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Flashcards in Membrane Potentials Deck (23):

Describe movement of Na, K and Cl in a typical mammalian cell.

Na out
K in
Cl out


What membrane protein dominates ionic permeability at rest?

K+ channels


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


Describe local current theory.

Positive charge repels other positive charges, spreading ions along the axon in both direction. Doesn't depolarise to the same as the original action potential.

Causes propagation of the action potential. The local current spread activates the next action potential.


Describe what the length constant is.

The distance that an action potentials travels before the potential different reduces to 37% of its original value.


What does the value of the length constant depend on?

Membrane resistance
Axon radius


Describe how voltage gated channels open

A change in membrane potential causes a conformational change to the protein, moving the positive residues which opens the pore.


During the spread of ions along the axon it initiates smaller peaks with a shorter upstroke. Why is the upstroke shorter?

Potassium channels are still open which causes increased resistance and decreased length constant.


Why can an action potential not move from right to left, even though the spread of ions is in both directions?

Inactivated sodium channels (refractory period)


Describe nodes of Ranvier

Approximately 1mm long.
Increased density of sodium channels enables the spread of local current further.
Very few channels are beneath the myelin.


Describe the difference in relation between velocity and diameter in myelinated and unmyelinated axons.

Myelinated: velocity is proportional to diameter.
Unmyelinated: velocity is proportional to the square root of diameter.