Term
| what is the Nernst potential? |
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Definition
the electrial diffusion potential level across a membrane that exactly opposes the net diffusion of a particular ion through the membran
K+ diffusion out, Na+ diffusing in
The greater the ratio of ions, the greater tendency for the ion to diffuse in one direction and the greater the Nernst potential required to prevent additional net diffusion |
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Term
Explain how membrane potentials are generated by movement of substsances across the cell membran |
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Definition
1. cellular proteins tend to be anions (inside cell)
2. Cations are attracted to electronegativity
3. ion channels regulate distribution of cations
4. Na/K pump maintains electrogenic gradient |
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Term
what kinds of ion channels are there in they hydrophobic membrane? |
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Definition
Passive (Na, K, Cl-)
Chemically gated
Voltage activated |
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Term
What does conductance of ions depend on? |
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Definition
1. # ion channels
2. size of ion
3. hydration (ions move slower)
Ohm's law voltage = current x resistance
K+ conductance is 100x Na+ conductance |
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Term
| [Na] and [K] inside vs outside of cell |
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Definition
[Na] higher outside
[K] higher inside |
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Term
| what is the excitable part of the neuron? |
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Definition
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Term
| What contributes to the normal resting membrane potential? |
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Definition
Potassium diffusion potential
-nernst potential = -94 mv
Na Diffusion - slight permeability of membrane to Na ions via Na-K leak channels
-Nernst potential for inside = +61 mv
Na-K pump
-pumps 3 Na to outside, 2 K to inside
-continual loss of + charges from inside (additional negativity)
membrane potential of -90mv |
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Term
Na+ channels
Characteristics? |
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Definition
2 gates: activation gate near outside of channel
Inactivation gate near inside
Activation gate close during resting potential
Both open during depolarization
Inactivation gate closed from +35 to -90mv (will not reopen until membrane potential returns to near original resting membrane potential) |
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Term
K+ (or Na-K) leak channels
More permeable to?
Contributes to? |
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Definition
K
resting membrane potential
requires no energy |
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Term
K+ channel
what kind of channel?
when does it open? |
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Definition
Voltage gated
opens when membrane potential approaches 0
also opens at same time Na+ channels close |
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Term
| why do the K channels not open until the Na+ channels close? |
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Definition
| there is a slight delay in the opening of potassium channels |
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Term
after the AP, what helps re-establish the concentration gradient?
What is it triggered by? |
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Definition
Na/K pump
Increased intracellular Na ions |
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Term
Ca++ channels
Type of channel?
Function?
Common in what types of muscle? |
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Definition
voltage gated
Ca++ channels open to allow Ca++ and Na into the cells
Slow
Cardiac and Smooth muscle |
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Term
Ca++ affects _______ at which Na+ channels will open.
How does Ca++ affect the Na channels? |
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Definition
voltage level
Ions bind to exterior surfaces of Na channel protein, positive charge of Ca++ alters the electrical state of the channel protein, altering voltage level required to open the Na gate (so increasing membrane potential, Na not as easily excited - good thing) |
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Term
| What happens where there is a low extracellular Ca++ concentration? (in regards to Na channels) |
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Definition
Na channels open prematurely
very small membrane potential increase to open Na channels
Nerve fiber is EXTREMELY excitable |
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Term
| what happens with a high extracellular Ca? |
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Definition
decrease Na membrane permeability
reduces excitability
Ca known as stabilizer |
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Term
Ca++ pump
pump Ca++ ___ to ? |
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Definition
most cells have them
Pumps Ca++ out of the cytosol to ECF or sarcoplasmic reticulum |
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Term
| What is Tetany? What is it caused by? |
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Definition
continued muscle contraction
can be caused by as little as a 50% decrease in Ca++ levles |
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Term
What are the signs of Hypocalcemic Tetany?
What would you test for? |
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Definition
muscle pain, cramps, stiffness, tingling of hands and feet
Trousseu's sign
Chvostek's sign
Blood test for Ca |
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Term
What is Trousseau's sign?
What is Chvostek's sign? |
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Definition
Inflate BP cuff above systolic pressure for 3 min
Contraction of fingers and hands (carpopedal spasm) indicates tetany
Chvosteks:
tap face just below temple where facial nerve emerges
Positive for tetany if: spasm of lip nose or face |
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Term
| If pateint has low serum Ca and High Serum Phosphate you must determine if these are due to? |
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Definition
hypoparathyroidism
Chronic renal failure |
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Term
| Tx for hypocalcemia tetany |
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Definition
Iv calcium gluconate
Vitamin D2 after Ca levels reached
Avoid food high in phosphorus |
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Term
cl- channels
Where is [Cl-] higher?
Permeability of channel during AP?
What can it be opened by? |
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Definition
Higher OUTSIDE
Permeability does NOT change during AP
Opened by GABA-A receptor binding (target for drugs to inhibit movement) |
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Term
| How does Cl- influx cause inhibition? |
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Definition
| Cl in negative, wants to move inside cell, inside of cell becomes more negative, harder to reach threshold |
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Term
what 3 factors determine membrane potential when the membrane is permeable to more than one ion?
