PSL300 Term Test 1

Cell membrane

phospholipid bilayer where lipid-soluble molecules and gases can diffuse easily, but NOT water-soluble molecules

simple diffusion

• small lipid-soluble molecules and gases pass through phospholipid bilayer or through pores

• goes down concentration gradient - from HIGH CONC to LOW CONC

facilitated diffusion

• no ATP required as it moves down the conc gradient

• requires carrier proteins

• down the concentration gradient

what are carrier proteins?

structures where solute binds to the transporter, causing a conformational change in the transporter → causes opening of one end and closure of previous end

• there’s a LIMITED # OF TRANSPORTERS so transporter system will saturate when conc of molecule exceeds # of available transporter proteins

active transport

• movement against concentration gradient

• requires energy derived from ATP hydrolysis

  • uses ion pumps (ATPases)

secondary active transport

• movement against concentration gradient

• uses energy stored in electrochemical gradient, often from primary active transport

ligand-gated channels

membrane proteins that open in response to the binding of a specific ligand (chemical agent), allowing ions to flow across the membrane

voltage-gated channel

membrane protein that opens in response to change in voltage across the membrane

vesicles

small membrane-bound sacs that transport materials within cells and to the cell membrane

exocytosis

• moving from inside to outside

• fusion of vesicles with membrane and released

  • used to release neurotransmitterse

exocytosis 1: “Kiss and Run”

release of vesicle contents occurs through a transient fusion pore that then rapidly re-closes to pinch the vesicle back off from the membrane

voltage sensing mechanism

• in the S4 segment of the protein

• S4 sticks out to the side of the protein, like a wing

• when membrane is polarized, positively charged wing is attracted downwards to negatively charged inner surface of membrane

• depolarization of membrane to -50mV can no longer hold S4 wing downwards, so it migrates up → allows ions to diffuse through it

pore loop

creates selectivity filter so that only specific molecules can diffuse through

depends on size AND charge

how often does exocytosis 1 happen before a vesicle is depleted?

occurs several times before vesicle is depleted because only parts of the contents are emptied in one ”kiss”

exocytosis 2: full exocytosis

• high levels of signaling

  • release contents of vesicles all at once by fusing with membrane

What disadvantage comes with exocytosis 2?

complete fusion with membrane can lead to loose membrane (think grandma’s skin)wha

what process counteracts exocytosis 2?

Endocytosis - pinching of cell membrane to create vesicle (brings cellular material into cell)

What 2 conditions must be met to generate a membrane potential?

1) create concentration gradient - enzyme ion pump must actively transport to create concentration gradient

2) semi-permeable membrane - allows one ion species to diffuse across membrane more than others

Na+/K+ pumps

3 Na+ ions pumped out and 2 K+ ions pumped in

THIS is what generates concentration gradient

resting membrane potential

-70mV for a typical neuron

what channel is open at resting MP?

K+ leaking channel is open → K+ can leak out of membrane across the concentration gradient

• makes membrane more negative on the inside

what is the main source of resting MP?

Leaking K+ from the K+ leakage channelcreates a negative internal environment.

electrochemical equilibrium

no net movement of ions across the membrane due to equal opposing forces of concentration gradient and electrical gradient

buildup of + charge on the outside repels diffusion of K+ 

equilibrium potential 

electrical work to repel outward cation diffusion equals chemical work of diffusion down concentration gradient

• forces are equal in magnitude but OPPOSITE in directionThis is the voltage at which there is no net movement of ions across the membrane, typically calculated using the Nernst equation.

what equation calculates the equilibrium potential?

Nernst Equation

• -90 mV is equilibrium for K+ - this means if K+ channels were open and nothing else, it would want membrane potential to be at -90 mV

• describes the balance between the chemical work of diffusion with electrical work of repulsion

what counteracts K+ leaving the cell?

the influx of Na+ counteracts K+ leaving the cell and makes the membrane slightly more positive

Na+ equilibrium potential

+60mV

At rest, membrane is not very permeable to sodium but under certain circumstances, permeability can be dominant and there will be net movement of Na+ into the cellwha

what happens when Na+ channels are open?

Na+ diffuses into the cell along the concentration gradient and the inside of the cell becomes more positive than the outside, leading to depolarization of the membrane potential.

Na+ channels

opens in response to depolarization to -55mVthre

Threshold potential

the critical level of depolarization that must be reached for an action potential to be initiated in a neuron

How does Na+ move from outside the cell to inside?

Change in MP allows for S4 segment wings to move upward, allowing Na+ ions in

The MORE depolarization, the MORE Na+ channels open → leads to rapid depolarization

When does the inactivation gate inside the Na+ gate close?

