2.3 - Graded Potentials and Action Potentials Flashcards
Where do graded potentials occur?
dendrites or cell body of neurons
How are graded potentials triggered?
by the opening or closing of ion channels
Why are graded potentials called graded?
because the amplitude of the potential is proportional to the strength of the triggering event
TRUE OR FALSE:
Graded potentials travel long distances through the neuron
FALSE
Short distances only
Why do graded potentials lose strength?
- current leak: some +ve charges leak back with the depolarization wave
- cytoplasmic resistance: organelles in cytoplasm restrict the flow of the current
How is the strength of the signal determined?
by the number of ions entering the cell
TRUE OR FALSE
Graded potentials diminish in strength as distance increases
TRUE
How is an action potential different from a graded potential?
- there is no volume control, action potential is either fired or not fired “on” or “off”
- they do not diminish in strength when travelling long distances, stays strong all the way
Action potentials start at the _____ , also known as ________
at the axon hillock, also known as the integrating center of the neuron also known as the trigger zone
How does the location of the trigger zone differ in various types of neurons?
sensory neurons (afferent) = TZ adjacent to receptor
efferent & interneurons = axon hillock and initial segment of the axon
what is the minimum depolarization necessary to trigger an action potential called?
threshold potential
What is excitability?
the ability for a neuron to respond to stimuli and fire an action potential
How can graded potentials lead to action potentials?
- spatial summation
- temporal summation
What are EPSPs and IPSPs?
EPSP: Excitatory Post Synaptic Potentials
- make the membrane potential less -ve
- brings membrane potential closer to threshold potential
- increase chance of exciting the axon to fire
IPSP: Inhibitory Post Synaptic Potential
- hyperpolarize cell membrane
- make more -ve
- decrease chance of firing AP
List the phases of the AP
- resting membrane potential: -70 mV
- depolarizing stimulus: reaches threshold potential
- rising phase: increases to max +30 mV (Na+ enters cell)
- repolarization: membrane potential travels down towards resting membrane potential (K+ leaves cell)
- hyperpolarization phase: MP overshoots resting potential
- resting membrane potential: repolarization back to resting potential
What is sodium activation?
- the opening of voltage gated Na+ channels which occurs once the threshold potential is exceeded
What sets the threshold potential for the initiation of the AP?
- sodium channel properties
- open at -55mV
List the steps of the rising phase
- resting membrane potential
- depolarizing stimulus
- voltage gated Na+ channels open at -55mV
- Na+ enters the cell
- causes further depolarization
- more Na+ channels open
What is the trigger for the closing of the inactivation gate of Na+?
when the membrane potential reached -55mV, triggered opening of activation channel and closing of inactivation channel
How does the membrane potential return to the resting level?
when K+ ions leave the cell: falling phase of the AP
The full gating potential for K+ channels happen at
30 mV
When does the refractory period happen?
During the hyperpolarizing phase
What are the 2 parts of the refractory period? Explain
absolute refractory period:
- last 1msec to 2msec
- no AP can be triggered, no matter how large the stimulus
relative refractory period
- really large stimulus is required to bring forth an AP
- threshold value moved closer to zero
- K+ channels are still open
Why can’t an AP be generated during the absolute refractory period?
- Na+ channels are inactivated
Why is the refractory period important?
sets the direction of the current flow, prevents temporal summation, prevents AP from going backwards
What determines how fast an AP can travel along the neuron?
- diameter of the axon
- resistance of the membrane to ion leakages
What is saltatory conduction?
- when action potentials jump from one node of Ranvier to the next
- allows rapid AP through small axons
