90 Questions
What is the main function of skeletal muscle?
Locomotion
Which type of muscle is controlled consciously?
Striated muscle
What is the property of muscles that allows them to generate tension?
Contractility
Where can smooth muscles typically be found?
In the walls of blood vessels and internal organs
Which type of muscle controls itself with the help of the nervous and endocrine systems?
Cardiac muscle
What is the capacity of a muscle to receive and respond to stimuli?
Excitability
What is the specialized organelle responsible for storing, releasing, and reuptake of ion calcium in a muscle fiber?
Sarcoplasmic Reticulum (SR)
Which protein makes up the thick filament in a sarcomere responsible for muscle contraction?
Myosin
What component of an actin filament is responsible for regulating the interaction between actin and myosin during muscle contraction?
Troponin
What allows for rapid transmission of the action potential into a muscle cell?
T-Tubules
What is the event that leads to a rapid change in membrane potential in a muscle cell?
Action Potential
According to the Sliding Filament Model, why does a muscle shorten or lengthen?
Due to thick and thin filaments sliding over each other
What is the primary function of the myofilaments in muscle contraction?
To shorten the sarcomere and cause muscle contraction
What is the primary role of the troponin complex within the thin filament?
To regulate the exposure of the actin binding sites on the thin filament
How do the mitochondria in slow-twitch muscle fibers differ from those in fast-twitch fibers?
Slow-twitch fibers have more mitochondria to provide a greater supply of ATP for prolonged activity
What is the primary function of the sarcoplasmic reticulum in muscle cells?
To regulate the uptake and release of calcium ions during contraction
What is the primary role of the myosin heads within the thick filament?
To act as binding sites for the actin filaments during contraction
What is the primary role of the connective tissue layers (epimysium, perimysium, and endomysium) surrounding the skeletal muscle fibers?
To provide strength, stability, and prevent tearing of the muscle during contraction
What happens once myosin binds with actin in muscle fiber action?
The myosin head tilts and pulls the actin filament.
What is the main function of Type I muscle fibers?
High aerobic endurance
Which type of muscle fiber is better for explosive activities?
Type IIb
What is the contractile speed of Type IIa muscle fibers?
50 ms
Which type of muscle fiber has low aerobic capacity and high anaerobic capacity?
Type IIb
How many fibers per motor neuron are present in Type IIa muscle fibers?
110-250
What are the three types of muscle found in the human body?
Skeletal, Smooth, and Cardiac
What is the primary function of skeletal muscle?
To enable voluntary movement of the body
Which of the following is NOT a property of muscle fibers?
Permeability
What is the name of the connective tissue that surrounds the entire muscle?
Epimysium
What is the name of the connective tissue that surrounds each fascicle (bundle) of muscle fibers?
Perimysium
What are the structures responsible for muscle contraction?
Myofilaments
What is the main property of skeletal muscle that enables it to perform its functions effectively?
Contractility
Which type of muscle is found in the walls of blood vessels and internal organs?
Smooth muscle
What is the primary function of the sarcoplasmic reticulum in muscle cells?
To store and release calcium ions
Which type of muscle fiber is better suited for explosive activities?
Type IIb (fast-twitch) muscle fibers
What is the name of the connective tissue that surrounds the entire muscle?
Epimysium
According to the Sliding Filament Model, why does a muscle shorten or lengthen?
Due to the movement of the myosin and actin filaments relative to each other
Which organelle acts as the power plant of ATP in muscle cells?
Mitochondria
In a sarcomere, what is the function of Troponin I within the thin filament?
Inhibition of actin-myosin interaction
What is the primary function of the light chain on a myosin molecule?
ATP binding
Which component of a myosin filament serves as an ATPase enzyme?
Myosin heads
What is the role of Tropomyosin in muscle contraction?
Inhibition of actin-myosin interaction
How do slow-twitch muscle fibers differ from fast-twitch fibers in terms of mitochondria?
Slow-twitch fibers have fewer mitochondria
What is the primary function of the sarcoplasmic reticulum in a muscle fiber?
To store, release, and reuptake calcium ions
According to the Sliding Filament Model, what is the primary reason a muscle shortens or lengthens?
The thick and thin filaments slide over each other without changing length
What is the primary role of the $t$-tubules in a muscle fiber?
To allow for rapid transmission of the action potential into the cell
What is the main component of the thin filament responsible for regulating the interaction between actin and myosin during muscle contraction?
Troponin
What is the primary function of the myosin heads within the thick filament?
To generate force and movement during muscle contraction
Which type of muscle fiber is better suited for explosive activities?
Type IIb (fast-twitch) muscle fibers
What is the role of ATP in muscle contraction according to the Sliding Filament Model?
Provides energy for muscle action
What is the immediate result of the cleavage of ATP by ATPase in muscle contraction?
Formation of ADP and Pi
In muscle contraction, what causes the actin filaments to slide toward the M line?
