Learning Outcomes - Week 3 - Excitable Tissues

Question 1

Explain what an excitable tissue is and name the major excitable tissue types.

Answer 1

Tissues that utilise electrical signals for communication are know as excitable tissues (reflecting their electrical excitability) and are the topic of this lesson.

Types are neurons, cardiac muscle, skeletal muscle, and smooth muscle

Question 2

Describe how to record the membrane potential in a single cell.

Answer 2

• voltmeter with fine leads - begin with leads outside the cell (would record potential difference of 0mV)
• Carefully place one of them into the cell and record the potential difference between ICF and ECF of the cell
• Find that the inside is negative compared to the outside (-80mV)

Question 3

Understand what the resting membrane potential is and that at rest the membrane is polarised.

Answer 3

The electrical potential of the membrane at rest is when there is no net exchange of charge between the ICF and ECF of the cell.

Resting membrane potential is -80mV (RMP)

Question 4

Define the terms depolarisation, repolarisation and hyperpolarisation and understand how these relate to the magnitude of the membrane potential.

Answer 4

• depolarisation = when the RMP decreases in magnitude (becomes less negative), then membrane becomes less polarised therefore termed depolarisation
• repolarisation = membrane potential increases in magnitude back towards the RMP (ICF becomes more negative), termed this movement as repolarisation
• hyperpolarisation = membrane potential increases in magnitude from the RMP (ICF becomes more negative), membrane becomes more polarised therefore termed hyperpolarisation

Question 5

Appreciate that graded potentials are caused by _______ and be familiar with the general characteristics of these potentials which are…?

Answer 5

stimuli (both excitatory and inhibitory)

Graded potentials are:
- relatively small changes to membrane potential (1-30mV)
- Fairly transient (lasting only 10’s of ms)
- named so as the size of the potential is not consistent but is directly proportional to the size of the stimulus. Thus the larger the stimulus the bigger the graded potential is.

Question 6

Describe the difference between a depolarising and hyperpolarising graded potential.

Answer 6

• Depolarising graded potentials = produced by EXCITATORY stimuli applied to the cell (shown in image) (observing a transient depolarisation of the membrane)
• hyperpolarising graded potential = graded potentials produced by an INHIBITORY stimulus (observing the opposite to the image and a transient hyperpolarisation of the membrane) (becomes more negative than the RMP)

Question 7

Understand the general features of an action potential.

When is it an action potential and not just a graded potential?

Answer 7

• large
• fast
• complex changes in the membrane potential elicited by a large excitatory stimulus. Once initiated they travel over the surface of the cell and consequently affect the whole cell.
• action potential is a graded potential which was large enough to reach threshold of -64mV

Question 8

Describe the three characteristic phases of an action potential and appreciate that these are named according to…?

Answer 8

1. Depolarising phase = period between THRESHOLD and the peak of the action potential (+30mV)
2. Repolarising phase = period between the peak of the action potential and the resting membrane potential
3. Hyperpolarising phase = Period where membrane becomes briefly more negative than RMP before returning to RMP

• the direction that the membrane potential is moving during that phase.

Question 9

Recall the typical membrane values at threshold and the peak of the action potential.

Answer 9

Peak = +30mV
Threshold = -65mV

Question 10

Explain the key differences between action potentials and graded potentials.

Answer 10

Action potentials:

• always same size (either whole thing or not at all)
• if threshold is reached we get entire sequence of peak, repolarisation, and hyperpolarisation
• last a few milliseconds in most cells (bigger but happen much faster than graded potentials)
• effect the entire cell (travel the length of the whole cell - known as action potential propagation)

Graded potentials:

• vary in size
• never reach threshold
• last tens of milliseconds (longer than action potentials)
• only effect part of cell (localised and not global)

Question 11

Understand the terms ‘all or none principle’, action potential propagation and conduction velocity.

Answer 11

All or none refers to the fact that an action potential only propagates if threshold is reached (-65mV)

At this point an action potential will occur and will be the same size every time with all three phases

Question 12

Explain frequency encoding and be able to describe how you could demonstrate this concept experimentally.

Answer 12

• It is not the size of the action potentials that the nervous system uses to encode information, but the frequency (i.e. number of action potentials per second). This is an important concept in excitable tissues and is referred to as frequency coding.
• Using the experiment in the image we can record the membrane potential of one neurone in response to a skin indentation produced by a probe.
• The magnitude and duration of the skin indentation is shown in black and the neurone response is shown in blue (due to duration of the experiment each action potential appears as a straight line)
• As the probe remains in contact for longer we get action potentials continuously throughout the length of the indentation but at the same frequency.
• As the probe presses further into the skin we see an increase in the frequency of the action potentials
• Therefore, in this example, the higher stimulus intensity is encoded by a higher frequency of action potentials

Question 13

Recall the normal intracellular and extracellular ion concentrations for the major anions and cations in mammals.

