BI0005 - Lecture 6 - Muscles
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What is skeletal muscle?
Skeletal muscle is attached to the bones and is responsible for movement. It is characterized by a hierarchy of smaller and smaller units.
Most skeletal muscles consist of...
...a bundle of long fibers running parallel to the length of the muscle. Each fiber is a single cell with multiple nuclei, reflecting its formation by the fusion of many embryonic cells.
A muscle fiber contains a bundle of...
...smaller myofibrils arranged longitudinally. The myofibrils, in turn, are composed of thin filaments and thick filaments.
What are thin filaments?
Thin filaments consist of two strands of actin and two strands of a regulatory protein coiled around one another.
What are thick filaments?
Thick filaments are staggered arrays of myosin molecules
What are striated muscles?
Skeletal muscle is also called striated muscle because the regular arrangement of the filaments creates a pattern of light and dark bands.
What is a sarcomere?
- Each repeating unit within a myofibril is a sarcomere, the basic contractile unit of the muscle.
- The boarders of the sarcomere are lined up in adjacent myofibrils and contribute to the striation visible with a light microscope.
Thin filaments are attached at the...
...Z lines and project toward the center of the sarcomere.
Thick filaments are attached at the...
...M llines centered in the sarcomere.
When the muscle fiber is at rest...
- ...thick and thin filaments only partially overlap.
- Near the edge of the sarcomere are only thin filaments, whereas the zone in the center contains only thick filaments.
- This arrangement is the key to how the sarcomere, and hence the whole muscle, contracts
What is the perimysium?
Perimysium is a sheath of connective tissue that groups muscle fibers into bundles (anywhere between 10 to 100 or more) or fascicles.
Mainly collagen and fibroblasts
What is the epimysium?
Epimysium is a layer of connective tissue, which ensheaths the entire muscle.
Mainly collagen and fibroblasts
What is a tendon?
A tendon (or sinew) is a tough band of fibrous connective tissue that usually connects muscle to bone and is capable of withstanding tension. Tendons are similar to ligaments and fasciae as they are all made of collagen except that ligaments join one bone to another bone, and fasciae connect muscles to other muscles.
What is a fascicle?
In anatomy, a fascicle is a bundle of skeletal muscle fibers surrounded by perimysium, a type of connective tissue.
What is actin?
A globular protein that links into chains, two of which twists helically about each other, forming microfilaments (actin filaments) in muscle and other kinds of cells.
What is myosin?
A type of protein filament that acts as a motor protein with actin filaments to cause cell contraction.
What is a sarcolemma?
The sarcolemma also called the myolemma, is the cell membrane of a striated muscle fiber cell.
What is sarcoplasmic reticulum?
A specialized endoplasmic reticulum that regulates the calcium concentration in the cytosol
What is the sliding-filament model?
According to the sliding filament model of muscle contraction, neither the thin filaments nor the thick filaments change in length when the sarcomere shortens; rather, the filaments slide past each other longitudinally, increasing the overlap of the thin and thick filaments.
What are T Tubules?
- Transverse tubules.
- They are infoldings of the plasma membrane through which the action potential spreads deep into the interior.
What is a calmodulin?
Calmodulin (CaM) (an abbreviation for CALcium-MODULated proteIN) is a calcium-binding messenger protein expressed in all eukaryotic cells.
What are the events that lead to contraction in striated muscles? Powerpoint
- 1. Muscle cell is excited (depolarised) by a nerve action potential
- 2. Depolarisation is carried into the depths of the cell through T-tubules.
- 3. Ca2+ is released from the sarcoplasmic reticulum.
- 4. Ca2+ binds to calmodulin
- 5. Ca2+-calmodulin complex activates myosin Light Chain Kinase.
- 6. Myosin is phosphorylated
- 7. Myosin binds to actin by cross bridges8. Actin flaments move closer together = contraction
How do muscle cells contract?
- Motor neurons cause muscle contraction by triggering release of Ca2+ into the cytosol of muscle cells with which they form synapses.
