Session 4 - Resting and Changing Membrane Potenatials

Question 1

Define a membrane potential

Answer 1

The electrical charge that exists across a membrane and is always expressed as the potential inside the cell relative to the extracellular solution

Question 2

What the membrane potentials for common excitable cells?

Answer 2

Skeletal Muscle Myoctyes -90mV
Cardiac Myocytes -80mV
Neurones -70mV
Smooth Muscle Myocytes -50mV

Question 3

What make the cell membrane selective?

Answer 3

The presence of selective transport proteins that control the flow of molecules (Particularly ions, if in relation to membrane potential)

Question 4

How is the resting potential setup?

Answer 4

Na+/K+ ATPase sets up the high [K+] internal. For most cells, open K+ channels dominate the membranes ionic permeability at rest and as such the K+ ions diffuse out along there concentration gradient. However, inside the cell there is a high concentration of A- ions (anions) which create a negative charge inside the cell. As K+ moves out of the cell the inside becomes more negative. Gradually the movement of K+ reaches an equilibrium and the concentration gradient is equal to the electrical gradient so there is no net movement

Question 5

What is the equilibrium potential and how is it calculated?

Answer 5

The equilibrium point at which the electrical and chemical gradients are balanced this is calculated via the Nernst Equation.

Question 6

Why isn’t the resting potential -95mV as this is the Ek value (equilibrium potential of K+)?

Answer 6

The membranes aren’t perfectly selective to K+ and as such some other ions are able to cross the barrier such as Na+ in small quatities. Generally speaking the lower the potential the lower the selectivity for K+

Question 7

Give 5 examples of how changing membrane potentials underlie many forms of signalling

Answer 7

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 actions by receptors
5) Postsynaptic actions of fast synaptic transmitters

Question 8

Define Depolarisation

Answer 8

A decrease in the size of the membrane potential from its normal value. (less negative)

Question 9

Define Hyperpolarisation

Answer 9

An increase in the size of membrane potential from its normal value (more positive)

Question 10

Define repolarisation

Answer 10

Increase in the membrane potential so the cell becomes less positive

Question 11

What effect does changing membrane permeability have on the membrane potential? What effect do K+/Cl-/Na+/Ca2+ have on it?

Answer 11

By increasing the membrane permeability to a particular ion it moves the membrane potential towards the equilibrium potential for the ion.
IE:
K+ and Cl- = Hyperpolarisation
Na+ and Ca2+ = Depolarisation

Question 12

What are the 3 main types of gating for ion channels?

Answer 12

Ligand gating
Voltage gating
Mechanical Gating (mechanoreceptors)

Question 13

What is the difference between fast synaptic transmission and slow synaptic transmission?

Answer 13

Slow synaptic transmission is where the receptor and channel separate proteins and as such transmission involves a GPCR protein and a messenger protein. Whereas in fast synaptic transmission the receptor (binding site) is on the channel.

Question 14

What happens at an excitatory synapse and give some examples of transmitters?

Answer 14

excitatory transmitters open ligand gated channels that cause membrane depolarisation. An Excitatory Post-Synaptic Potential (EPSP)
This can be graded through the amount of transmitter
ACh and Glutamate

Question 15

What happens at an inhibitory synapse and give some examples of some transmitters.

Answer 15

Transmitters open ligand gated channels that cause membrane hyperpolarisation. IE. permeable to K+ and Cl-. This creates an Inhibitory Post-Synaptic Potential (IPSP).
Examples: Glycine and GABA

Question 16

What are the two main types of slow synaptic transmission?

Answer 16

1) Direct G Protein gating: This is where the g protein stimulated the channel protein. It is localised and quite rapid
2) G Protein via intracellular messenger/protein kinase: This is where the G protein stimulates an enzyme which in turn has a cascade ultimately stimulating the Channel protein. It occurs throughout the cell and amplification can occur via a cascade

Question 17

What other factors influence membrane potential?

Answer 17

1) Changes in ion concentration (such as in hyperkalaemia makes membrane potential more positive)
2) Electrogenic pumps (Na+/K+ ATPase)