exam 1

a barrier to outside of cell which is achieved by the____?

a barrier to outside of cell which is achieved by the____?

cell membrane -> defines intra- vs. extra-cellular spaces

cytosol?

aqueous solution (liquid) inside the cell

cytoplasm?

everything between nucleus and plasma membrane (i.e. cytosol + organelles)

phospholipid bilayer?

hydrophilic + hydrophobic

hydrophilic?

interacts with water/environment; H2O lover, polar; phosphate head

Hydrophobic?

interacts with water/environment; H2O lover, polar; lipid tail

Cellular membrane?

Membrane plays a critical role in communication between cells 

Membrane is dynamic; protein components change and move around = critical for neuroadaptations

cytoskeleton?

protein scaffold

• localization and interconnection between organelles

• gives shape to neurons or glia

• dynamically modulated (it plays role in the plasticity of neurons)

cytoskeleton parts?

Microtubule – conveyor belt

Intermediate filament – cell structure

Microfilament – membrane scaffold

mitochondria?

Major site of ATP production, O₂ utilization, and CO₂ formation  - THE POWERHOUSE OF THE CELL!

rough ER

Ribosomes synthesize proteins

smooth ER

Stores enzymes to make lipids and steroids

Stores Ca++ for cellular processes

golgi apparatus?

• Concentrates, modifies, and sorts proteins arriving from RER

• Packages proteins in vesicles

nucleus?

contains the DNA

nuclear envelope?

surrounds nucleus

neurons?

cells that send and receive information in the form of chemical/electrical signals

compartmentalization/functional segregation of neurons?

Dendrites = input

• receive signal

Body = protein production

Initial Segment = integration (axon hillock)

• where neuron decides to send signal or not

Axon = conduction/propagation

• of AP of signal

Terminal = output

• where neuron sends message

Afferents?

• go “to” CNS from PNS

• sensory neurons

Efferents?

• efferents go “away from”; CNS to PNS

• motor neurons

Interneurons?

remain in the same area (don’t project anywhere; not afferent or efferent since don’t move

neurotransmitters ex?

dopamine, norepinephrine, serotonin, GABA, glutamate, etc

Excitatory neurotransmitters?

encourage a target cell to take action

Inhibitory neurotransmitters

decrease chances of the target cell taking action

Modulatory neurotransmitters

send messages to many neurons at the same time, or even communicate with other neurotransmitters

glial?

provide structural support in nervous system, make myelin, immune (immune survalience) & neuronal functions (neurogenesis & apoptosis)

Myelin?

protective fatty coating of axons to increase the speed at which they transmit messages

myelin producing glia are?

1. Oligodendroglia (CNS)= Myelinates many axons

2. Schwann cells (PNS)= Only myelinates 1 axon

ependymal cells?

Specialized cells that line the ventricular system of the brain and play a key role in the production of cerebrospinal fluid (CSF) in brain and spinal cord.

atrocsytes

• star-shaped cells

• assist in the transfer of nutrients and waste

  • wrapped around blood vessels

  • also take up and release ions, thus making them important for neurotransmission

atoms?

• neutral particles

•  # protons = # electrons

not all atoms are stable?

• atoms want to have 8 electrons in their outer shells to be stable. 

***2 electrons in outer shell if only one shell (hydrogen)

cations (+)?

lose an electron they now have a positive charge

anion (-)?

gain an electron they now have a negative charge

“like attracts like”?

Polar molecules attract other polar molecules and repel non-polar molecules

water dissolves polar molecules?

salt & sugar cuz polar

hydrophilic dissolves?

polar and dissolve in water

hydrophobic dissolves?

nonpolar and do not dissolve in water

hydrophobic molecules can go through membrane?

yes go straight through (O2, CO2, N2)

hydrophilic molecules can go straight through membrane?

no, blocked by lipid tails

• ions: Cl-, K+, Na+

• small uncharged polar molecules: H2O, ethanol

• large uncharged polar molecules: glucose, sucrose

intra vs extra cellular contents?

1. there is a difference in the concentration of specific ions and proteins across the cell membrane 

2. the cell membrane is selectively permeable to some of these ions

compare ion concentrations intra & extra cellular?

intra: K+ & anion (-) high

• neg banana

extra: Na+ & Cl- , Ca2+high

• salty milk

resting membrane potential (-65 or -70 mV)?

Relative net ionic differences between inside and outside of cell with outside defined at 0

2 forces establishes membrane potential?

1) diffusion

2) electrical charge

diffusion (force #1)?

the movement of molecules down a concentration gradient

requires:

1. Concentration Gradient (H to L)

2. The ability to move (channels)

resting membrane potential is determined by?

resting membrane potential is determined by selective permeability AND the direction of the concentration gradient. 

electrical charge (force #2)?

opposite charges attract and like charges repeles attract and like charges repel

ohm’s law?

