Lecture 2 - Neurons and action potential

Question 1

What is the way nerves communicate called?

Answer 1

Electrochemical communication
Because within each nerve cell we have the electricity part, and how neurons communicate with each other is the chemical part

Question 2

How do sensory and motor neurones differ?

Answer 2

THe main difference is just where the cell body lives ( which is where the DNA,mitochondira is stored and all of the organelles that keep the cell going )

Question 3

What are the components of a neurone? Draw this out labelled with explanation of each of the components.

Answer 3

Cell body- Decides whether there has been enough net signal to pass on another signal down its axon, passed on at its terminals to the next neruon

Axon hillock -When information is received at the dendrites, it causes changes in the electrical properties of the cell, and the axon hillock is a region where those changes are integrated to determine if the incoming signals are strong enough for the neutron to initiate its own action potential- process called summation

Axon- This is the long protrusion. This is where the action potential travels down- contains the axon terminal which links to other neurons

Axon terminals - End of the axon, situated close to the dendrites of another neuron + communicate with other neurons at specialised regions called synapses

Dendrites - All over the cell body - picking up signals from other neurons

Myelin Sheeth - Axons usually covered in a lipid rich imsulatroy material called myelin

Myelin helps to speed up the propagation of electrical signals down the axon, prevents current from leaking out of the axon

Nodes of ranvier - These are gaps in the myelin

Question 4

Explain the neuronal membrane - using key terminology

Answer 4

Neuronal membrane= Membrane that encapsulates the entirety of the nerve cell

-Every cell in the body is surrounded by a membrane that separates fluid inside the cell (intracellular) from fluid outside the cell (extracellular)

-This neuronal membrane is a phospholipid bilayer

This are two layers of fat, which means it keeps fluids on the inside and keeps fluids on the outside (as fat Is hydrophobic)

Nothing can get in, nothing can get out

UNLESS!!!!!!

Neurons contain special proteins in
their membranes (ion pumps and ion
channels) that control the movement
of ions into and out of the cell, so lets things in and out

Basically - A neuron membrane isa phospholipid bilayer surrounding a neuron, containing embedded proteins like ion channels and pumps that separate the internal and external environments and enable electrical signaling.

Question 5

Explanation of what an ion is

Answer 5

• An ion is a charged particle
• When an atom is attracted to another atom because it has an unequal number of electrons/ protons
• If you have more protons then it is positively charged, if you have more electron then it is negatively charged
• Ions formed by the loss of electrons are represtend which with a plus, ions formed by the gain of electrons represented with a minus
• Cation = positively charged, anions = negatively chargerd
• “an atom or molecule whose total number of electrons is not equal to its total number of protons, giving the ion a net positive or a net negative charge”
• eg salt is made up of sodium ( Na+ )and chloride (Cl-)- hence why the positive and negative ions bond happily

Question 6

How are neurons let into and out of a cell?

Answer 6

Through Ion chanels which are selective for particular ions but also ions pumps

( some ion chanels stay open all of the time, while others open only in response to specific stimuli/ signals like the binding of a neurotransmitter)

The movement of ions into and out of the cell is what causes the electrical signal

Question 7

What are the different types of ion channels?

Answer 7

• Ion Chanels can be resting (open all the time)
• Ion Chanels can be voltage gated ( only open when the inside of the cell is at a particular voltage with repect to the outside of the cell)
• Ion Chanels can be ligand gated ( eg kind of like a lock and a key) eg neurotransmitter
• Ion chanels can be mechanicaly gated ( eg through touch)

Question 8

What is the intracellular and extra cellar enviroment?

Answer 8

-The intracellular environment and extracellular environment are each filled with fluid, and suspended in that fluid are charged particles called ions

-These ions create an important roll in creating the conditions just right for an action potential to occur

-This unequal distribution of ions is maintained in multiple ways

Question 9

What are the types of ions contained within the intracellular and extracellular enviroment?

Answer 9

The intracellular and extracellular
fluid contain different types of ions:
– Sodium (Na+)
– Potassium (K+)
– Chloride (Cl-)
– Large negative ions (A-)- stuck inside the cell that can’t move anywhere. Basically all the stuck inside the cell and cannot move eg nucleus, mitochondria, DNA- have a net charge which is negative

So cell is already negatively charged on the inside, without anything happening at all

Question 10

What are the two forces that determine the movement of ions into and out of the cell?

Answer 10

• Concentration (High-Low density)
• Electrical (Negative ←–> Postive )

Question 11

What is an explanation for the concentration gradient?

