Excitable membranes Flashcards
Wk 7
What is the membrane potential caused by?
movements of different ions via concentration and electrical forces
Describe the basis of NA+/K+ pump
important for nerve function
3NA out of cell and 2K into cell using ATP as energy.
What are the functions of NA+ K+ pump?
Very important nerve function
Responsible for establishing and maintaining the NA and K concertation gradient
How much of the brain’s energy does the Na+K+ pump use?
25%
Are cells positively or negatively charged compared to the ECF?
negatively
What is the charge of the inside of the cell compared to the outside?
Inside= -70mV
Outisde = 0mV
What is the equilibrium potential of any cell dependant on?
valence and intra/extra cellular concentrations
What is the cell membrane impermeable to at rest?
Na+ and fairly impermeable to K+
What is the equilibrium potential?
Value in which electrical force is equal and opposite to chemical driving force
What do changes in intracellular or extracellular ion alter?
concentration force of the ion.
What is the rule for ion movement?
ion will move in the direction that brings membrane potential closer to the equilibrium potential for that ion.
What do changes in permeability also affect
ion movement across the movement
Are Na and Cl ion concentrations higher on the outside or inside of the cell?
Higher on outside
is K ion concentration higher on the
Higher on inside
What are three important things to consider in ion and plasma membrane transport?
Ion concentrations are different inside/outside of the cell
Electrical forces (opposites attract)
Ion channels allow ions to move – passive and gated
How do mechanically-gated channels open?
in response to the physical deformation of receptors
How do voltage-gated channels open?
in response to changes in MP
What type of a channel is chemically gated?
Ligand-gated channels
How are chemically gated channels opened?
by binding of specific neurotransmitter
How do passive ion channels open?
They are always open?
What is the resting membrane potential generated by?
differences in ionic makeup of ICF and ECF and Differential permeability of the Na and K plasma membrane.
Are neuron membranes more permeable to K or Na?
K
What is the intracellular concentration of Na?
15mM
What is the intracellular concentration of K?
140mM
What is the intracellular concentration of Cl?
4mM
What is the extracellular concentration of Cl?
115mM
What is the extracellular concentartion of Na?
145mM
What is the extracellualr concentartion of K?
4mM
In what ways to ions move?
Ions move from high to low
Positively charged anions move towards negative areas
What do ions create as they move?
A flow of electrical currents that change the membrane potentia
Why are many cells more permable to K?
Most membranes have more K leak channels than Na leak channels = more permeable to K
Where are leak channels found?
nearly all cells
Where are ligand-gated channels found?
Dendrites’ of sensory neurons (pain receptor’s)
Dendrites and cell bodies of interneurons and motor neurons
Where are volatge gated channels found?
axons of all neurons
Where are mechanically gated channels found?
Dendrites of some sensory neurons such as touch receptors, pressure receptors and some pain receptors
How do mechanically gated channels work?
The force from the physical stimulus disports the channels from the resting position causing the gate to become opened
what is the role of voltage-gated channels?
Participate in the generation and conduction of action potentials in axons.
How is resting membrane potential created?
Due to a small build up of negative ions in the cytosol and an equal buildup of positive ins in ECF.
The greater the difference in charge across the membrane
= larger membrane potential
What is the resting membrane potential?
is an electrical potential difference (voltage) that exists across the plasma membrane of an
excitable cell under resting conditions.
What is the resting membrane potential range for neurons?
-40 - -90 V average = -70
What does the negative sign in -70 (for example) indicate)?
the inside of the cell is negative relative to the outside.
What do you call a cell that exhibits membrane potential?
polarised
What does extracellular fluid have a higher concentration of?
Na and Cl
What is the result of more K leak channels?
the number of potassium ions that diffuse down their concentration gradient out of the cell into the ECF is greater than the number of sodium ions that diffuse down their concentration gradient from the ECF into the cell.
What is a major contributor to negative resting membrane potential
Inability of most anions to leave the cell
What are the three major factors that contribute to resting membrane potential?
Unequal Distribution of ions in ECF and cytosol
The inability of most anions to leave the cell
Electrogenic nature of Na and K ATPases
What occurs if the membrane becomes polarised? What is this called?
inside becomes more negative
= hyperpolarising graded potential
What occurs if the membrane becomes less polarised? What is this called?
inside becomes more positive = depolarising graded potential
Where are graded potentials most commonly found and why?
mechanically gated and ligand-gated are mostly in dendrites of sensory neurons or interneurons and motor neurons = most are in dendrites and the cell body of a neuron.
