Flashcards in Physiology Chapter 2-Action potentials Deck (39):
What are action potentials?
Brief, rapid, large (100mV) changes in membrane potential during which potential actually reverses, so that the inside of the excitable cell transiently becomes more positive than the outside.
How are action potentials propagated?
In a non-decremental fashion. They do not diminish in strength as they travel from their site of initiation throughout the remainder of the cell membrane.
How do graded potentials initiate action potentials?
Typically, the portion of the excitable membrane where graded potentials are produced in response to a triggering event does not undergo action potentials.
Instead, the graded potential, by chemical or electrical means, brings about depolarization of adjacent portions of the membrane where action potentials can take place.
What are the basic steps of an action potential?
Depolarization proceeds slowly at first, until it reaches threshold (-50mV).
At threshold potential, an explosive depolarization takes place.
The inside of the cell becomes positive relative to the outside.
Just as rapidly, the membrane repolarizes.
Often, a hyperpolarization occurs.
Resting potential is then restored.
What are basic characteristics of action potentials?
The duration of an action potential is always the same in a given excitable cell.
In a nerve cell, the action potential only lasts 1 millisecond, but lasts longer in muscles, depending on the muscle type.
Often an action potential is called a spike.
If threshold potential is not reached, no action potential takes place.
What is an overshoot?
The portion of the action potential during which the potential is reversed.
What ion makes the greatest contribution to resting potential?
Which two channels are the root cause of carrying action potentials?
voltage gated sodium and potassium
What are the two gates for the voltage gated channels?
An activation gate and inactivation gate.
What are the three different conformation that the voltage-gated channels can adopt?
Open: both gates open
Closed but capable of opening: activation gate closed, inactivation gate open
Closed and not capable of opening: Inactivation gate closed, activation gate open.
What occurs when reaching threshold potential, in what concerns the sodium voltage-gated channels?
The channels open and the concentration and electrical gradients of sodium favour its movement inside the cell. This opening of channels leads to an explosive increase in sodium permeability as the membrane swiftly becomes 600x as permeable to sodium as to potassium.
Sodium pushes the membrane potential towards its eqb. potential.
At the peak, sodium channels start to close and inactivate, the permeability then lowers to resting value.
What causes the sodium channels to close?
When the membrane potential reaches threshold, two closely related events take place in each sodium channel.
The activation gates are are triggered to open rapidly in response to the depolarization converting the channel to the open state.
This channel opening actually triggers the process of channel closing but, this process is much slower.
How do the voltage-gated potassium channels open?
Simultaneously with the inactivation of sodium channels the voltage-gated channels start to slowly open at the peak of action potential.
This opening is a delayed response triggered by the initial depolarization to threshold.
What are the three action potential related events that occur at threshold?
1- The rapid opening of the sodium activation gates leading to the positive peak.
2- The slow closing of the sodium inactivation gates which halts sodium entry after a brief time delay.
3- The slow opening of the potassium gates.
What does the opening of the potassium gates do to its membrane permeability?
Becomes 300x as permeable as sodium at resting potential.
How is the concentration gradient from resting potential restored?
The sodium-potassium ATPase pump restores this in the long run but, not after each action potential. This takes time and another action potential can fire off even if the original gradient isn't restored. The exchange of sodium and potassium causes large changes in membrane potential but not a large change in the ICF and ECF concentrations.
What does a single nerve cell, a neuron, consist of?
The cell body, the dendrites and the axon.
What are the dendrites?
Numerous projections from the cell body that receive signals from other nerve cells.
What is the input zone of the nerve cell?
The cell body and dendrites.
This is the region where graded potentials are produced in response to triggering events.
What is the axon?
A single, elongated, extension that conducts action potentials away from the cell body and eventually terminates at other cells.
What are collaterals?
Side branches that come of the axon.
What is the axon hillock?
The first portion of the axon, in conjunction with the region of the cell body where the axon emerges from the soma, is called the axon hillock.
The axon hillock is is the zone where action potentials can be triggered by a graded potential of sufficient magnitude.
How are the action potentials conducted from the axon hillock?
They are conducted from the axon hillock to the typically branched ending at the axon terminals which release chemical messengers that simultaneously influence numerous other cells with which they come into close association.
Functionally, the axon is the conducting zone and the axon terminals constitute the output zone.
What are the two methods of action potential propagation?
Contiguous and saltatory conduction.
How does contiguous conduction occur?
Involves the spread of the action potential along every patch of membrane down the length of the axon.
The membrane at the axon hillock is at the peak of an action potential.
The rest of the axon is inactive (atm).
The depolarization of the remainder of the axon is accomplished by local current flow between the area already undergoing an action potential and the adjacent inactive area.
This brings the inactive areas to threshold and this cycles.
Once an action potential is initiated in one part of a nerve cell membrane, a self-perpetuating cycle is initiated so that the action potential is propagated along the rest of the fibre automatically.
What is a refractory period?
Prevents the propagation of new action potentials along area that have just undergone action potentials.
What are the two different refractory periods?
Absolute and relative refractory periods.
What is the absolute refractory period?
Once the voltage-gated sodium channels have flipped to their open, or activated, state, they cannot be triggered to open again in response to another depolarizing triggering event.
What is the relative refractory period?
During which, a second action potential can be produced only by a triggering event considerably stronger than is usually necessary. This is during the hyperpolarization part of a action potential.
What else do refractory periods do?
Put an upper limit on the frequency of action potentials.
What does a stronger stimulus produce, in what concerns action potentials?
Not a larger action potential but, a greater number of action potentials per second.
What does the velocity with which an action potential travels down the axon depend on?
Whether the fibre is myelinated and the diameter of the fibre.
Where does saltatory conduction take place?
What are the benefits of myelination?
Faster propagation and less energy consumption due to less ion fluxes because they are confined to nodal regions.
What is myelin?
Composed primarily of lipids and ensheathes the fibres. Acts as an insulator.
What are oligodendrocytes?
Myeling forming cells in the brain and spinal cord of the CNS.
What are Schwann cells?
Myelin forming cells in the peripheral nervous system.
What are the nodes of Ranvier?
Spaces where the axonal membrane is bare and exposed to the ECF. Allows current flow from an active node to an adjacent one.