Current = Potential * Conductance

Current = flow of electrical charge

Potential = membrane potential

Conductance = resistance, existence of ion channels

(If potential increases, current flow increases; If resistance increases (e.g. fewer ion channels = less conductance), current flow decreases)

how would you increase conductance of a neuron? What would happen to ionic current?

increase ion channels; more channels so more ions come through

how would you increase potential of a neuron? What would happen to ionic current?

depolarize neuron; increases as Na+ comes into cell because of diffusion & electric charge force

equilibrium potential?

diffusion & electrical potential is balanced out; ions still move=1 comes in as 1 leaves cell (no net movement)

Nernst equation?

If the concentration difference across the membrane is known an equilibrium potential can be calculated for any ion.(only use for single ions)

relative permeability?

The ability of an ion to cross the membrane can be expressed relative to the ability of all other ions.

• ion that can cross more readily=high “relative permeability” to an ion that passes less readily

• Flow of ion(s) with high relative permeability contribute more to the membrane potential; i.e. membrane potential will be close to the ion’s equilibrium potential.

Goldman equation?

Relative permeability serves to weight the contribution of an individual ion to the membrane potential which is mathematically incorporated into the Goldman equation as P_ion

• use for multiple ions

Channel?

passively allow ions to cross membrane; no ATP

pump?

 actively (requires energy) move ions across membrane; needs ATP; major determinant in concentration gradients (ex: 3 Na⁺ out for every 2 K⁺⁾

Co-Transporters and Exchangers?

use concentration gradient (H to L) of one molecule to move another molecule against concentration gradient (L to H)

Three major  factors influence ion movement through channels?

1. # channels: determined by gene expressions

2. single channel current (phosphorylation: channel open more=more ions come through)

3. probability of channel opening: intrinsic gating properties and modulation

resting membrane potential summary?

1. Membrane has a large relative permeability to K+ versus Na+

2. Membrane is impermeable to bulk of intracellular proteins (net anionic charge)

3. Na/K Pump is electrogenic : (3 Na⁺ out of cell and 2 K⁺ in)

action potential?

wave of depolarizing current that flows down the axon to the terminals

-brief (1/1000 sec reversal of membrane potential)

pre-synaptic?

neuron before the synapse (sending the message)

synapse?

space between two neurons

post-synaptic neuron?

neuron after the synapse (receiving the message)

EPSP = excitatory post-synaptic potential?

• slight depolarization in membrane (closer to threshold)

• due to influx of positively charged ions (typically Na+)

IPSP = inhibitory post-synaptic potential?

• Slight hyperpolarization in membrane

• due to influx of negatively charged ions (typically Cl-) or efflux of positively charged ions (K+)

initial segment/axon hillock?

• Ions from IPSP/EPSP diffuse along surface of the membrane

• Ions (depolarization/hyperpolarization) waves reach axon hillock

voltage-gated Na+ channels open based on?

Open/inactivate/close and reset based on membrane voltage

why is the axon hillock the trigger zone for AP?

Large number of voltage-gated Na+ channels at the axon hillock make it a “trigger zone” for the beginning of an AP

Any given neuron can receive information (i.e. PSPs) from thousands of synaptic inputs. What could make certain PSPs more likely to influence if the neuron fires an AP?

if Na+ came in closer to hillock; bigger channels size

What could make certain PSPs more likely to influence if the neuron fires an AP?

1. Their size (e.g. a lot of Na+ influx)

2. Their distance from the axon hillock

3. Their frequency

4. All of the above

All of the above

1. more Na+ come in

2. closer to axon hillock

3. cuz Na+ keeps building up

typical action potential?

1) resting state

2) rising phase

3) overshoot

4) falling phase

5) undershoot/hyperpolarization

1) resting state?

activation gats on Na+ & K+ channels are closed & the membranes resting potential is maintained

2) rising phase

rapid depolarization of the membrane (+)

1. vg Na+ channels open, vg K+ closed, Na+ comes in making inside more pos w/ respect to outside

3) overshoot peak (+30mV)

reaching about 40mV this is the points at which the inside of the neuron is positive with respect to the outside

4) falling phase

rapid repolariztion (becomes more neg)

• vg Na+ inactivate block Na+, activation on most K+ channels open permitting K+ exit cell so cell inside becomes neg

5) undershoot/hyperpolarization

the point at which the cell is more negative with respect to the outside than at resting phase. 

There is a gradual restoration of the resting potential following undershoot.

• both gates of Na+ channels are closed but the activation gates on some K+ channels still open; as these gates close most K+ channels & inactivation gates open on Na+ channels=>resting

Two types of voltage-gated ion channels mediate changes in membrane permeability during the AP?

