Chapter 9: Muscles and Muscle Tissue Flashcards
Where is skeletal muscle tissue found?
packaged into skeletal muscles
What are skeletal muscles?
Organs attached to bones and skin
Skeletal muscle fibers: description
longest of all muscles and have striations (stripes)
Other name for skeletal muscle
voluntary muscle: can be consciously controlled
Skeletal muscle characteristics
contract rapidly, tire easily, powerful
Skeletal muscle key words
skeletal, striated, voluntary
Where is smooth muscle tissue found?
in wall of hollow organs
ex. stomach, urinary bladder, airways
Smooth muscle appearance
not striated
Smooth muscle characteristics
involuntary: cannot be controlled consciously
Smooth muscle key words
visceral, nonstriated, involuntary
Where is cardiac muscle tissue found
only in heart (makes up bulk of heart)
Cardiac muscle appearance
striated (striped appearance)
Cardiac muscle characteristics
involuntary: cannot be controlled consciously
contracts at steady rate due to heart’s own pacemaker, but nervous system can increase rate
Cardiac muscle key words
cardiac, striated, involuntary
Sarcolemma: what is it
muscle fiber plasma membrane
Sarcoplasmic Reticulum (SR): what is it
network of endoplasmic reticulum tubules surrounding each myofibril
Sarcoplasmic Reticulum (SR): what is its purpose
regulation of intracellular calcium ion levels (Ca+2)
stores and releases Ca+2: essential for muscle contractions
Sarcoplasm: what is it
muscle fiber cytoplasms
Sarcoplasm: important items in it
- glycosomes: store glucose as glycogen for muscle fiber to make ATP
- myoglobin: oxygen-storing protein, provides oxygen to make ATP
Myofilaments: what are they
feature of myofibrils (densely packed, rodlike elements)
What are myofilaments composed of
proteins’ actin (thin filaments) and myosin (thick filaments), found within the sarcomere
Sarcomere: what is it
- feature of myofibrils
- smallest contractile unit (functional unit) of muscle fiber
Where do individual sarcomeres align?
end to end along myofibril, like boxcars of a train
4 major steps in skeletal muscle contraction
- Events at neuromuscular junction
- Muscle fiber excitation
- Excitation-contraction coupling
- Cross-bridge cycling
- Events at neuromuscular junction
motor neuron releases acetylcholine which stimulates skeletal muscle fiber, causing a depolarization called end plate potential (EPP)
- Muscle fiber excitation
EPP triggers an AP that travels across sarcolemma of muscle fiber
- Excitation-contraction coupling
AP in sarcolemma causes release of calcium ions from SR, calcium binds troponin which moves tropomyosin and exposes myosin- binding sites for binding with actin
- Cross-bridge cycling
actin-myosin cross-bridge cycling results in muscle contraction
Key components of step 4 in skeletal muscle contraction
- “Grab rope” = cross bridge formation
- “Pull” = powerstroke
- “Let go” = cross bridge detachment
- “Reach forward” = cocking myosin head
“Grab rope”
- cross bridge formation
- energized myosin head attaches to an actin myofilament, forming a cross bridge (attached high energy state)
“Pull”
- Power stroke
- ADP and P released and myosin head pivots and bends, changing to its bent low-energy state.
- AS a results it pulls actin filament towards the M line (middle)
“Let go”
- cross bridge detachment
- after ATP attaches to myosin, link between myosin and actin weakens, and myosin head detaches
“Reach forward”
- cocking myosin head
- as ATP is hydrolyzed (broken down) to ADP and Phosphate, myosin head returns to its prestrike high -energy, or cocked position
Neurotransmitter released in skeletal muscle contraction
AP in neuron triggers release of neurotransmitter acetylcholine (ACh) which tells skeletal muscles to contract
Rigor Mortis: what is it and when does it occur
- 3-4 hours after death, muscles begin to stiffen; peak rigidity about 12 hours postmortem
Rigor Mortis: why does it occur
- in dying muscle cells, intracellular calcium levels increase resulting in cross bridge formation
- ATP needed for cross bridge detachment; upon death ATP no longer produced
Rigor Mortis; how does it occur
myosin head stays bound to actin causing constant state of contraction
What causes rigor mortis to stop
muscles stay contracted until muscle proteins break down, causing cross bridge to break
What is recruitment
process of activating more motor units to increase strength of muscle contraction (faster and more prolonged)
Another name for recruitment
multiple unit summation
How does recruitment work
works on size principle: motor units of small muscle fibers recruited first; as stimulus intensity increases, motor units innervating increasingly larger muscle fibers get recruited
Muscles are never 100% relaxed, what is this phenomenon commonly known as
muscle tone: all muscles are constantly in a slightly contracted state
Why does this occur? (muscles never fully relaxed)
due to spinal reflexes (involuntary response to stimuli mediated by spinal cord): stimuli for muscle tone comes from stretch receptors in muscles
Purpose of muscles never relaxed phenomenon
- keep muscles firm, healthy, and ready to respond to stimuli
- does not result in active movement of body
What molecules are involved in direct phosphorylation?