Answer 11

Ions move through diffusion - they will always try and distribute themselves evenly and will move from an area of high concentration to an area of low concentration — called concentration gradient

Question 12

What is an explanation for the electrical gradient?

Answer 12

Electrical gradient - ions will always try to distribute the electrical charge so that there is a balance between positive and negative ions, so that the charge is neutral eg positive ions will move to an area dominated by negative ions and visa versa

Question 13

What is the resting membrane potential like in terms of sodium?

Answer 13

At rest, sodium ion channels are closed. Therefore, sodium is not free to move across the membrane

Question 14

What is the resting membrane potential like in terms of potassium?

Answer 14

• However, there is a class of potassium ion channels that are always open (Resting ion Chanel). This allows potassium to move into and out of
  the cell. - so at rest it is always open , so potassium is on the move all of the time
• Potassium (K+) is attracted into the cell because the cell is more negative inside compared to outside
  (electrical force).
• Potassium is attracted out of the cell because there is less potassium outside the cell (concentration force)- So through the concentration gradient
• You’ve got to have an equilibrium between the forces thats pulling potassium into the cell, and force pushing the potassium out of the cell
• At some point, those forces are going to be equal and opposite = Equalibrium

If you were just to take potassium into account, the equilibrium potential would be -90mv (millivolts). ( so would leave us with a net negative charge still inside the cell)

Question 15

Explain the sodium potassium pump.

Answer 15

-Another thing that helps to regulate how much sodium and potassium there is on eitherside of the cell membrane is the sodium-potassium pump

-Helps if there is any sodium inside the cell

-Enzyme that continuously pumps sodium out of the cell and pumps potassium ions into the cell

• cell. The cell is therefore negatively charged.
• The Na/K pump causes this imbalance by continually pumping pushing out 3 positive sodium ions out of the cell and pumping 2 positive potassium ions into the
  cell.
• The Na/K pump causes an increased potassium concentration inside the cell, but an increased sodium concentration outside the cell.
• Particularity important after there has been an activation to rebalance everything inside the cell, but it is active pretty much all of the time

Question 16

What is the overall resting membrane potential, taking everything into account?

Answer 16

So potassium is much more concentrated inside the cell, and sodium much more concentrated outside the cell

Overall resting membrane potential ( taking all of the ions into account ) = -65mv

Question 17

What cases the resting membrane potential to become an action potential?

Answer 17

-If the neuron is stimulated ie there is an influx of positiviy/ positive ions (and if its big enough and reaches threshold) then the resting potential becomes an action potential

-So if the equilibrium is upset + Excitatory stimulation is large enough

Question 18

Define what an action potential is

Answer 18

-Action potential = activity in an area of the neuronal membrane

-An action potential isa rapid, temporary change in the electrical potential across a neuron’s membrane that serves as a signal

-Can also refer to action potential as a neuron firing

• The changes in the electrical activity of a neuron that give rise to an action potential can be explained by the movement of ions into and out of the cell

Question 19

What happens if the net charge in the axon reaches threshold?

Answer 19

An action potential is generated at the axon hillock, if the net change is above threshold (-50mV). ( basically the net charge moves from -65 to -50, a change of 15 positive mv) - the action potential is then propogated down the axon.

Question 20

What happens with sodium and the rise of action potential ( depolarisation)

Answer 20

When a cell Is stimulated above the threshold (-50mv), sodium ion channels open ( as there are sodium ion chanels that are voltage gated and therefore open at -50mv)

Sodium is attracted into the cell because there is more sodium outside of the cell (concentration force)

Sodium is also attracted into the cell because the cell is more negatively charged (electrical force)

Therefore, this influx cause the cell to be more postive

So, this stage is known as depolarisation, as the membrane is becoming less negative

This depolarisation goes even further, going past 0, and making the cell positive

Question 21

What happens with Potassium/ Sodium and the fall of action potential (Repolarisation)?

Answer 21

• At + 40mv , the voltage gated potassium channels open ( as we have two sets of ion Charles for potassium we have the resting Chanel which is always open and the voltage gated ion chanels)
• As there is more potassium inside the cell then outside the cell and that the inside of the cell is now more positive potassium rushes out ( so travels along electrochemical gradient)
• At the peak , the sodium gated ion channels shut, so only potassium is able to move
• There is a downward deflection as the inside of the cell Is becoming more negative again

Question 22

What happens with hyperpolarisation?