Closes shortly after the channel opens and prevents further influx of Na+ ions, which is crucial for the repolarization phase of the action potential.

What ion channel is needed to generate an Action Potential?

When Na+ channel opens, membrane is depolarized and MP surges towards the equilibrium potential for Na to initiate an action potential.

When does the MP reach +60 mv?

Never reaches because inactivation gates close, meaning no more Na+ comes in to depolarize. Only K+ leakage channels are activated so lots of K+ leaves the cell

Subthreshold stimulus

stimulus that causes depolarization LESS than 15mV

some channels open but not enough to overcome the K+ channels

Threshold stimulus

Enough depolarization to generate AP

aka enough Na+ channels are open

Suprathreshold stimuluss

causes MORE than enough depolarization and produces an AP

Frequency coding

The process by which the strength of a stimulus is represented by the frequency of action potentials

aka higher frequency means stronger stimulus

What is a refractory period?

When a neuron is unable to fire another AP due to the inactivation of Na+ channels after an AP has occurred

What happens during the refractory period?

Channels reconfigure to their original state and membrane becomes excitable againA

Absolute Refractory Period vs Relative Refractory Period

Absolute: ALL voltage-gated Na+ channels are inactivated

• no stimulus can generate an AP

Relative: SOME channels are reconfigured

• Na+ channels restore themselves at different speeds so some will reconfigure faster than others

• can generate a small AP

• some Na+ channels are reconfigured and becoming active

Depolarization block

Depolarization that results in permanent Na+ inactivation and membrane remains in absolute refractory state

After-hyperpolarization

The phase following an action potential where the membrane potential becomes more negative than the resting potential

Passive current vs Action Potential

Passive current: current that flows through membranes without generating an AP and weakens with distance

Active Potential: a rapid change in membrane potential that constitutes the electrical impulse along neurons

Impulse conduction

AP propagates from its original site to adjacent patches

Excitable cell

A cell that can generate APs due to change in membrane potential

Neurons are the excitable cells that conduct impulses

What does lambda measure?

Length constant to determine how far we can carry the signal before it dies off

• the better the lambda, the longer the potential differences will be carried out without losing from its original value

• Conduction speed of AP depends on lambda

What reduces the amount of current lost?

1) Increasing diameter of an axon (wider straw is better)

2) Increasing membrane resistance (wrap straw w/ tape)

What are the 3 things lambda depends on?

R_i - internal resistance

R_o - extracellular fluid resistance

R_m - membrane resistance

What does myelination do?

• Increases conduction velocity (think wrapping straw w/ tape so juice doesn’t leak out)

  • only 20% of cells are myelinated because myelination takes up a lot of space

What are glial cells?

Schwann cells and oligodendrocytes are specialized glial cells that produce myelin sheets to wrap around axons → results in increased membrane resistance and reduces leakage of current out of the membrane

What are Nodes of Ranvier?

• gaps between the glial cells where the AP Is generated

• Current can cross the membrane at Node of Ranvier

  • this is where the voltage-gated Na+ channels are located, which helps generate an AP

What happens if there’s no myelination?

Slowing down of signals between the brain and the body, which can lead to diseases like MS where there are issues in fine-motor control

What is saltatory conduction?

AP jumps from one node to the next

• ex. Node 1 current strong enough to generate threshold in the next 5-10 nodes to fire an AP

• Furthest node will serve as the depolarizing force for the next few nodes

• Think student passing messages on in a line and the same message is being passed on

Passive transmission

Electrical signals being transmitted without a new AP being generated

• Think students in a line and first student yells so the last student will hear it the least loud

Unmyelinated axons

No myelin sheath → lots of current leakage and slows down conduction velocity

Remak bundle

Non-myelinating Schwann cells that surround unmyelinated axons

What happens at an axon terminal?

AP continues all the way to the axon terminal and releases contents of vesicles via exocytosis

What happens during a refractory period?

AP cannot be regenerated because Na+ channels are all inactivated 

Which direction does AP go?

Only goes forward toward the axon terminal - region behind is undergoing refractory period so AP cannot be fired

Synapse

The synapse forms the functional association of a neuron with another neuronE

Difference between electrical synapse and chemical synapse

Electrical: signal transmitted directly from 1 cell to the next WITHOUT release of neurotransmitter

Chemical: depolarizing current results in the release of neurotransmitters into the synapse

• Acts as the processing station - neurotransmitter does not always release

What triggers the release of neurotransmitters at the axon terminal?

Influx of Calcium ions

• depolarization opens voltage-gated Calcium channels and calcium diffuses into the bouton to trigger vesicle exocytosis 

• Vesicles are usually docked and lined up with the membrane but with calcium, it triggers the vesicle to fuse with external membrane to release contents into the synapse