Week 3 - Excitable Tissues

Question 1

T/F: it is important that there is effective communication between spatially separated cells, tissues, and organs

Failure of these communication systems can have quite ________ _________

Answer 1

True

Dramatic consequences

Question 2

What do some tissues synthesise and then secrete into the blood stream?

How do these get to their target tissue?

Answer 2

Hormones

Circulation via the blood stream

Question 3

Communication mediated by hormones is relatively slow?

Answer 3

Yes

Question 4

Which system does hormonal communication form the basis of?

Answer 4

Endocrine system

Question 5

How is electrical communication faster than hormonal?

Answer 5

electrical signals travel along cells and can be readily transferred from cell to cell.

These electrical signals travel very quickly and consequently constitute a much more rapid form of communication

Question 6

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

Answer 6

excitable tissues

Question 7

What are the major excitable tissues?

Answer 7

neurones and all three varieties of muscle (skeletal, cardiac and smooth).

Question 8

T/F: a number of other cells types have been shown to exhibit electrically excitable properties that underpin their physiology

Answer 8

True

Question 9

electrical activity in the nervous system determines things like…

T/F: Therefore, in many respects, excitable tissues really define what we, as humans, ‘are’.

Answer 9

consciousness, memory External link icon and personality.

True

Question 10

The resting membrane potential is…

Answer 10

the membrane potential that you observe in any excitable cell that is simply sitting around, minding its own business and not being influenced in any way by other cells (i.e. it is at rest).

Question 11

What is the usual magnitude of the membrane potential of cells?

Does it vary from cell to cell?

Answer 11

-80mV

Yes

Question 12

Define polarised in terms of a cell

Answer 12

Cell at rest is -80mV, more negative than outside therefore is polarised

Question 13

Graded potentials are observed wherever an…?

Answer 13

excitable tissue cell is subjected to an excitatory or inhibitory stimulus

Question 14

T/F: Graded potential effect the entire cell/produce global effects?

Answer 14

False - only effect part of the cell and produce local effects

Question 15

Two main types of graded potentials?

Answer 15

Depolarising graded potentials

Hyperpolarising graded potentials

Question 16

What is the difference between a graded potential and an action potential?

Answer 16

An action potential is a graded potential which was large enough to reach a value referred to as threshold (varies from cell to cell but is around -65mV)

Question 17

If a stimulus is too small to reach threshold what do we get?

Answer 17

A depolarising graded potential

Question 18

If the stimulus is big enough so that the depolarising graded potential reaches threshold then we get….?

Answer 18

An action potential.

Question 19

The speed of action potential propagation is fairly rapid and usually referred to as?

Answer 19

conduction velocity

Question 20

Different cells have different conduction velocities but these are usually in the order of…?

Answer 20

0.5 - 130 m.sec-1

Question 21

What is the secret to the very rapid form of communication that is enabled by excitable tissues?

Answer 21

The speed at which the electrical signals travel along cells

Question 22

What are action potentials travelling along excitable tissues responsible for?

Answer 22

every sensation, thought and movement we make because these large, fast changes in the membrane potential encode all the information that is communicated in nervous and muscle tissue.

Question 23

How do action potentials encode information?

Answer 23

The frequency of action potentials. (Frequency is usually expressed in Hertz (Hz) which means number of cycles per second. So when we talk about frequency in excitable tissues we are talking about the number of action potentials per second.)

Question 24

How do we distinguish between a large deformation in the skin and a small one?

Answer 24

Frequency of action potentials (large deformation will yield a higher frequency of action potentials than a small one)

Question 25

The changes in the membrane potential in excitable cells are all ultimately brought about by the diffusion of _______ across the plasma membrane.

Answer 25

ions

Question 26

How is diffusion of ions possible?

Answer 26

Diffusion is possible because there are marked differences in the intracellular and extracellular concentrations of a number of ions.

Question 27

Note that the concentrations of Na+, Ca2+ and Cl- are all much higher in the extracellular fluid whilst K+ has a much higher intracellular concentration - How is this maintained?