1. Inactivating Voltage-gated Na⁺ Channel

2. Delayed (Rectifying) Voltage-gated K⁺ Channel

vg Na+ channels

• closed but can opening at rest

• open at threshold to peak potential (-50mV to +30mV); Na+ rapidly comes in

• closed & inactivated (can’t open) from peak to resting potential (+30mV to =70mV) ; ball & chain blocks opening

vg potassium channels

• closed: @ resting potential delayed opening until threshold closed until peak potential (-70mV to +30mV);

• open: from peak potential until hyperpolarization (+30mV to -80mV)

ion channels at rest

• non-gated K+ open: permeable to K+ cuz always open

• vg Na+ closed

• vg K+ closed

ions channels at rising stage

• non-gated K+ open: permeable to K+ cuz always open

• vg Na+ open: cuz rapid depolarization (+)

• vg K+ triggered

ions channels at falling stage

• non-gated K+ open: permeable to K+ cuz always open

• vg Na+ channels inactivated

• vg K+ channels closed

ion channels at resting stage

• non-gated K+ open: permeable to K+ cuz always open

• vg Na+ channels reset/closed

• vg K+ channels closed

What do you think would happen if we use a drug to block the activity of voltage-gated Na+ channels?

1. There would be no action potentials

2. There would be more action potentials

3. Nothing would happen

4. All of the above

1. There would be no action potentials; need Na+ vg channels to open in rising stage & depolarize (+)

Tetrodotoxin (TTX) from pufferfish affects which ion?

Blockade of vg-Na+ channels = blockade of APs

all or none?

need to reach threshold at axon hillock then fire AP; if not then no AP

how does increase generator potential impact AP frequency?

input more EPSP increased frequency (#) of AP NOT THE SIZE

absolute refractory period?

Neuron CANNOT fire another action potential at this time cuz…

• Vg-Na+ channels are already open (-55mV to +30mV) OR (rising phase)

• Vg-Na+ channels are inactivated +30mV to -65mV (peak to falling stage)

relative refractory period?

Vg-Na+ channels are reset

Neuron is hyperpolarized! (K+ leaks out cuz vg K+ channels don’t close until -80mV)

Neuron can fire another action potential at this time, but it takes more EPSPs since -80mV is more neg than resting potential (-65mV)

Why can AP only go in 1 direction?

in a absolute refractory period the vg Na+ channels are inactivated for the rest of the AP

what impacts conductance speed?

1. diameter

2. insulation

diameter larger vs smaller in conductance speed?

Larger diameter is faster (less axial resistance) less resistance agsnt ion movement

what if Na+ leak?

As Na+ ions enter the axon they diffuse randomly down their concentration gradient => If ions “leak” (through pumps/channels/exchangers) from the axon, then it will take longer for a depolarization wave to reach the next vg-Na+ channels

insulation in conductance speed?

Myelin serves to insulate axon:

reduces leakage - stronger repulsion down axon - faster conduction (saltatory conduction)

myelination?

 speeds up electrical signal (creates greater membrane resistance)

Saltatory conduction:?

“jumping” of electrical signal from node to node

voltage-clamp electrophysiology?

• Electrode to measure membrane potential (i.e. voltage)

• This is the voltage clamp amplifier. It considers the membrane potential and injects current to achieve a specified membrane (i.e. voltage)

• Measuring current flowing across the membrane 

Using Voltage Clamp to Hyperpolarize the membrane exp1

-Inject current to hyper polarize the membrane – moving the membrane to -130mv

Measure current flow in and out of membrane. 

• Initial capacitive current 

  • during voltage step only 

  • due to rapid change in voltage of the membrane

• Then nothing. No ion movement across the membrane.

why does using Voltage Clamp to Hyperpolarize the membrane results make sense? exp1

•  voltage-gated Na+ channels don’t open for Na+ to pass into cell until threshold (i.e. -55mV)

• voltage-gated K+ channels don’t fully open until the membrane is depolarized

So, at anything less then -55mV, there is no notable passage of ions across the membrane (just some leaking K+ channels and the Na+/K+ pump)

Using Voltage Clamp to Depolarize the membrane? exp1

from -65 mV(resting) to 0 mV

Measure current flow in and out of membrane. 

• Initial capacitive current 

  • during voltage step only 

  • Then immediate fast inward current (Na+)

  • Followed by a delayed lasting outward current

why does Using Voltage Clamp to Depolarize the membrane results make sense? exp 1

• depolarization of the membrane opens Na+ channels and Na+ floods into the cell

• This is followed by the opening of voltage-gated K+ channels and K+ flowing out of the cell

• Voltage-gated K+ channels are highly heterogenous – having different voltage-regulated and temporal dynamics. 

• This experiment shows an inward followed by outward ion movement that is voltage dependent (relies on depolarization).