40 Questions
Which enzyme breaks down acetylcholine (ACh) to permit skeletal muscle relaxation?
Acetylcholinesterase (AChE)
What prevents cross-bridge formation during muscle relaxation?
Tropomyosin-troponin complexes covering the myosin binding sites on actin
When does a muscle fiber develop its greatest tension?
When there is an optimal overlap between thick and thin filaments
Which factor does NOT affect maximum muscle tension (force)?
Type of muscle fiber involved
Approximately how many muscle fibers are in a motor unit that controls eye movements?
10 - 20
What term refers to the brief contraction in response to a single action potential?
Twitch contraction
Which phenomenon describes larger contractions resulting from stimuli arriving at different times?
Wave summation
How long is the refractory period for skeletal muscle?
5 ms
What do smooth muscle filaments attach to and stretch from one to another?
Dense bodies
What functions like Z discs in smooth muscle?
Dense bodies
What activates myosin light chain kinase (MLCK) during smooth muscle contraction?
Calcium ions binding to calmodulin
Which of the following causes relaxation of smooth muscle in airways and some blood vessel walls?
Epinephrine
What type of invaginations contain Ca2+ in smooth muscle?
Caveolae
Which of the following is NOT a typical response trigger for smooth muscle fibers?
Action potentials from the somatic nervous system
Which proteins prevent myosin from binding to actin in a relaxed muscle?
Troponin and tropomyosin
What role does myosin play in muscle contraction?
Hydrolyzes ATP to generate energy
During the contraction cycle, which step is associated with the detachment of myosin from actin?
Binding of ATP to myosin
Which process enables the release of more Ca2+ into the sarcoplasmic reticulum (SR)?
Binding of Ca2+ to calsequestrin
What triggers the removal of tropomyosin from myosin-binding sites on actin?
Binding of Ca2+ to troponin
What is the primary factor that allows muscle contraction to occur?
Exposure of myosin-binding sites on actin
Which of the following is correctly associated with the 'power stroke' phase of the contraction cycle?
Rotation of myosin cross-bridges towards the center of the sarcomere
Which bands form the striations visible in skeletal muscle fibers?
A bands and I bands
What occurs during a concentric isotonic contraction?
The muscle shortens to produce movement
Which type of muscle fiber is characterized by high myoglobin content and numerous mitochondria?
Red muscle fibers
Which muscles have a high proportion of fast glycolytic fibers?
Muscles of the shoulders and arms
What happens to fast glycolytic fibers during aerobic exercise?
They transform into fast oxidative-glycolytic fibers
What is the primary function of connexins in cardiac muscle?
To allow action potentials to spread from one cardiac cell to the next
Where is visceral (single unit) smooth muscle commonly found?
In walls of small arteries and veins
Which type of skeletal muscle fiber is predominant in postural muscles of the neck, back, and legs?
Slow oxidative fibers
What characteristic is associated with isometric contraction?
The muscle tension is generated without changing the muscle length
Which type of muscle has long cylindrical fibers with multiple peripherally located nuclei?
Skeletal
What regulates the contraction of cardiac muscle?
Acetylcholine and norepinephrine released by autonomic motor neurons
Which type of muscle tissue can regenerate via pericytes?
Smooth
Where are intercalated discs found?
Cardiac muscle
Which muscle type is voluntary and controlled by the somatic nervous system?
Skeletal
What is the primary function of the skeletal muscle pump?
To aid the heart in venous return
What feature is unique to skeletal muscle under microscopic examination?
Striations with multiple peripherally located nuclei
Which layer of connective tissue surrounds individual muscle fibers?
Endomysium
Which part of the sarcomere contains only thick filaments?
H zone
What is the role of somatic motor neurons in skeletal muscle?
Stimulate skeletal muscle to contract
Study Notes
Skeletal Muscle Relaxation
- Two processes permit skeletal muscle relaxation:
- ACh is broken down by acetylcholinesterase (AChE)
- Ca2+ active transport pumps
- As Ca2+ levels drop, tropomyosin-troponin complexes slide back over the myosin binding sites on actin, preventing cross-bridge formation.