Conformational change in the myosin head
What is the purpose of the troponin-tropomyosin complex in muscle contraction?
Lifting tropomyosin off active sites on actin filaments
Where does the energy that powers the power stroke in muscle contraction come from?
Stored ADP
What initiates muscle action by lifting tropomyosin off active sites on actin filaments according to the Sliding Filament Model?
Binding of calcium ions to troponin
Which type of muscle fiber is best suited for long-distance running or cycling?
Type I
What happens to calcium ions during muscle relaxation?
Calcium is pumped out of the sarcoplasm to the sarcoplasmic reticulum
Which of the following is true about Type IIb muscle fibers?
They have fast contractile speed and high anaerobic capacity
What is the approximate contractile speed of Type I muscle fibers?
110 ms
How many muscle fibers are typically innervated by a single motor neuron for Type IIa fibers?
300-800
What is the primary function of Type IIa muscle fibers?
Moderate-intensity activities requiring both aerobic and anaerobic metabolism
What is the primary function of the Troponin complex within the thin filament?
Regulates the concentration of calcium ions in the muscle fiber
What component of a myosin filament serves as an ATPase enzyme?
Myosin heads
Which of the following is true about Type IIb muscle fibers?
High anaerobic capacity and low aerobic capacity
What is the main role of the Elasticity property of muscles?
Returning to original shape after being stretched or contracted
What causes actin filaments to slide toward the M line during muscle contraction?
Binding of myosin heads to actin
What distinguishes smooth muscles from skeletal muscles?
Involuntary control and non-striated appearance
What is the primary role of the $t$-tubules in a muscle fiber?
To carry the depolarization from action potentials to the interior of the fiber
Which protein makes up the thick filament in a sarcomere responsible for muscle contraction?
Myosin
What is the primary function of the sarcoplasmic reticulum in muscle cells?
To store and release calcium ions for muscle contraction
What is the primary function of the myofilaments in muscle contraction?
To interact and slide past each other, generating the force for muscle contraction
Which type of muscle fiber is better suited for explosive activities?
Type IIb (fast-twitch) muscle fibers
What is the primary role of the light chain on a myosin molecule?
To act as an ATPase enzyme, powering the contraction process
What is the primary function of the sarcoplasmic reticulum in a muscle fiber?
To store and release calcium ions for muscle contraction
What is the primary role of the t-tubules in a muscle fiber?
To allow for rapid transmission of the action potential into the cell
According to the Sliding Filament Model, what causes the actin filaments to slide toward the M line during muscle contraction?
The binding of myosin heads to actin filaments and the power stroke
What is the primary function of the troponin complex within the thin filament?
To regulate the interaction between actin and myosin during muscle contraction
What is the primary function of the myosin heads within the thick filament?
To bind to actin filaments and undergo a power stroke during muscle contraction
What is the immediate result of the cleavage of ATP by ATPase in muscle contraction?
The release of energy for the power stroke
What is the main difference between Type I and Type II muscle fibers?
Type I fibers have high aerobic endurance, while Type II fibers are better for anaerobic or explosive activities.
Which type of muscle fiber has a high anaerobic capacity and motor unit strength?
Type IIb muscle fibers
What is a characteristic of Type IIa muscle fibers compared to Type I and Type IIb fibers?
Type IIa fibers have moderate aerobic capacity and fatigue resistance.
Which statement best describes the function of Type IIb muscle fibers?
Low aerobic capacity and fatigue resistance
In terms of the sarcoplasmic reticulum, how do Type IIa muscle fibers differ from Type I muscle fibers?
Type IIa muscles store more calcium in the sarcoplasmic reticulum compared to Type I muscles.
What differentiates Type IIa muscle fibers from Type IIb muscle fibers?
Mitochondrial density
What is the role of ATP in the initial stages of muscle contraction according to the Sliding Filament Model?
ATP is cleaved by ATPase, energizing the myosin heads in a 'cocked' position
What is the primary function of the troponin-tropomyosin complex during muscle contraction?
To cover and uncover the active sites on actin filaments
What is the primary source of energy that powers the 'power stroke' during muscle contraction?
The stored energy from the 'cocked' myosin head conformation
What is the primary event that initiates the muscle contraction process?
Release of calcium ions from the sarcoplasmic reticulum
What is the primary result of the 'power stroke' during muscle contraction?
The actin filaments slide toward the M line
What is the primary function of the ATPase enzyme on the myosin heads?
To cleave ATP, energizing the myosin heads
Study Notes
Muscle Structure and Function
- A muscle fiber is enclosed by a plasma membrane called the sarcolemma.
- The sarcoplasmic reticulum (SR) is a specialized organelle responsible for storing, releasing, and reuptake ion calcium.
- T-tubules allow for rapid transmission of the action potential into the cell and play an important role in regulating cellular calcium concentration.
Myofibril Structure
- Myofibrils are made up of sarcomeres, the smallest functional units of a muscle.
