Membrane Potential

Question 1

What is membrane potential?

Answer 1

an electrical potential (voltage) difference across their plasma membrane

Question 2

What is the purpose of a membrane potential?

Answer 2

Membrane Potential provides the basis of signalling in the nervous system as well as in many other types of cells

Question 3

What is the membrane potential of cardiac and skeletal cells?

Answer 3

-80mv to -95 mV

Question 4

What is the membrane potential of nerve cells?

Answer 4

– 50 to – 75 mV

Question 5

How is resting potential set up?

Answer 5

For most cells, open K+ channels dominate the membrane ionic permeability at rest. There is chemical diffusion out of the cell by K+ ions, until electrical gradient becomes large enough for K+ to move back in down electrical gradient. Equilibrium reached

Question 6

Why does membrane potential differ in cardiac muscle and nerve cells compared to values worked out from the Nerst Equation?

Answer 6

Resting potential close to E(k) but not exactly due to membrane not being perfectly selective for K+

Question 7

Why does membrane potential differ in smooth muscle and skeletal muscle cells compared to values worked out from the Nerst Equation?

Answer 7

Smooth muscle cells: Lower selectivity for K+ and increased contribution from other channels

Skeletal muscle: Many Cl- and K+ channels open in resting membrane so resting potential close to both E(Cl) and E(k)

Question 8

How do changes in membrane potential underlie many forms of signalling between and within cells?

Answer 8

1. Action potentials in nerve and muscle cells
2. Triggering and control of muscle contraction
3. Control of secretion of hormones and neurotransmitters
4. Transduction of sensory information into electrical activity by receptors
5. Postsynaptic actions of fast synaptic transmitters

Question 9

What is depolarisation?

Answer 9

cell interior becomes less negative e.g. a change from – 70 mV to – 50 mV

Question 10

What is hyperpolarisation?

Answer 10

Cell interior becomes more negative e.g. a change from – 70 mV to – 90 mV

Question 11

What are the equilibrium potentials for the common physiological ions?

Answer 11

K+ = - 95 mV

Cl- = - 96 mV

Na+= + 70 mV

Ca2+= + 122 mV

Question 12

How do nicotinic acetylcholine receptors work?

Answer 12

1. Have an intrinsic ion channel
2. Opened by binding of acetylcholine
3. Channel lets Na+ and K+ through, but not anions
4. Moves the membrane potential towards 0 mV - intermediate between the equilibrium potential of sodium and potassium

Question 13

What is ligand gating?

Answer 13

-The channel opens or closes in response to binding of a chemical ligand e.g. channels at synapses that respond to extracellular transmitters or channels that respond to intracellular messengers

Question 14

What is voltage gating?

Answer 14

Channel opens or closes in response to changes in membrane potential e.g. channels involved in action potentials

Question 15

What is mechanical gating?

Answer 15

Channel opens or closes in response to membrane deformation e.g. Channels in mechanoreceptors: carotid sinus stretch receptors, hair cells

Question 16

Describe how the ears hair cell stretch receptors work

Answer 16

1) Potassium channel closes in cuticular gate
2) Membrane depolarises
3) Calcium channel opens
4) Release of neurotransmitter which goes on to fuse to basement membrane to afferent nerve
5) Neurotransmitter binds to receptor at post synaptic plate and generates an action potential that goes to the CNS for interpretation

Question 17

What is fast synaptic transmission?

Answer 17

In fast synaptic transmission, the receptor protein is also an ion channel and transmitter binding causes the channel to open

Question 18

Explain how excitatory synapses work?

Answer 18

Excitatory transmitters open ligand-gated channels that cause membrane depolarization.

They can be permeable to Na+, Ca2+, sometimes cations in general (nAChR).

The resulting change in membrane potential is called an Excitatory post-synaptic potential (EPSP)

Question 19

How are excitatory post-synaptic potentials different to action potentials?

Answer 19

1. Longer time course than AP (around 20ms)
2. Graded with amount of transmitter
3. Transmitters include Acetylcholine and Glutamate

Question 20

What are inhibitory synapses?

Answer 20

• Inhibitory transmitters open ligand-gated channels that cause hyperpolarization
• Permeable to K+ or Cl-
• Transmitters include Glycine, GABA

Question 21

What is direct G-Protein gating?

Answer 21

• localised and quite rapid
  1) ion binds to receptor
  2) lateral diffusion of G-protein to channel
  3) binding of G-protein to channel
  4) channel opens

Question 22

What is gating via an intracellular messenger?

Answer 22

1) ion binds to receptor
2) G-protein binds to enzyme in membrane
3) Signalling cascade activated
4) Intracellular messenger or protein kinase activated
5) Causes channel to open -effects throughout cell -amplification by cascade

Question 23

How does extracellular K+ concentration influence membrane potential?

Answer 23

extracellular K+ concentration is ~4.5 mM normally. Sometimes altered in clinical situations. Can alter membrane excitability, e.g. in heart

Question 24

How do electrogenic pumps influence membrane potential?

Answer 24

Na/K- ATPase is indirectly is responsible for the entire membrane potential, because it sets up and maintains the ionic gradients