Length Tension Relationship
- The graph displays how tension developed varies with different sarcomere lengths.
- A muscle fiber develops its greatest tension when there is an optimal overlap between thick and thin filaments.
Control of Muscle Tension
- Maximum tension (force) is dependent on:
- Rate of nerve impulses at NMJ (frequency of stimulation)
- Amount of stretch before contraction
- Nutrient and O2 availability
- Number of muscle fibers that are contracting (motor unit size)
Motor Units
- A motor unit consists of a somatic motor neuron and all the skeletal muscle fibers it stimulates.
- Motor unit size varies:
- Voice production: 2-3 muscle fibers/motor unit
- Eye movements: 10-20 muscle fibers/motor unit
- Limbs: 2000-3000 muscle fibers/motor unit
Twitch Contraction - Myogram
- A brief contraction in response to a single action potential
- Refractory Period: period of lost excitability, muscle cannot be excited again during this time
- Skeletal muscle: 5 ms
- Cardiac muscle: 300 ms
Frequency of Stimulation
- The phenomenon in which stimuli arrive at different times causing larger contractions is called wave summation.
- This occurs when additional Ca2+ is released from SR and levels of sarcoplasmic Ca2+ are still high from the previous stimulus.
Zones and Bands of a Sarcomere
- The striations of skeletal muscle are formed by alternating darker A bands and lighter I bands.
Skeletal Muscle Proteins
- Myofibrils are built from three kinds of proteins:
- Contractile proteins
- Regulatory proteins
- Structural proteins
Structure of Thick and Thin Filaments
- Contractile proteins:
- Myosin (thick filaments): convert ATP to energy of motion
- Actin (thin filaments): site where a myosin head attaches
Regulatory Proteins
- Troponin and Tropomyosin:
- In relaxed muscle, myosin is blocked from binding to actin
- Calcium ion binding to troponin moves tropomyosin away from myosin-binding sites, allowing muscle contraction
The Sliding Filament Mechanism
- With exposure of the myosin binding sites on actin (thin filaments) - in the presence of Ca2+ and ATP - the thick and thin filaments "slide" on one another and the sarcomere is shortened.
The Contraction (Cross-Bridge) Cycle
- Consists of 4 steps:
- ATP hydrolysis: myosin heads hydrolyze ATP and become reoriented and energized
- Detachment of myosin from actin
- Formation of cross-bridges: myosin heads bind to actin, forming crossbridges
- Power stroke: myosin crossbridges rotate toward center of the sarcomere (power stroke)
Calcium is Key to Muscle Contraction
- Inside SR, a calcium binding protein called calsequestrin binds Ca2+, enabling more Ca2+ to be taken up into the SR
- Ca2+ release channels: open and closed
Muscle Tissue
- Types of Muscle:
- Skeletal
- Cardiac
- Smooth
- Properties of Muscle:
- Microscopic appearance and features
- Location
- Fiber diameter
- Fiber length
- Nervous control
- Contraction regulated by
- Capacity for regeneration
Muscle Pump and Connective Tissues
- Skeletal Muscle Pump: aids the heart in venous return and relies on the presence of valves in veins
- Fascia: dense sheet or broad band of irregular connective tissue that surrounds muscles and other organs of the body
- Tendon: cord that attaches a muscle to a bone
Within Myofibrils are Filaments and Sarcomeres
- Filaments: thick and thin
- Sarcomeres: compartments of arranged filaments - basic functional unit of a myofibril
- Z discs: separate one sarcomere from the next
- A band: darker middle part of the sarcomere, thick and thin filaments overlap
- I band: lighter, contains only thin filaments
- H zone: center of each A band which contains only thick filaments
- M line: supporting proteins that hold the thick filaments together in the H zone
Learn about the zones and bands of a sarcomere, the types of skeletal muscle proteins, and the structure of thick and thin filaments. Understand the roles of contractile, regulatory, and structural proteins.
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