Quiz 2 Flashcards
there are 2 aqueous compartments with different concentrations of NaCl and separated by a membrane equally permeable to Na and Cl
- what will happen to the concentration of Na and Cl in each compartment
- will there be a difference in the electrical potential between the 2 compartments (membrane potential)
- Na and Cl moves towards the compartment with lower concentration until they are equal
- After diffusion the compartment that was originally lower in concentration will be more negative and the other more positive (diffusion potential), this is because Cl diffuses through water faster -> electrical potential will cause the ions to move
diffusion potential
- stabilizes when potential difference balances the greater mobility of Cl-Na & Cl cross membrane at same rate
- the diffusion creates the potential, the potential influences the diffusion
- dependent upon the concentration gradient
- will continue as long as there is a concentration gradient between compartments
- eventually it will equilibriate and stop
equilibrium potential* ex
- Cl travels down its concentration gradient
- Na cannot cross
- Diffusion of Cl creates a neg charge which will cause the opposition of Cl diffusion
- Diffusion moves Cl from 1->2
- Electropotential moves Cl from 2->1
- electropotential prevents Cl from moving down is gradient
- equilibrium is achieved when electropotential is exactly sufficient to balance the force of diffusion and there is NO NET FLUX of Cl across membrane
nernst equation
- each permeant ion has a unique equilibrium potential that is primarily due to the concentration gradient of that ion across the membrane
- E=RT/zF ln(ion1/ion2)
- R=gas constant
- T= absolute temp (K)
- F=Faradays constant
- z=valence electrons
- assuming T is 20C…E= 58/z log(ion1/ion2)mV
effect of electric potential
- electric potential is zero- Cl moves high to low
- electric potential is exactly the same as the force of diffusion -> equilibrium
- electric potential is greater than equilibrium potential it offsets diffusion- Cl moves away from neg charge
steady state
- since the permeability is greater for one ion than the other, the membrane potential is close to the equilibrium potential for the dominant ion
- resting membrane potential
- sum of all ionic currents is equal to zero in steady state
- no net current (sodium leaving is equal to potassium coming in -> sum of ionic currents is zero)
- permeability ratio is .02
equiliirum
-achieved when electrical potential is exactly sufficient to balance the force of diffusion and there is no net flux of Cl across the membrane
membrane potential in most cells
- 70mV
- Em
- this is only one membrane potential but can be multiple equilibrium potential
membrane potential is determine by
- the intracellular and extracellular concentrartion of the permeant ions
- determines equilibrium potential for each ion - the permeability (P) of the membrane for each ion
- determines relative influence of each ion on E
- in cells in the resting state P potassium > P sodium
- therefore, K is the dominant ion and the membrane potential is close to E
- if the membrane is permeable to an ion and Eion is not equal to Emembrane then the ion will cross the membrane pulling E membrane towards E ion
- there is only one membrane potential
goldman equation
- Em= 58 log (concen of potassium out +b(Sodium out)/(potassium in +b(sodium in)))
- where b=Pna/Pk
Typically at rest, K permeability is 50 times greater than Na permeability (.02) and Em=-70
if b suddenly becomes equal to 20, what happens to Em
- when b becomes much bigger Na concentration becomes more relevant than K
- Em becomes +50mv because its closer to Na Em
membrane potential becomes less neg
depolarize
-less polarized
cell membrane potential becomes more negative
hyperpolarize
cell membrane returns to normal
repolarization
equilibrium potential
- the membrane potential that exactly balances the force of diffusion for some ion crossing the membrane (no net flux)
- unique potential for each ion
- tries to stay constant
- based on log of ratio of concentration
20 or 37
58 and 61
electrical current and movement of ions across membrane
- the amount of current carried by an ion through a membrane (Iion) is equal to ease of movement through the ion channels multiplied by the driving force for diffusion for the ion (Em-Eion)
- I(ion)=g(ion) (Em-Eion)
membrane potential
Em
equilibrium potential
Eion
membrane permeability
- determined by the properties of the aqueous ion channels in the plasma membrane
- can change easily
1. ion selectivity of channel
2. channel conductance (gs)- index of ion flux through the membrane (opposite of resistance)
3. the average proportion of the time individual channels are open (Po)
4. channel number (N) or density (number per area)- functional density vs. anatomical density - determined by the density and conductance of the channels in the membrane that are selective for that ion
cell signaling with membrane potential
cell tries to maintain a constant equilibrium potential for the ions and relative permeability can change quickly
- use this to signal in a cell
- changing membrane potentials signal to the cell
membrane potential becomes positive
overshoot
graded potentials
- increase in amplitude with increase in stimulus strength (amplitude is graded)
- membrane potential is changing due to change in permeability in channels in the membrane
- can be hyperpolarizing or depolarizing
- localized bc its not regenerative
- spatially and temporally summate- two graded potentials come together and make a larger one or smaller one (if opposing)
