NS: POTENTIALS & TRANSMISSION Flashcards
rate of diffusion through a membrane depends on:
- magnitude of concentration gradient: ↑ [gradient] ↑ ∆ of diffusion
- permeability of membrane: ↑ permeability ↑ ∆ of diffusion
- SA of membrane: ↑ SA ↑ ∆ of diffusion
- mol weight of substance: ↑ mol weight ↓ ∆ of diffusion
- distance (thickness) over which diffusion takes place: ↑ distance ↓ ∆ of diffusion
membrane potential (MP)
separation of opposite charges across the PM
* separated charges form a layer around PM
* greater separation = larger MP
- EMP for K+
- EMP for Na+
- RMP
- GP
- AP
equilibrium potential
force exerted by electrical gradient balances force exerted by [gradient]
cell creates charge separation through:
1. establishing & maintaining [gradient] for key ions (K+/Na+)
2. ions diffuse through membrane down [gradient]
3. diffusion through membrane results in charge separation creating MP (electrical gradient)
4. net diffusion continues until force exerted by electrical gradient exactly balances force exerted by [gradient]
5. the potential that would exist at this equilibrium = equilibrium potential
equilibrium potential for K+ (EK+= —90mV)
1. [gradient] moves K+ out of cell
2. outside of cell becomes more +
3. electrical gradient moves K+ into cell
4. electrical gradient counterbalances [gradient]
5. no further net movement of K+ occurs ➞ EK+ = —90mV
equilibrium potential for Na+ (ENa+ = +60 mV)
1. [gradient] moves Na+ into cell
2. inside of cell becomes more +
3. electrical gradient moves Na+ out of cell
4. electrical gradient counterbalances [gradient]
5. no further net movement of Na+ occurs ➞ ENa+ = +60 mV
resting membrane potential
- −70mV
- K+ much more permeable than to Na+
- RMP is much closer to K+ EP because K+ b/c of higher permeability
- established from balance of passive leak channels & active Na+/K+ ATPase pumps
- neither ion is at equilibrium
- [gradients] & permeabilities remain constant
- RMP established by these forces remaining constant
transmembrane transport
- Na+/K+ leak channels ➞ ions diffuse down [gradients] (passive)
-
Na+/K+ ATPase ➞ establish unequal distribution of Na+/K+ inside/outside of cell
- active
- pumps 3 Na+ out for every 2 K+ in
- ICF: ↓ [Na+] & ↑ [K+]
depolarization
change in membrane polarization to more + values than RMP
* less polar
* upward deflection on graph = ↓ potential
* -50 mV ➞ -30 mV
* Na+ channels open & Na+ enters the cell
repolarization
return to MP after depolarization
* ~ +30 mV ➞ -70 mV
* K+ channels open & K+ leaves cell
hyperpolarization
change in membrane to more - values than RMP
* more polarized
* closer to K+ EP
* downward deflection = ↑ in potential
* ~ -70 mv ➞ -80 mV
graded potential
local changes in MP at varying grades/degrees of magnitude/strength
* triggering even does not exceed threshold potential (~ 50mV)
* decay with distance
* acts through neurotransmitter post-synaptic cells
* additive
* charge spreads in both directions
* size correlates with size of stimulus
* spread of passive current flow:
* current: any flow of electrical charge
* resistance: hindrance to electrical charge movement
action potential
brief all-or-nothing reversal in MP
- cannot stop
- initiated by rapid changes in membrane permeability to Na+/K+
-
depolarization (rising phase) = Na+ in
- more rapid than K+ channels (0.5ms)
- at threshold: Na+ activation gates open & Na+ permeability rises
-
repolarization (falling phase) = K+ out
- Na+ channels close & K+ channels open
- hyperpolarization = K+ continuing to exit cell past RMP
AP propagation
- propagation: transmitting/spreading signal
- AP propagates when depolarizing current spreads & causes adjacent regions to depolarize
- conducted through axons
- contiguous (continuous) conduction
- saltatory conduction
contiguous (continuous) conduction
propagation of AP in unmyelinated fibers
* touching ➞ next to in sequence
* slower than saltatory conduction
saltatory conduction
propagation of AP in myelinated axons by jumping from node to node
- rapid impulse jumping between myelin sheath gaps ➞ skip myelinated sections
- much much faster than coniguous
-
myelin: multilayered sheath of PM derived from specialized glial cells that wrap around axon fibers & act as insulator for current flow
- schwaann cells: myelin-forming glial cells in PNS
- oligodendrocytes: myelin-forming glial cells in CNS
