Muscles Flashcards
How many skeletal muscles do humans have?
~660 skeletal muscles
What is the average muscle mass in adult females?
13-30 kg
What is the average muscle mass in adult males?
18-40 kg
What is the diameter range of muscle fibers?
10-100 μm
What is the length range of muscle fibers?
1-500 mm
What are the three layers of connective tissue surrounding muscle fibers?
- Epimysium: Encloses muscles. (The connective tissue)
- Perimysium: Surrounds the groups of fibres that form muscle fascicles.
- Endomysium: Encloses each muscle fibre, and it consists of Extracellular matrix (made of collagen).
What is the basic contractile unit of muscle?
Sarcomere: Consists of myofilaments within myofibrils which are arranged in a series of repeating units.
What type of proteins are troponin and tropomyosin classified as?
Regulatory proteins
What are the two types of myofilaments found in myofibrils?
- Thick filaments
- Thin filaments
What proteins make up thick and thin filaments?
Thick Filaments: Myosin and Myosin-binding proteins (C protein, H protein, M protein, myomesin).
Thin filaments: Actin, tropomyosin, and troponin complex (TN-T, TN-I, TN-C).
What is the primary function of dystrophin in muscle fibers?
Force transmission
What are the three types of muscle contractions?
- Eccentric contraction: muscle generates force when lengthening. (Where muscle force is at its greatest.
- Isometric contraction: Muscle generates force while length stays the same.
- Concentric contraction: Muscle shortens and generates force.
In which type of contraction does the muscle generate force while lengthening?
Eccentric contraction
What is myosin?
A myosin molecule is composed of six proteins:
- Two myosin heavy chains (MyHC)
- One essential myosin light chain (MLC) on each MyHC
- One regulatory myosin light chain (MLC) on each MyHC
MyHC contains ATP- and actin-binding sites (essential for muscle contraction)
What is the role of titin in muscle fibers?
Acts as a molecular spring permitting return of stretched sarcomeres to ‘optimal’ length.
What occurs during the power stroke in the cross-bridge cycle?
Dissociation of Pi results in 5-10 nm displacement
True or False: Muscle force is greatest during concentric contractions.
False
What is the relationship between force and velocity during muscle contraction?
Speed of contraction decreases with increasing resistance
What does the sarcoplasmic reticulum do in muscle fibers?
Surrounds myofibrils and stores calcium
What is the average resting myoplasmic calcium concentration?
~0.1 µM
What is the maximum contraction myoplasmic calcium concentration?
~2 µM
What is the delay between action potential and initiation of muscle contraction?
~20 ms
What slows down the relaxation after fatiguing contractions?
Decrease in ATP concentration
What provides the framework for actin and myosin filaments in the sarcomere?
Cytoskeletal proteins
What links to the endomysium?
Structural Intracellular proteins of muscle fibre link to the endomysium via focal adhesions. (This is important for force transmission).
What is the Contractile machinery of the muscle?
- Muscle fibres (10-100 μm) are covered by sarcolemma (≈7.5 nm thick).
- T-tubules are invaginations of sarcolemma.
- Sarcoplasmic reticulum is a network of tubes surrounding myofibrils.
- Sarcoplasm is fluid enclosed within the fibre.
- Myofibrils (~1 μm) are packed bundles of myofilaments running along the fibre.
What do the different filaments in the sarcomere do?
- Thin filaments at one end project into the sarcomere while another connects the Z line.
-Thin filaments contain troponin and tropomyosin proteins that participate in blocking and unblocking of thin filaments (regulatory proteins). - Thick filaments (myosin) are in the centre of sarcomere and overlap thin filaments from both sides
Muscle Contraction:
Myosin heads drag thin filaments from both ends towards each other
•The distance between Z lines shortens (sarcomere shortens)
•Shortening of sarcomeres in series add up.
•Example
–10-cm fibre contains ~4,000,000 sarcomeres
–Each sarcomere shortening by 1 µm shortens the fibre to 6 cm.
How is force produced: the cross-bridge cycle
- ATP hydrolysis increases affinity of myosin for actin.
- Following Ca++ release from SR myosin binds actin.
- ATP binds to myosin decreasing its affinity for actin.
- Myosin dissociates from actin, ATP is hydrolysed.
Why does eccentric contraction generate more force than concentric?
Eccentric - all associated cross-bridges contribute.
Concentric - “power stroke” cross-bridges contribute.
What slows down the relaxation after fatiguing contractions?
Possible cause of slowing relaxation is a decrease in concentration ATP leading to impaired dissociation of these metabolites from the myosin.
Where is the muscle contractile force transmitted through?
Through the connective tissue: Endomysium, perimysium, epimysium, tendons).