Answer 13

Question 14

Understand the functional significance of ion selective channels, resting channels and gated ion channels.

Answer 14

• ions are dissolved in the aqueous solutions of the intracellular and extracellular fluids and are unable to move through the lipid bilayer
• they move through specialised membrane spanning proteins called ION CHANNELS
• Many of these channels only permit the movement of one type of ion, therefore said to be ION SELECTIVE

Question 15

Explain the difference between voltage-gated, ligand-gated and stretch-gated ion channels and be able to provide an example where each of these has a physiologically-important role.

Answer 15

1. Voltage-gated ion channels

• state of the gate determined by the membrane potential
• have a molecular sensor that measures the membrane potential and opens or closes the gate depending on its value
• voltage-gated Na+ and K+ channels play important role in the changes in the membrane potential associated with the action potential

2. Ligand-gated channels

• gated by binding of chemicals to a receptor closely associated with the channel
• nature of the interaction between the chemical (ligand) and receptor enables a high degree of specificity in controlling ion channel opening
• important in a wide variety of physiological systems (acute sense of smell) and implicated in number of diseases + the actions of many therapeutic drugs
• The changes in membrane permeability enabled by ion channels are also responsible for many forms of graded potentials as will see subsequently.

3. Stretch-gated Ion Channels

• The gating of some channels is regulated by the degree of stretch exerted on the membrane in which they are embedded.
• The mechanical deformation associated with the stretch produces a conformational (shape) change that opens the gate and allows ions to move through the channel.

Question 16

Describe the ionic-basis of the resting membrane potential.

Answer 16

Ions flow down their concentration gradients and what balances this movement to provide equilibrium is the electrical gradient

Question 17

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

Answer 17

• we do not live in a perfect world (no kidding :-) and at rest the membrane is not only permeable to K+ but is slightly leaky with small numbers of Na+ ions also able to move across the membrane. This means that in the real world the membrane is freely permeable to K+ and very slightly permeable to Na+ at rest.
• concentration gradient for Na+ results in movement of Na+ into cell also
• In addition to K+ leaving cell making more negative, Na+ moves into cell making less negative
• Therefore, slightly leaky membrane results in RMP being slightly less negative than the equilibrium for K+ because of the contamination by Na+
• RMP trying to reach the equilibrium potential for K+ but being prevented from doing so by Na+ leaking into the cell.

Question 18

Understand the ionic-basis of the action potential with particular reference to the changes in membrane permeability that occur at threshold, the peak of the action potential and during the hyperpolarising phase.

Answer 18

1. Threshold:

• high concentration of Na+ outside and low concentration inside the concentration gradient favours movement of Na+ inward
• negative ICF has electrical gradient that attracts positive Na+
• voltage-gated Na+ channels closed a RMP, but open when the membrane potential reaches threshold
• When membrane potential reaches threshold voltage-gated channels open and membrane suddenly becomes highly permeable to Na+
• Na+ then rushes in, down it’s electrical and concentration gradient
• positive charge pouring into cell causing it to become less negative (membrane depolarises
• depolarisation opens more voltage-gated Na+ channels and more Na+ enters (positive feedback loop - responsible for explosive nature of depolarising phase of action potential)
• equilibrium potential for Na+ is +56mV therefore Na+ inward flow continues past 0mV membrane potential and peaks at +30mV

DEPOLARISING PHASE IS A DIRECT CONSEQUENCE OF NA+ INFLUX

2. Peak:

• when approaching peak of action potential, voltage-gated Na+ channels close
• at this point voltage-gated K+ channels open (different from passive K+ channels)
• both concentration and electrical gradient now favours movement of K+ out of the cell)
• net effect is that Na+ influx rapidly decreases and K+ leaves cell along concentration and electrical gradient.
• This result in efflux of positive charge, action potential peaks, then rapidly repolarises

REPOLARISATION PHASE IS A DIRECT CONSEQUENCE OF K+ EFFLUX

3. Hyperpolarisation:

• toward end of action potential many cells exhibit a marked undershoot where membrane potential is more negative the RMP (called hyperpolarisation phase)
• consequence of slow closing of K+ channels and they remain open in addition to resting K+ channels responsible for RMP, therefore the permeability of the membrane to K+ is actually higher than it is at rest.

