Action Potential Flashcards
Explain the phases of the action potential
Rising (depolarizing) phase–>Overshoot phase (reversal to positive membrane potential) then occurs.
Falling (repolarizing) phase.
The undershoot (afterhyperpolarization) phase contains a (refractory) period during which it is not possible for the axon to have an action potential, and then a (relative refractory ) period when it requires greater stimulation for the axon to produce another AP.
The undershoot then dissipates and the membrane potential returns to resting.
Voltage and time dependent change in Na+ and K+ conductance in AP’s & steps of AP
First, a stimulus depolarizes the voltage sensitive membrane potential above threshold. Then, Sodium does the depolarization phase by rushing into the cell through a sodium channel (reduced sodium = smaller & slower AP) & makes the curve more positive = Overshoot phase (reversal to positive membrane potential) then occurs. After this, Potassium leaves the cell down its gradient through potassium channels & hyperpolarizes it & makes the curve negative = falling phase. After this, Sodium channels close & then Potassium channels close afterward; therefore, positive charge via Potassium is leaving the cell after the Sodium channel closes = undershoot. After this, Sodium channels become inactivated** as Potassium channels begin to close–this marks the absoloute refractory period when the cell does not fire in response to depolarizing stimulation since the Sodium channels are not sensitive to voltage changes. Once the sodium channels become active again (we are still in hyperpolarization) you can do an AP, but you need to depolarize more than you normally would = Relative refractory period.
Draw or explain the shape of the action potential
Horizontal–>Rise to a peak–>Fall below horizontal–>Positive horizontalish slope
See pg. 82
How voltage-sensitive mechanisms are measured
Through voltage clamp recordings by Hodgkin & Huxley.The amount of current that must be injected by the voltage clamp to keep the membrane voltage constant is a MEASURE OF THE CURRENT FLOWING ACROSS THE MEMBRANE! The experiment determined that Sodium & Potassium conductances change with time and membrane potential.
Drugs that selectively block Na+ and K+ currents also selectively block the early and late currents respectively, demonstrating that these are Na+ and K+ currents, respectively. TTX (tetrodotoxin) blocks sodium channels without affecting K+ channels (you cannot fire AP’s). TTX is obtained from the puffer fish.TEA (tetraethylammonium bromide) blocks K+ channels without affecting Na+ channels.
Describe the process of action potential propagation
Current is translated down the membrane of the neuron, causing depolarization of the channels down the neuron in a passive way. Passive current flow can go in both directions, but it wont because of the refractory period.
The absolute refractory period is important because it ensures that AP’s travel only in 1 direction.
Define hyper-polarization & depolarization
Hyper-polarization occurs when you drive current out of the cell & depolarization occurs when you drive current into the cell
APs are generatedby three different voltage-sensitive mechanisms:
(1) activation of Na+ conductance
(2) activation of K+ conductance
(3) **inactivation of voltage gated Na+ and K+ conductance
AP is driven by
Membrane potential changes, activation of Na channels, activation of K channels, **inactivation of Na channels, **inactivation & closure of K channels
Describe myelin
Most axons are covered in membranes from glial cells = myelin, they wrap around many times = myelination. This increased insulation reduces leak of current & increases efficiency. Myelination decreases the capacitance ( C = A/ d, myelin increases distance, d is total thickness), & increases conduction velocity of neurons. Oligodendrocytes in CNS and Schwan cells in the PNS.
Nodes of ravnier are gaps in the myelin sheath that contain Sodium & Potassium channels. Therefore, AP channels are generated & localized to the nodes of ravnier (so # of channels reduced).
Saltatory conduction = AP, jumps from node to node.
Conduction velocity is higher in myelinated nerves.
Discuss Multiple Sclerosis
Multiple sclerosis: a demyelinating disease, disrupts saltatory conduction slowing and even blocking the transmission of nerve impulses in the CNS (box D, p.59). Demyelination increases the capacitance of the membrane and thereby slows or even blocks AP propagation.
Discuss myelination & conduction velocity. Does myelination increase membrane resistance?
Conduction velocity is directly related to axon diameter and the amount of myelin. Myelination increases membrane resistance. Small-diameter fibers will be slow-conducting fibers & large diameter fibers will be fast conducting.
Myelination both increases the velocity of the nerve impulses along the axon and minimizes the number of charges that cross the membrane during an impulse, thereby minimizing the energy required by Na+ , K+ -ATPase to re-establish the relative concentration gradients for Na+ and K+.
Conduction velocity is directly related to axon ____ and the amount of myelin. The fastest conducting unmyelinated neurons have the ______ diameter. Myelination ______ membrane resistance. Small-diameter fibers will be _____ conducting fibers.
Conduction velocity is directly related to axon diameter and the amount of myelin. The fastest conducting unmyelinated neurons have the largest diameter. Myelination increases membrane resistance. Small-diameter fibers will be slow-conducting fibers.
The voltage clamp technique is used to measure ionic currents induced when a neuron:
is hyperpolarized
is depolarized to a level below threshold for AP production.
is depolarized to a level above threshold for AP production.