What are the imporant ions? |
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Definition
1. polarity of electrical charge for each ion
2. permeability of the membrane
3. concentrations of respective ions
Na, K, Cl |
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Term
What equation is used to calculate membrane potential across membrane when there is more than one ion involved?
What ion has the largest effect on the equation? |
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Definition
Goldman - Hodgkin-Katz
K - most permeable ion |
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Term
| What contributes to the establishment and maintenance of nerve membrane resting potentials? |
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Definition
Na/K pump (electrogenic)
Leakage across [ ] gradient (calculated by Nernst)
Contribution of K diffusion: -94
Contribution of Na diffusion: +61
Contribution of Na/K pump: -4 |
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Term
| Outline the action of the AP |
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Definition
Membrane stimulated
Na+ channels open for influx of Na
Rise in membrane potential
More Na+ channels open
Depolarization
at +35 Na channels close, K channels open
K efflux back to resting potential: Repolarizaiton |
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Term
| What are the phases of nerve APs and what happens in each? |
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Definition
Resting state Em = -90 mv
-Na+ gates closed
Slowly rising phase
-voltage activated Na gates open
-Na+ influx
Threshold between -70 and -50
Rapid rising Phase
-more Na gates open
-K gates open
-K efflux
-Na inactivation gates begin to close
Peak (past 0 mv)
-Na overwhelms cell
-ABSOLUTE refractory period
EArly repolarizaiton
-Inactivation Na gates close
-Conductance of K>Na
-K gates remain open
Hyperpolarization
-Prolonged efflux of K
-RELATIVE refractory period
Na/K pump restores Em |
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Term
| How are APs propagaed along nerve fibers? |
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Definition
Propagation:
Na+ influx depolarizes adjacent membrane causing local current
-Unmyelinated-proportional to diameter
-Myelinated - Saltatory conduction |
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Term
Once the AP begins
Is there a single direction of propagation? |
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Definition
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Term
| Where are nerve fibers typically stimulated? |
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Definition
At the soma - more sensitive
Can be stimulated in middle though |
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Term
| What is the safety factor? |
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Definition
| AP strength/Excitability threshold |
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Term
| What factors can cause conduction failure? |
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Definition
Low safety factor
Local anesthetics
Pressure on fiber
hypoxia (reduces ATP available)
demyelination
Inhibitory actions (NTs, Cl influx, High extracell Ca) |
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Term
| What are the nodes of ranvier? |
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Definition
| small areas of uninsulated areas between schwann cells |
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Term
Saltatory conduction: what is it?
why is this important? |
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Definition
AP only occurs at nodes of ranvier
-AP jumps from one node to next
-ION flux (Na channels) at nodes causes AP
-Electric current between nodes
Important because: jumping increases speed and conserves energy for the axon |
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Term
Heart muscle fibers. Does the membrane repolarize immediately after depolarization?
What does this cause? |
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Definition
NO, plateau for many milliseconsds
Prolonged depolarization, contraction lasts longer |
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Term
| what causes the plateau in heart muscle? |
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Definition
Fast Na channels
Voltage activated Ca-Na SLOW channels |
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Term
what is the resting membrane potential of the heart?
Why is this important? |
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Definition
-60-70 mv
Not enough to cause the Na and Ca channels to totally close. Some Na and Ca ions flow inward which increases membrane permeability - AP generated |
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Term
| How do local anesthetics work? |
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Definition
decrease excitability by decreasing rate of depol and repol
Inhibit Na influx through nerve membrane via Na channel - AP does not reach threshold |
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Term
MS - caused by?
affects what neural areas? |
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Definition
Demyelination
Optic nerve, corticobulbar/spinal tracts/ cerebellar tracts, MLF (head and eye movement), posterior spinal cord columns |
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Term
What NT are inhibitory?
What are excitatory? |
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Definition
Inhib: DA, glycine, GABA, 5HT
Excit: ACh, NE, Glutamate |
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Term
| what causes Huntington's disease? |
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Definition
Loss of GABA secreting neurons
spontaneous outbursts from globus pallidus and substantia nigra
-choreiform movements, rigidity, Akinesia
Genetic |
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Term
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Definition
loss of DA neurons in substantia nigra
Lack of Da causes understim of movement
direct DA path inhibited (normally initiate movement)
Indirect path activated (normally inhibits movement) |
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Term
Myasthenia gravis
caused by?
symptoms?
Tx? |
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Definition
affects communication between motor neuron and muscle cell
antibodies destroy ACh receptors in NMJ
Symptoms:
muscle weakness
fatigue
ptosis, diplopia
ACh esterase inhibitors - allows more Ach to build up in NMJ |
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Term
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Definition
snare proteins allow vesicle filled with Ach bind w/ membrane and release NT
Botulism cleaves SNARE proteins, ACH can't be released ionto NMJ |
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