- nodes of ranvier = gaps in myelin sheath containing high densities of voltage-gated Na+/K+ channels
-
multiple sclerosis (MS): autoimmune disease that attacks myelin sheaths➞ slow transmission of impulse in affected neurons
- symptoms:
- vision
- pain
- speech
- memory
- coordination
- symptoms:
refractory periord
-
absolute refractory period: brief period during spike
- repolarization: voltage-gated Na+ inactivation gates close
- 2nd spike cannot be generated
-
relative refractory period: brief period following a spike
- below RMP: voltage-gated Na+ channel inactivation gates open
- capable of opening in response to depolarization
- during hyperpolarization a higher intensity stimulus is needed
- prevents backward current flow
- AP cannot be initiated in a region that has just undergone AP
- ensures 1-way propagation & limits frequency
neuron anatomy
- soma = cell body
- dendrites = input zones ➞ receive incoming signal
- axon hillock = trigger zone ➞ initiates AP
- axon terminals = output zone ➞ release neurotransmitter in response to AP propagation
convergence
many neural inputs ➞ 1 output
divergence
1 neural input ➞ many outputs
synapse
junction between 2 neurons or 1 neuron + 1 muscle or gland that enables 1 cell to electrically/biochemically influence another cell
-
electrical synapse: direct electrical signals through gap junctions
- gap junctions: made of multiple connexins (proteins) ➞ only small mol
- chemical synapse: chemical messenger transmits info 1 way across synaptic cleft
neurotransmitter removal
- degradation: enzymatic breakdown (ex: AChE)
- transport: active transport back into preseynpatic cleft via transporter ions (re-uptake)
- diffusion: transmitter simply transfuses away from synaptic terminal
effect: tetanus blocks vesilucar fusion
blocks transmission
effect: cocain blocks reuptake of dopamine
prolongs transmission
effect: SSRIs block reuptake of serotonin
prolongs transmission
effect: insecticides block degradation of ACh
prolongs transmission
effect: curare blocks post-synaptic action of ACh at neurotransmitter junction
blocks transmission
effect: THC is an antagonist for the endogenous cannabinoid receptor
prolongs transmission
synaptic transmission
- AP propagation in presynaptic neuron
- Ca2+ entry into presynaptic knob
- release neurotransmitter by exocytosis
- binding of neurotransmitter to postsynaptic receptor (target cell can be muscles, gland, or other neurons)
- opening of specific ion channels in sub-synaptic membrane
- presynaptic release: voltage-gated Ca2+ channels activate synaptic release
- post synaptic response: postsynaptic receptors & postsynaptic potentials (PSPs)
postsynaptic graded potentials
excitatory postsynaptic potential (EPSP): depolarizing potential brings MP back towards threshold
- glutamate (Glu) & ACh
inhibitory postsynaptic potential (IPSP): hyperpolarizing potential that brings MP away from threshold
- gamma-aminobutyric acid (GABA) & glycine (Gly)
temporal summation: additive effect of PSPs occurring close together in time (EPSP or IPSP)
spatial summation: additive effect of EPSPs occurring together on nearby parts of same cell
cancellation summation: EPSP & IPSP cancel each other out
presynaptic inhibition: inhibitory presynaptic neuron inhibits a postsynaptic neuron that acts as an excitatory presynaptic neuron for a target cell
EPSP
excitatory postsynaptic potential = depolarizing potential that brings MP back towards threshold
* glutamate (Glu) & ACh
* postsynaptic graded potential
IPSP
inhibitory postsynaptic potential = hyperpolarizing potential that brings MP away from threshold
- gamma-amino butyric acid (GABA) & glycine (Gly)
- postsynaptic graded potential
temporal summation
additive effect of PSPs occurring close together in time (EPSP or IPSP)
* postsynaptic graded potential
spatial summation
additive effect of EPSPs occurring together on nearby parts of same cell
* postsynaptic graded potential
cancellation summation
postsynaptic graded potential where EPSP & IPSP cancel each other out
presynaptic inhibition
inhibitory presynaptic neuron inhibits a postsynaptic neuron that acts as an excitatory presynaptic neuron for a target cell
Na+/K+ movement: EP
- [gradient] moves Na+ in & K+ out
- electrical gradient moves Na+ out & K+ in
[Na+/K+] ICF
↓ [Na+]
↑ [K+]
[Na+/K+] ECF
↑ [Na+]
↓ [K+]
Na+/K+ movement: active pumping
3 Na+ out
2 K+ in
