Membrane & Muscle Flashcards
What are 4 functions of the plasma membrane?
1) Separate ICF from ECF
2) Selective barrier to diffusion
3) Selective transporter of nutrients and ions
4) Interpret and carry signals
What do proteins function as in the plasma membrane?
Channels, carriers, receptors, and enzymes
What are important factors for passive transport?
- Lipid solubility
- Concentration gradient
- Size
Does the rate of passive transport depend on temperature?
Nope
What are 4 characteristics of carrier-mediated transport?
1) Specificity
2) Saturation (waiting time)
3) Competition
4) Temperature-sensitivity
What are the 2 types of carrier-mediated transport?
1) Passive (facilitated diffusion)
2) Active (“pumps”)
What is primary active transport?
When the mechanism uses ATP directly
What is secondary active transport?
When the mechanism is powered by a mechanism that directly uses ATP
What is the relationship between solutes and water concentration?
Inverse – less solutes means a higher concentration of water
Define isotonic in regards to a cell
- A solution in which the cell retains its original volume because it does not gain or lose water
- Impermeable solutes = 300 mOsm
Define isosmotic
A solution in which the TOTAL solutes (impermeable and permeable) = 300 mOsm
Do permeable solutes contribute to the dilution of water?
Nope
What happens when concentration gradient equals electrical gradient?
The ion will stop moving down its concentration gradient
What is resting membrane potential?
A dynamic steady state in which inward current is exactly matched by outward current
What impact will increasing extracellular potassium have on Em?
It will raise Em, resulting in depolarization
What impact will increasing permeability of potassium have on Em?
It will decrease Em, resulting in hyperpolarization
What impact will increasing permeability of sodium have on Em?
Raise Em, resulting in depolarization
What impact will decreasing sodium potassium pumping have on Em?
Raise Em, resulting in depolarization
What impact will increasing the permeability of chlorine have on Em?
No impact
Uncharged and nonpolar molecules are highly ___ soluble
Lipid
Charge and polar molecules are highly ___ soluble
Water
What is equilibrium potential?
When the concentration gradient and the net electrical gradient become balanced
What direction is the sodium gradient?
Into the cell
What direction is the potassium gradient?
Outside of the cell
What causes Em to change?
Any change in ionic current
When do aquaporins close?
Never
When do voltage-gated sodium channels open?
Upon depolarization
What is the speed of the response from a voltage-gated sodium channel?
Very fast
How long does it take for voltage-gated sodium channels to self-inactivate?
Very short time
What happens when a cell becomes depolarized?
1) Potassium current increases through aquaporin channels, tending to repolarize
2) Voltage-gated sodium channels open, producing a sodium current
3) If the current of sodium is more than the potassium current an action potential starts
What does the peak of an action potential represent?
When the sodium current equals the potassium current
When do voltage-gated potassium channels open?
Upon depolarization
What is the speed of the response from a voltage-gated potassium channel?
Slow
How long do voltage-gated potassium channels stay open?
As long as depolarization persists
What is the membrane considered during the downstroke (repolarization)?
Refractory
What happens with progressive repolarization?
- Sodium channels reset
- Potassium current decreases as Em approaches Ek
- Potassium channels begin to close
- Excitability is restored
What is the absolute refractory period?
When not enough sodium channels are open to make the sodium current greater than the potassium current, results in total inexcitability
What is the relative refractory period?
When there are less sodium channels and/or more potassium channels open than normal, the threshold is higher than normal and can still be excited
What impact does increasing extracellular potassium have on an action potential?
Potassium from inside the cell with leak outside, inhibiting repolarization
What effect does decreasing temperature have on an action potential?
Stops the Na/K pump, which stops membranes from getting excited
What effect does increasing extracellular calcium have on an action potential?
Calcium stabilizes sodium channels, so more depolarization would be needed to excite
What effect does local anesthetic have on an action potential?
“Super calcium”; will cause threshold to increase dramatically
What are 3 characteristics of local circuits?
1) Depolarize adjacent membranes, causing an action potential
2) Decay with distance
3) Slow
What is the velocity of conduction determined by?
Spatial extent of local circuits
What is the relationship between threshold and velocity of conduction?
Inverse
What is the relationship between height and velocity of conduction?
Direct
What is the relationship between membrane resistance and velocity of conduction?
Direct
What is the relationship between fibre diameter and velocity of conduction?
Direct
Why does decreasing diameter result in a lower velocity?
Decreasing the diameter means an increase in axoplasmic resistance
What is the immediate effect of temperature on rate of activation of channels?
Cooling decreases fluidity of membranes which slows opening of sodium channels
What is the progressive effect of temperature on rate of activation of channels?
