biology and physiology Flashcards
what are action potentials
Differences in ion concentrations across the nerve cell membrane provide the potential energy required to transmit nerve impulses.
plasma membrane potential is due to…
due to the separation of electrical charges across the cell membrane
What allows charge separation across the plasma membrane to happen
action of the NA=/k+ atoase activekly transporting na and k+ in different directions across the membrane
na/k ATpase role
replenish the K+ lost from the cell
remove the Na+ accumulated within the cell
Na+/K+ ATPase mechanism
- Binding of cytoplasmic Na+ to the protein stimulates phosphorylation by ATP.
- Phosphorylation causes the protein to change the conformation.
- The conformational change expels Na+ to the outside and extracellular K+ binds.
- K+ binding triggers release of a phosphate group.
- Loss of phosphate restores original conformation.
- K+ is released and Na+ sites are receptive again. The cycle repeats.
Role= replenish the K+ lost from the cell
=remove the Na+ accumulated within the cell
resting membrane potential
Due to the relatively high membrane permeability of K+.
Inside and outside the cell= electrical neutrality
Outside Na+ balanced mainly by Cl-.
Inside K+ balanced mainly by A-.
Other anions present= PO4-, Cl-
leak channels
negatively charged ions line up on the other side of the membrane
K+ leaks out
separation of charges constitues to the membrane potential
Electrochemical equilibrium
Concentration gradient tends to drive K+ out of the cell but the negative charge inside attracts K+ back in.
Equilibrium is established where the electrical potential balances the chemical potential.
chemical gradient can be balanced by an electrical potential
what produces chemical force and what is this equal and opposite to in order to reach equilibrium potential
chemical force produced by the concentration gradient
chemical force is equal and opposite to THE ELECTRICAL FORCE EXPERIENCED BY AN ION VIA A VOLTAGE
nerst equation
enrst equation describes equilibrium potential
The voltage across the membrane is proportional to the ratio of the ion concentrations on either side of the membrane.
goldmans equation
a more realistic approxmatuon of memrbane potential
Effect of external K+ concentration on membrane potential of skeletal muscles
Increase in external potasiunnion concentration increases membrane potential
Doing the log of external potassium concentration with membrane potential give a linear relationship which suggests that
Increasing extra cellular concentration depolarises the cell
what controls the membrane potential
changes in ion permeability due to opening and closing of protein ion channels in the membrane
neuronal signalling
Rapid changes in membrane potential (action potential)
Neuronal firing
Rapid depolarization caused by opening of voltage gated cation channels.
4 types of ion channels:
- voltage gated
- Ligand gated (extracellular ligand)
- Ligand-gated (intracellular ligand)
- Stress activated
Initiating an AP
Initial change in membrane potential (depolarization) is required.
Threshold comes from the opening of small capacitance ligand-gated cation channels.
(Nicotinic acetylcholine receptors at skeletal NMJ,
5HT3 receptors and P2X at CNS synapses +smooth muscle)
what causes the initial depolarization
Opening of small capacitance cation channel.
Graded responses.
stages of an action potential
- if local potential change;graded potential reaches the threshold potential na channels open
na channels open and influx of calcium causing depolarization
repolarization-na channel closes and efflux of potassium channels to reverse the increase in membrane potnetial
hyperpolarization-K+ channel remain open after potential reaches resting level
oubain
na/K+ atpase inhibitor
Reduces the size of the AP progressively until the membrane Na+ gradient is reduced to the point where the APs can be initiated but fail.
tetrodotoxin
blocks neuronal VGSC
abolishes AP
action potential refractory period is due to….
due to the inactivation of voltage gated na channels
during this period, another stimulus given to the neuron (no matter how strong) will not lead to a second action potential
Propagation of action potential along an axon. why propagation goes forward and not backwards
At rest the membrane is polarised.
Depolarization due to AP sets up local circuit currents in both directions.
In ‘backwards’ direction, Na+ channels are in refractory phase and another AP cannot be generated.
AP moves forward.
speed of conductance
regulated by
Temp
Axon diameter [thicker the axon, lower the longitudinal resistance, faster the conductance.]
[membranes have high capacitance= charge storage]
myelination
what is greater at increasing the speed of conductance myelination or increasing diameter
insulating the axon with myelin is a more efficient way to increasse AP conduction velocity than increasing diameter
Glia
Oligodendocytes wrap multiple CNS axons with myelin.
nn cells apply myelin to single peripheral axons.
saltory conductance
is the propagation of action potentials along myelinated axons from one node of Ranvier to the next node, increasing the conduction velocity of action potentials.
