1
Q
- integrated proteins that span the cell membrane
* When open, permit the passage of certain ions
A
Ion Channels
2
Q
• Ion channels permit the passage of some ions, but not others
• Selectivity is based:
o channel size
o distribution of charges that line it
o size and charge of ions
o how much water the ion attracts and holds around it
A
Ion Channel Selectivity
3
Q
- ion channels have gates controlled by voltage (differences in membrane potential)
- location: axon hillock, unmyelinated axons, along the nodes of Ranvier in myelinated axons
- responsible for generation and propagation of action potentials (outgoing signals from neurons)
A
Voltage-gated
4
Q
- ion channels (chemically-gated channels) are opened or closed by hormones, second messengers, or neurotransmitters
- location: dendrites, cell body
- responsible for synaptic potentials (incoming signals to neurons)
A
Ligand-gated
5
Q
- ion channels (leakage channels) are always open
- location: cell membrane on dendrites, cell body and axon
- responsible for resting membrane potential
A
Non-Gated
6
Q
- Potential difference generated across a membrane because of a concentration difference of an ion
- Can be generated only if the membrane is permeable to the ion
- SIZE depends on the size of the concentration gradient
- SIGN depends on whether the diffusing ion is positively or negatively charged
- Created by the diffusion of very few ions
- Do not result in changes in concentration of the diffusing ions
A
Diffusion Potential
7
Q
• Diffusion potential that exactly balances (opposes) the tendency for diffusion caused by a concentration difference
A
Equilibrium Potential
8
Q
- Chemical and electrical driving forces that act on an ion are equal and opposite
- No net diffusion of the ion occurs
A
Electrochemical Equilibrium
9
Q
• In the presence of a nondiffusible ion, diffusible ions distribute themselves so that at equilibrium their concentration ratios are equal
A
Gibbs-Donnan Equilibrium
10
Q
• Used to calculate the equilibrium potential at a given concentration difference of a permeable ion across a cell membrane
A
Nernst Equation
11
Q
• Calculates membrane potential on the inside of a membrane when a membrane is permeable to several different ions
A
Goldman-Hodgkin-Katz Equation
12
Q
- Measured potential difference across the cell membrane in millivolts (mV)
- Expressed as the intracellular potential relative to the extracellular potential
- A resting membrane potential of -70 mV means 70 mV, cell negative
A
Resting Membrane Potential (RMP)
13
Q
• Any change in which membrane voltage shifts to a less negative value
A
Depolarization
14
Q
generation of a nonpropagated response
A
Local potential
15
Q
generation of a propagated response
A
Action potential
16
Q
- Vary in magnitude (voltage) according to the strength of the stimulus
- A more intense or prolonged stimulus opens more ion gates than a weaker stimulus
A
Local potentials are GRADED
17
Q
- Get weaker as they spread from the point of stimulation
- As Na spreads out under the plasma membrane and depolarizes it, K flows out
- Prevents local potentials from having any long-distance effects
A
Local potentials are DECREMENTAL
18
Q
• if stimulation ceases, K diffusion out of the cell quickly returns the membrane voltage to its resting potential
A
Local potentials are REVERSIBLE
19
Q
- Alteration in the membrane potential of a cell resulting from activation at the synapse
- If the intracellular voltage increases, it is called an excitatory post-synaptic potential (EPSP)
- If the intracellular voltage decreases, it is called an inhibitory post-synaptic potential (EPSP)
A
SYNAPTIC POTENTIALS
20
Q
- Transmembrane potential difference produced in sensory receptors
- Occurs generally as depolarization resulting from inward current flow
- Influx of current will often bring the membrane potential of the sensory receptor towards threshold for triggering an action potential
A
GENERATOR/RECEPTOR POTENTIALS
21
Q
- Passive spread of charge inside a neuron due to local changes in ionic conductance
- Ionic charge enters in one location and dissipates to others, losing intensity as it spreads (graded response)
A
ELECTROTONIC POTENTIALS
22
Q
- Rapid changes in the membrane potential that spread along the nerve fiber
- To conduct a nerve signal, the action potential moves along the nerve fiber until it comes to the fiber’s end
A
Action Potential
23
Q
- If a stimulus depolarizes the neuron to threshold, the neuron fires at its maximum voltage
- If threshold is not reached, the neuron does not fire at all
- Above threshold, stronger stimuli do not produce stronger action potentials (not graded)
A
Action potentials follow ALL-OR-NONE LAW.
24
Q
- Action potentials do not get weaker with distance
- An action potential at the end of a nerve fiber will be just as strong as an action potential in the trigger zone up to a meter away
A
Action potentials are NON DECREMENTAL.
25
* If a neuron reaches threshold, the action potential goes to completion
* Action potentials cannot be stopped once it begins
Action potentials are IRREVERSIBLE.
26
* Resting membrane potential before the action potential begins
* Membrane is polarized because of the -90 mV membrane potential
Resting Stage
27
* When the threshold potential (-65 mV) is reached, the voltage-gated Na+ channels overwhelm K+ and other channels
* Membrane suddenly becomes permeable to sodium ions
Depolarization
28
• in large nerve fibers, the great excess of positive sodium ions causes the membrane potential to overshoot beyond the zero level
Overshoot
29
o Necessary actor in both depolarization and repolarization of the nerve membrane
Voltage-Gated Sodium Channel
30
* Sodium channels begin to close and the potassium channels open more than normal
* Rapid diffusion of potassium ions to the exterior re-establishes resting membrane potential
Repolarization
31
o also plays an important role in increasing the rapidity of repolarization of the membrane
Voltage-Gated Potassium Channel
32
* K+ conductance remains higher than at rest after closure of the Na+ channels
* Membrane potential is driven very close to the K+ equilibrium potential
After hyperpolarization (undershoot)
33
• Depolarization process travels over the entire membrane if conditions are right, or it does not travel at all if conditions are not right
All-or-Nothing Principle
34
Factors Affecting Conduction Velocity
1. Fiber Size
- Increasing the diameter of a nerve fiber results in decreased internal resistance and faster conduction velocity
2. Myelination
• Myelin acts as an insulator around nerve axons and increases conduction velocity
• Myelinated nerves exhibit saltatory conduction
35
• Time periods after an action potential, during which a new stimulus cannot be readily elicited
Refractory Periods
36
* Another action potential cannot be elicited, no matter how large the stimulus
* Coincides with almost the entire duration of the action potential
Absolute Refractory Period
37
* inactivation gates of the Na+ channel are closed when the membrane potential is depolarized and remain closed until repolarization occurs
* No action potential can occur until the inactivation gates open
Ionic Basis of Absolute Refractory Period
38
* Begins at the end of the absolute refractory period and continues until the membrane potential returns to the resting level
* Action potential can be elicited only if a larger than usual inward current is provided
Relative Refractory Period
39
* K+ conductance is higher than at rest
* Membrane potential is closer to the K+ equilibrium potential and farther from threshold
* More inward current is required to bring the membrane to threshold
Ionic Basis of Relative Refractory Period
40
* Cell membrane is held at a depolarized level such that the threshold potential is passed without firing an action potential
* Occurs because depolarization closes inactivation gates on the Na+ channels
Accommodation
Phisiology Hope
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