Chap 12 Study guide:

What are the structural components of a skeletal muscle fiber?

A skeletal muscle fiber contains myofibrils, which are bundles of myofilaments including thin filaments (actin, troponin, tropomyosin) and thick filaments (myosin).

What is the functional unit of a muscle fiber?

The functional unit of a muscle fiber is the sarcomere, which shortens during contraction.

Describe the sliding filament model.

During contraction, actin filaments slide toward the center of the sarcomere, while myosin heads form crossbridges with actin, pulling the filaments inward using ATP.

What is the structure of the thick filaments?

Thick filaments are composed of myosin that binds to actin and ATP.

What is the structure of the thin filaments?

Thin filaments consist of actin, troponin, and tropomyosin, with troponin binding to calcium and tropomyosin blocking actin sites.

What happens at the neuromuscular junction (NMJ)?

A motor neuron releases acetylcholine (ACh), which binds to receptors on the motor end plate, triggering an action potential in the muscle fiber.

Explain excitation-contraction coupling.

An action potential travels along the sarcolemma and down T-tubules, triggering the release of Ca²⁺ from the sarcoplasmic reticulum which binds to troponin and initiates the crossbridge cycle.

What stimulates the release of Ca²⁺ from the sarcoplasmic reticulum?

The action potential traveling down the T-tubule stimulates the sarcoplasmic reticulum to release calcium ions (Ca²⁺).

Describe the crossbridge cycle.

Myosin binds to actin, performing a power stroke to pull actin, ATP then binds to myosin, breaking the bond, and ATP hydrolysis resets the myosin head.

What causes the 'latent period'?

The latent period is the delay between the action potential and muscle twitch, occurring due to the time needed for Ca²⁺ release and crossbridge setup.

How are muscle fibers classified?

Muscle fibers are classified based on their speed (fast or slow) and ATP source (oxidative or glycolytic).

What are the 3 types of skeletal muscle fibers?

1. Slow-twitch oxidative – endurance, fatigue-resistant; 2. Fast-twitch oxidative – mix of strength and endurance; 3. Fast-twitch glycolytic – strong, quick fatigue.

What is a motor unit?

A motor unit is one motor neuron and all the muscle fibers it activates.

How are motor units recruited?

Small motor units are recruited first for light tasks, and larger units are added as more force is needed.

What’s the difference between isotonic and isometric contractions?

Isotonic contractions involve the muscle shortening to move a load, while isometric contractions involve tension without shortening the muscle.

• Isotonic: force > load → muscle shortens and moves load
  
  

• Isometric: load > force → tension, but muscle does not shorten
  
  

How does summation lead to tetanus?

When rapid action potentials overlap, twitches stack, leading to fused tetanus or sustained contraction.

Rapid APs overlap → twitches stack → fused tetanus (sustained contraction)

What ends a muscle contraction?

Muscle contraction ends when acetylcholine is removed from the synapse and calcium ions (Ca²⁺) are pumped back into the sarcoplasmic reticulum.

Contrast skeletal, smooth, and cardiac muscle.

Feature	Skeletal	Smooth	Cardiac
Control	Voluntary	Involuntary	Involuntary
Striated?	Yes	No	Yes
Nuclei	Multinucleated	Single nucleus	Single nucleus
Speed	Fast	Slow	Moderate
Connections	None	Gap junctions	Gap junctions

What is acetylcholine?

Acetylcholine is a neurotransmitter that plays a crucial role in communication at the neuromuscular junction.

What is the function of myofilaments?

Myofilaments (actin and myosin) are responsible for muscle contraction.

What is a sarcomere?

A sarcomere is the functional unit of contraction in a muscle fiber.

What is the sarcolemma?

The sarcolemma is the muscle cell membrane.

What is the role of the sarcoplasmic reticulum?

The sarcoplasmic reticulum stores and releases calcium ions (Ca²⁺) essential for muscle contraction.

What are T-tubules?

T-tubules help transmit action potentials into the muscle fiber.

What is the motor end plate?

The motor end plate is part of the sarcolemma that receives acetylcholine.

What is a tendon?

A tendon connects muscle to bone.

What is recruitment in muscle physiology?

Recruitment refers to adding more motor units to generate more force during muscle contraction.

What is 'force/tension' in muscle physiology?

Force or tension refers to the strength produced by a muscle during contraction.

What do troponin and tropomyosin do?

Troponin and tropomyosin regulate the binding of actin and myosin during muscle contraction.

What distinguishes oxidative from glycolytic fibers?

Oxidative fibers utilize oxygen for energy and are long-lasting, whereas glycolytic fibers primarily rely on anaerobic processes and are suited for short bursts of activity.

What is the difference between fast-twitch and slow-twitch fibers?

Fast-twitch fibers are designed for strength and quick energy release, while slow-twitch fibers are designed for endurance and sustained activities.

Additional Terms

Additional Terms Recap

• Acetylcholine – Acetylcholine is the neurotransmitter released at the neuromuscular junction that stimulates muscle contraction.

  
  

• Fascicle – bundle of muscle fibers
  
  

• Myofilament – actin or myosin strand
  
  

• Myofibril – bundle of myofilaments inside fiber
  
  

• Sarcomere – functional unit of contraction
  
  

• Sarcolemma – muscle cell membrane
  
  

• Sarcoplasmic reticulum – stores/release Ca²⁺
  
  

• T-tubule – helps transmit AP into fiber
  
  
  

Additional terms

• Motor end plate – part of sarcolemma that receives ACh
  
  
  

• Tendon – connects muscle to bone
  
  
  

• Recruitment – adding more motor units for more force
  
  
  

• Force/Tension – muscle strength
  
  
  

• Troponin & Tropomyosin – regulate actin-myosin binding
  
  
  

• Oxidative vs. glycolytic – oxygen vs. no oxygen energy sources
  
  
  

• Fast-twitch vs. Slow-twitch – strength vs. endurance