Lecture 3 Flashcards
In the past
though folk knew other organs were made of cells, they were reluctant to say the brain was as they had no proof
Golgi stain
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
Neuron structure
Soma
2 types of processes: dendrites and an axon
At the end of the axpn is the axon terminal or terminal bouton
releases neurotransmitter
Cell specialisation
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
Transmission of information in the body (3 ways)
(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
Measuring the membrane potential
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
Ions and Ion Channels
Ion is a charged molecule or atom
Cations are +ve
Anions are -ve
Electrostatic pressure
Attractive force between molecules with opposite charge or repulsive force between those of the same charge
Ion channels
Specialized protein molecules which sit in the cells membrane. They have a pore through which specific ions can enter or leave the cell
Leak channel
An ion channel which is in the membrane and has a pore thats always open
Positively charged ions
Movovalent
Sodium
Potassium
Divalent
Calcium
Magnesium
Negatively charged ions
monovalent ions
chloride
Salts
All exist in water
Outside water form salts
K+, Na+, Ca2+, Mg2+
Cl-
Intra/extracellular fluid
Intra - inside cells
extra - outside cells
The sodium potassium pump
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
The potassium leak channel
is always open. K+ is free to enter or leave the cell all the time
The force of diffucion
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
The maintenance of the resting membrane potential
(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
The number of K+ leak channels
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.
Receptors
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
Depolarization
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
Hyperpolarization
Membrane potential becomes more negative than usual
Influx of anions such as Cl-
Graded changes in the membrane potential
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
Voltage gated channels
Have an electrically gated pore. When this threshold is reached, open allowing a rapid influx of ions.
Voltage gates Na+ channel
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
What opens voltage-gated Na+ channels
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
Change in voltage during an action potential
-40 to +40mV
The action potential
how long
overshoot
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
Voltage gated K+ channels
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
The action potential order
(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
Experiment to determine the action potenital
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
5 proteins of an action potential
(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
Synaptic transmission
(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
Conduction of the action potential (5 things)
(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
