Flashcards in Membranes and receptors 3 Deck (46):
What is the normal intracellular concentration of Na+?
What is the normal intracellular concentration of Cl-+?
What is the normal intracellular concentration of Ca2+?
What is the normal intracellular concentration of K+?
What is the normal extracellular concentration of Na+?
What is the normal extracellular concentration of Cl-?
What is the normal extracellular concentration of Ca2+?
What is the normal extracellular concentration of K+?
How is the resting membrane potential set up and maintained?
At rest the membrane has open K+ channels, so is selectively permeable to K+. K+ will begin to diffuse out of the cell down its concentration gradient. Since anions cannot follow the cell will become negatively charged inside. This membrane potential will oppose the outward movement of K+ and the system will come to an equilibrium.
What is meant by membrane depolarisation?
A decrease in the membrane potential from its normal value, so that the inside of the cell becomes less negative.
What is meant by membrane hyperpolarisation?
An increase in the membrane potential from its normal value, so that the inside of the cell becomes more negative.
List the main ion-specific channels in the plasma membrane:
How can the resting membrane potential of a cell be measured?
Using a very fine glass micropipette - a microelectrode - that can penetrate cell membranes. This is filled with conducting solution (KCl) and allows you to measure the difference in voltage between inside the cell and the extracellular fluid.
What are the range of values found for the resting membrane potential of cells? Which cells have the largest resting membrane potential?
In animal cells: -20mV to -90mV. Skeletal and cardiac muscle have the largest resting membrane potential.
What is selective permeability? How do cells achieve different selective permeabilities?
Selective permeability is the difference in permeability to ions that cell membranes exhibit. This depends on which type of channels are open in the membrane.
Define equiliubrium potential for an ion.
The membrane potential where the net flow through any open channels is 0. In other words, the chemical and electrical forces are in balance.
How do you calculate the equilibrium potential for an ion?
It can be calculated using the Nernst equation, using the intra- and extr-cellular concentrations of the ion.
Explain the mechanism that leads to depolarisation.
Depolarisation is when the membrane potential become less negative. This happens when the membrane permeability for ions with a positive equilibrium potential are increased relative to those with a negative equilibirum potential and therefore the membrane potential shift towards a more positive membrane potential.
Explain the mechanism that leads to hyperpolarisation.
Hyperpolarisation is when the membrane potential become more negative. This happens when the membrane permeability for ions with a negative equilibrium potential are increased relative to those with a positive equilibirum potential (as compared to that seen at resting membrane potential) and therefore the membrane potential shift towards a more negative membrane potential.
How can changes in ion channel activity lead to changes in membrane potential?
It alters the selective permeability of the membrane and therefore the flow of ions across the membrane.
List some examples of how membrane potential changes are used to signal within and between cells.
1. action potentials in nerve and muscle cells
2. Triggering and control of muscle contraction
3. Control and secretion of hormones
4. Transduction of sensory information into electrical activity by receptors
5. Postsynaptic actions of fast synaptic transmitters
How can ligand-gated channels give rise to synaptic potentials?
In fast synaptic transmission the receptors are ligand-gated ion channels. Therefore depolarising transmitters open these ion channels (Na+ or Ca2+ or several cation channels) and cause depolarisation of the synaptic membrane and triggering an excitatory post-synaptic potential (EPSP).
What is the range of resting membrane potentials that is seen in nerve cells?
-50 to -75mv.
List three characteristics of ion channels:
1. Selectivity - they only let through one (or a few) ion species. Channels selective for Na+, K+, Ca2+, Cl- and with non-selective cation permeability are known.
2. Gating - the channel can be open or closed by a conformation change in the protein.
3. A high rate of ion flow - this is ALWAYS down the electrochemical gradient for the ion.
Why is the resting membrane potential not the equilibrium potential of K+?
Although open K+ channels dominate the resting permeability of many cells, so the resting membrane potential is quite close to Ek, other types of channels are also open (the membrane is not perfectly selective) and so the resting membrane potential is a little less negative than -95mV.
In skeletal muscle the resting membrane potential is also highly permeable to another ion than K+. Which ion? Therefore where does the resting membrane potential lie?
Cl-. Therefore the resting membrane potential lies close to both Ek and Ecl.
What effect does increasing the extracellular concentration of K+ have on the equilibrium potential for K+ and what effect does this have on the resting membrane potential??
It makes the Ek more positive and therefore the resting membrane potential also becomes more positive.
What will be the effect on the membrane potential of opening Na+ or Ca2+ channels?
Depolarisation occurs -their equilibrium potentials are positive and therefore opening more of their channels will result in the membrane potential becoming closer to these positive equilibrium potentials and the membrane potential will become more positive.
What will be the effect on the membrane potential of opening K+ or (more usually) Cl- channels?
Hyperpolarisation - their equilibrium potentials are negative and therefore opening more of their channels will result in the membrane potential becoming close to these negative equilibrium potentials and the membrane potential will become more negative.
When channels for more than one ion species are open what determines how much each of the species will contribute to the membrane potential?
Depends upon the relative permeability for each ion. This depends upon:
1. The number of available channels for that ion
2. How easily these channels let the ion through
This can be approximated using the Goldman-Hodgkin-Katz (GHK) equation.
What are the two main ways in which channel opening/closing is controlled?
1. Ligand gating
2. Voltage gating
3. Mechanical gating
How does a ligand-gated mechanism open/close a channel?
Binding of a chemical ligand, which may be an extracellular transmitter or an intracellular messenger.
How does a voltage-gated mechanism open/close a channel?
It opens or closes in response to changes in membrane potential.
How can ligand-gated channels give rise to suppression of synaptic potentials?
Hyperpolarising transmitters can open ligand-gated channels (these channels are the receptor), selective for K+ or Cl-. These transmitters therefore lead to hyperpolarisation of the synaptic membrane and therefore inhibit synaptic transmission. This membrane potential change is called inhibbitory postsynaptic ptoential (IPSP)
How does slow synaptic transmission differ from fast synaptic transmission?
In slow synaptic transmission the receptors are not ligand-gated channels. Instead they signal to the channel via a GTP-protein. This causes a longer delay in synaptic transmission.
What are the two types of channel signalling that can occur in slow synaptic transmission?
1. GTP-binding protein moves within the membrane from the GPCR to the ion channel.
2. GTP-binding protein creates a signalling cascade which results in an intracellular messenger or protein kinase activating the ion channel.
What type of ligand-gated channels do GABA and glycine bind to on the synaptic membrane? What effect do they have?
They bind to ligand-gated chloride channels and therefore have a hyperpolarising inhibitory effect on post-synaptic transmission.
What is roughly the membrane potential of cardiac muscle?
What is roughly the membrane potential of nerve cells?
What is roughly the membrane potential of smooth muscle?
What is roughly the membrane potential of skeletal muscle?
Give an example of an ion channel that is not very selective:
Nicotine acetylcholine receptors. They have an intrinsic channel which is opened upon binding of ACh. This channel lets through Na+ and K+, but not anions. Na+ has a depolarising effect, whereas K+ has a hyperpolarising effect, therefore together they move the membrane potential towards 0mV (an intermediate between Ena and Ek).
How does a mechanical-gated ion channel operate?
It opens or closes in response to membrane deformation e.g. channels in mechanoreceptors: carotid sinus stretch receptors, hair cells.
List the different type of synaptic connections that can occur:
1. nerve cell-nerve cell
2. nerve cell- muscle cell
3. nerve cell - gland cell
4. sensory cell- nerve cell
Describe an excitatory post-synaptic potential.
It occurs over a longer time course than an action potential. The potential is graded with the amount of transmitter.