Lecture 3

Question 1

In the past

Answer 1

though folk knew other organs were made of cells, they were reluctant to say the brain was as they had no proof

Question 2

Golgi stain

Answer 2

is silver nitrate and potassium chromate
Was found by Camillo Golgi and Santiago Cajal who won the Nobel in 1906
It forms silver chromate chrystals over 2% of cells and stains them black

Question 3

Neuron structure

Answer 3

Soma

2 types of processes: dendrites and an axon

At the end of the axpn is the axon terminal or terminal bouton

releases neurotransmitter

Question 4

Cell specialisation

Answer 4

Cells specialised. Some specialised for moving an organism. But when? WHen to move?

Thus neural cells evolved to work out when to move and to convey that signal

Sensory and transmission

Question 5

Transmission of information in the body (3 ways)

Answer 5

(1) diffusion. Sense, release chemical - doesnt work across large distances
(2) microtubuals - would take days to pass down an arm
(3) electrical transmission - much faster

Question 6

Measuring the membrane potential

Answer 6

Measure with a glass micropipet filled with solutions which conduct charge and insert it into cell

Use a voltmeter to measure the difference in charge between 2 points (mV)

reference electrode is placed in extracellular fluid. This is designated as ground and set to have 0 mV (it ont because it has positively charged but set that as reference

Result: is between -40 and -90mV in nerve and muscle

Question 7

Ions and Ion Channels

Answer 7

Ion is a charged molecule or atom

Cations are +ve
Anions are -ve

Question 8

Electrostatic pressure

Answer 8

Attractive force between molecules with opposite charge or repulsive force between those of the same charge

Question 9

Ion channels

Answer 9

Specialized protein molecules which sit in the cells membrane. They have a pore through which specific ions can enter or leave the cell

Question 10

Leak channel

Answer 10

An ion channel which is in the membrane and has a pore thats always open

Question 11

Positively charged ions

Answer 11

Movovalent
Sodium
Potassium

Divalent
Calcium
Magnesium

Question 12

Negatively charged ions

Answer 12

monovalent ions

chloride

Question 13

Salts

Answer 13

All exist in water
Outside water form salts

K+, Na+, Ca2+, Mg2+

Cl-

Question 14

Intra/extracellular fluid

Answer 14

Intra - inside cells

extra - outside cells

Question 15

The sodium potassium pump

Answer 15

Protein in cell mebrane

Binds 3 NA= and an ATP

ATP splits to ADP (which leaves) and P (which stays bound to the channel)

This changes the shape of the channel. Drives thr NA+ through

These are released to the extracellular space and the new shape of the channel allows 2 K+ to bind

The phosphate is released from the channel into the intracellular space

This causes the channel to change shape once more, releasing the 2K+ inside the cell

Net = 3Na+ out for 2K in

Causes:

K= ions to be 30x more concentrated inside the cell than out

Na+ ions to be 15x more concentrated outside the cell than in
This only changes of the cell dies

Question 16

The potassium leak channel

Answer 16

is always open. K+ is free to enter or leave the cell all the time

Question 17

The force of diffucion

Answer 17

if there is a concentration gradient and no forces or barriers to prevent free movement of molecules, then the molecules will move, on average, from regions of high concentration to those of low concentration

Question 18

The maintenance of the resting membrane potential

Answer 18

(1) The cell membrane is impermiable to ions
(2) potassium is able to freely enter and leave the cell through the potassium leak channel
(3) the Na/K pump results in 30x more potassium inside and 15x more potassium outside the cell
(4) potassium leaves the cell via the leak channels downa concentration gradient
(5) eventually the electrostatic force and diffusional force match one another and a balance is reached

This sets up a resting membrane potential of between -40 and -90mV. The cell is negatively chrged relative to the outside of the cell.

Though millions of K+ ions must leave the cell for it to fall to -90mV, this amount is less than 0.0001% of the K+ ions in the cell

Question 19

The number of K+ leak channels

Answer 19

The closer to -90mV the membrane will be

However, there are channels for other ions. SO it usually less negative. The more K+ channels a cell has relative to other ion channels, the closed to -90mV it will be.

-90 mV is the balance between diffusion and electrostatic forces when only K+ is concerned.

It is the number of K+ channels that determines the exact voltage across the membrane. The Na/K pump always creates the same concentration gradient.

