*Pharmacology - 3 (5) Flashcards
What is depolarisation?
The membrane potential becomes less negative (more positive)
What is hyper polarisation?
Membrane potential becomes more negative
What in general causes changes in membrane potential?
Ionic movements
Name for movement of ion into a cell?
Influx
Name for movement of ion out of a cell?
Efflux
What drives the passive movement of an ion through an ion channel?
The electrochemical gradient
What are required to allow movement of ions across membranes?
Ion channels (Transporters)
What aids the movement of Na+?
Sodium-selective channels
In what direction does Na+ flow?
Why? (3)
Inwardly
The concentration gradient is inwards
The electrical gradient is inwards
The membrane potential is negative to the equilibrium potential for Na+
What is the membrane potential?
The difference in electrical potential between the interior and anterior aspects of the cell
What is the approximate membrane potential for many neurones?
-80mV
What is the equilibrium potential?
membrane potential where the net flow through any open channels is 0.
What is the driving force for Na+ influx equal to?
(Vm - Ena)
Since the driving force for Na+ is negative, in what direction does Na+ move?
Inwards
What is conductance?
The degree to which an object conducts electricity
Ina = (sodium current)?
Ina = gNa (Vm - Ena)
Na+ current = Na+ conductance X driving force
What enables the movement of K+?
Potassium-selective channels
In what direction does K+ flow?
Why? (2)
Outwards
the concentration gradient is outwards (has an energy that exceeds that of the electrical gradient (which is inwards))
The membrane potential is positive to the equilibrium potential for K+
What is the equilibrium potential of Na+?
+60mV
What is the equilibrium potential of K+?
-100mV
What is the driving force of K+ efflux equal to?
(Vm - Ek)
In terms of the driving force, why does K+ move inwards?
The driving force is positive (+20mV)
Does K+ channel opening cause hyper polarisation or depolarisation?
Hyperplorisation
Does Na+ channel opening cause hyper polarisation or depolarisation?
Depolarisation
What are some factors that can open gated ion channels?
Membrane voltage (voltage-gated ion channels) Chemical substances (ligand-gated ion channels) Physical stimuli e.g. mechanical, thermal
What are the 2 ion channels responsible for the action of action potential in neurones?
Voltage-activated Na+ channels (Nav) - depolarising
Voltage-activated K+ channels (Kv) - hyper polarising
What side of a cell membrane (inside or outside) is positively and negatively charged?
Inside = negative Outside = positive
How many glycoprotein subunits make up the K+ channels?
4
How many glycoprotein subunits make up the Na+ channels?
1 (acts as if there is 4 separate parts)
When is an action potential?
the change in electrical potential associated with the passage of an impulse along the membrane of a muscle cell or nerve cell.
When does an action potential occur in a neurone?
When a neutron sends information down an axon
What are the parts of the action potential in a neurone?
Resting potential ->
Stimulus causes the sensory nerves to depolarise to threshold (this is when the action potential begins)
Upstroke
Downstroke
What is the positive part of an action potential called?
Overshoot
What is the negative part of an action potential called?
Undershoot
What causes undershot in neurones?
The potassium channels do not cause instantaneously after the action potential
What happens to the polarity of the nerve cell membrane during action potentials?
it is reversed momentarily (about 2msec)
What allows signals along a long distance of a neurone?
Action potential propagate along nerve cell axons with constant magnitude and velocity
What prolongs for longer during an action potential, sodium or potassium conductance?
Sodium conductance
Are cations positive or negative?
Positive
What activates Na+ and K+ channels?
Membrane depolarisation
Are Na+ or K+ channels activated quicker?
Na+ channels are activated rapidly
K+ channel are activated with a slight delay
Is the activation of na+ channels positive or negative feedback?
Positive - the opening of a few channels causes further channels to open causing further depolarisation
Is the activation of k+ channels positive or negative feedback?
Negative feedback - outward movement of K+ causes depolarisation which turns off the stimulus for opening
What are the 3 possible states of a Na+ channel and what causes the occurrence of each?
Open state - triggered by depolarisation
Inactivated state - triggered by maintained depolarisation non-conducting)
Closed state - repolarisation (is able to be reopened)
What 2 gates does the voltage-activated Na+ channel contain?
The activation gate
The inactivation gate
What is the refractory period?
a period immediately following stimulation during which a nerve or muscle is unresponsive to further stimulation.
What is the absolute refractory period?
The period during repolarisation were no stimulus, however strong, can elicit a second action potential as all Na+ channels are inactivated
What is the relative refractory period?
The period were the action potential undershoots and a stronger that normal stimulus may elicit a second action potential (mixed population of inactivated and closed channels)
When does passive potential occur?
When the depolarisation does not reach threshold - membrane is temporarily depolarised but this does not reach threshold meaning an action potential is not produced, the current spreads passively to adjacent parts of the membrane but decays with time and distance
why do passive signals not spread far from their site of origin?
The nerve cell membrane is “leaky” and therefore current is lost across the membrane
What is the length constant (λ)?
a mathematical constant used to quantify the distance that a graded electric potential will travel along a neurite via passive electrical conduction
How is passive conduction related to the action potential?
Has a roll in propagating the action potential
How is the length constant related to the local current spread?
The longer the length constant the greater the local current spread
How does the local current spread impact not he AP conduction velocity?
Greater local current spread increases AP conduction velocity
Strategies to increase passive current spread (and thus action potential velocity)? (2)
Decrease axoplasm resistance (ri) - possibly by increasing axon diameter Increase rm (membrane resistance) - possibly by adding insulating material e.g. myelin
What cells in the PNS produce the myelin sheath?
Schwann cells
What cells in the CNS produce the myelin sheath?
Oligodendrocytes
What is the area between myelin sheaths on an axon called?
Nodes of Ranvier
What is saltatory conduction?
conduction in which the nerve impulse jumps from one node of Ranvier to the next.
What type of conduction occurs in unmyelinated axons?
Passive spread
What type of conduction occurs in myelinated axons?
Saltatory conduction