- spreads passively to neighboring membranes, NOT regenerative- the potential doesnt regenerate -> it gets weaker by distance
- decrease in amplitude as a function of distance traveled along the membrane in an excitable tissue
ex. of graded potential
BETA CELL AT REST
-creates insulin in response to a increase in glucose (transported into the cell through facilitated diffusion carrier protein)
-low glucose -> low metabolism -> low ATP
-transmembrane integral protein (potassium channel) senses the ATP (ligand gated channel)-> ATP bound- close, ATP not bound- open
-relative permeability of K is high when ATP is low -> membrane potential is negative from K leaving
BETA CELL SECRETES INSULIN
-high glucose in plasma -> high metabolism -> high ATP
-K gated channel closes
-decrease in permeability to K -> membrane potential increases (depolarizes)
-voltage gated calcium channel opens due to depolarization -> calcium enters -> exocytosis of cell -> releases insulin into blood stream
excitable cells
a cell that has voltage gated channels
-cells are excitable when gates are closed (at rest) (not inactivated)
neuron
- cell body and dendrites- receives information and integrate it through temporal and spatial summation of graded potentials
- decides to fire a action potential
- conduction of action potential travels down the axon and causes the release of neurotransmitters -> stimulates next cell
action potential
- if you depolarize an excitable cell it will give a graded depolarizing potential
- depolarizes until it reaches threshold potential- cell generations all or none action potential
- large, short (varies) change in membrane potential
characteristics of an action potential
- triggered by depolarization
- threshold depolarization is required
- all or none events
- at peak the membrane potential reverses sign- cell becomes inside positive (overshoot)
- at the end the membrane potential is transiently more negative (undershoot)
- after a neuron fires an action potential, it cannot generate another one for a brief period (absolute refractory period)
- propagate along axon without decrement
action potentials are dependent on
- voltage gated channels, specially voltage gated sodium channels
- depolarization begins to open voltage gated sodium channels
- sodium rushes in and membrane potential changes rapidly
- equilibrium potential is similar to sodium and b=20
- sodium permeability decreases at peak while potassium increases
depolarization triggers 3 things
- activates voltage gated sodium channels- fast
- inactivates voltage gated sodium channels- causes them to close but slow* (at peak)
- activates voltage gated potassium channels- open and close slow (at peak)
depolarization cycle
hodgken cycle
- depolarization causes the opening of voltage gated sodium channels
- this causes more sodium to enter which causes depolarization
- this causes more gates to open
- positive feedback
- stops at threshold
activating and deactivating gates
- at rest the activation sodium channel is closed, the inactivation is open, and K gate is closed
- activation gate opens in response to depolarization
- potassium gates havnt opened yet bc slow
- inactivation gates havnt closed yet bc slow
undershoot
- hyperpolarization
- sodium channels are inactivated (slow to close)
- potassium channels are still open (bc slow to close)
absolute refractory period
- even if the cell were to depolarize the gates are inactivated (bc they are slow to close)
- arnt enough closed gates to respond
- dependent only upon sodium channels
difference between inactivated and closed
- both wont let sodium through
- closed will respond to depolarization but inactive wont (until it open)
relative refractory period
- only a larger than normal stimulus can initiate a new action potential
- dependent upon both the number of sodium channels that have closed and the number of potassium channels that are still open
- once the cell repolarizes the sodium gates will begin to close (but not completely at rest)
regenerable
- action potentials are regenerable because of positive feedback
- neighboring gates are reaching threshold- all or none
- each patch of the membrane is firing its own action potential as it goes down
myelinated axon
- insulates to prevent current ion from leaving the cell
- shwan cells- produces myelin
- node of ranvier- gaps
- blocks the flow of ions into and out of the axons- the only place the current can flow is in the nodes of ranvier
- action potential skips from node to node- saltatory conduction
how do cells pick up information from the environment and respond appropriately
- fundamental problem for all life
- cells need to translate an external signal into an internal change
- cells can use electrical or chemical signals to relay messages, sometimes over long distances
- similar messages used in all forms of multicellular life
cells communicate over short distances- local
- gap junctions
- contact dependent signals
- paracrine signals
- autocrine signal (same cell)
gap junctions
- form direct cytoplasmic connections between adjacent cells
- electrical communication
- localized
- express proteins in their cells called connexins- form channels (pores) between the two cells -> allow for diffusion between the cells
- *electrical synapse
contact-dependent signals
- signal transduction proteins
- require interaction between membrane molecules on two cells
- matches up and starts intercellular messaging
autocrine signals
- act on the same cell that secreted them
- release signals that bind to receptors on the outside of the same cell
- signal being transfer via extracellular fluid
- local
