Topic 2 electrical signals

Question 1

what was benjamin Franklins kite experiment?

Answer 1

• he attached a long metal wire to a kite made of silk and flew it in an electrical storm
• this drew electricity from the storm clouds and charged a leyden jar
• this proved that storm clouds carry electricity and that electricity passes through objects and not along surfaces

Question 2

who coined the term positive and negative charge and electric battery?

Answer 2

benjamin franklin

Question 3

who found out that electricity makes animal muscles move and how?

Answer 3

Luigi Galvani

Discovered that if you electrically stimulate a dead frogs sciatic nerve its legs will twitch

Question 4

Who found out that electricity makes human muscles contract and how?

Answer 4

Giovanni Aldini found out that electricity makes human corpses twitch
in some cases they were even able to make the bodies sit up

Question 5

what are the largest axons in the world?

Answer 5

squid giant axons

Question 6

what is the resting potential?

Answer 6

-70 mv

Question 7

what is another word for a voltemeter to measure action potentials?

Answer 7

oscilloscope

Question 8

how does the thickness of a membrane impact the attraction between particles

Answer 8

• the thinner the membrane the more the differently charged molecules on either side of the membrane are attracted to each other

Question 9

What is a definition of the equilibrium potential?

Answer 9

The transmembrane voltage at which electrochemical forces counterbalance so that there is no net ion flow across the membrane

Question 10

what equation can calculate the equilibrium potential

Answer 10

The nernst equation

Question 11

What is the nernst equation for a monovalent ion?

Answer 11

Equilibrium potential = 61.5log (conc. outside/conc. inside)

Question 12

What is the resting distributions of ions across a membrane?

Answer 12

K+ = mostly inside the cell
Na+= mostly outside the cell
Cl-= mostly outside the cell
other anions- = mostly inside the cell

Question 13

what are the two different types of pottasium channels?

Answer 13

• pottasium leak channels which are continuously open and help set the resting membrane potential
• voltage gated pottasium channels, opened at +30mv
• both channels k+ goes from inside to outside the cell

Question 14

what are the two types of sodium channels

Answer 14

• sodium leak channels. The effect of these is very small and much lower than the effect of the pottasium leak channels
• voltage gated sodium channels
• both channels na+ goes from outside to inside the cell

Question 15

what is the relative permeability of pottasium to sodium?

Answer 15

around 65:1

Question 16

what equation would you use to calculate the cell potential when the cell is permeable to more than one ion?

Answer 16

The goldman-hodgkin-katz equation

- this takes into account the permeability of each ion

Question 17

What four factors contribute to the resting membrane potential?

Answer 17

1. Selective permeability of the membrane to pottasium ions
2. Pottasium ions freely move across the membrane down the concentration gradient. The pottasium equilibrium potential is -90mv.
3. The membrane is slightly permeable to sodium ions which shifts the membrane potential from -90mv to -70mv.

Question 18

what is the positive feedback effect that occurs before the action potential is generated?

Answer 18

• the membrane potential is slightly depolarised
• this opens voltage gated NA+ channels
• na+ floods in which further depolarises the membrane
• this continues until an action potential is generated

Question 19

what are the concentration changes needed in order to depolarise the membrane?

Answer 19

• the intracellular concentration of na+ only needs to increase by 1:600,000
• the intracellular concentration of k+ only needs to decrease by 1:5,000,000

Question 20

what is an absoloute refractory period?

Answer 20

• when a neuron is incapable of responding to another stimulus, no matter how strong

Question 21

what is a relative refractory period?

Answer 21

only very strong stimuli can generate an action potential

- during this period some sodium gates are inactivated

Question 22

how do action potentials code information?

Answer 22

• which neurons fire
• frequency of firing
• timing of firing

Question 23

what are the three reasons that signal decays along the axon?

Answer 23

• membrane resistance (due to the number of ion channels. The inverse of permeability so the less permeable the membrane is the greater the membrane resistance)
• axial resistance (due to the diameter of the axon)
• membrane capacitance (where positive and negative ions are attracted to each other across the membrane)

Question 24

describe membrane capacitance

Answer 24

• oppositly charged particles are electrostatically attracted to each other across membranes
• the charging of the membrane is measured as capacitance
• the thicker the membrane, the less capacitance there is
• therefore the current that is travelling down the axon is slower as the charge is initially neutralised when it is attracted to the membrane

Question 25

what is the effect of a larger capacitance?

