Session 3

Question 0

Explain the concept of selective permeability and explain how the selective permeability of cell membranes arises

Answer 0

Permeability of membranes to ions occurs by way of channel proteins.
The ion channels are characterised by selectivity (lets through one or a few ion species), gating (open or closed by a conformational change), and down the electrochemical gradient for that ion.
Depending on what types of channel are open, resting membrane potential can be selectively permeable to certain ion species.

Question 1

Outline what a membrane potential is, how the resting potential of a cell may be measured and the range of values found

Answer 1

Membrane potential - electrical potential difference across the membrane, voltage of the inside of the cell relative to the outside
Measured by a micropipette (a microelectrode) that penetrates the cell membrane
Animal cells: -20mV - -90mV (largest in skeletal and cardiac)
Nerve cells: -50mV - -75mV

Question 2

Describe how the resting potential is set up given the distribution of ions across cell membranes

Answer 2

At rest the membrane has open K+ ion channels so is selectively permeable to K+. K+ will diffuse out of the cell making the inside negatively charged. This membrane potential will oppose the outward movement of K+ and the system will come into equilibrium.

Question 3

Explain the term equilibrium potential for an ion and calculate its value from the ionic concentrations on either side of the membrane

Answer 3

E(ion) - theoretical value that states what the membrane potential would be if perfectly selective to that ion (electrical and diffusional forces balance)
Nernst equation:
V - membrane potential, R - gas constant (8.314), T - temperature in K (310), Z - valency, F - Faraday’s number (96500), [ion]o - outside concentration of ion, [ion]i - inside concentration of ion

Question 4

Define depolarisation and hyperpolarisation, and explain the mechanisms that may lead to each of these

Answer 4

Depolarisation - a decrease in membrane potential, less negative
Hyperpolarisation - an increase in membrane potential, more negative
Depolarise - open Na+ or Ca2+ channels
Hyperpolarise - open K+ or Cl- channels

Question 5

Explain how changes in ion channel activity can lead to changes in membrane potential and outline some of the roles of the membrane potential in signalling within and between cells

Answer 5

When channels for more than one ion species are open these ions will contribute to the membrane potential.
Channel opening is controlled by ligand gating (open or close by binding of a chemical ligand), voltage gating (open or close in response to changes in voltage) and mechanical gating (open or close in response to membrane deformation)

Question 6

Outline how ligand-gated channels can give rise to synaptic potentials

Answer 6

Fast synaptic transmission - receptor is also a ligand gated ion channel –> excitatory post synaptic potential (EPSP) - channels selected for Na+ or Ca2+, or inhibitory post synaptic potential (IPSP) - channels selected for K+ or Cl-
Slow synaptic transmission - receptor is not itself an ion channel but signals to the channel involving a G-protein coupled receptor –> within the membrane or by way of an intracellular messenger