Lecture 3B. Action Potential Flashcards
Action potential
- neurons undergo brief, but large, rapid changes in membrane potential that is propagated down the length of the axon
Voltage-gated ion channels
- protein pores embedded in cell membrane
- channel opens in response to specific voltage
- permits ions of specific size & charge to pass through
- think toll booths
Ion channel gating: What are the two discrete states?
- Open: conducting/passing ions
- Closed: non-conducting
- Switching states requires conformational change: change in voltage and binding of ligand (chemical)
________ ________ changes the membrane potential.
Ion movement
At rest, the membrane is ________.
POLARIZED; charge difference, inside neuron more negative
At rest, the membrane is ________.
POLARIZED
________ is a decrease in membrane potential.
Depolarization; reduction in membrane potential (charge difference), voltage going up (> -70 mV)
________ & ________ increase in membrane potential.
Repolarization & Hyperpolarization; increase in charge difference, more negatively charged (< -70 mV) ex. - 80 mV
What are the steps of the action potential?
- Axon membrane potential
- Threshold voltage reached
- Depolarization
- Repolarization
- Hyperpolarization
Axon membrane potential
- membrane potential-70 mV at rest
- Na+ ion channels control the membrane potential at rest
Threshold voltage is reached
- if summation of local potentials reaches the threshold (-55 mV), large numbers of Na+ channels open and Na+ rushes into the cell very quickly, because of voltage gated ion channels
In step 2, what happens to the dendrites?
receives a signal; when neurotransmitters bind to receptors on postsynaptic neuronal membranes, it produces local potential
- voltage change that spreads passively across POSTSYNAPTIC neuron’s cell membrane
What are the different types of local potentials?
- Excitatory postsynaptic potential (EPSP): produces a small local depolarization
- “You should turn on”
- increase voltage
- Inhibitory postsynaptic potential (IPSP): produces a small hyperpolarization
- “Don’t do anything”
- negatively charged
Neuron receptors receive information from thousands of synaptic connections at any given instant. What determines whether an action potential is fired?
Balance between the number of excitatory and inhibitory signals; threshold (-55 mV); all or none phenomenon
Where does the integration of EPSPs and IPSPs occur?
axon hillock
Depolarization
“rising phase”
- if summation of local potentials reaches the threshold, large numbers of Na+ channels open up and Na+ rushes out very quickly (threshold voltage reached)
- causes rapid change in membrane potential from -50 mV to +40 mV (rising phase of action potential)
- when membrane potential = +40 mV, voltage-gated Na+ channels close and cannot be opened for a fixed period — the absolute refractory period, cannot create new action potential
Repolarization
“falling phase”
- at +40 mV, voltage-gated Na+ channels close & voltage-gated K+ channels open
- K+ ions flow out of cell
Hyperpolarization
- too much K+, leaves neuron
- refractory
Now there’s too much Na+ in the cell and too much K+ outside. How do you think the cell “resets” itself?
sodium potassium pump; restore normality
The ins and outs of an action potential
- rising phase: inward sodium current (depolarization)
- falling phase: outward potassium current (repolarization/hyperpolarization)
- generated at the axon hillock
- move along the axon because Na+ ions spread passively to near by regions, which changes the membrane potential to threshold, which opens more Na+ channels
What factors influence conduction velocity?
spread of action potential along membrane
- dependent upon axon structure
- temperature (cold = longer for signals to move)
- diameter of axon (wider = faster electronic signal
- myelination (more myelin = faster)
Action potential propagation
- in myelinated axons, regeneration of the action potential occurs only at the nodes of ranvier
- conduction seems to jump along the axon — saltatory conduction
- less energy is needed because ion channels are only at the nodes
Maximum speed of conduction in myelinated vertebrae axons ________.
120 m/s (270 mph); expressway
Small, unmyelinated axon of human autonomic nervous system ________.
1 m/s (2.25 mph); under construction, need ion channels