IT IS THE SLOW CLOSING OF THE VOLTAGE-GATED K+ CHANNELS THAT IS RESPONSIBLE FOR THE HYPERPOLARISATION PHASE OF THE ACTION POTENTIAL

Question 19

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

Answer 19

• Using a voltmeter and two tiny electrodes you could measure the membrane potential of 5 cells bathed in a solution with varying concentrations of K+.
• As the concentration of K+ increases the membrane potential will become less negative - a result of the K+ moving down it’s concentration gradient, into the cells, taking with it it’s positive charge and increasing the cells membrane potential.
• you would then assume that the movement of K+ is what is causing the change in membrane potential
• to confirm this statement you would run the same experiment with other ions such as Cl- or Ca+ - you would see that there is no change to the RMP therefore there is not movement of these ions across the membrane proving it is K+ that is causing the change in membrane potential

Question 20

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

Answer 20

• Using a voltmeter and two tiny electrodes you could measure the membrane potential of 5 cells bathed in a solution with varying concentrations of Na+.
• As the concentration of Na+ increases the membrane potential will become less negative - a result of the Na+ moving down it’s concentration gradient, into the cells, taking with it it’s positive charge and increasing the cells membrane potential.
• you would then assume that the movement of Na+ is what is causing the change in membrane potential
• to confirm this statement you would run the same experiment with other ions such as Cl- or Ca+ - you would see that there is no change to the RMP therefore there is not movement of these ions across the membrane proving it is Na+ that is causing the change in membrane potential

Question 21

Describe the relationship between ion concentration and membrane potential from the graphical representation of experimental data and be able to interpret the significance of the relationship.

Answer 21

As ion concentration (Na+ or K+) increases outside the cell, the membrane potential of the cell becomes less negative. Significant as it demonstrates the movement of these ions down their concentration gradient across the membrane in order to cause a graded potential until it reaches threshold (-65mV) and propagates an action potential

Question 22

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+.

Answer 22

(Refer to lab for week 3)

Use the shortened version - must have in and out of both ions, relative amount of both ions to one another, must use triple brackets in calculator, must always use +61 and not -61 for cations like Nernst

Question 23

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

Answer 23

Goldman equation takes into account all ions like sodium and potassium with their difference in permeability across the membrane, whereas Nernst equation considers only the membrane potential of one ion.

Question 24

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

Answer 24

Unmyelinated axons:

• propagation is simple
• presence of AP the membrane triggers opening of voltage-gated Na+ channels in the adjacent membrane.
• sensor detects rapid depolarisation and open as programmed.
• Result - Na+ rushes into the cell and initiates the action potential further along the axon
• close appostion of voltage-gated Na+ channels enable downstream to see the action potential in the adjacent membrane and effectively replicate it in the portion of membrane where they are located

conduction velocity of unmyelinated axons is relatively slow and typically in the range of 0.5 - 2.5 m.sec-1 in mammals.

Myelinated axons:

• wrapped in thick myelin sheath with gaps (nodes of Ranvier)
• AP propogation along gaps is relatively slow as in unmyelinated axons
• Heavy insulation formed by myelin sheath allow voltage-gated Na+ channels to detect the voltage of the AP in the adjacent node
• results in the AP jumping instantaneously from node to node (called SALTATORY CONDUCTION)

the conduction velocity of action potentials is very much quicker in myelinated axons (12 - 130 m.sec-1).

Question 25

Understand the difference between the absolute refractory period and the relative refractory period and the ionic-basis of each.

Answer 25

Refractory (meaning inactive) refers to the region of the axon that the AP has just passed through becomes inactive therefore the AP can only pass in one direction (like a cool down period).

• During refractory period it is difficult to initiate AP therefore AP can only go in one direction
• REFRACTORY PERIOD is when an AP of reduced amplitude is allowed to propagate as it is outside the absolute refractory period
• AP’s gradually get larger as they are stimulated further away from the absolute refractory period

ABSOLUTE REFRACTORY PERIOD:

• when a stimulus has been applied to the cell but fails to produce AP at all because the area of the cell hasn’t had enough time after the last AP has propagated through it.

RELATIVE REFRACTORY PERIOD:

• As we increase the delay between our two stimuli and move out of the absolute refractory period we see a small action potential that gradually increases in size as the delay increases. This period is known as the relative refractory period and is characterised by an action potential of reduced amplitude.
• caused by gradual recovery of the voltage-gated Na+ channels from inactivation - as more and more come out of inactivation more and more open up allowing more Na+ to flow into the cell

Question 26

What is responsible for the absolute and relative refractory periods?

Answer 26

It is Na+ channel inactivation that is responsible for the absolute refractory period and the progressive recovery from Na+ channel inactivation that results in the relative refractory period.

Question 27

Refractory periods have two important physiological consequences: What are they?

Answer 27

1. They ensure that action potentials travel along axons in one direction. Refractory periods mean that the part of the membrane where the action potential has just been is inactive for a short period of time.
2. They limit the action potential frequency in neurones. Stimuli that produce action potentials much above 300 Hz (300 action potentials per second) produce inter stimulus intervals that are starting to get into the refractory period and are therefore blocked.

Question 28

How do local anaesthetics work?

Answer 28

Local anaesthetics prevent action potential propagation by blocking voltage-gated sodium channels. In the presence of this blockage sodium is unable to enter the cell and so the depolarising phase cannot occur and the action potential is prevented from occurring.