W2 Excitable cells Flashcards
Nervous systems
System of communication that allows an organism to react rapidly and modifiable to changes in its environment.
The nervous system collects information and then outputs the information to the next level of information processing, into the decision-making parts of the brain.
Sensory nerve
senses the temperature of the surface, transfers information to the spinal cord then the information goes to the motor neurons and body reacts. Two meters of transfer, 1 on the sensory nerve, 1m down the motor nerve.
Electical activity provides
a rapid, reliable and flexible means for neurons to receive integrate and transmit signals. Chemical messengers and receptors between and within cells provide much more flexibility (for inhibition).
Graded potential
variable size, local signals not propagated over long distance, they generated on the dendrite, short axon, so they don’t need an action potential. They can travel both ways down the neuron, however they tend to only go one way. Graded potential code by size, vary by the strength of the stimulus.
Action potentials
= fixed size, all or nothing signals that travel along the axon, can only travel one way down the axon. Action potentials code by frequency, as they are of a unit size.
Resting membrane potential
= Negative resting potential is an absolute requirement for a functioning nervous system.
= Resting potential is caused by a selectively permeable membrane, unequal distribution of charged molecules/ion, physical force. Higher concentration of sodium outside.
1-Diffusion
Ions in solution are in constant motion and tend to distribute themselves evenly so that there is net movement of ions from regions of ‘high’ concentration to regions of ‘low’ concentration.
Selective and unequal membrane
Channels confer slectivity and pumps assist unequal change distribution
2- Electrical
electrical field cause ions to move opposite charges attract and like charges repel. Because Ions are charged movement of ions gives rise to an electric current. How much current will flow dependent upon = electrical potential (voltage), electrical conductance, relative inability of an electrical charge to migrate
Sophisticated channel
Selectivity, unequal distribution, channesl are passive and pumps are active thye need energy (ATP)
Ion pumps
in the membrane set up the ionic concentration gradients found in neurons. Important Ion pumps = Na+/K+ ATPase, Ca2+ pumps (not just in the plasma membrane). Without ion pumps, the resting membrane potential would not exist and the brain would not function. But the effect of pump inhibitors takes some time to work.
Na+/K+ channels
exchanges internal sodium for extracellular potassium, notice it is moving these ions against their concentration gradients and therefore it requires energy (provided by the breakdown of ATP) to do this, this pump probably uses up ~70% of ATP in the brain!!! -Ca pump transports Ca out of neurones, maintaining low intracellular ca is important because (1) Ca is a signalling ion, changes in ca concentration are detected by many proteins/enzymes and are used to control various cellular functions, (2) high intracellular Ca is toxic, kills neurones.
Equilibrium potentials
Ionic gradients influence membrane potential by determining equilibrium potentials Eion. Eion is the membrane potential that would be achieved in a neuron if the membrane were selectively permeable to that ion. Concentration gradient + Electrical force equal = equilibrium electrostatic forces.
Nernst Equation
= used to calculate the equilibrium potential (Eion) for an ion. Takes into account concentration of ion outside and inside the cell.
Eion = 2.303 (RT/zF) x log ( (ion)o / (ion)i
Goldman equation
Neurons do not have resting Vm et Eion for K+. The resting membrane is also permeable to others ion (Na+). To estimate real Vm you need the Goldman.
Vm = 61.54 mV log (Pk[K+]o + Pna [Na+]o)/(Pk [K+]i + Pna[Na+]i)
Differente faces of action potentials
1-Rising phase = rapid depolarization of the membrane.
2-Overshoot = where membrane potential is above 0, In this case +40mv.
3-Faling Phase = Phase-rapid ‘repolarization’ of the membrane, note that it goes more –ve than the starting resting membrane potential.
4-Undershoot or ‘after hyperpolarization’ = gradually ‘declines’ so that membrane potential comes back to ‘resting’ levels.
Properties of the Action potential
Transient, rapid adn reversible change in membrane potential from -ve to +ve. Different types of excitable cell may have different types of action potential. Neuron AP is often triggered by Na+ permeability increase.
AP’s or ‘spikes’ generated by a cell. = all of the same size and duration. do not decrease as conducted down the axon
Na>K
more positive, rising face of action potassium. Closing potassium force sodium channels to get NA out = following face K>Na.
gK > gNa
Resting membrane
Useful poisons = Tetraethylammonium
TEA. K+ channels = small molecules, binds to potassium channels, blocks them.
Useful poisons = Lidocaine
Na+ channels = local anesthesia, blocks sodium channels.
Useful channels = Tetrodotoxin
found in puffer fish (fugu.) Na+ channels. Important (it can kil), if large amount of sodium channels is disactivated the brain will shut down.
Why are neurons myelinated
prevents current loss along the axon and increase the space constant.
thicker axon with myelination, huge brains if all axons were myelinated. They need energy to myelinated.
Space constant
is distance from site of depolarization where it has fallen to 37%
