Muscles (Topic 10) Flashcards
Characteristics of skeletal muscle (2)
- Multiple nuclei
- long fiber
- cylindrical in shape
- striated (has sarcomeres)
- fibers (cells) are individually innervated
Characteristics of cardiac muscle (2)
- single nucleus (most)
- branching
- striated = has sarcomeres
- cells communicate via gap junctions
Characteristics of smooth muscle (2)
- single nucleus
- short & narrow
- spindle shaped
- not striated = no sarcomeres
- Some have gap junctions
Structure of skeletal muscle 1 -> 5 (biggest to smallest) (3)
- muscle
- fiber bundles (fascicle)
- muscle fiber = cell
- myofibril
- myofilament (thick and thin)
Do muscle fibers have a lot or little mitochondria? (4)
a lot
What is the sarcoplasmic reticulum? (4)
a modified smooth ER in muscle fiber (cells)
What are extensions of the plasma membrane? (4)
Transverse tubules (T-tubules)
What are extensions of the plasma membrane? (4)
Transverse tubules (T-tubules)
What is another name for the plasma membrane in skeletal muscle? (4)
sarcolemma
What is another name for the cytoplasm in skeletal muscle? (4)
sarcoplasm
What is another name for the smooth ER in skeletal muscle? (4)
sarcoplasmic reticulum
What do filaments contain? (5)
- actin (thin)
- myosin (thick)
What is actin? (5)
-Active sites covered by tropomyosin.
What is tropomyosin? (5)
covers the binding sites and is held in place by troponin
What does troponin do? (5)
Troponin changes conformation in the presence of Ca2+ and moves tropomyosin away from the binding sites. (controls tropomyosin)
What is myosin? (5)
- 2 heavy and 4 light chains
- Cross-bridge heads bind to actin and to ATP
What is the structure of a sarcomere? (6)
- made of thick and thin filaments
- from Z-line to Z-line
- m-line in center
- titin
What does the m-line do? (6)
anchors the thick filament
What does titin do? (6)=
connect thick filaments to the z-line
How do you distinguish the A-band? (6)
a-band always corresponds to the length of the thick filament
How do you distinguish the H-zone? (6)
- only myosin (thick), no actin (thin)
How do you distinguish the I-band? (6)
only actin (thin, no myosin (thick)
T/F During contraction thick filaments get shorter (7)
false
T/F During contraction thin filaments get shorter (7)
false
What happens during contraction of skeletal muscle? (7)
the thin filaments move toward the in-line
- the thin slides over the thick filaments
- (A-band stays the same)
- (I-band and H-zone is reduced)
What is a motor unit? (9)
a motor neuron and all of the fibers it innervates
Where are motor units located? (9)
scattered throughout a muscle
What is a neuromuscular junction? (9)
synapse between the motor neuron and the muscle fiber
How can strength of contraction be varied? (9)
by recruiting more or fewer motor neurons
What is the only mechanism by which an action potential is initiated in skeletal muscle? (10)
Stimulation of the nerve fibers to a skeletal muscle
What do the synaptic vesicles in skeletal muscle contain? (10)
the neurotransmitter acetylcholine (ACh)
Are the junctions in skeletal muscle excitatory or inhibitory? (10)
excitatory
What is the purpose of t-tubules? (12)
gets the plasma membrane, close to the sarcoplasmic reticulum
- gives a way to get the action potential to the sarcoplasmic reticulum
What happens if calcium levels go down? (12)
- Less would be bound to troponin
- tropomyosin goes back in place covering binding sites
- myosin and actin cannot interact
- muscles are relaxed
What is cross-bridge cycling? (16)
myosin heads “walk” across actin and pulls the thin filaments towards the m-line in both directions
Characteristics/things in the cross-bridge cycle (16)
- At rest: ATP is hydrolyzed but products not yet released: energy still contained
- Binding to actin releases energy (as movement) and products
- New ATP binds myosin and causes release from actin
Explain the steps of cross-bridge cycling (16)
-ATP hydrolyzes to ADP
- infinity increases, head moves upright, and attaches to actin binding sites
- ADP leaves and head flexes
- actin is pulled
- ATP comes back and head detaches
- repeat
Why would there be a period of time where the cross-bridge cycle freezes after death (rigor mortis)? (16)
there is no ATP
- ATP is needed to lower affinity and break the crossbridge
- so when there is no ATP, cross bridges get stuck
How many action potentials is needed to contract a muscle fiber? (18)
one action potential
What does an action potential in muscle fibers lead to? (18)
leads to tension
What is muscle tension? (19)
force generated by a muscle fiber