Cooling slows rate of Na/K pumping, which means a loss of sodium and potassium gradients, resulting in depolarization, which raises threshold, decrease action potential height, and decreases membrane resistance. These altogether decrease conduction velocity
Why can a “backward” current flow NOT reexcite old active areas?
Because these areas are in a refractory period
What are 5 events of neuromuscular transmission?
1) Release of acetylcholine
2) Acetylcholine diffuses across cleft
3) Acetylcholine binds to Ach receptors
4) End-plate potential spreads via local circuits to adjacent excitable sarcolemma
5) Hydrolysis of Ach
What causes acetylcholine to be released?
Motor action potential arrives at nerve terminal, depolarizes, activates voltage-gated calcium channels, causing a calcium influx, which causes packets of Ach to be released
What happens when Ach binds to receptors?
Permeability of sodium AND potassium increases
How much does end-plate potential depolarize to?
Threshold
What happens when voltage-gated calcium channels are blocked?
Voluntary muscles (lungs) will no longer work
Is a membrane more permeable to K or Na?
K
How does a membrane being more permeable to potassium impact Em?
Em will be affected more dramatically by changes in K permeability than by changes in Na permeability
Is the sodium/potassium needed to keep up concentration gradients after every action potential and why?
No because very few sodium and potassium ions are needed to cause an action potential
What is the charge of a cell that is experiencing an action potential?
Positive
What is the charge of a cell that is at resting potential?
Negative
Does the original action potential travel along a membrane?
No, it triggers an identical new action potential in the adjacent area
What must happen for a sodium channel to be triggered a second time?
Resting potential must be restored and the channels must be reset to their original positions
How are refractory period and action potentials related?
The longer the refractory period the greater delay between action potentials
Describe the all-or-none fashion of an action potential
An excitable membrane responds to a triggering event with either a maximal AP, or it does not respond with an AP at all
What is the effect of a stronger stimulus on an AP, and what will it not do?
- It will produce a greater number of AP’s per second
- It will not produce a larger AP
What causes striations?
Acto-myosin organization
Between skeletal and cardiac muscle, which one controls voluntary movement and which one controls involuntary movement?
- Skeletal – voluntary
- Cardiac – involuntary
Which types of muscle have striations?
Skeletal & cardiac
Does smooth muscle control voluntary or involuntary movement?
Involuntary
How is skeletal muscle attached to the skeleton?
Tendons
Define isometric
Force generated without change in length
Define isotonic in regards to skeletal muscle
Length change at steady force
What is concentric contraction?
Tension during shortening motion
What is eccentric contraction?
Tension during lengthening motion
What is each muscle cell/fiber innervated by and where?
By an alpha motor neuron in a neuromuscular junction or “end plate”
Where is the efferent motor neuron cell body found?
Ventral horn of spinal cord
Where is the afferent motor neuron found?
Dorsal horn
How do neurons leave the spinal cord and what do they form?
- By the ventral root
- Form mixed peripheral nerves
What is a motor unit?
A motor nerve and all the muscle fibers it innervates
What does it mean when a motor unit is described as a functional contractile unit?
All of the fibers contract synchrously
How is motor unit size related to muscle function?
- The motor unit gets larger as more fibers are needed
- Large motor units are needed for limbs that facilitate large movements with high force
Where are neurotransmitters released?
Neuromuscular junctions
What is epimysium and what does it do?
Epimysium is a mixture of elastin and collagen that surrounds muscle and prevents it from falling apart
What is a fasciculus?
A bundle of muscle fibers
What does perimysium surround?
Fasciculus’
What does endomysium surround?
Muscle fibers
What are myofibrils?
Bundles of actin and myosin filaments running along the long axis of cells
What are the Z lines?
The beginning and ending of a sarcomere
What is the I band composed of, and is it light or dark?
- Actin filaments
- Light
What is the A band composed of, and is it light or dark?
- Myosin and actin filaments
- Dark
What is the H band composed of and where is it found?
- Only myosin
- Centre of sarcomere
What does the M line do?
Organize and align thick filaments
What are myofibrils surrounded by?
Calcium ion rich sarcoplasmic reticulum
What is a T tubule?
Essentially the plasma membrane, hollow and filled with ECF
What direction do actin filaments extend and from where?
Inward from the Z-disk
What does it mean when myosin filaments are described as bipolar?
The heads are oriented in opposite directions at each end of the sarcomere
Describe the organization of a thick filament
- Hexamer with 2 heavy chains and 2 pairs of light chains
- Heavy chain has coiled tail with 2 globular heads
What is a cross bridge?
