Term
| Functions of Skeletal Muscle |
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Definition
Body movement
Move bones, make facial expressions, speak, breathe, swallow
Maintenance of posture
Stabilize joints, maintain body position
Protection and support
Package internal organs and hold them in place
Regulating elimination of materials
Circular sphincters control passage of material at orifices
Heat production
Help maintain body temperature |
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Term
| Characteristics of Skeletal Muscle Tissue |
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Definition
Excitability: ability to respond to a stimulus by changing electrical membrane potential
Conductivity: involves sending an electrical change down the length of the cell membrane
Contractility: exhibited when filaments slide past each other
Enables muscle to cause movement
Elasticity: ability to return to original length following a lengthening or shortening
Extensibility: ability to be stretched |
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Term
| Hierarchy of muscle structures |
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Definition
What is the hierarchy of structures in a muscle?
A whole muscle contains many fascicles
A fascicle consists of many muscle fibers
A muscle fiber is a muscle cell
In addition to the muscle cells, a skeletal muscle contains nerves, blood vessels, and connective tissue
[image] |
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Term
| Gross Anatomy of Skeletal Muscle |
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Definition
Connective tissue components
Three concentric layers of wrapping
Epimysium
Dense irregular connective tissue wrapping whole muscle
Perimysium
Dense irregular connective tissue wrapping fascicle
Houses many blood vessels and nerves
Endomysium
Areolar connective tissue wrapping individual fiber
Delicate layer for electrical insulation, capillary support, binding of neighboring cells |
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Term
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Definition
Attachments of muscle to bone (or to skin or to another muscle) can be tendons or aponeuroses
Tendon: cordlike structure of dense regular connective tissue
Aponeurosis: thin, flattened sheet of dense irregular tissue |
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Term
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Definition
Sheet of dense irregular connective tissue
Located external to epimysium
Separates different muscles while binding them together
Contains nerves, blood vessels, and lymph vessels |
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Term
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Definition
Areolar and adipose tissue
Located superficial to deep fascia
Separates muscles from skin |
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Term
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Definition
Skeletal muscle has extensive blood vessels
Deliver oxygen and nutrients, removing waste products
Skeletal muscle is innervated my somatic neurons
Axons of neurons branch, terminate at neuromuscular junctions
Can allow for voluntary control of contraction |
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Term
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Definition
Sarcoplasm (cytoplasm)
Has typical organelles plus contractile proteins and other specializations
Multiple nuclei (individual cells are multinucleated)
Cell is formed in embryo when multiple myoblasts fuse
Some nearby myoblasts become undifferentiated satellite cells for support and repair of muscle fibers
Sarcolemma (plasma membrane)
Has T-tubules (transverse tubules) that extend deep into the cell
Sarcolemma and its T-tubules have voltage-gated ion channels that allow for conduction of electrical signals
The interior of T-tubules is part of the extracellular environment |
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Term
| Development of Skeletal Muscle |
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Definition
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Term
| Structure of Muscle Fiber |
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Definition
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Term
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Definition
Myofibrils (hundreds to thousands per cell)
Bundles of myofilaments (contractile proteins) enclosed by sarcoplasmic reticulum
Make up most of the cell’s volume
Sarcoplasmic reticulum
Internal membrane complex similar to smooth endoplasmic reticulum
Terminal cisternae: blind sacs of sarcoplasmic reticulum
Serve as reservoirs for calcium ions
Combine in twos with central T-tubule to form triads |
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Term
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Definition
Has a pump – sarcoplasmic endoplasmic reticulum calcium ATPase (SERCA) that import Ca2+ into sarcoplasmic reticulum where it binds to calmodulin and calquestrin
Has channels (Ryanodine receptors) that allow Ca2+ to be released into surrounding sarcoplasm to trigger contraction |
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Term
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Definition
Thick filaments
Consist of bundles of many myosin protein molecules
Myosin has ATPase activity and when it hydrolyzes ATP, it is in a high energy state with a high affinity for actin
Each myosin molecule has two heads and two intertwined tails
Heads have binding site for actin of thin filaments and ATPase site
Heads point toward ends of the filament
