INTS 8: Membrane Potentials

Question 1

Why does the membrane potential exist?

Give an example of some common membrane potentials and their concentration inside and outside cells

Answer 1

• the membrane potential exists due to the differences in concentration of ions on either side of the membrane

Question 2

Define diffusion

Answer 2

• the movement of molecules (or ions) down their concentration gradient until equilibrium is achieved

Question 3

Define the electrochemical gradient

Answer 3

• an electrochemical gradient shows an electrochemical potential difference
• allowing an ion to move across a membrane
• comprises of two parts:
• the chemical gradient: difference in concentration across the membrane
• the electrical gradient: difference in charge across a membrane

Question 4

How do neuronal membranes allow ions to diffuse across the membrane?

Answer 4

• neuronal membranes contain channels

Question 5

Describe the properties of membrane channels

Describe the different types of membrane channels

Look at images

Answer 5

• channels are proteins embedded in the neuronal membranes that are selectively permeable to one ion
• they allow passive diffusion of ions down their electrochemical gradient
• no energy is required
• two types:
  1. voltage-gated: open in response to changes in the membrane potential
  2. ligand-gated: open in response to binding of a chemical messenger or ligand

Question 6

What is the action potential and when and where can they be produced?

Answer 6

• the action potential is a specialised potential
• it can be produced in the neurons when the membrane potential reaches a certain level
• which is called the threshold potential

Question 7

What is the equilibrium potential?

Answer 7

• when the diffusion potential (or driving force) balances or opposes the tendency of ions to move down their concentration gradients
• the movement of ions across the membrane creates a diffusion potential
• each potential is unique to each ion and is determined by a set of ion concentrations on either side of the membrane

Question 8

Describe the general principle of the equilibrium potential for potassium (K⁺)

Answer 8

• there are two compartments, inside and outside, (check image) with sodium and potassium ions and a concentration gradient
• if a potassium channel is added, the potassium ions will be diffuse down the concentration gradient and move outside the cell
• a net negative charge develops inside the cell
• initially, the net force is diffusion down the concentration gradient, but as the inside fluid becomes more negative, the positive potassium ions are pulled back in by electrical force
• eventually the electrical force counterblances the diffusion force
• this is the equilibrium potential
• electrical and diffusion forces are equal and opposite
• no net movement of potassium ions into the channel
• the charge difference between the two sides is the equilibrium potential and for potassium, it is approx -80mV

Question 9

What is the resting potential?

Answer 9

-70mV (negative)

Question 10

List the five phases of the action potential

• how long does this take?

Answer 10

1. Rising phase
2. Peak phase
3. Falling phase
4. Hyperpolarisation
5. Refractory period
   - this whole process is rapid and takes around 4ms

Question 11

Describe how an action potential is generated, using Na⁺ as an example

Answer 11

• when the permeability of the membrane to ions, such as sodium ions, changes, the membrane potential is altered
• if the potential reaches the threshold, an action potential can be generated
• this threshold is around -55mV, less negative than resting potential
• this depolarisation occurs when sodium ion channels open and sodium ions move across the membrane into the cell
• this depolarisation is rapid and the membrane potential eventually reverses to about +20mV
• the inactivation of sodium channels prevents further influx of sodium ions
• the opening of potassium ion channels allow efflux of potasssium ions out of the cell, repolarising the membrane potential
• there is an overshoot when the membrane potential becomes more polarised than the resting potential and the potential eventually becomes resting state

Question 12

Why does the refractory period occur?

Explain absolute refractory period and relative refractory period

Use sodium and potassium ions as an example

Answer 12

• the refractory period is due to the inactivation of sodium ion channels and the speed with which potassium channels close
• while the sodium channels are inactivated, depolarisation will not open them
• when most of the channels are in this state, this is the absolute refractory period
• following this period, enough Na+ channels are in the active state and they are capable of being activated by depolarisation
• however, because the speed with which voltage-gated potassium channels close is slower, a large stimulus is required to induce a further action potential
• this is relative refractory period

Question 13

What are the distinct states of the voltage-gated Na+ channel?

When does each occur?

Answer 13

• closed: when the membrane is at the resting membrane potential
• open: during the depolarisation phase
• inactivated: after initial opening, the channels rapidly adopt a special inactive conformation, in which they cannot open even though the membrane is still depolarised

Question 14

Why does the pore of a channel (e.g. sodium ion channel) have an inactivated state when depolarised?

Answer 14

• once the membrane is depolarised, the channel opens, but the inactivated conformation is more stable
• so, after a brief period spent in the open conformation, the channel becomes temporarily inactivated and cannot open until the membrane is re-polarised again

Question 15

Once an action potential has started to progress, can it travel backwards?

Answer 15

• it has to continue in the same direction, travelling only forawrd from the site of polarisation
• the channel inactivation prevents the advancing front of depolarisation from spreading backwards

Question 16

What can increase the speed with which action potentials are transmitted along the axon?

Answer 16

• the presence of myelin
• it is wrapped around the axon

Question 17

Where is myelin produced?

Answer 17

• glial cells
• the Schwann cell in the peripheral nervous system
• one cell can myelinate only a single axon
• oligodendrocyte in the central nervous system
• can myelinate many axons

Question 18

What are the unmyelinated portions of myelinated axons called?

Answer 18

• node of Ranvier

Question 19

What diseases are loss of myelin a hallmark of?

Answer 19

• some neurodegenerative diseases
• e.g. multiple sclerosis

Question 20

Why do action potentials travel at great speed along myelinated axons?

What speed?

Answer 20

• due to saltatory conduction
• myelinated axons conduct speeds up to 120m/s
• unmyelinated axons conduct speeds of up to 5m/s

Question 21

Study these images on myelinated and unmyelinated axons

Answer 21

Question 22

Examine this image of the classification of nerve fibres by their size and conduction velocity

Answer 22

Question 23

What determines whether the post-synaptic potential is excitatory or inhibitory?

Answer 23

• the type of ion channel which opens on the post-synaptic membrane

Question 24

Describe excitatory post-synaptic potentials (EPSP)

Answer 24

• these post-synaptic potentials bring the membrane potential closer to the firing threshold by making it less polarised
• i.e. they increase the likelihood of an action potential being generated
• these are mediated by the influx of Na+ ions

Question 25

Describe inhibitory post-synaptic potentials (IPSP)

Answer 25

- these post-synaptic potentials take the membrane potential further away from the firing threshold by making it more polaised - i.e. they decrease the likelihood of an action potential being generated - these are mediated by the influx of Cl- ions

Question 26

What is temporospatial summation?

Answer 26

- a single post-synaptic potential will not bring the membrane potential to the firing threshold - but the cumulative effect of many potentials may generate an action potential - this is temporospatial summation - it relies on receiving synapses being close together and also receiving input within a short time frame

Question 27

Give examples of excitatory neurotransmitters

Answer 27

Question 28

Give examples of inhibitory neurotransmitters

Answer 28