The interaction between myosin heads with thin filaments
Describe the troponin-tropomyosin complex
- Tropomyosin binds actin and blocks myosin head binding
- Troponin T binds tropomyosin
- Troponin I facilitates inhibition myosin binding
- Troponin C binds calcium ions, changes shape, and causes releases of troponin I and exposes myosin binding sites
Describe the sliding filament theory
Muscle contraction is caused by the sliding of actin filaments past myosin filaments with no change in filament length
What gets pulled together in the sliding filament theory?
Z-lines
In the crossbridge cycle, does actin or myosin move?
Actin
How does elevating cellular calcium enable cross bridge cycling?
Calcium binds troponin C, causing release of troponin I, and movement of tropomyosin to expose myosin binding sites
Describe excitation-contraction coupling
1) Motor neuron releases synaptic vesicle at end plate
2) Ach from vesicles activates channels enriched on the facing sarcolemma
3) Short AP transmitted along the sarcolemma and down T tubules
4) AP changes conformation of DHPR leading to calcium release from SR terminal cisternae via RYR into myoplasm
5) Increased calcium promotes actin-myosin interaction (binds troponin C to move tropomyosin), thus inducing a twitch
6) Intracellular calcium re-sequestered to the SR via SERCA to cause relaxation
What is DHPR?
A protein in the T tubule
What is RYR?
A protein in the terminal cisternae
What does an AP in the T tubule cause?
A conformational change in DHPR
What does a DHPR shape change cause?
The RYR to open and allow SR calcium to be released
What is SERCA?
A calcium-ATPase pump that re-sequesters intracellular calcium
What is passive tension?
Resistance to stretch of a muscle at rest
What is active tension?
New tension is developed after the muscle is stimulated
What is Lo?
The optimal length to develop active tension
What is the ideal Lo?
2.0 - 2.2 micrometers
What is Vo (maximum shortening velocity)?
Velocity in the absence of a load
What is Po (maximum isometric tension)?
Load at which stimulated muscle no longer shortens
What is the relationship between load and shortening?
Inverse – increasing load decreases shortening
What type of skeletal muscle is considered type 2?
Fast twitch
How is a fiber determined to be fast or slow?
Innervation
What type of activity is high for fast fibers?
Glycolytic
What type of activity is high for slow fibers?
Oxidative
Between fast and slow fibers, which has more mitochondria?
Slow
Between fast and slow fibers, which has more capillaries?
Slow
Between fast and slow fibers, which has a more extensive SR?
Fast
Between fast and slow fibers, which is innervated by alpha 1 motor neurons?
Fast
Between fast and slow fibers, which is innervated by alpha 2 motor neurons?
Slow
Describe the diameter, excitability, and conduction velocity of alpha 1 motor neurons
- Large diameter
- Low excitability
- Very fast conduction velocity
Describe the diameter, excitability, and conduction velocity of alpha 2 motor neurons
- Small diameter
- High excitability
- Fast conduction velocity
Between fast and slow fibers, which fatigues faster?
Fast
Between fast and slow fibers, which is used for sustained activities?
Slow
What are 3 energy sources used during contraction?
1) ATP
2) Creatine phosphate
3) Glycogen & glucose
What is oxygen debt?
When rate of energy use exceeds rate of production by oxidative metabolism
How is oxygen debt payed back?
Increased respiration and cardiac work during recovery
Can the ATP pool ever be reduced to zero?
No
How is the force of contraction increased?
Recruiting more fibers
How are cardiac muscle cells connected?
End-to-end
Do smooth muscle cells have sarcomeres?
Nope
Do smooth muscle cells have neuromuscular junctions?
No
What are the 2 types of smooth muscle cells?
1) Phasic (single unit)
2) Tonic (multi-unit)
Describe phasic smooth muscle cells
- Cells contract together and are normally relaxed
- Cells coupled for free flow of ions and AP
Describe tonic smooth muscle cells
- Cells operate independently, normally contracted
- Each cell innervated by a nerve ending
- Little cell-cell communication
What are muscle fibres made up of?
Myofibrils
What are the outer limits of the A band determined by?
The 2 ends of the thick filaments
What make up cross bridges?
Myosin heads
What happens when troponin is not bound to calcium?
It stabilizes tropomyosin in its blocking position over actin’s cross-bridge binding sites
What happens when troponin is bound to calcium?
The calcium’s shape changes, which allows tropomyosin to slip away from it’s blocking position
When the sarcomere shortens, do the thick and thin filaments decrease in length?
No
Where are T tubules found?
Every junction of an A band and I band
What do terminal cisternae store?
Calcium
How many muscle fibres does one axon terminal supply?
One
As a motor unit increases in size, what happens to its ability to become activated?
The ability decreases (gets harder to activate)
