Lecture 10/22 Flashcards
Trileptil
Blocks Na+ gated channels, not permitting depolarization. Only affects excitable cells (only those cells have Na+ channels).
Nerst Equation Purpose
Use when you follow flow of ion across membrane.
Used to calculate the electrochemical equilibrium across the cell membrane.
Parallel Conductance Equation Purpose
Use when you follow flow of two ions across membrane.
Used to calculate the electrochemical equilibrium across the cell membrane.
Axon Hillock
Where action potential starts. Where there is a massive amount of voltage-gated Na+ channels. The chemically gated Na+ channels are located at dendrites.
It is a cone-shaped area of the cell body.
Axolemma
Plasma membrane of axons.
Axoplasm
Cytoplasm of axon.
Phases of Action Potential: Depolarization
When the cell reaches threshold, depolarization occurs. Tons of Na+ voltage-gated channels open and Na+ comes swarming in.
Phases of Action Potential: Repolarization
At the height of depolarization, repolarization begins. Na+ channels close and K+ channels open with K+ beginning to leave the cell to bring membrane potential to rest.
However, too many K+ may leave thus resulting in hyperpolarization. When hyperpolarized, no action potential can occur.
Refractory Periods
A period within the action potential when voltage-gated sodium channels are inactivated and a new action potential cannot be initiated.
Prevent action potentials from going in another direction. Keeps action potential unidirectional.
Absolute Refractory Period - absolutely no way to have another action potential during this period, as one is already occurring at the moment. Reached when cell begins to go into repolarization.
Relative Refractory Period - if you have a stimulus large enough, you can actually initiate another action potential during this period. Must be higher than normal stimulus. Reached when cell goes into hyperpolarization.
Central Nervous System vs. Peripheral Nervous System
CNS controls brain and spinal cord. PNS controls everything else, including nerves connected to brain and spinal cord.
Nodes of Ranvier
In between myelin on axons, there are spaces called nodes of Ranvier. When no myelin is present, nodes of Ranvier occupy the entire length of axon.
Saltatory Conduction
Conduction across an axon that contains myelin.
Oligodendrocytes make myelin in the CNS. Schwann cells do the same thing in the PNS.
Myelin helps to increase speed of nerve conduction. In a myeliniated axon, flow of sodium into cell will only occur at the nodes of Ranvier. Na+ travels to the next node of Ranvier skipping over the preceding myelin. Once enough Na+ reaches the next NoR, those Na+ gated channels open and the process repeats. This is the faster method of two ways to process a nerve conduction.
Continuous Propogation
Conduction across an axon that is unmyelinated. You instead have NoR at every spot of the axon, forcing the axon to open that many more Na+ channels to conduct an AP.
What Happens at End of Axon
Ca+2 flows in through Ca+2 voltage-gated channels to release neurotransmitters from vesicles which travel across the synapse to the connecting cell.
Very few synapses involve direct contact. Its usually pure communication.
Multiple Sclerosis
Condition where our body one day starts to attack oligodendrocytes and schwann cells. Happens in women more than men, northeast region more than south. It’s an autoimmune disease.
Speed of Action Potentials
The propagation speed is dependent on the physical properties of the axonal membrane. Action potentials travel fastest along the widest axons; the narrower the axon, the slower the movement.
The inward Na+ current depolarizes the interior of the axonal membrane, which then opens additional voltage-gated channels. The process is repeated along the entire axonal length. The movement of positive charge through the cell interior is dependent on the relationship between resistance to diffusion through the middle of the cell interior (RM) and along the cell the length (RL).
- If the RM were increased by increasing the axonal membrane without altering RL, the movement of positive charge along the axonal membrane would increase, as would the propagation speed of the action potential.
Another means by which propagation velocities can be increased is through the myelination of the axonal membrane. Schwann and oligodendritic cells extend their cell membrane to wrap tightly around the outside of the axonal membrane. The cytoplasm is squeezed out, leaving only the layers of the lipid bulayer membrane containing the protein myelin. Between adjacent glial or oligodendrocyte cells are nodes of Ranvier. Their the sites where action potential occurs. This means that the action potential is propagated only at these nodes that allow inward Na+ currents (Saltatory conduction).
- These nodes dramatically increase speed of AP. It takes less time for positive charge to diffuse along the inside of the cell membrane than it does to open voltage-gated Na+ channels. The nodes must be placed precisely apart so that they increase conduction velocity without being too far apart and causing the AP to fail.