Answer 27

This situation is maintained by pumps such as the Na/K exchange pump that transports K+ into the cell and Na+ out of the cell against their concentration gradients using the energy derived from the breakdown of ATP.

Question 28

What type of channel is this?

Answer 28

Ligand-gated channel

Question 29

What type of channel is this?

Answer 29

Voltage-gated channel

Question 30

What type of channel is this?

Answer 30

Stretch-gated channel

Question 31

a small number of ion channels are described as __________ __________ (or passive channels) to indicate that they are not gated. Consequently these channels are open most of the time and are not significantly affected by the types of stimuli that open gated-channels

Answer 31

resting channels

Question 32

These resting channels are very important in the formation of the…?

Answer 32

Resting membrane potential

Question 33

At rest, the membrane of an excitable tissues cell is freely permeable to _____ because of the presence of _______ _______ that are selectively permeable to K+. These channels are not ______ and therefore are open all the time. Armed with this vital piece of information we should know be in a position to see why the resting membrane potential is ________.

Answer 33

K+

resting channels

gated

negative

Question 34

If we have a high concentration of K+ inside the cell and low concentration outside the cell where can we expect the K+ ions to flow from and to?

Answer 34

From inside to outside, down it’s concentration gradient

Question 35

What does the mass exodus of K+ ions out of the cell do to the electrical potential of the cell from rest?

What happens when the membrane potential gets to a certain point of negativity?

Answer 35

K+ ions take with them their positive charge leaving negative charge thus making the membrane potential more negative

Start to attract the ions back into the cell and is called an electrical potential

Question 36

Eventually a point of equilibrium is reached with an ion leaving or entering the cell where the _____________ ________ is exactly balanced by the ____________ ________

What does this then result in?

Why does this happen?

Answer 36

concentration gradient

electrical gradient

No net movement of ions occurs and the membrane potential stabilises

Ion concentrations do not become equal but the electrical gradient is sufficiently larfe to effectively balance the existing concentration gradient

Question 37

So the negative resting membrane potential is simply a consequence of fact that (at rest) the membrane is….?

Answer 37

Permeable to K+ ions and the concentration of K+ is much higher inside the cell than outside.

Question 38

If we know the concentrations of the ions inside and outside (that have a selectively permeable membrane channel) what do we use to calculate the what?

Answer 38

Nernst equation to calculate the potential at which equilibrium is reached (equilibrium potential)

Question 39

T/F: some neuroglia have a resting membrane potential that is exactly the same as the equilibrium potential for K+, but most excitable tissues cells have a resting membrane potential around -80 mV.

Answer 39

True

Question 40

What is the equilibrium potential for K+?

This is slightly less negative than the equilibrium potential for K+. How do we explain the 16 mV difference?

Answer 40

-96.9mV

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.

Question 41

In neurones, action potentials appear to be initiated at…?

Why?

Does this part of the membrane reach threshold first? What does this mean?

Answer 41

a region of the axon just adjacent to the axon hillock that is known as the axon initial segment

this part of the membrane has a lower threshold than the rest of the cell

Yes. Action potential is initiated here.

Question 42

What direction does the action potential travel in a neurone from the start?

Answer 42

travels down the axon, away from the soma and towards the axon terminals

Question 43

One of the variables that determines the conduction velocity of an axon is whether or not it has a _______ ________.

Answer 43

myelin sheath

Question 44

What experiment can you use to demonstrate the refractory period?

Answer 44

If you stimulate a neurone with a stimulus (S1) that is sufficiently intense to produce an action potential and then after a few milliseconds stimulate it again (S2) you observe two normal action potentials.

However if you reduce the interval (delay) between the two stimuli (using the adjacent +/- buttons) then you can observe what we refer to as refractory periods. Button to reduce the interval between the two stimuli in animation Button to increase the interval between the two stimuli in animation

(interactive animation showing the impact of changing the delay between two stimuli on the height of the second action potential)