Answer 22

• The cell becomes even more negative than It was before, which is called hyoerpolorisation - really important as the action potential can only go in one direction
• The sodium-potassium pump can help restore this balance

Question 23

What is propogation?

Answer 23

When an action potential occurs in a neuron, it affects the next bit of the neuron — propogation

• Its gets propagated down the axon all the way to the end of the axon - the pre-synaptic terminal

Question 24

What is myelin sheath, where is it located, how does it work, what does it with?

Answer 24

Some axons are coveted in a fatty substance called myelin/oligodencytes ( produced by glial cells). They wrap around the neruon, it insulates the neuron. It means that instead of having to propagate it a bunch of times along the axon, it can jump between those fatty cells eg only needs to repropogate its self a few times. The action potential repropogates at the nodes of ranveir, which are the gaps between the glial cells, propelling the axon potential down the axon

So myelinated axons can conduct the axon potential faster than unmyelinated axons by saltatory conduction ( jumping between the nodes) and myelin prevents current leaking out the axon

Question 25

What is it called with myelinated vs unmyelinated neurons?

Answer 25

Unmyleinated axons do it by propagation, whereas myelinated axons do it by salutatory conduction

Question 26

What are the problems with lack of propogation?

Answer 26

-eg Motor neurons disease - as signals dont move fast enough

Question 27

Action potentials are...

Answer 27

ALL Or NOTHING EVENTS

Question 28

Explain the concept of all or nothing events

Answer 28

- Every action potential looks the same, they are all or nothing - There aren't small/ large action potentials, they all look the same - How do we know a stimulus is bigger than another stimulus— strength of the stimulus is denoted by the firing rate .eg if you have really really big stimulus, going to have lots of action potential. If you have a lack of stimulsus, going to have less action potential. eg think about your tonic firing rate, neurons are always active, but if someone thumps you for example you would have lots of action potentials - Neurons can fire many action potentials per second

Question 29

Explain how the process of propogation down the axon works

Answer 29

- Axon potential migrates towards the axon terminal - Action potential fires, the positive charge associated with the first action potential spreads to adjacent region, and this area reaches its threshhold and fires an action potential . The area before this that fired an action potential a millisecond ago cannot become depolarised now as its refractory as its sodium channels are innactivated - Therefore action potential propogates In only one direction like a fuse - This continues to spread to next region. The area that first fired an action potential has returned to its resting potential - too far away to be influenced by current action potential

Question 30

How do we move from the electrical part to the chemical part?

Answer 30

So, we've explained the electricity part, but this electricity does not jump from one neuron to the next, so there must be some other mechanism whereby this signal is passed to the next neuron — The chemical part

Question 31

Explain the checmial process of passing the electrical signal to the next neuron

Answer 31

- The action potential which has come down the axon, opens v**oltage gated calcium chanels (Ca++) ,** which causes calcium to come into the presynaptic terminal, fuse with the synaptic vesicles (sacks filled with neurotransmitters), and cause them to move towards the terminal, fuse with the terminal membrane and dump their contents into synapse - The neurotransmitter molecules float across the cleft and bind with receptors sitting on ion channels in the postsynaptic membrane - because the neurotransmitters are key to the ion Chanels lock, they are called **ligand-gated** ions -- Those neurotransmitters can either have an excitatory or inhibitory affect - if they are excitatory, they will open positive ion chanels in the post synaptic membrane or if inhibitory, will open negatively charged ion chanels

Question 32

What system is crucial for the nervous system to work?

Answer 32

The whole nervous system works through excitation and inhabitation - The balance between excitation and inhabitation is critical for proper nerve function

Question 33

What happens when excitation and inhibition breaks down?

Answer 33

When this breaks down, it leads to conditions such as eplisleys - eg the neurones becoming too active resulting in seizures

Question 34

What happens with ion channels that are selective for positive or negative ion?

Answer 34

if those ion chanles are selective for positive ions, it will let positivity in- excitatory postsynaptic potential EPSP ( graded potentials ) -making the cell more postive If the ion channel lets in a negative ion, then it will have an inhibtiaty effect ( Inhibitary post-synaptic potential) IPSP — makes inside of the cell more negative

Question 35

How are IPSPS and EPSPS different to action potentials?

Answer 35

different to action potentials, they are graded potentials and different to all or nothing events

Question 36

What does the axon hillock need to do with IPSP and EPSPS?

Answer 36

The axon hillock needs to decide if putting together all of the EPSPs and IPSPs from all other different synapses on that cell, is there enough net postive charge change to start of an action potential ( it needs 50mv)