Membrane Potentials Flashcards
What membrane protein dominates ionic permeability at rest?
K+ channels
Describe movement of Na, K and Cl in a typical mammalian cell.
Na out
K in
Cl out
Why does smooth muscle have a lower membrane potential than nerve or cardiac muscle cells?
Cardiac muscle and nerve cells are near Ek because the K+ channel predominates, but their membranes aren’t perfect.
Smooth muscle has a lower selectivity for potassium as they have a larger contribution from other channels. (-50mV).
Describe some of the functions of changing membrane potentials.
Action potentials in nerves and muscles
Trigger and control muscle contraction
Secreting hormones and neurotransmitters
Transduce sensory to electric activity by receptors
Postsynaptic action of fast synaptic transmission
Describe the difference between depolarisation and hyperpolarisation.
Depol - decreased membrane potential from normal, less negative interior. E.g. Movement of K/Cl
Hyperpol - increased membrane potential from normal, more negative interior. E.g. Movement of Na/Ca
Describe the different types of gated channels.
Ligand gated - can respond to intracellular ligands e.g. IP3 in the endoplasmic reticulum (second messenger) which activates a calcium channel.
Voltage-gated - action potentials.
Mechanical - sensitive to a degree of stretch e.g in the carotid sinus.
Describe the difference between a fast and slow synapse.
Fast - e.g. Nicotinic receptors. Are ion channels.
Causes mild depolarisation for approximately 20ms, graded with the amount of neurotransmitter. ESPS e.g. Glutamate (calcium) or acetylcholine (sodium). ISPS e.g. Glycine (spinal cord)/GABA (chloride)
Slow - receptors and channels are different proteins. E.g. G-coupled receptors
Direct G-coupled gating is localised and quite rapid.
Or gating can be via an intracellular messenger.
The signal can then be spread through the cell, amplified by a cascade
What feature of a membrane does conductance rely on?
The number of open ion channels
Describe the change to membrane proteins in an action potential and the ion movement that accompanies this.
(Diagrams are ok)
Depolarisation to the threshold initiates an action potential by opening sodium channels, causing sodium to enter the cell. This creates a positive feedback loop, with more depolarisation causing more Na channels to open. They then inactivate, causing a sharp drop in sodium moving into the cell.
Voltage gated potassium channels have delayed opening to stay open after returning the membrane potential. As K channels close the potential returns to rest.
Where in a neuron is there the highest density of ion channels?
Axon hillock
Describe the absolute refractory period
Nearly all sodium channels are in the inactivated space, so it is impossible to make another action potential.
Describe the relative refractory period.
Sodium channel recovery. Excitability returns to normal as the number in the inactivated state decreases. An action potential can be stimulated if enough channels have recovered.
Describe accommodation.
If a cell depolarises sufficiently slowly it will pass the threshold without firing an action potential.
The longer a stimulus takes, the larger the depolarisation necessary to initiate an action potential.
A long stimulation makes the peak less positive because sodium channels aren’t open long enough to enter the feedback loop but still become positive.
Describe the basic structure of a sodium channel.
Pore in the centre
S4 voltage sensor with positive residues
One subunit
Describe the basic structure of a potassium channel
Pore in the centre
S4 voltage sensor with positive residues
Four subunits