Question 20

Receptors

Answer 20

Protiens in the membrane that are sensitive to specific features of the extracellular environment

eg

The presence of molecules (via chemical interactions)

physical pressure (movement, touch)

electrical pressure (voltage)

temperature

PH

Electromagnetic radiation (light)

Typically these cause the receptor to change shape (conformational change) or catalyze a reaction. These then launch g protein intracellular signaling cascades:

receptor > g protein > enzyme > activates effect

effect might be change in gene expression or movement

Neurons do use this too but also use receptor proteins to control their membrane potential. This allows for electrical signaling.

This means:

stimuli > receptor > ion channel open or close > hyper/depolarization

Question 21

Depolarization

Answer 21

Membrane potential becomes less negative than usual

An acute influx of positive ions (like Na+) through a receptor channel can cause a change in resting potential

Question 22

Hyperpolarization

Answer 22

Membrane potential becomes more negative than usual

Influx of anions such as Cl-

Question 23

Graded changes in the membrane potential

Answer 23

Graded changes are short lived because of leak channels in the membrane. These are always active and ddetermine the resting membrane

If positive ions enter, K+ will leave
If negative ions enter, Cl- will leave

Question 24

Voltage gated channels

Answer 24

Have an electrically gated pore. When this threshold is reached, open allowing a rapid influx of ions.

Question 25

Voltage gates Na+ channel

Answer 25

Initiates and propagates the action potential

Closed when membrane potential is lower than -40mV

Has a ball and chain that quickly blocks the channel

This INACTIVATION lasts until the membrane potential drops to -60 mV

takes about half a ms to get to this inactivation point

Question 26

What opens voltage-gated Na+ channels

Answer 26

Receptor is triggered and opens, allowing Na+ in

IF thershold is reached (over -40mV) this opens a voltage gated Na+ channel

This makes it more positive casuing a cascade of Na+ voltage gated channels opening

Question 27

Change in voltage during an action potential

Answer 27

-40 to +40mV

Question 28

The action potential

how long

overshoot

Answer 28

Is a brief electrical impulse that is the basis for conduction of information by neurons

returns to normal within 2 msec

there is an initial overshoot where it becomes more negative than normal

Question 29

Voltage gated K+ channels

Answer 29

if it were just left to leak channels the cell would not repolarize for ages

When the cell is more positive than 0mV they open

Allow rapid outflow of K+ so cut the repolarization time down

important to have this mechanism because if not

(1) would take ages
(2) if you just had more leakjs, they would always be open and you would not be able to reach threshold potentials

Question 30

The action potential order

Answer 30

(1) receptor activated and opens pore allowing Na+ in
(2) if this raises the membrane potential to over the -40 mV threshold potential, this causes voltage gated Na+ channels to open and there is a rapid influx of Na+ ions
(3) the voltage gated K+ channels open at positive voltage inside the cell
(4) the Na+ channels become refractory and no more Na+ is allowed in
(5) K+ contiinues to leave the cell, the membrane potential becomes more negative
(6) the K+ channels close and Na+ channels reset
(7) the extra K+ outside the cell causes the membrane potential to be hyperpolarised temporarily

(*) this K+ diffuses away and the resting potential is restored to -70

Question 31

Experiment to determine the action potenital

Answer 31

done on giant quid

Wire electrode placed in extracellular fluid

Glass micro electrode is inserted into the axon

Oscilloscope turns the electrical fluctuations into visible signals

Question 32

5 proteins of an action potential

Answer 32

(1) NA/K transporter - creates Na/K concentration gradients
(2) Leak potassium channels - always open, create an electrical potential; the resting membrane potential
(3) voltage gates Na+ channel - initiates and propagates the action potential
(4) voltage gates K+ channel - to restore the resting membrane potential
(5) voltage gated calcium channel - located at the end of the axon, the axon terminal and cases the release of neurotransmitter containing vesicles

Question 33

Synaptic transmission

Answer 33

(1) action potential arrives at end of axon
(2) voltage gated Ca2+ channels open and Ca2+ enters the axon terminal
(3) Ca2+ entry causes neurotransmitter containing vesicles to release their contents into the synaptic cleft via exocytosis
(4) neurotransmitter diffuses across the cleft and binds to ligard gated ion channels on the post synaptic membrane

Question 34

Conduction of the action potential (5 things)

Answer 34

(1) movement is referred to as conduction of the action potential
(2) unidirectional
(3) size of the action potential is constant
(4) The all-or-none law states that the action potential occurs or does not and once triggered, will propagate down the axon without growing or shrinking until it reaches the end
(5) the rate law states that the strength of the stimulus is coded for by the rate of firing of the axon