- result of contraction
How long do muscle action potentials last? (18)
1-2 ms
- muscle contraction is about 100 ms
What is a twitch? (19)
response of a single fiber to a single action potential
What are the three phases to twitch contraction? (19)
- latent period
- contraction phase
- relaxation phase
What is the latent period in a twitch contraction? (19)
from action potential to beginning of tension increase
- not instantaneous because everything takes time
What is the contraction phase in a twitch contraction? (19)
from beginning of tension increase to tension peak
- tension develops due to crossbridge cycling
- depends on high calcium levels in the cytoplasm
What is the relaxation phase in a twitch contraction? (19)
from tension peak back to relaxed state
- tension decreases because calcium is being pumped back into the sarcoplasmic reticulum
Isometric vs. Isotonic contraction (20)
isometric- muscle stays the same length; no shortening (no movement; still tension)
ex: plank; wall-sit; pull-up hold
isotonic- muscles change length; shortening (movement and tension)
ex: weight lifting
How can a load be lifted? (20)
has to do enough cross-bridge cycling, to produce enough tension, to lift the load (tension must outweigh the load)
How does a twitch change as a load gets heavier? why? (21)
- the distance of shortening and time gets shorter (b/c b/c Heavier load increases delay and reduces duration of twitch, velocity and extent of shortening)
- latent period gets longer (b/c Shortening occurs only when the number of cross-bridges attached to actin is enough to oppose the load; TAKES MORE CYCLING TO LIFT THE LOAD SO TAKES LONGER)
- lower shortening velocity
What is the relationship between load and shortening velocity? (22)
as load increases, shortening velocity decreases
When would shortening velocity be its highest? (22)
when there is no load
Do your muscles still lengthen (produce tension) if you try to lift something that’s too heavy? (22)
yes
What happens if a stimulus is applied before relaxation? (23)
- there is more calcium in the cytoplasm than there would have been from a single action potential because the cell did not go back to resting
- it induces a contractile response with a peak tension greater than that produced in a single twitch (summation)
What is tetanus? (23)
a maintained contraction in response to repetitive stimulation
unfused vs fused tetanus (23)
- unfused fluctuates up and down; never hits 0
- fused is really high and steady; never hits 0 (max crossbridge cycling you can get, max tension)
How does tetanus occur? (23)
- 3-5 x stronger than twitch because Ca2+ remains high for longer
When is max tension reached? (23)
Maximum reached when Ca2+ remains at troponin-saturating concentrations
What is passive tension? (24)
- Due to Titin: spring-like protein attached to myosin
- Muscle returns to passive length after stretched and released
- ex: stretching
- HAS NOTHING TO DO WITH CROSS BRIDGE CYCLING
What is active tension? (24)
- due to cross-bridge cycling
- ex:
How does starting length affect active tension? (24)
- most tension produced if starting at 100% relaxed length (like 90 degree bent arm) because gap between thick and thin is small and no overlap so lots of room for cycling
- if muscle length is really short, you get less tension because thin filament are already overlapped over thick so there is not much room for crossbridge cycling
- if muscle length is really long, you get far less tension because thin filament are not overlapped over thick at all so it is hard for cycling to occur due to the gap
What do skeletal muscle disorders affect? (25)
Can affect muscles or their control
What is poliomyelitis? (25)
Virus that destroys motor neurons
- effect: death or paralysis due to not being able to contract muscles
- vaccine created by Jonas Salk in 1955
What is Hypocalcemic tetany? (25)
Involuntary tetanic contraction of skeletal muscles.
- low calcium level in the EXTRAcellular fluid
- membrane potential changes that makes it easier to hit threshold
What is muscular dystrophy? (26)
- Common genetic disease 1:3500 males (but less frequent in females).
- progressive degeneration of skeletal and cardiac muscle fibers.
- leading to death from respiratory or cardiac failure.
- Caused by the absence or defect of one or more proteins that link the outermost myofibrils to the sarcolemma.
What is myasthenia gravis? (26)
- 1:7500 Americans, more common in women than in men.
- Destruction of nicotinic ACh receptor proteins of the motor end plate by antibodies of a person’s own immune system.
