Pharmacology Part 6 Flashcards
Define depolarization?
the membrane potential becomes less negative (or even positive)
Define hyperpolarization?
the membrane potential becomes more negative
What causes change in membrane potential?
Ionic movement across the membrane
What does the direction of change in potential depend upon?
> The direction of the movement of the ion [into the cell (influx), or out of the cell (efflux)]
> The charge carried by the ion (positive, or negative)
What does movement of ions require?
requires ion channels (or transporters)
What is passive movement of an ion through a channel driven by?
Electrochemical gradient
What happens in response to the opening of cell membrane Na selective channels?
Na flows inwardly;
-concentration gradient is inward
- electrochemical gradient is inward
- the membrane potential is about -80mV which id nrgative to the equilibrium potential for Na which is +60mV
- driving force is Vm-Ena
What happens in response to opening of cell membrane K-selective channels?
K+ flows outwardly :
- concentration gradient is outward and has an energy which exceeds that of the electrical gradient, which is inward
- Quantitatively, the membrane potential (Vm, approximately -80 mV for many neurones) is positive to the equilibrium potential for K+ (i.e. EK, about -100 mV) given by the Nernst equation.
- The driving force for K+ efflux is simply (Vm - EK). When positive (in this case +20 mV, note the double negative) outward movement of K+ occurs
Describe the action potential in neurones?
brief electrical signals in which the polarity of the nerve cell membrane is momentarily (about 2 msec) reversed
How to action potentials propagate along neurones?
propagate along nerve cell axons with constant magnitude and velocity (for a given axon) allowing signalling over long distances
When are neurone action potentials generated?
when threshold is reached. They are ‘all or none’
What is the ionic basis for action potential?
Inflow of sodium causes hyperpolarization and outflow of K depolarisation
Are Na+ and K+ channels selective?
Both are highly selective
How are Na+ and K+ channels activated?
activated by membrane depolarization – Na+ channels rapidly; K+ channels with a slight delay
Describe the activation of Na+ channels?
self-reinforcing – opening of a few channels causes further channels to open, causes increased depolarization
This is positive feedback
Describe the activation of K+ channels
self-limiting –
outward movement of K+ causes repolarization which turns off the stimulus for opening
This is negative feedback
What happens to Na channels during depolarisation
initially open rapidly in response to depolarization, they quickly enter a non-conducting, inactivated state during maintained depolarization
What is the importance of depolarisation?
required for the channel to enter the closed state in readiness for opening and the generation of a further action potential
What does inactivation of Na channels contribute to?
repolarizing phase of the action potential and is responsible for the absolute refractory period
What is the absolute refractory period?
no stimulus, however strong, can elicit a second action potential (all Na+ channels inactivated)
What is the relative refractory period?
a stronger than normal stimulus may elicit a second action potential (mixed population of inactivated and closed channels, plus the membrane is hyperpolarized)
What is the issue with conducting impulses in axons?
nerve cell membrane is ‘leaky’ (i.e. not a perfect insulator): passive signals do not spread far from their site of origin due to current loss across the membrane
Describe passive conduction and action potential velocity?
factor in the propagation of the action potential (AP)
The longer the length constant (λ) the greater the local current spread
Greater local current spread increases AP conduction velocity
What are the strategies to increase passive current spread?
> Decrease ri (axial resistance of the axoplasm) by increasing axon diameter
> Increase rm (membrane resistance ) by adding insulating material (myelin) provided by Schwann cells in the PNS and oligodendrocytes in the CNS)
What kind of axons have the fasted conduction?
Myelinated axons is much faster than nonmyelinated axons of same diameter
What is saltatory conduction in myelinated axons?
Action potential jumps from one node of ranvier to the next via voltage activate Na channels