Thin filaments
Consist mostly of two twisted strands of fibrous actin (F-actin)
Each strand is a necklace of hundreds of actin globules (G-actin)
Each G-actin has a myosin binding site to which myosin heads attach during contraction
Troponin and tropomyosin are regulatory proteins of thin filament
Tropomyosin: twisted string-like protein covering the myosin-binding site on actin in a noncontracting muscle
Troponin: globular protein attached to tropomyosin
Troponin T binds tropomyosin, Troponin I binds actin and inhibits contraction, Troponin C binds Ca++
When Ca2+ binds to troponin it pulls tropomyosin off actin allowing contraction |
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Term
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Definition
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Term
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Definition
Myofilaments arranged in repeating units, sarcomeres
Composed of overlapping thick and thin filaments
Delineated at both ends by Z discs
Specialized proteins perpendicular to myofilaments
Anchors for thin filaments
The positions of thin and thick filaments give rise to alternating I-bands and A-bands
[image] |
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Term
| Organization of Sarcomere |
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Definition
I bands
Light-appearing regions that contain only thin filaments
Bisected by Z disc
Get smaller when muscle contracts (can disappear with maximal contraction)
A band
Dark-appearing region that contains thick filaments and overlapping thin filaments
Contains H zone and M line
Makes up central region of sarcomere
H zone: central portion of A band
Only thick filaments present; no thin filament overlap
Disappears with maximal muscle contraction
M line: middle of H zone
Protein meshwork structure
Attachment site for thick filaments
[image]
[image] |
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Term
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Definition
Connectin
Extends from Z disc to M line
Stabilizes thick filaments and has “springlike” properties (passive tension)
Dystrophin
Anchors some myofibrils to sarcolemma proteins
A large protein that connects thin filaments to β-dystroglycan in the sarcolemma by syntrophins in complex with sarcoglycans
It provides a scaffold for the myofibrils
Abnormalities of this protein cause muscular dystrophy |
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Term
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Definition
Titin (largest known protein) anchors myosin and is important in skeletal muscle elasticity
Actinin binds actin to the Z lines
Desmin attaches the Z lines to the sarcolemma |
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Term
| Mitochondria and other structures associated with energy production |
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Definition
Muscle fibers have abundant mitochondria for aerobic ATP production
Myoglobin within cells allows storage of oxygen used for aerobic ATP production
Glycogen is stored for when fuel is needed quickly
Creatinine phosphate can quickly give up its phosphate group to help replenish ATP supply |
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Term
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Definition
A motor neuron and all the fibers it controls. Axons of motor neurons from spinal cord (or brain) innervate numerous muscle fibers
The number of fibers a neuron innervates varies
Small motor units have less than five muscle fibers
Allow for precise control of force output
Large motor units have thousands of muscle fibers
Allow for production of large amount of force (but not precise control)
Fibers of a motor unit are dispersed throughout the muscle (not just in one clustered compartment) |
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Term
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Definition
Location where motor neuron innervates muscle
Usually mid-region of muscle fiber
Has synaptic knob, synaptic cleft, motor end plate[image]
[image] |
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Term
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Definition
Expanded tip of the motor neuron axon
Houses synaptic vesicles
Small sacs filled with neurotransmitter acetylcholine (ACh)
Has Ca2+ pumps in plasma membrane
Establish calcium gradient, with more outside the neuron
Has voltage-gated Ca2+ channels in membrane
Ca2+ flows into cell (down concentration gradient) if channels open |
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Term
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Definition
Specialized region of sarcolemma with numerous folds
Has many ACh receptors[image]
Plasma membrane protein channels
Opened by binding of ACh
Allow Na+ entry and K+ exit |
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Term
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Definition
Narrow fluid-filled space
Separates synaptic knob from motor end plate
Acetylcholinesterase resides here
Enzyme that breaks down ACh molecules |
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Term
| Resting Membrane Potential |
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Definition
Fluid inside cell is negative compared to fluid outside cell
RMP of muscle cell is about –90 mV
RMP set by leak channels and Na+/K+ pumps (voltage-gated channels are closed) |
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Term
| Skeletal Muscle Contraction |
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Definition
Calcium enters synaptic knob
Nerve signal travels down axon, opens voltage-gated Ca2+ channels
