W2 Excitable cells

Question 1

Nervous systems

Answer 1

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.

Question 2

Sensory nerve

Answer 2

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.

Question 3

Electical activity provides

Answer 3

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).

Question 4

Graded potential

Answer 4

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.

Question 5

Action potentials

Answer 5

= 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.

Question 6

Resting membrane potential

Answer 6

= 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.

Question 7

1-Diffusion

Answer 7

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.

Question 8

Selective and unequal membrane

Answer 8

Channels confer slectivity and pumps assist unequal change distribution

Question 9

2- Electrical

Answer 9

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

Question 10

Sophisticated channel

Answer 10

Selectivity, unequal distribution, channesl are passive and pumps are active thye need energy (ATP)

Question 11

Ion pumps

Answer 11

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.

Question 12

Na+/K+ channels

Answer 12

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.

Question 13

Equilibrium potentials

Answer 13

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.

Question 14

Nernst Equation

Answer 14

= 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

Question 15

Goldman equation

Answer 15

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)

Question 16

Differente faces of action potentials

Answer 16

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.

Question 17

Properties of the Action potential

Answer 17

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

Question 18

Na>K

Answer 18

more positive, rising face of action potassium. Closing potassium force sodium channels to get NA out = following face K>Na.

Question 19

gK > gNa

Answer 19

Resting membrane

Question 20

Useful poisons = Tetraethylammonium

Answer 20

TEA. K+ channels = small molecules, binds to potassium channels, blocks them.

Question 21

Useful poisons = Lidocaine

Answer 21

Na+ channels = local anesthesia, blocks sodium channels.

Question 22

Useful channels = Tetrodotoxin

Answer 22

found in puffer fish (fugu.) Na+ channels. Important (it can kil), if large amount of sodium channels is disactivated the brain will shut down.

Question 23

Why are neurons myelinated

Answer 23

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.

Question 24

Space constant

Answer 24

is distance from site of depolarization where it has fallen to 37%

Question 25

What influences action potential conduction speed

Answer 25

Axon diameter, permeability and myelination

Question 26

What do dendrites do ?

Answer 26

Mostly encode information with graded potentials.

Question 27

The frequency of the action potentials is dependent on

Answer 27

the size of the depolarizing stimulus. = the stronger the stimulus (ie. The more positive) the higher the frequency. This provides a way to encode stimulation intensity in the nervous system.

Question 28

Graded potentials

Answer 28

have non stander varying amplitude, they can depolarise to different Vm, however action potentials go to the same value. Most graded potentials are in the dendrite. 2 main recepter for graded potentials = glutamite recepter = rotropic recepters and channels, they bind clutimate and open the channel, and is sulective for sodium.

Question 29

Factors infleunceing Cv (Conduction velocety) - diamter

Answer 29

Resistance to current flow is invesely proportional to cross-sectional area of the axon.
Leaky (permeability) of membrane

Question 30

Electrical synapses

Answer 30

rapid, retinal neuronal, few other adult CNS neurons (glial junctions), cardiac muscle, smooth muscle.

Question 31

Factors infleuncing Cy

Answer 31

Prevents current loss along the axon by increase Rm and increases the Space constant.
Space constant is distance from site of depolarization where it has fallen to 37%.

Thicker axon with myelination, huge brains if all axons were myelinated, they need nergery to myelinated.

Question 32

Another way to increase conduction velocity, wihtout simply increasing diamter…

Answer 32

Is by insulating the axon this is done by Myelin.

Question 33

Myelin

Answer 33

made up by wrappin gthe plasma membrane of glia support cells round the axon.

Schwann cells in peripheral nervous system, oligodendrocytes in CNS. There are breaks in the myelin sheath where ion channels involved in generation of action potentials are concentrated, and where action potentials will therefore be generated…Node of Ranvier.

Question 34

Unmyelinated small axons

Answer 34

The space constant Rm/Ri so the benefit of a high membrane resistance is eucedby the high internal resistance. Metabolic and volume costs of myelination.

Ion channels have restricted distribution, Saltatory conduction. Myelin increases spead and distance that action potentials travels