Nerve 2 Flashcards
Describe the resting membrane potential
At rest :
Inside cell : negative
Outside cell: positive
Membrane potential about -70 mV
Nerve signals are transmitted by ___________________.
Action potentials
What are action potentials
Rapid changes in the membrane potential that spread rapidly along the nerve fiber membrane οΏΌ
What causes the changes in the membrane potential?
The movement of ions in and out of the cell
An action potential has three phases
Depolarization
Overshoot
Repolarization
What is meant by ion gating in axons?
We know that the changes in membrane potential are caused by changes in the net flow of ions through ion channels in the membrane.
These gates can open or close the ion channels in response to particular stimuli
When ion channels are closed , plasma membrane is _________ permeable to an ion.
Less
When ion channels are open , plasma membrane is _________ permeable to an ion.
More
At what reading is the resting state
-70 mV
What channel is closed during the resting state
Voltage-gated Na and K channels
Depolarization is characterized by what
Entry/infux of sodium (Na)
Steps of depolarization
During depolarization, we said the membrane potential depolarizes from -70 mV to +30 mV. Does the membrane potential reach more than +30mV?
NO because these voltage-gated Na channels open for a LIMITED time.
Describe the voltage-gated sodium channel during
- resting (-70 mV)
- activated (-70 to +35 mV)
- Inactivated (+35 to -70 mV)
Resting :
- activation gate: closed
- inactivation gate: open
Activated:
- activation gate: open
- inactivation gate: open
Inactivated:
- activation gate: open
- inactivation gate: closed
NOTE: the selectivity filter
Steps of repolarization
True or false
Voltage-gated sodium Na channel has 3 states
True
True or false
Voltage-gated potassium K+ channel has 3 states
False
2 states only (resting and slow activation)
What is afterhyperpolarization ?
What channels are closed or open?
More negative than resting membrane potential
Na channels are CLOSED
K channels are still OPEN
The length of time that Na and K channels stay open is ______________ of the strength of the depolarization stimulus
Independent
The change from -70 mV to +30 mV and back to -70 mV lasts only about ______ msec
3 msec
The image of action potential on an oscilloscope screen looks like what shape?
Spike
NOTE: action potentials, therefore are called SPIKE POTENTIALS
What is threshold?
The minimum stimulus required for generation of action potential
Do the voltage-regulated gates open, when the depolarization is below threshold value?
NO, threshold must be reached in order to generate an action potential and open gates.
What happens once the stimulus reaches threshold?
Action potential is produced
Comment on the relationship between stimulus and response (action, potential generation)οΏΌ
Weak stimulus < threshold = no response
Strong stimulus > threshold = response
Action potentials are all-or-none.
What does this mean ?
A stimulus normally either produces a full-sized action potential or fails to produce one.
The amplitude of action potential is constant, regardless of the strength of the stimulus ; therefore, a stronger stimulus cannot produce an action potential of greater amplitude.
True or false
Stronger stimulus can produce an action potential of greater amplitude
False
The amplitude of action potential is constant, regardless of the strength of the stimulus
How can the nerve transmit different strengths of stimuli ?
(See picture)
- Weak stimulus - no AP
- Strong stimulus - few AP
- Stronger stimulus - many AP
When is the cell membrane refractory ?
When it is producing an action potential
During refractory periods, the cell membrane is in either one of two situations. Explain
It is either :
- completely unable to fire an action potential
OR
- requires a much stronger stimulation than usual
There are two types of refractory periods.
Absolute
Relative
Absolute refractory period
The time during which the membrane cannot respond to any other stimulus NO MATTER HOW STRONG IT IS
Therefore, a SECOND action potential is NOT possible
During absolute refractory period , a second action potential is not possible as the membrane cannot respond to any stimulus.
Why??
Large fraction of Na channels are inactivated = cannot be reopened until the membrane is repolarized
βBall and chainβ effect
Explain the βball and chainβ action during absolute refractory period
First we know that the :
- Na voltage-gated ion channel is closed at resting membrane potential, and it opens in response to a threshold level of depolarization where ions required for action potential diffuse.
SOβ¦
After a brief period, the channel is inactivated by the βball and chainβ portion of a polypeptide chain.
This inactivation ends after a fixed period when the ball leaves the mouth of the channel. Hence, no action potential is produced/ no response to any stimuli.
Relative refractory period
The time during which an axon can be stimulated with sufficiently very strong stimulus.
So, the cell CAN fire a second action potential, BUT a stimulus stronger than normal is required.
Why are we able to fire an action potential with a strong stimulus during the relative refractory period BUT not during the absolute refractory period ?
Because during the relative refractory period:
- some Na channels are still inactivated
- enough Na channels in the closed state
- K+ channels are open
Explain the local ionic disturbance at the end of an action potential.
The Na+/K+ pump reset the chemical equilibrium at the end of the action potential by pumping 3 Na+ out of the cell and 2K+ in the cell, restoring the resting ionic distribution.
Explain the conduction of action potential in an UNMYELINATED axon
Conduction of action potential in a MYELINATED axon
Myelin sheath provides ______________ for the axon , preventing the movement of Na and K+ through the membrane
Insulation
The myelin sheath has interruptions called
Nodes of Ranvier
Na+ channels are __________________________ at the nodes but they are almost _____________ in the regions of axon membranes between the nodes
Highly concentrated
Absent
What is meant by Saltatory conduction
Can it occur in unmyelinated axons?
Action potential only occurs at nodes of Ranvier
(Saltatory conduction describes the way an electrical impulse skips from node to node down the full length of an axon)
NO, only in myelinated axons
Compare the conduction rate of myelinated vs unmyelinated axons
Myelinated : fast (100 m/sec )
Unmyelinated: slow (1 m/sec)
What is the relationship between conduction velocity and nerve diameters?
As the diameter DECREASES, the conduction velocity DECREASES.
Therefore, the relationship is DIRECTLY PROPORTIONAL
True or false
Multiple sclerosis (MS) is a demyelinating autoimmune disease.
What occurs in multiple sclerosis??
True
- Myelin is destroyed by immune cells
- Slow or no transmission of action potential
- Difficulty in performing routine tasks
What are local anesthetics?
Drugs that REVERSIBLY bind to Na+ channels in the axon membrane, preventing these channels from opening ( NO DEPOLARIZATION)
- Block the ability of sensory axons to produce action potentials
- Reduce membrane excitability = nerve impulses fail to pass along the anesthetized nerves
Examples of local anesthetics
Lidocaine
Procaine