Ca2+ diffuses into synaptic knob
Ca2+ binds to proteins on surface of [image]synaptic vesicles
1. Neuromuscular Junction: Excitation of Skeletal Muscle Fiber:
Release of ACh from synaptic vesicles and subsequent binding of ACh to ACh receptors on motor end plate.Vesicles merge with cell membrane at synaptic knob: exocytosis
Thousands of ACh molecules released from about 300 vesicles
2. Sarcolemma, T-tubules, and sarcoplasmic reticulum: excitation-contraction coupling:
ACh binding triggers propagation of an action potential along the sarcolemma and T-tubules to the sarcoplasmic reticulum, which is stimulated to release Ca2+
3. Sarcomere: Crossbridge Cycling:
Ca2+ binding to troponin triggers sliding of thin filaments past thick filaments of sarcomeres. As sarcomeres shorten, the muscle contracts |
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Term
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Definition
Autoimmune disease, primarily in women
Antibodies bind to ACh receptors in neuromuscular junctions
Receptors removed from muscle fiber by endocytosis
Results in decreased muscle stimulation
Rapid fatigue and muscle weakness
Eye and facial muscles often involved first
May be followed by swallowing problems, limb weakness |
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Term
| End-Plate Potential (EPP) |
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Definition
ACh receptors are chemically gated channels that open when ACh binds to them
Na+ diffuses into the cell through the channels (while a little K+ diffuses out)
Cell membrane briefly becomes less negative at the end plate region
EPP is local but it does lead to the opening of voltage-gated ion channels in the adjacent region of the sarcolemma |
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Term
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Definition
becomes less negative, eventually becomes +30 mV
This results in the opening of adjacent voltage-gated Na+ channels and more Na+ entry
A chain reaction occurs as depolarization is propagated down the membrane and T-tubules |
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Term
| K+ diffuses out of the cell |
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Definition
Cell repolarizes: returns to –90mV
Repolarization is then propagated down the membrane and T-tubules
While the cell is depolarizing and repolarizing it is in a refractory period—unable to respond to another stimulation |
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Term
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Definition
[image]
Release of Ca2+ from the sarcoplasmic reticulum
The spreading action potential causes a conformational change in the voltage-sensitive dihydropyridine (DHP)receptors on the sarcolemma to cause an unblocking of the RyR and Ca++ efflux from the sarcoplasmic reticulum
(Action potential opens voltage-gated Ca2+ channels of sarcoplasmic reticulum)
Ca2+ diffuses out of cisternae into sarcoplasm
Ca2+ interacts with myofilaments triggering contraction |
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Term
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Definition
When Ca2+ binds to troponin C, it triggers crossbridge cycling
Troponin and tropomyosin move so actin is exposed.
Crossbridge formation
Myosin head attaches to exposed binding site on actin
Power stroke
Myosin head pulls thin filament toward center of sarcomere
ADP and Pi released
3) Release of myosin head
ATP binds to myosin head causing its release from actin
4) Reset myosin head
ATP split into ADP and Pi by myosin ATPase
Provides energy to “cock” the myosin head
[image]
Cycling continues as long as Ca2+ and ATP are present
Results in sarcomere shortening as Z discs move closer together
Narrowing (or disappearance) of H zone and I band
Thick and thin filaments remain the same length but slide past each other |
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Term
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Definition
Spastic paralysis caused by toxin from Clostridium tetani
Blocks release of inhibitory neurotransmitter in spinal cord, resulting in overstimulation of muscles
Vaccination prevents this life-threatening condition |
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Term
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Definition
Muscular paralysis caused by toxin from Clostridium botulinum
Prevents release of ACh at synaptic knobs
Although toxin ingestion can be life-threatening, careful injections of it can treat spasticity (e.g., due to cerebral palsy) or can be used for cosmetic purposes (diminishing wrinkles) |
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Term
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Definition
Termination of nerve signal and ACh release from motor neuron
Hydrolysis of ACh by acetylcholinesterase
Closure of ACh receptor causes cessation of end plate potential
No further action potential generation
Closure of calcium channels in sarcoplasmic reticulum
Return of Ca2+ to sarcoplasmic reticulum by pumps
Return of troponin to original shape
Return of tropomyosin blockade of actin’s myosin binding sites
Return of muscle to original position due to its elasticity |
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Term
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Definition
Muscle cells have only a little ATP in storage
Stored ATP is spent after about 5 seconds of intense exertion
Three ways to generate ATP in skeletal muscle fiber
Immediate supply via phosphate transfer
Short-term supply via glycolysis
Long-term supply via aerobic cellular respiration |
