cell excitability

Question 1

what is the nervous system?

Answer 1

a system of communication that allows an organism to react rapidly and modifiably to changes in its environment

Question 2

how do neurons function?

Answer 2

• collect information
• integrate information
• produce an output

Question 3

what are electrical signals that neurons produce?

Answer 3

• action potentials: fixed size, all or nothing signals that propagate along axon, usually unidirectional
• graded potentials: variable size, local signals not propagated along long distances, pass both ways along neuronal membrane

Question 4

how do neurons encode information?

Answer 4

APs are coded by frequency as they are of a unit size
graded potentials are coded by size and vary according to strength of stimulus

Question 5

what is resting membrane potential of neuron?

Answer 5

-70mv
negative membrane potential is a requirement for a functioning nervous system

Question 6

why do neurons have a resting potential?

Answer 6

inevitable consequence of:
- selectively permeable membrane
- unequal distribution of charged molecules/ions
- physical forces

Question 7

why are neurons selective and unequal?

Answer 7

• ion channels (selectively permeable)
• ion pumps (assist unequal charge distribution)

Question 8

what are the physical forces controlling ion movements?

Answer 8

diffusion and electrical

Question 9

examples of important ion pumps

Answer 9

Na+/K+ ATPase (creates high conc of Na+ outside membrane and high conc of K+ inside membrane)
Ca2+ pumps (creates gradient of Ca2+ across plasmamembrane and other membranes)

Question 10

how are equilibrium potentials established?

Answer 10

ionic gradient influence membrane potential by determining equilibrium potential
diffusion across the membrane establishes electrical charges on both sides of the membrane, eventually the charges equillibriate n

Question 11

what is equilibrium potential?

Answer 11

the electrical potential difference across the cell that exactly balances the conc gradient for an ion
electrostatic forces = diffusional forces

Question 12

how to calculate equilibrium potential for an ion?

Answer 12

nernst equation

Question 13

what ions is the neuronal membrane permeable to at rest?

Answer 13

very permeable to K+ (slightly permeable to others)

Question 14

is equilibrium potential for the main ions positive or negative?

Answer 14

K+ = negative
Na+ = positive
Ca2+ = positive
Cl- = negative

Question 15

how to calculate membrane potential?

Answer 15

the goldman equation

Question 16

what value does membrane potential reach during an action potential?

Answer 16

up to 40mV

Question 17

how long does an action potential last?

Answer 17

2 - 4 ms
limitation on the frequency of action potentials

Question 18

properties of action potentials?

Answer 18

• transient (up and down), rapid and reversible change in membrane potential from -ve to +ve
• different types of cell may have different types of action potentials

Question 19

what is membrane permeable to at rest?

Answer 19

mainly permeable to K+ (leaky potassium channels)

Question 20

what is membrane permeable to during depolarization?

Answer 20

permeable to Na+ (sodium channels open)

Question 21

what is membrane permeable to during repolarization?

Answer 21

permeable to K+ (sodium channels close, potassium channels open)

Question 22

structure of voltage-gated Na+ channels

Answer 22

• consists of 4 subunits
• several transmembrane domains
• one subunit crosses the membrane 6 times
• loop creates distance between transmembrane domains s5 and s6
• s4 contains many positively charged amino acids, it is the voltage sensor which sense the change in membrane potential

Question 23

how to voltage-gated Na+ channels open in response to depolarization?

Answer 23

• change in membrane potential causes the positive charges of the polypeptide chain to move from one side of the membrane to the other
• causing conformational changes of the entire protein and opens the pore

Question 24

how does Na+ channel inactivation occur?

Answer 24

• occurs quickly ~1ms
• de-inactivation must occur before channels can be activated again

Question 25

how can poisons which implicate ion channels be useful?

Answer 25

studying ion channels, toxins which block specific ion channels can be used to see if activity of neuron is effective

Question 26

what are the factors influencing conduction velocity (speed of AP propagation)?

Answer 26

diameter of axon
myelination

Question 27

how does diameter of axon influence conduction velocity?

Answer 27

the larger the diameter, the faster the speed of action potential
resistance to current flow is inversely proportional to cross-sectional area of the axon

Question 28

how does myelination influence conduction velocity?

Answer 28

myelinated axon increases speed of action potential
prevents current loss along the axon (from leaky channels)

Question 29

why are there so many unmyelinated small axons?

Answer 29

• the space constant is equal to the ratio of membrane resistance and resistance inside axons, so benefit of a high membrane resistance is reduced by the high internal resistance
• metabolic and volume costs of myelination

Question 30

how do dendrites encode information?

Answer 30

graded potentials (action potentials are initiated at axon hillock)

Question 31

how do action potentials encode information?

Answer 31

by frequency of action potentials
high stimulus intensity = high frequency of action potentials

Question 32

what are the types of graded potentials?

Answer 32

excitatory (depolarizing) or inhibitory (hyperpolarizing)

Question 33

what is GABA?

Answer 33

GABA in a inhibitory neurotransmitter
GABA receptors are selective for chloride which hyperpolarizes the membrane

Question 34

what is glutamate?

Answer 34

glutamate is an excitatory neurotransmitter
glutamate receptors are selective for sodium which depolarizes the membrane

Question 35

what are electrical synapses?

Answer 35

junctions in which the positive and negative ions can travel freely between these gaps and move to the neighboring neurons (rapid)
they can be seen using fluorescent dye