- creatine phosphate (CP) + ADP
Direct phosphorylation equation (result)
- CP + ADP –> ATP
Products: 1 ATP per CP, creatine - creatine phosphate in muscle fibers donates phosphate to make ATP from ADP
Direct phosphorylation: energy source
Creatine phosphate
Direct phosphorylation: oxygen use
none
Direct phosphorylation: duration of energy provided
15 seconds
What molecules are involved in anaerobic pathway?
- Glycolysis ( energy source- glucose- broken down via glycolysis to make ATP)
- lactic acid formation
Anaerobic pathway: oxygen use
- none
- in absence of O2 (anaerobic) lactic acid is the end product
- glycolysis does not require oxygen, making it part of the anaerobic pathway
Anaerobic pathway: products
2 ATP per glucose, lactic acid
Anaerobic pathway: duration of energy provided
30-40 seconds or slightly more
Aerobic pathway: energy source
- glucose, pyruvic acid, free fatty acids from adipose tissue, amino acids from protein catabolism
Aerobic pathway: oxygen use
required
Aerobic pathway: products
32 ATP per glucose, CO2, H2O
Aerobic pathway: duration of energy provided
hours
Aerobic pathway vs others
- further breakdown of glucose following glycolysis
- makes significantly more ATP, but required O2
Skeletal vs. Smooth muscle: cell shape
Smooth muscle fiber: spindle-shaped, thin and short
Skeletal muscle fiber: cylindrical, wider, longer
Skeletal vs. Smooth muscle: sources of calcium for muscle contraction
Smooth muscle: extracellular fluid from SR
Skeletal muscle: Sarcoplasmic Reticulum (SR)
Organization/Orientation of muscle tissue: Smooth muscle
- unitary muscle in walls of hollow organs, multiunit muscle in intrinsic eye muscles, airways, large arteries
- lacks connective tissue sheaths, contains endomysium only
- arranged in sheets or layers; typically found in walls of hollow organs, with 2 primary layers: longitudinal and circular
Smooth muscle tissue: longitudinal layer
fibers run parallel to long axis of organ; contraction causes organ to shorten
Smooth muscle tissue: circular layer
fibers run around circumference of organ; contraction causes lumen (inside cavity) of organ to constrict
Organization/orientation of skeletal muscle tissue
- attached to bones or (some facial muscles) to skin; composed of long fibers organized into bundles; myofibers (contain several myofibrils)
- each skeletal muscle surrounded by 3 layers of connective tissue: epimysium, perimysium, and endomysium
Mechanism of muscle contraction (filaments and cross-bridge cycling): Smooth muscle
involves actin and myosin sliding filaments and cross-bridge cycling in a slower, more sustained manner, smooth muscle filaments are arranged in a crisscross pattern allowing contraction in multiple directions
Mechanism of muscle contraction (filaments and cross-bridge cycling): Skeletal muscle
uses sliding filament model where actin and myosin filaments form cross-bridges and slide past each other
* myosin heads attach to actin binding sites, creating contraction force through ATP-fueled cycles
Calcium ion involvement: smooth muscle
- Ca2+ comes from both the SR and extracellular sources with influx occurring through caveolae and voltage-gated channels on the sarcolemma
- contraction can be stimulated by mechanical, hormonal, or neural inputs
Calcium ion involvement: skeletal muscle
Ca2+ released from the SR in response to an AP initiated by ACh binding at neuromuscular junction
Regulatory proteins: smooth muscle
- no troponin complex; instead, protein called calmodulin binds calcium ion; calcium ion binds to calmodulin not troponin
- activated calmodulin leads to cascade of events that results in myosin activation and cross-bridge formation
Regulatory proteins: skeletal muscle
tropomyosin and troponin: regulatory proteins bound to actin; block actin binding site until muscle contraction is stimulated