- A sarcomere is composed of filaments of two proteins, myosin and actin, which are responsible for muscle contraction.
- Myosin is a thick filament with a globular head at one end.
- An actin filament is composed of actin, tropomyosin, and troponin. It is attached to a Z disk.
Muscle Contraction
- Action potential is a rapid change in membrane potential that travels rapidly along the membrane.
- It is caused by the inflow of Na+ (highly concentrated outside the cell) and the outflow of K+ (highly concentrated inside the cell), causing a change in the voltage of the cell, producing a spike in the action potential.
The Sliding Filament Model
- Proposed in the early 1950s by two British biologists, Hugh Huxley and Andrew Huxley.
- The theory proposes that a muscle shortens or lengthens because thick and thin filaments slide over each other without changing length.
Muscle Fiber Action
- Muscle action is initiated by a nerve impulse.
- If the cell receives the right stimulus, an action potential occurs, which releases stored Ca2+ ions.
- Ca2+ ions bind with troponin, which lifts the tropomyosin molecules off the active sites on the actin filament.
- These open sites allow the myosin heads to bind to them.
- Energy for muscle action is provided when the myosin head binds to ATP.
- ATPase on the myosin head splits the ATP into a usable energy source.
Muscle Fiber Types
- Type I (Red) fibers: have high aerobic endurance and are suited to low-intensity endurance activities.
- Type II (White) fibers: are better for anaerobic or explosive activities.
- Type IIa fibers: are mixed oxidative-glycolytic fibers.
- Type IIb fibers: are glycolytic fibers.
Muscle Properties
- Contractility: the ability to contract or shorten, allowing muscles to generate tension.
- Extensibility: the property of muscles that allows them to be stretched or lengthened beyond their resting length.
- Elasticity: the ability to return to its original shape after being stretched or contracted.
- Excitability: the capacity to receive and respond to stimuli (from the motor neuron, neurotransmitters, hormone, etc).
Muscle Types
- Skeletal muscle: primary function is to enable movement of the body; attached to the bones of the skeleton.
- Smooth muscle: involuntary muscle; controlled unconsciously; found in the walls of blood vessels and internal organs.
- Cardiac muscle: controls itself with help from nervous and endocrine systems; only found in the heart.### Myofilaments
- Myofilaments are responsible for muscle contraction
- Thin filaments consist of Actin, Troponin complex, and Tropomyosin
- Thick filaments consist of Myosin molecules, with a tail and two globular heads that can bind both ATP and Actin
- Myosin heads function as an ATPase enzyme, using ATP as an energy source for contraction
Actin Filament (Thin Filament)
- Composed of ACTIN, TROPOMYOSIN, and TROPONIN COMPLEX
- 2 helical strands of actin protein intertwined with 2 helical strands of tropomyosin protein
- Troponin complex consists of three globular protein subunits (C, I, and T)
- TnC binds calcium, TnI is inhibitory, and TnT binds tropomyosin
Organelles of the Muscle Cell (Fiber)
- MITOCHONDRIA: Power plant of ATP, provides myofibrils with large amounts of energy for muscle contraction
- SARCOPLASMIC RETICULUM: Specialized endoplasmic reticulum, regulates calcium storage, release, and reuptake, important for muscle contraction
Muscle Fiber
- Surrounded by layers of connective tissue (epimysium, perimysium, and endomysium) for strength and stability
Muscle Fiber Action
- Muscle contraction occurs when myosin binds with actin, causing actin filaments to slide across each other
- Muscle action ends when calcium is pumped out of the sarcoplasm to the sarcoplasmic reticulum for storage
Types of Muscle Fibers
- Skeletal muscles contain both Type I and Type II fibers
- Type I (Red): High aerobic endurance, suited for low-intensity endurance activities
- Type II (White): Better for anaerobic or explosive activities, divided into Type IIa and Type IIb
Type I Muscle Fibers
- Oxidative Fiber, high aerobic capacity and fatigue resistance
- Rich in mitochondria, slow contractile speed (110 ms)
- Found in muscles with slow, prolonged activity, 10–180 fibers per motor neuron
Type IIa Muscle Fibers
- Mixed oxidative-glycolytic fiber, moderate aerobic capacity and fatigue resistance
- High anaerobic capacity, fast contractile speed (50 ms)
- Highly developed sarcoplasmic reticulum, 300–800 fibers per motor neuron
Type IIb Muscle Fibers
- Glycolytic Fiber, low aerobic capacity and fatigue resistance
- High anaerobic capacity and motor unit strength, fast contractile speed (50 ms)
- Highly developed sarcoplasmic reticulum, 300–800 fibers per motor neuron
Learn about the basic components of skeletal muscle, muscle functions, organization, types of muscles and muscle fibers, as well as the physiological events of muscle contraction in this lecture by Marcus Machado, MSc, PhD. Test your knowledge with this quiz!
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