paracrine signals
- are secreted by one cell and diffuse to adjacent cells
- local
- signal being transfer via extracellular fluid
- doesnt go into the circulatory system
hormonal signaling
- signal being transferred through the plasma
- goes through the circulatory system
- long distances
endocrine cells
- release chemicals into the blood stream
- goes to every other cell in the body (only cells with the specific receptor respond)
- long distance
electrical communication
- Withing a cell
- action potentials
- graded potentials
- Between cells
- electrical synaptic transmission
- gap junction
chemical communication
- Within a cell
- cell signaling (intracellular signaling molecules and pathways)
- Between cells
- chemical synaptic transmission
- endocrine system
- specialized paracrine
electrical synaptic transmission
electrical signals move between cells through gap junctions
chemical synaptic transmission
- the electrical signal from the action potential in the presynaptic neuron axon is converted into a chemical signal at the synapse
- involves 2 cells (sometimes more)
- requires a receptor and ligand (first messenger)
- chemical message is relayed within the cell and translated into a cellular change using signal amplification
- specialized paracrine
presynaptic cell
always a neuron
postsynaptic cell
- neuron
- muscle cell
- cardiac conducting system cell
- endocrine cell
- target cell
synaptic terminal bouton
- the end of the presynaptic cell
- does not contain voltage gated Na or K channels
- region that releases neurotransmitters
neurotransmitter release
- action potential depolarizes the axon terminal
- graded depolarization opens Ca channels and Ca enters cell
- Ca binds to snare proteins (synaptotagmin)
- synaptic vesicles release neurotransmitters into the synaptic cleft- exocytosis (slowest step)
- neurotransmitters binds to ligand gated receptors on the postsynaptic cell -> cellular response
excitation of the synaptic terminal
- the action potential coming down is stopped by the Na and K gates
- excitation in the synaptic terminal itself is a graded depolarization that spread from the axon into the terminal
- > activates Ca channels
three ways to stop an action potential
- some mechanism that transmits the neurotransmitters back up into the cell- can be done by a support cell or cells in the synaptic terminal
- enzymes preset to breakdown the neurotransmitter- (AChE breaks down ACh) -> recycles the ACh to be used again
- neurotransmitters diffuse away from the synaptic cleft (slow)
saltatory conduction of graded potentials occurs more rapidly in myelinated axons compared to unmyelinated axons
- false
- there are no saltatory conductions of graded potentials
- there are no saltatory conduction in an unmyelinated axon!
in neurons, activation of ligand-gated channels that generate hyperpolarizing graded potentials will inhibit the formation of an action potential
- true
- getting farther away from threshold
- inhibits
the extracellular and intracellular compartments are in osmotic equilibrium, but chemical and electrical disequilibrium
- true
- cell are in osmotic equilibrium (or else they would be changing sizes)
- cells are in chemical and electrical equilibrium in order to cell signal
receptor signaling
- chemical
- endogenous and exogenous ligands can activate receptors
- **cellular response depends on the receptor not the ligand
- chemical message is relayed within the cell and translated into a cellular change using signal amplification
second messengers
- the first messenger (ligand) binds and initiates an amplification process that produces other chemicals inside the cell
- the other chemicals that are signaling molecules are second messengers
4 types of Receptors
- receptor-channel
- G protein-coupled receptor
- receptor enzyme
- integrin receptor
receptor-channel
- ligand binding opens or closes the channel
- ligand gated channel
- ex. ATP gated potassium channel in the beta cell
- *the channel is the receptor
- fast!!
G protein-coupled receptor
- ligand binding opens an ion channel or alters enzyme activity
- membrane proteins that interact with GPCR’s
- ligand binding causes a conformational change
- contains 3 subunits that dissociate upon activation- alpha, beta gamma
- alpha subunit drops GDP and picks up GTP
- GDP goes around the cell and activates things
- different subunits are active in different signaling pathways
- diverse, common
receptor-enzyme
-ligand binding activates an intracellular enzyme
integrin receptor
- ligand binding alters enzymes or the cytoskeleton
- integrated into the cytoskeleton of the cell
- causes changes in the structure of the cell
Nicotinic acetylcholine receptor (nAChR)
- receptor-channel ex.
- monovalent
- when ACh binds the channel opens and ion can flow
- change in permeability in the membrane to Na and K -> depolarization graded potential -> electrical signal -> voltage gated channels are signaled
- Na driving force > K driving force- Na will have a stronger electrochemical gradient
- skeletal muscles cells -> muscle contraction
- non selective (monovalent) cation-specific ion channel
- depolarizing graded potential
- nicotine is an exogenous agonist to this receptor
antagonist
-blocks receptor activity
agonist
-activates the receptor
Muscarinic (mAChR)
- G protein coupled receptor ex.
- Ach binds to receptor and activates it releasing the g protein (betta and gamma)
- Subunits activate ligand gated potassium channels
- permeability to potassium increases -> change in membrane potential -> inhibitory response due to hyperpolarizing graded potential
- muscerin is the exogenous agonist