Chapter 12 [ EXAM #1 ] Flashcards
action potentials (3)
To get electrical signal from dendrites and cell body to axon terminal
Rapid change in membrane potential that once starts moves along length of excitable membrane without loss of strength: “all or none” (-60mV does not work)
- Neurons and muscle cells
Voltage gated Na+ channels (VGSCs) open at axon’s initial segment
- Depolarizes and triggers more to open
- Action potential is nerve impulse: domino effect along membrane
threshold (4)
Depolarization of membrane to certain level: threshold (~60mV in neurons)
Result of local graded potentials produced by >1 stimuli
Locally summed at axon hillock
Spreads to initial segment
- VGSCs in initial segment open
- Chain reaction along membrane
threshold all-or-none principle (3)
Triggered when threshold reached -55mV: graded potentials result of stimuli added together
or not (subthreshold): only produces graded potential below threshold -55mV
Action Potential Generation (the 4 steps)
- ) Depolarization to threshold (-60mV)
- ) Rapid Depolarization: Activation of VGSCs (Na+ channels)
- ) Repolarization: Inactivation of VGSCs and activation of K+ channels (Kv)
- ) Hyperpolarization and return to normal resting potential and permeability
Channels involved in action potential generation
voltage gated Na+ (VGSCs) and K+ (Kv) channels
At resting potential, what channels are open?
Only leak channels are open at this phase
1.) Depolarization to threshold (-60mV)
local currents, graded potential summation at axon hillock
2.) Rapid Depolarization (3)
Activation of VGSCs (Na+ channels)
At threshold Na+ channel activation gates open, influx of Na+ occurs, opens more Na+ channels (up to +30mV)
Positive feedback loop
Is sodium positive or negative charge?
positive
3.) Repolarization (4)
Inactivation of VGSCs (close) and activation of K+ channels (open) (Kv)
Occurs at +30mV
Kv channels open slowly
K+ flows out of neuron
- Hyperpolarization (6)
return to normal
Kv channels begin to close at RMP (~ 70mV)
Kv finish closing slowly (~90 mV)
returns to rest (AP is over)
Na+/K+ ATPase works to restore ion concentrations
Na+ channels reset (ready)
Define refractory period
time between action potential start until normal resting potential regained
2nd action potential can not form in response to normal threshold stimulus
Absolute refractory period
Can not respond to any further stimulation
Maximum VGSCs open or inactivated
K+ flow-opposing force
Relative refractory period
Can produce another action potential in response to larger-than-normal stimuli….why?
Some VGSCs have returned to resting state
Propagation of action potentials
Propagation: message passed along axon by repeating same events over and over (open voltage-gated channels etc.)
Can be continuous or saltatory
From point of generation (axon hillock and initial segment) to axon terminal action potential keeps its strength
Where does continuous propagation occur
unmyelinated axons
is continuous propagation fast or slow
slow (2mph)
how does continuous propagation operate?
local Na+ current opens VGSCs in next axon segment
every axon segment affected
message is “retold” over and over until it reaches synapse
why can’t continuous propagation travel backward?
refractory periods
where does saltatory propagation occur?
myelinated axons of PNS and CNS
is saltatory propagation fast or slow?
fast (250 mph)
does saltatory propagation require more or less ATP than continuous propagation?
saltatory propagation uses less ATP than saltatory propagation
how does saltatory propagation operate?
local currents skip internodes, depolarize nodes of Ranvier
how does axon diameter affect absolute refractory period?
smaller axon diameter = longer abs. refract. period
what prevents continuous propagation in saltatory propagation?
myelin wrapped around axons = greater resistance to ion flow (few VGSCs)
what are type A nerve fibers?
largest
myelinated
urgent 250mph
somatic sensory and motor fibers
what are type B and C nerve fibers?
smallest
myelinated and unmyelinated
slow 2mph
visceral sensory and motor fibers serving organs
some somatic pain
what are synapses
non-direct cell-to-cell attachment
most neuron-neuron and all neuron-cell
where does NT travel in the context of post- and pre-synapse?
from the pre-synaptic cell to the post-synaptic
what is the chemical called that is released from the pre-synaptic cell?
neurotransmitter
what is an excitatory neurotransmitter?
depolarization results
promotes action potential
what is an inhibitory neurotransmitter?
hyperpolarization results
opposed action potential
what is the function of synaptic vesicles?
to store neurotransmitters
what is a cholinergic synapse?
a synapse that releases acetylcholine (ACh)
where can you find cholinergic synapses?