Answer 25

- a larger capacitance means that a greater concentration change is needed to create the same potential difference across the membrane

Question 26

what is the length constant and how does it relate to membrane resistance and axial resistance?

Answer 26

- it shows the distance that a graded electric potential will travel via passive electrical conduction - it is the square root of (membrane resistance/axial resistance) - this means that the lower the axial resistance the further the potential can travel - and the higher the membrane resistance the further the potential can travel

Question 27

what is the time constant?

Answer 27

- this shows the speed at which the membrane can be charged - It is defined as the amount of time it takes for the change in potential to reach 63% of its final value. - time constant = membrane resistance * membrane capacitance - this shows that the lower the membrane resistance and the lower the capacitance the faster the membrane can be charged

Question 28

how does myelin work in terms of capacitance?

Answer 28

- the fatty insulating layer decreases the build up of charge (capacitance) across the membrane - this therefore decreases the time constant so the action potential travels faster along the axon

Question 29

how many times faster is the action potential in myelinated axons?

Answer 29

- around 50 times faster

Question 30

how does axon diameter influence speed of action potential?

Answer 30

the greater the diameter the faster the action potential

Question 31

what is the relationship between conduction velocity and axon diameter in myelinated and unmyelinated axons?

Answer 31

in unmyelinated axons conduction velocity is proportional to axon diameter in myelinated axons (under 1 micron) an increase of 10 microns axon diameter increased the conduction velocity of 70 m/s if the axon diameter is below 1 micron, unmyelinated axons are faster, if the diameter is above 1 micron, myelinated axons are faster

Question 32

how can you measure the amount of myelination of a neuron

Answer 32

you can calculate the myelin G-factor myelin g-factor = diameter of axon / (diameter of axon + myelin)

Question 33

what is the average an optimal g factor of myelin?

Answer 33

between 0.5 and 0.9 optimally its 0.77 the thicker the myelination the lower the g factor number

Question 34

what is the evolution of myelin

Answer 34

it has evolved independently many times over the course of evolution this shows how important it is for given conduction speed, myelinated axons take up less space than non-myelinated axons which is important in a size constrained brain

Question 35

what is the name of conduction in myelinated axons?

Answer 35

saltatory conduction

Question 36

what are the nodes and internodes of myelinated axons?

Answer 36

gaps in the myelin sheath are known as nodes myelinated areas are known as internodes

Question 37

what is recent research suggesting as an additional function of mylination

Answer 37

it is thought that the myelinating oligodendrocytes are involved in providing energy to axons this is because axons can often be far away from their cell body and local cappileries SAAB ET AL 2016 - glucose gets converted into pyruvate and lactate in oligodendricites - these are then shuttled to axons by monocarboxylate transporters such as MCT1 and MCT2 LEE ET AL 2016 - removing MCT1 from oligodendrocytes can lead to axonal damage and loss of neurons OTHER RESEARCH - a decrease in MCT1 is implicated in the pathology of ALS and MS

Question 38

describe multiple sclerosis pathology

Answer 38

- immune cells enter the brain and attack myelin - loss of myelin stops neurons from firing action potentials properly - at first the axons get remyelinated by the pool of oligodendrocyte progenitors in the brain but after some time remylination fails and the neurons die

Question 39

describe neuronal channelopathies

Answer 39

- a channelopathy is a disease following the dysfunction of a channel - this can be due to a genetic mutation or an autoimmune disease leading to an attack on the channel - voltage gated sodium channelopathies can be fatal and lead to numerous disease states - channelopathies associated with voltage gated sodium or pottasium channels often lead to epilesy syndrome - different mutations within the same channel can cause either a gain of function or loss of function

Question 40

write the nersnt equation

Answer 40

equilibrium potential = E0 - (RT)/(zF) ln Q ``` E = reduction potential E^0 = standard potential R = universal gas constant T = temperature in kelvin z = ion charge (moles of electrons) F = Faraday constant Q = reaction quotient ```