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Neuroscience Lecture 4: Action potentials and neural firing
Terms in this set (39)
The resting potential arises from what two activities?
-The sodium‐potassium pump
What are Leaky channels?
Some potassium channels in the plasma membrane are "leaky" allowing a slow diffusion of K+(potassium) out of the cell.
What do the sodium-potassium pumps have to do with resting potential? What kind of charge is then on the inside?
-This pushes only two potassium ions (K+) into the cell for every three sodium ions
(Na+) it pumps out of the cell so its activity results in a loss of positive charges within the cell making it negative on the inside.
During the Resting Potential, what kind of charge is inside/outside?
More negative inside than outside.
What is Resting potential? How much is the charge at this point?
-Also called Resting Membrane Potential
-Standing potential inside a neuron is -65 microvolts
-Higher concentration of Sodium ions(NA) on the outside of the cell and more Potassium ions(K) inside the cell
-Sodium pump is closed
-Potassium pump is "leaking" Potassium(K)
-Sodium-potassium pump is pushing 3 Sodium ions(Na) out and bringing in 2 Potassium ions(K).
What does the Sodium/potassium pump do? What is the result?
-Actively transport sodium ions to the outside of the membrane and potassium ions to the inside of the membrane
-3 sodium ions are pumped out for every 2 potassium ion that go in
-The result or "net" is negative in the cell
What does a neuron need to fire?
-Protein Anions (A-)
-Its intracellular potential increases by 15 micro
volts, will cause cell to fire
Structure of a Neuron: How a signal travels
1. At the dendrite the incoming
signals arrive (incoming currents)
2. At the soma current
are finally integrated.
3. At the axon hillock action potential
are generated if the potential crosses the
4. The axon transmits (transports) the
action potential to distant sites(down the axon)
5. At the synapses are the outgoing
signals transmitted onto the
dendrites of the target
What are the multiple states of an Ion channel?
Open (ions can pass) or closed (no ions can
-"All or none" must cross threshold value for it to occur
-Happens as a chain reaction
-Also called nerve impulse
-A brief fluctuation in membrane potential caused by rapid opening and closing of voltage-gated ion channels
-Action potentials sweep like a wave along axons to transfer information from one place to another in the nervous system
From beginning to end, how long does the action potential last?
-A change in membrane potential from the normal resting value (-65 mV) to a less negative value.
-Once this depolarization reaches a threshold (about -40 to -55 µV), it causes action potential
Explain the chain reaction of Action Potential: Puncture foot
1. Sodium channels open and Na rushes in.
2. This depolarizes the cell and causes resting potential (-65 µV) to increase(more positive) called the "rising phase"
3.This change continues until it reaches a peak of 40 mV
4. It continues to rise until the inside of the neuron is positively charged with respect to the outside causing "overshoot."
5. Next is the "falling phase" where sodium channels become inactivate and Potassium rushes in. This causes a rapid depolarization until the membrane is more negative than the resting potential.
6. Causes an "undershoot" where the membrane potential is more negative than at rest.
What are Ion channels?
Ion channels are protein molecules with pores that allow ions of specified size and charge to pass through.
A property of many ion channels, making them open or closed in response to specific signals, such as membrane voltage or the presence of neurotransmitters.
-A property of ion channels that are selectively permeable to some ions and not to others.
-Ex: Potassium channels are selectively permeable to K.
-Sodium channels are selectively permeable to Na.
-A protein that transports ions across a membrane at the expense of ATP.
-Ex: Transporting (Na) and (K) from the inside of the neuron to the outside.
Ionic movements through channels are influenced by two things, what are they?
-A movement of ions from regions of high concentration to regions of low concentration.
-This will cause ions to be pushed through channels in the membrane
-Ex: Adding a teaspoon of milk to a cup of tea. The milk tends to evenly spread though the tea solution.
-The rate of movement of electrical charge.
-Opposite charges attract and like charges repel.
-Ex: Na+ toward negative and Cl- toward positive.
-Also called Voltage
-An important factor in determining how much electricity will flow
-Is the force exerted on a charged particle and it reflects the difference in charge between the anode and the cathode.
-More current will flow as this difference is increased
-The voltage across a cell membrane.
-Sometimes at rest and sometimes not.
-Also called equilibrium potential
-The electrical potential difference that exactly balances an ionic concentration gradient
Voltage gated sodium channels, what are they permeable to and what are they gated by?
A membrane protein forming a pore that is permeable to Na+ ions and gated by depolarization of the membrane.
Action potential - rising phase
-When the inside of the membrane has a negative electrical potential, there is a large driving force of Na+ ions.
-Therefore, Na+ ions rush into the cell through the open sodium channels, causing the membrane to rapidly depolarize.
Action potential - overshoot
The part of the action potential when the membrane potential is more positive than 0 mV.
Action potential - falling phase
-The behavior of two types of channel contributes to the falling phase.
-First, the voltage-gated sodium channels are inactive.
-Second, the voltage-gated potassium channels finally open.
-There is a great driving force on K+ ions when the membrane is strongly depolarized.
-Therefore, K+ ions rush out of the cell through the open channels, causing the membrane potential to become negative again.
Action potential - undershoot
-The open voltage-gated potassium channels add to the resting potassium membrane permeability.
-Because there is very little sodium permeability, there is a cause of hyperpolarization relative to the resting membrane potential until the voltage-gated potassium channels close again.
Absolute refractory period
-Happens when sodium channels inactive when the membrane becomes strongly depolarized (repolarization)
-They cannot be activated again, and another action potential cannot be generated, until the membrane potential goes sufficiently negative to deactivate the channels.
Relative refractory period
-The membrane potential stays hyperpolarized until the voltage-gated potassium channels close since they lag.
-Therefore, more depolarizing current is required to bring the membrane potential to threshold.
The "undershoot" since membrane is hyperpolarized
A specialized junction where a narrow gap between two cells is spanned by protein channels (connexons) that allow ions to pass directly from one cell to another.
-Think of example with a garden hose
-The same principles apply to positive current spreading down the axon ahead of the action potential
-There are two paths that a positive charge can take:
-One: Down the inside of the axon
-Two: Across the axonal membrane
-If the axon is narrow and there are many open membrane pores, most of the current will flow out across the membrane(bad)
-If the axon is wide and there are few open membrane pores, most of the current will flow down inside the axon.
-The farther the current goes down the axon, the farther ahead of the action potential the membrane will be depolarized and the faster the action potential will propagate.
-Therefore, action potential conduction velocity increases with increasing axonal diameter.
-The propagation of an action potential down a myelinatd axon.
Nodes of Ranvier
A space between two consecutive myelin sheaths where an axon comes in contact with extracellular fluid.
-A human genetic disease caused by alterations in the structure and function of sodium channels
-Inactivation of sodium channels, prolonging the action potential
-Generalized epilepsy with febrile seizures
What are Cations? Which ones are they considered?
-Sodium (K+) and potassium(Na+)
What are Anions?
Negative ions. Chloride(Cl-)
True/false: Increasing extracellular potassium depolarizes neurons.
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