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Term
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Definition
Immediate supply of ATP: phosphate (Pi) transfer
Myokinase transfers Pi from one ADP to another
Creatine kinase transfers Pi from creatine phosphate to ADP |
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Term
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Definition
Does not require oxygen
Glucose (from muscle’s glycogen or through blood) is converted to two pyruvate molecules
2 ATP released per glucose molecule
Occurs in cytosol |
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Term
| Aerobic Cellular Respiration |
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Definition
Requires oxygen
Occurs within mitochondria
Pyruvate oxidized to carbon dioxide
Transfer of chemical bond energy to NADH and FADH2
Energy used to generate ATP by oxidative phosphorylation
Produces a net of 30 ATP
Triglycerides can also be used as fuel to produce ATP
More ATP from triglycerides with longer fatty acid chains |
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Term
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Definition
For a 50-meter sprint (less than 10 seconds)
ATP supplied primarily by phosphate transfer system
For a 400-meter sprint (less than a minute)
ATP supplied primarily by glycolysis after first few seconds
For a 1500-meter run (more than a minute)
ATP supplied primarily by aerobic processes after first minute |
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Term
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Definition
Amount of additional oxygen needed after exercise to restore pre-exercise conditions
Additional oxygen required to
Replace oxygen on hemoglobin and myoglobin
Replenish glycogen
Replenish ATP and creatine phosphate
Convert lactic acid back to glucose |
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Term
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Definition
Differences in power, speed, and duration
Power related to diameter of muscle fiber (larger are more powerful)
Speed and duration related to type of myosin ATPase (some catalyze faster), quickness of action potential propagation, and quickness of Ca2+ release and reuptake by sarcoplasmic reticulum
Fast-twitch fibers are more powerful and have quicker and briefer contractions than slow twitch fibers.
Speed of contraction determined by Vmax of myosin ATPase.
High Vmax (fast, white)
All fibers in a particular motor unit will be of the same type, i.e., fast or slow. |
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Term
| Oxidative vs Glycolytic Fibers |
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Definition
Oxidative fibers (fatigue-resistant) use aerobic cellular respiration
Extensive capillaries
Many mitochondria
Large supply of myoglobin (red fibers)
Smaller diameter
Glycolytic fibers (fatigable) use anaerobic cellular respiration
Fewer capillaries
Fewer mitochondria
Smaller supply of myoglobin (white fibers)
Large glycogen reserves |
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Term
| 3 types of Skeletal Muscle |
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Definition
1. Slow oxidative fibers (type I)
Contractions are slower and less powerful
High endurance since ATP supplied aerobically
About half the diameter of other fibers, red in color due to myoglobin
2. Fast oxidative fibers (type IIa, intermediate)
Contractions are fast and powerful
Primarily aerobic respiration, but delivery of oxygen lower
Intermediate size, light red in color
3. Fast glycolytic fibers (type IIb, fast anaerobic)
Contractions are fast and powerful
Contractions are brief, as ATP production is primarily anaerobic
Largest size, white in color due to lack of myoglobin
Hand muscles have a high percentage of fast glycolytic fibers for quickness
Back muscles have a high percentage of slow oxidative fibers to continually maintain postural support |
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Term
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Definition
Long-distance runners
Higher proportion of slow-oxidative fibers in legs
Sprinters
Higher percentage of fast glycolytic fibers
Determined primarily by genes
Determined partially by training |
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Term
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Definition
Tension is the force generated when a muscle is stimulated to contract
Lab experiments measure tension and graph it (myogram) |
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Term
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Definition
A twitch is a brief contraction to a single stimulus
The minimum voltage that triggers a twitch is threshold stimulus
Periods of the twitch
Latent period
Time after stimulus but before contraction begins
No change in tension
Contraction period
Time when tension is increasing
Begins as power strokes pull thin filaments
Relaxation period
Time when tension is decreasing to baseline
Begins with release of crossbridges
Generally lasts a little longer than contraction period |
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Term
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Definition
Recruitment is also called multiple motor unit summation
It explains how muscles exhibit varying degrees of force
Recruit just a few motor units to lift a pencil
Recruit many motor units to lift a suitcase
Above a certain voltage, all units are recruited, and so maximum contraction occurs (regardless of how much higher voltage is) |
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