What does graded mean?
electrical signals vary in amplitude depending on the strength of the stimulus.
What is the generation of Action Potentials?
Sequence of rapidly occurring events that decrease and reverse membrane potential
restores to resting state
What occurs in the depolarising phase?
Negative membrane becomes less negative –> reaches zero –> becomes positive
What is the first step in the Generation of Action Potential?
Ist = Na voltage-gated channel = depolarisation
What is the second step in the Generation of Action Potential?
2nd - K voltage-gated channels open =. k flows out = repolarising phase
What are the two signals that produce changes in the membrane potential?
Graded potentials (incoming signals over short distances) and Action potentials (long distances)
How do neurons communicate?
Via changes in the resting membrane potential.
what changes resting membrane potential?
changes in: the concentration of ions across the membrane and membrane permeability
What is a graded potential?
Incoming signals over short distances that create localised changes in membrane potential
What are the features of a graded potential?
Magnitude varies with stimulus strength
Stronger stimulus –> more voltage change = farther current flow
Flow decays quickly
Triggered by a stimulus that opens gated-ion channels
what mV does the membrane potential change to when it uses Action Potentials?
–70mV to +30mV
What channel opens and closes to allow for the occurrence of action potentials?
voltage-gated channels
What is the opening of voltage-gated channels in Action Potential due to?
The depolarising graded potential.
What are the two main ionic events that occur in Action potentials?
A brief increase in Na permeability (flows in)
Delayed and slightly longer increase in K permeability (K out)
What are the three resulting phases of Action Potentials?
Depolarisation
Repolarisation
Hyperpolarisation
what is threshold potential? normal value?
- a certain membrane potential that must be reached in order to open VG {Na^2) channels
- -50mV
What occurs in depolarisation?
Excitory postsynaptic potential reaches threshold (-50mV) –>activation of gates Na channel –> influx of Na –> MP depolarisation (more positive)
what occurs in repolarisation?
- nactivation gates of VG {Na^+ c)hannels close and delayed opening of VG {K^+} channels
- {K^+} efflux drives repolarisation (MP becoming -)
- RMP restored, VG {Na^+}$ channels activation gates close and inactivation gates open
What occurs in hyperpolarisation?
- $\small\ce{K^+}$ efflux continues and drives MP past RMP of -70mV
- VG $\small\ce{K^+}$ ion channels close (all VG ion channels closed)
- ion movement via passive channels restores RMP
What is an absolute refractory period?
period during which neuron cannot respond to another stimulus and fire another AP
What is an relative refractory period?
period during which neuron can fire another AP but only if stimulus is strong enough
What are postsynaptic potentials?
Graded potentials generated in postsynaptic neuron
What are the two types of postsynaptic potentials?
excitatory = depolarsing = closer to threshold potnetial
inhibitory = hyperolarisng = further from 0MV
what does the amount of neurotransmitters at the synaptic cleft depend on?
Frequency of Action Potentials
What are the 4 groups of neurotransmitters?
Acetylcholine (Ach)
Catecholamines
idolamines
Amino acids
where does Acetylcholine act at?
neuromuscular junction
somes ANS/CNS synapse
What are the types of catecholamines?
epinephrine, norepinephrine and dopamine
Where do catecholamines act?
at adrenergic receptors
What are the two idolamines?
serotonin and histamine
what are the two amino acids in neurotransmitters?
glutamate and GABA
What two types of receptors doe neurotransmitters act on?
Ionotropic (LGICs) and Metabotropic (GPCRs)
What are the properties of action of ionotropic receptors?
immediate and brief
What is the mechanism of action of ionotropic receptors?
ligand binding activates channel
What are the properties of action and mechanism of action for metatropic receprtors (GPCRs)
properties
indirect (act through 2nd messengers)
- complex
- slow, and often prolonged
Mechanims
EPSPs and IPSPs summate to influence postsynaptic neuron
What is hypokalemia? what are its effects?
low K in ECF
decreased K = greater electrochemical gradient driving K efflux = hyperpolarisation = MP more negative
What are excitatory receptors channels for (inotropic receptors)
Small cations
Na influx = most depolarisation
What are inhibitory receptors channels for (inotropic receptors)
Cl- influx = hyperpolarisation
What is teh role of summation in synaptic integrtion?
EPSPs and IPSPs add (summate) influence to postsynaptic neurones
In synpatic integration: Ap will only result id
Excitatory Postsynaptic potentials are present