- Theoretical treatments: anything that increases a synapse (increase ACh, inhibit ACE, increase receptor response with agonist, etc)
Where do you find smooth muscle? (27)
organs/intestines/bladder/ etc
Where do you find cardiac muscle? (27)
heart
What is the structure of smooth muscle? What does it not have that skeletal muscle does? (28)
- Cells spindle-shaped, and much shorter than skeletal muscle fibers.
- Single nucleus, can divide throughout the life of an individual.
- Have thick and thin filaments (have tropomyosin, but NO troponin, has calmodulin instead)
- Thin filaments are anchored. (have dense bodies, NOT z-lines
- No myofibrils or sarcomeres, so no banding pattern. Contraction does occur by a sliding-filament mechanism
Steps of smooth muscle contraction (29)
- increase in calcium
- calcium binds to calmodulin and activates it
- activates calmodulin myosin light-chain kinase (MLCK)
- myosin head gets phosphorylated (shape changes to active and upright) allowing for cross-bridge cycling
- AS LONG AS MYOSIN HEAD IS PHOSPHORYLATED, CYCLING CAN OCCUR
What stops cross-bridge cycling in smooth muscle? (29)
un-phosphorylation of the myosin head by myosin light-chain phosphatase (MLCP)
compare and contrast smooth and skeletal muscle contraction (29)
Similarities:
- both cycle when Ca is high
- stops cycling when Ca is low
Differences:
- Skeletal: Ca binds to troponin and moves tropomyosin, opening binding sites when Ca is high
- smooth: Myosin head phosphorylates and cycles when Ca is high, dephosphorylate when calcium is low
What is the source of calcium for skeletal muscle? (31)
One sources of Ca2+:
- Sarcoplasmic reticulum
What is the source of calcium for smooth muscle? (31)
Two sources of Ca2+:
- Sarcoplasmic reticulum
- Extracellular Ca2+ entering the cell through plasma-membrane Ca2+ channels.
What is latch state? (32)
- Cross bridges become dephosphorylated while still attached to actin.
- Can maintain tension in an almost rigor-like state.
- Tension can be maintain for a long period of time with very little ATP usage.
What can influence smooth muscle to contract? (33)
- spontaneous electrical activity in the plasma membrane of the muscle cell
- neurotransmitters released by autonomic neurons
- hormones
- locally induced changes in chemical composition of the extracellular fluid surrounding the cell
- stretch
What influences a skeletal muscle to contract? (33)
- a signal from a motor neuron
What is pacemaker potential? (34)
spontaneous depolarization to threshold
What are slow waves? (34)
periodic fluctuations in membrane potential (still need stimulus to hit threshold)
How many fibers may be affected by a single axon? (35)
multiple
How many neurons may send signal to a single muscle cell? (35)
multiple
Can smooth muscle be inhibitory? (35)
yes, not always excitatory
What local factors can influence smooth muscle contraction? (36)
- Paracrine signals
- Acidity
- O2 and CO2 levels
- Osmolarity
- Ion composition of the extracellular fluid
What does stretching smooth muscle do? (35)
Stretching opens mechanosensitive ion channels, leading to membrane depolarization. In hollow organs like stomach.
What are the types of smooth muscle? (37)
- single-unit
- multi-unit
What is a single-unit smooth muscle? (37)
- has gap-junctions
- when one cell contracts, all of them contract
What is a multi-unit smooth muscle? (37)
- has none (or few) gap-junctions
- all cells respond individually
What is the structure of cardiac muscle? (38)
- Cells have one (rarely two) nucleus that is centrally located.
- Striated, contraction through sliding filament mechanism, with troponin.
- Branching cells joined by intercalated discs (Desmosomes, Gap junctions, Electrically couple cells to transmit stimulus)
What is the structure of cardiac muscle? (38)
- Cells have one (rarely two) nucleus that is centrally located.
- Striated, contraction through sliding filament mechanism, with troponin.
- Branching cells joined by intercalated discs (Desmosomes, Gap junctions, Electrically couple cells to transmit stimulus)
What do desmosomes do? (38)
holds cells together
What does high levels of calcium do in cardiac muscle? (39)
Ca binds to troponin and moves tropomyosin, opening binding sites when Ca is high
Cardiac muscle contraction steps (40)
- AP propagates along cell membrane
- AP stimulates L-type channel and Ca comes in
- can stimulate ryanodine receptors (to release more Ca) or bind to troponin (to open tropomyosin and cause cycling)
- Ca goes back to reticulum or extracellular fluid
