C11 12-14 and 17 Flashcards
Types of membrane potentials
Resting, graded, action
What are graded potentials?
Short-lived, localized changes in membrane potential that can be either hyperpolarization or depolarization.
Why do graded potentials happen?
A change (stimulus) in the neuron’s environment opens gated ion channels (ligand).
Where do graded potentials happen?
Dendrites/cell body
What is an action potential?
(Aka nerve impulse). A brief reversal of membrane potential. Involves a transient increase in Na+ permeability followed by restoration of Na+ impermeability and then a short lived increase in K+ permeability.
When and where is an AP generated?
An AP is generated only when adequately stimulated by local currents from a graded potential. In most neurons, the transition from local GP to AP happens at the axon hillock.
How is an AP generated?
A threshold stimulus opens voltage-gated sodium channels. Sodium ions diffuse into the axon depolarizing it to +30mV. Voltage-gated sodium channels close and voltage-gated potassium channels open. Potassium ions diffuse out of the axon repolarizing it
How is an AP propogated along a neuron?
Local currents of an area undergoing depolarization cause depolarization of the forward adjacent area. Repolarization chases the depolarization down the length of the axon.
- Resting state
All gated Na+ and K+ channels are closed. RMP
- Depolarization
Local currents depolarize the axon membrane. Voltage gated Na+ open and Na+ rushes in causing more gates to open. All Na+ channels open, and MP ends up at +30mV.
(When threshold is reached (-50/55), depolarization becomes self generating)
This lasts only 1 msec
- Repolarization
Na+ channels inactivate and K+ channels open.
The inactivation gates of Na+ close and membrane permeability declines to resting levels. Na+ stops entering the cell.
Voltage gated K+ open and K+ rushes out of the cell following its electrochemical gradient.
- Hyperpolarization
Some K+ channels remain open and Na+ channels reset. Increased K+ permeability lasts longer than needed to restore the resting state and as a result a hyperpolarizatin is seen. Na+ activation gates close and inactivation gates open. Resting electrical conditions are restored, but ionic conditions are not. Sod-Pot pumps redistribute ions.
depolarizations
interior of the cell becomes less negative (addition of Na+) (excitatory)
hyperpolarizations
interior of the cell becomes more negative (Loss of K+ or Cl- into cell) (inhibitory)
How can the CNS determine whether a stimulus is weak or strong?
AP’s are independent of stimulus strength - they are all alike in amplitude. So, the frequency of AP’s tells the CNS if the stimuli is strong or weak.
What contributes to conduction velocity?
Diameter of axons, myelination, temperature
Saltatory conduction
Where: myelinated axons How: Current can pass through the membrane only through myelin sheath gaps where most of the voltage gated Na+ channels are concentrated. AP are generated only at the gaps, about 30 times faster than continuous conduction.
Continuous conduction
Where: nonmyelinated axons. How: The AP is generated at each point along the axon (slower than saltatory)
Bare plasma membrane conduction
Where: membranes lacking voltage gated channels What happens: Voltage decays because current leaks across channel. (no regeneration of current by voltage gated channels)
Absolute refractory period
Period following stimulation during which no additional action potential can be evoked. The neuron cannot respond to another stimulus, no matter how strong.
Relative refractory period
Follows the absolute refractory period; interval when a threshold for AP stimulation is markedly elevated. A stimulus that would normally generate an AP is no longer sufficient, but an exceptionally stong stimulus can reopen the Na+ channels.
Spatial summation
Multiple locations
Temporal summation
Single location, multiple times
Summation
Accumulation of effects
Action Potential definition
A large, transient depolarization event, including polarity reversal, that is conducted along a nerve fiber