NMJ
neuron-neuron in PNS
parasympathetic neuromuscular and neuroglandular synapses
brain
what are the four steps occurring in a cholinergic synapse?
AP arrives and depolarizes at synaptic terminal
Ca2+ enters synaptic terminal, triggering exocytosis of ACh
ACh binds to receptors and depolarizes the postsynaptic membrane
ACh removed by AChE
what is synaptic delay
time between arrival of action potential and post-synapic effect
reflects time of Ca2+ influx and NT release
less synapses = short synaptic delay
more synapses = more synaptic delay
what does Ca2+ do?
causes release of vesicles that release ACh
what is synaptic fatigue? what does it depend on?
NT cannot recycle fast enough to meet demands
depends on RER production in cell body and axoplasmic transport
where is norepinephrine released?
adrenergic synapses
CNS & sympathetic nerves
what is the role of norepinephrine?
excitatory and deploarizing effect
where is dopamine found?
CNS
what is the role of dopamine?
excitatory or inhibitory depending on the synapse
is dopamine depleted or increased in Parkinson’s?
depleted
is dopamine depleted or increased with cocaine usage?
increased
where is serotonin found?
CNS
what is the function of serotonin?
alters emotional states (e.g. chronic depression)
effects depend on receptors
what is substance P?
pain
what is the function of amino acid based peptide NTs?
substance P and pain nerves
what is the function of opioids?
endorphins and pain reduction
what is the function of neuromodulators?
affect pre- and post-synaptic cells
rate of NT synthesis
what are the direct effects NTs have on membrane potential?
NT binds chemically to gated ion channel, channel opens
Ionotropic: alters ion flow through membrane
fast effect compared to indirect
what is Ionotropic? is it a direct or indirect effect on membrane potential?
direct effect
alters ion flow through membrane
what are the indirect effects NTs have on membrane potential?
NT bind to membrane proteins linked to G-proteins
Metabotropic: NT receptor coupled to G protein
slow effect compared to direct
function of G protein?
triggers intracellular signaling cascade, turns on enzymes, etc.
can the same NT be excitatory at some synapses and inhibitory at others?
yes
how does lipid soluble gas enter the cell?
diffusion
what does lipid soluble gas bind to?
enzymes in cytosol
what is the role of lipid soluble gas?
it’s the second messenger production that affects post-synaptic cell activity
what are the two gases at certain brain synapses?
NO and CO
how many stimulus is needed for action potential formation?
> 1
what determines form of action potential?
sum of all stimuli
role of axon hillock
integrates (sums) stimuli, determines effect on initial segment
what are the two types of summation?
temporal summation
spatial summation
define temporal summation
summation of stimuli that occur at same synapse but at different times
if a second stimulus arrives before the effects of the first has worn off, it is called…
temporal summation
does temporal summation result in more or less NT released by presynaptic terminal?
more NT
do graded depolarizations (or hyperpolarizations) increase or decrease in temporal summation?
increase
define spatial summation
summation of stimuli at multiple synapses at same time
what two things does the amount of depolarization depend on in spatial summation?
number of synapses active at same time
how far away from axon hillock the synapse is
define postsynaptic potentials (PSP)
graded potentials develop in postsynaptic membrane when NT binds
define excitatory PSP (EPSP)
NT causes membrane depolarization
Na+ flows in
makes more neg
define inhibitory PSP (IPSP)
NT causes hyperpolarization
makes more positive
define facilitation
a neuron whose membrane potential shifts closer to threshold
if EPSPs > IPSPs, but below threshold what is the result
no AP
if EPSPs > IPSPs and reaches threshold what is the result
AP produced
rates of action potentials are limited by what two things?
refractory periods and NT availability
what determines how the message is interpreted in action potentials?
frequency
define neuronal pools
connected neurons in CNS responsible for a specific function
define neural circuit
wiring patterns of synaptic connection in neuronal pools
what is a diverging circuit?
one neuron synapses on many post-synaptic cells
amplifies signal
tree-effect
one brain neuron stimulating many muscle fibers
what is converging circuit?
several neurons synapse of one post-synaptic
effective stimulation or inhibition
primarily sensory
what is reverberating circuit
branches from later neurons to former ones
recycles impulses
prolongs response to stimulus
short term memory, normal breathing
what is serial processing?
info relayed in stepwise manner from one neuron to another
straight line, single file
spine reflexes, sensory to brain
what is parallel processing
divergence
several neurons process info at same time side by side