- The arrival of an action potential at the synaptic terminal of a motor neuron causes release of the neurotransmitter acetyl-choline.
- Binding of acetylcholine to receptors on the muscle fiber leads to a depolarization, triggering an action potential.
- Within the muscle fiber, the action potential spreads deep into the interior, following infoldings of the plasma membrane called transverse tubules.
- From the T tubules, the action potential spreads even farther, entering a specialized endoplasmic reticulum, the sarcoplasmic reticulum.
- With the SR, the action potential opens Ca2+ channels alowing Ca2+ stored in the interor of th SR to enter the cytosol.
- Finally, Ca2+ binds to the troponin complex, triggering contraction of the muscle fiber.
Individual muscle of the body are composed of different types of skeletal muscle fibres.
What are they?
- Type 1 fibres (slow-oxidative) - 'slow-twitch', fatigue-resistant
- Type 2A fibres (fast-oxidative) - 'fast-twitch', less fatigue-resistant than Type 1
- Type 2B fibres (fast-glycoloytic) - 'fast-twitch', fatigue easily
What's the difference between fast-twitch fibers and slow-twitch fibers?
- Fast fibers are used for brief, rapid, powerful contractions.
- Slow fibers, often found in muscles that maintain posture, can sustain long contractions.
- A slow fiber has less sarcoplasmic reticulum and pumps Ca2+ more slowly than a fast fiber.
- Because Ca2+ remains in the cytosol longer, a muscle twitch in a slow fiber lasts about five times as long as one in a fast fiber.
Most muscles of the body contain all three types of skeletal muscle fibres.
The exact mixture of fiber types depends on (and reflects) the function of the muscle.
1. Postural muscles of the nck, back and legs are rich in Type 1 fibres.
2. Muscles of the shoulders and arms which usually act for short periods of time to produce large amounts of tension are rich in type 2B fibres
3. Leg muscles are also used for walking and running and are rich in both type 1 and type 2B
What are intercalcated discs?
- Intercalated discs are microscopic identifying features of cardiac muscle.
- Cardiac muscle consists of individual heart muscle cells (cardiomyocytes) connected by intercalated discs to work as a single functional organ or syncytium.
- By contrast, skeletal muscle consists of multinucleated muscle fibers and exhibit no intercalated discs.
- Intercalated discs support synchronised contraction of cardiac tissue.
- They occur at the Z line of the sarcomere and can be visualized easily when observing a longitudinal section of the tissue.
Where does smooth (non-striated) muscle occur?
- The walls of hollow organs
- The walls of blood vessels
- The iris of the eye
- The skin - attached to hair follicles
- The nipples
Briefly describe smooth muscle structure.
- Smooth muscle in vertebrates is found mainly in the walls of hollow organs, such as blood vessels and organs of the digestive tract.
- Smooth muscle cells lack striations because their actin and myosin filaments are not regularly arrayed along the length of the cell.
- Insteaed, the thick filaments arescattered throught the cytoplasm, and the thin filaments are attached to structures called dense bodies, some of which are tehter to the plasma membrane.
- There is less myosin that in striated muscle fibers, and the myosin is not associated with specific actin strands.
- Some smooth muscle cells contract only when stimulated by neurons of the autonomic nervous system
- Others can generate action potentials without input from neurons - they are electrically coupled to one another.
- Smooth muscles contract and relax more slowly than striated muscles
How do smooth muscles contract?
- Although smooth muscle contraction is regulated by Ca2+, the mechanism for regulation is different from that in skeletal and cardiac muscle.
- Smooth muscle cells have no troponin comple or T tubules, and their sarcoplasmic reticulum is not well developed.
- During an action potential, Ca2+ enters the cyosol mainly through the plasma membrane.
- Calcium ions cause contraction by binding to the protein calmodulin, which activates an enzyme that phosphorylates the myosin head, enabling cross bridge activity.
What is a troponin complex?
the regulatory proteins that control the position of tropomyosin on the thin filament
What is tropomyosin ?
The regulatory protein that blocks the myosin-binding sites on the actin molecules .
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