concentrations for major anions and cations in mammals

A
20
Q

What is the functional significance of ion selective channels, resting channels and gated ion channels?

A

Bc ions are dissolved in aqueous ECF and ICF, can’t move through hydrophobic core of lipid bilayer so they move through these channels instead.

21
Q

Explain voltage-gated ion channels and an example

A
  • State of the gate is determined by MP
  • Has a molecular sensor that measures MP
  • Opens/closes depending on value
  • E.g: VGNa+ and K+ channels in AP
22
Q

Explain ligand-gated ion channels and an example

A
  • Gated by binding of chemicals to a receptor closely associated with the channel
  • Interaction between ligand/chemical and receptor enables high specificity in controlling ion channel opening
  • E.g: acute sense of smell
  • Important in a wide variety of physiological systems and are implicated in a number of diseases and actions of therapeutic drugs
23
Q

Explain stretch-gated ion channels and an example

A
  • Regulated by degree of stretch (mechanical deformation) exerted on membrane where they’re embedded
  • Stetch produces a conformational shape change, opens the gate so ions pass
  • Involved in initiating GP associated with sensory stimuli
24
Q

Explain resting channels

A
  • Ungated
  • Open most of the time, not affected by stimuli
  • Important for RMP
25
Q

Describe the ionic-basis of the RMP

A
  • At rest, membrane is freely permeable to K+ due to resting channels
  • Higher concentration of K+ inside the cell and lower conc outside.
  • K+ flows out of the cell down the concentration gradient
26
Q

Explain why the resting membrane potential is slightly less negative than the equilibrium potential for K+

A
  • Membrane is slightly leaky, so some Na+ passes through
  • Making it slightly less negative
27
Q

Ionic basis of action potential at threshold

A
  • [Na] out > [Na] in. Therefore Na+ goes into the cell down conc gradient
  • Inside of cell is negative, so EG attracts Na+ into the cell
  • VGNa+ channels are closed at RMP, but are programmed to open during threshold.
  • At threshold, membrane has increased permeability to Na+, so Na rushes into cell along CG and EG
  • Membrane depolarises, as positive charge flows into cell. Inside of cell becomes less negative.
  • Depolarisation causes more VGNaC to open and more Na goes into cell
  • This positive feedback loop is responsible for the explosive nature of the depolarising phase of the action potential. Because the equilibrium potential for Na+ is around +56 mV, Na+ continues to flow into the cell even after the membrane potential reaches 0 mV and in fact continues on until it peaks at around +30 mV.
28
Q

Ionic basis of AP at peak

A
  1. VGNaC close, some VGKC open
  2. CG and EG favours movement of K+ out of cell
  3. Net effect: influx of Na+ rapidly declines, more K+ leaving the cell along CG and EG
  4. As a result of declining influx of positive charge (Na+) and an increase in efflux of positive charge (K+), AP peaks and rapidly repolarises
  5. Repolarising phase is a direct consequence of K+ efflux
29
Q

Ionic basis of hyperpolarising phase

A
  • Consequence of slow closing of VGK+C
  • Those channels + resting K+ channels causes MP to be higher than RMP
  • Slow closing of VGKC responsible for hyperpolarising phase
30
Q

Describe an experiment that could do to demonstrate that K+ is responsible for the resting membrane potential.

A
  • Investigate the effects of varying conc of K+ in the solution bathing the cells on RMP
  • Measure the RMP of 5 cells by placing a moveable electrode inside each of the cells in turn
  • (In prac book)
31
Q

Describe an experimental approach that you could use to demonstrate that Na+ influx is responsible for the depolarising phase of the action potential.

A

Investigate the effects of varying extracellular [Na+] on the magnitude of the depolarising phase of the AP
(Prac book)

32
Q

Understand what the Goldman-Hodgkin-Katz equation is and how to use it to predict the membrane potential if you know the relative permeability of the membrane to K+ and Na+.

A
33
Q

Explain why the Goldman-Hodgkin-Katz equation provides a more accurate prediction of membrane potential that the Nernst equation.

A

Nernst eq assumes that channels are only permeable to 1 specific ion species. While this equation takes into account that channels may be permeable to more than 1 ion species.

34
Q

Explain why the Goldman-Hodgkin-Katz equation provides a more accurate prediction of membrane potential that the Nernst equation.

A

Nernst eq assumes that channels are only permeable to 1 specific ion species. While this equation takes into account that channels may be permeable to more than 1 ion species.

35
Q

Explain the mechanisms of action potentials

A
  1. Initiation: AP initiated at axon initial segment (adjacent to axon hillock). This is because this part of the membrane has a lower threshold
  2. Propagation: AP travels down the axon, away from the soma to the axon terminal.
36
Q

Explain how the mechanisms of action potential propagation differ between myelinated and unmyelinated axons and the implication that this has for conduction velocity.

A

Unmyelinated
* AP in membrane causes VGNaC to open in adjacent membrane
* Na+ goes into cell and initiates AP further along axon
* Conduction velocity is quite slow (0.5-2.5 m/s)
Myelinated
* Wrapped in thick myelin sheath
* Node of ranvier: gaps in myelin sheath, axon is exposed
* Heavy insulation from myelin enables VGNa+C to detect the voltage of AP in adjacent node (AP can jump from node to node)
* Called Saltatory conduction
* Therefore, slow propagation of AP along nodes, but very rapid saltatory communication between nodes
* Causes conduction velocity to be much quicker (12-130 m/sec)

37
Q

What are refactory periods?

A
  • Refactory period keeps AP moving in 1 direction
  • Region that AP has just passed through becomes refactory (inactive) for a short time
  • Difficult to initiate AP in this part (hence 1 direction)
38
Q

Exaplin the ionic basis for absolute refactory periods

A
  • When the second stimulus is applied, cell fails to produce AP
    Caused by:
  • VGNaC hae opened then closed again
  • They get “jammed”
  • Called Na+ channel inactivation
39
Q

Explain the ionic basis of relative refactory period

A
  • Small AP that gradually increases in size as the delay between stimuli increases
  • AP with reduced amplitude
  • Caused by gradual recovery of VGNaC
  • As more and more of these Na+ channels come out of inactivation, more and more of them can be opened by the stimulus and more and more Na+ can flow into the cell. Because Na+ influx is responsible for the depolarising phase of the action potential the greater the influx of Na+ the bigger the action potential.

LQB388: Medical Physiology 1 (6 decks)

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