Skeletal Muscle Structure and Proteins

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Questions and Answers

Which enzyme breaks down acetylcholine (ACh) to permit skeletal muscle relaxation?

Cholinesterase

Acetylcholinase (ACh)

Carboxylase

Acetylcholinesterase (AChE) (correct)

What prevents cross-bridge formation during muscle relaxation?

Tropomyosin-troponin complexes covering the myosin binding sites on actin (correct)

Binding of Ca2+ to actin

Inhibition of acetylcholinesterase

Low ATP levels

When does a muscle fiber develop its greatest tension?

When there is an optimal overlap between thick and thin filaments (correct)

When thick filaments outnumber thin filaments

When Ca2+ concentration is zero

When there is no overlap between thick and thin filaments

Which factor does NOT affect maximum muscle tension (force)?

Type of muscle fiber involved Signup and view all the answers

Approximately how many muscle fibers are in a motor unit that controls eye movements?

10 - 20 Signup and view all the answers

What term refers to the brief contraction in response to a single action potential?

Twitch contraction Signup and view all the answers

Which phenomenon describes larger contractions resulting from stimuli arriving at different times?

Wave summation Signup and view all the answers

How long is the refractory period for skeletal muscle?

5 ms Signup and view all the answers

What do smooth muscle filaments attach to and stretch from one to another?

Dense bodies Signup and view all the answers

What functions like Z discs in smooth muscle?

Dense bodies Signup and view all the answers

What activates myosin light chain kinase (MLCK) during smooth muscle contraction?

Calcium ions binding to calmodulin Signup and view all the answers

Which of the following causes relaxation of smooth muscle in airways and some blood vessel walls?

Epinephrine Signup and view all the answers

What type of invaginations contain Ca2+ in smooth muscle?

Caveolae Signup and view all the answers

Which of the following is NOT a typical response trigger for smooth muscle fibers?

Action potentials from the somatic nervous system Signup and view all the answers

Which proteins prevent myosin from binding to actin in a relaxed muscle?

Troponin and tropomyosin Signup and view all the answers

What role does myosin play in muscle contraction?

Hydrolyzes ATP to generate energy Signup and view all the answers

During the contraction cycle, which step is associated with the detachment of myosin from actin?

Binding of ATP to myosin Signup and view all the answers

Which process enables the release of more Ca2+ into the sarcoplasmic reticulum (SR)?

Binding of Ca2+ to calsequestrin Signup and view all the answers

What triggers the removal of tropomyosin from myosin-binding sites on actin?

Binding of Ca2+ to troponin Signup and view all the answers

What is the primary factor that allows muscle contraction to occur?

Exposure of myosin-binding sites on actin Signup and view all the answers

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 Signup and view all the answers

Which bands form the striations visible in skeletal muscle fibers?

A bands and I bands Signup and view all the answers

What occurs during a concentric isotonic contraction?

The muscle shortens to produce movement Signup and view all the answers

Which type of muscle fiber is characterized by high myoglobin content and numerous mitochondria?

Red muscle fibers Signup and view all the answers

Which muscles have a high proportion of fast glycolytic fibers?

Muscles of the shoulders and arms Signup and view all the answers

What happens to fast glycolytic fibers during aerobic exercise?

They transform into fast oxidative-glycolytic fibers Signup and view all the answers

What is the primary function of connexins in cardiac muscle?

To allow action potentials to spread from one cardiac cell to the next Signup and view all the answers

Where is visceral (single unit) smooth muscle commonly found?

In walls of small arteries and veins Signup and view all the answers

Which type of skeletal muscle fiber is predominant in postural muscles of the neck, back, and legs?

Slow oxidative fibers Signup and view all the answers

What characteristic is associated with isometric contraction?

The muscle tension is generated without changing the muscle length Signup and view all the answers

Which type of muscle has long cylindrical fibers with multiple peripherally located nuclei?

Skeletal Signup and view all the answers

What regulates the contraction of cardiac muscle?

Acetylcholine and norepinephrine released by autonomic motor neurons Signup and view all the answers

Which type of muscle tissue can regenerate via pericytes?

Smooth Signup and view all the answers

Where are intercalated discs found?

Cardiac muscle Signup and view all the answers

Which muscle type is voluntary and controlled by the somatic nervous system?

Skeletal Signup and view all the answers

What is the primary function of the skeletal muscle pump?

To aid the heart in venous return Signup and view all the answers

What feature is unique to skeletal muscle under microscopic examination?

Striations with multiple peripherally located nuclei Signup and view all the answers

Which layer of connective tissue surrounds individual muscle fibers?

Endomysium Signup and view all the answers

Which part of the sarcomere contains only thick filaments?

H zone Signup and view all the answers

What is the role of somatic motor neurons in skeletal muscle?

Stimulate skeletal muscle to contract Signup and view all the answers

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:
1. ATP hydrolysis: myosin heads hydrolyze ATP and become reoriented and energized
2. Detachment of myosin from actin
3. Formation of cross-bridges: myosin heads bind to actin, forming crossbridges
4. 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

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Description

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.