Muscle Physiology Quiz

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Questions and Answers

What is the primary source of ATP during aerobic endurance activities?

Krebs cycle (correct)

Direct phosphorylation

Fatty acid oxidation

Anaerobic glycolysis

In which situation is oxygen NOT required for energy production?

Aerobic endurance activities

Anaerobic endurance activities (correct)

Rigor mortis

Post-exercise recovery

What causes rigor mortis to occur after death?

Calcium ions leaking from the sarcoplasmic reticulum (correct)

Depletion of calcium ions in muscle fibers

Increase in ATP production

Accumulation of creatine phosphate

Which muscle type is primarily involved in sustained contractions, known for being fatigue resistant?

Slow oxidative fibers Signup and view all the answers

What is one of the reasons for heavy breathing after prolonged strenuous exercise?

To replace oxygen in muscle myoglobin Signup and view all the answers

What is the maximum duration skeletal muscles can maintain maximal contraction using ATP and creatine phosphate?

15 seconds Signup and view all the answers

Which of the following factors contributes to muscle fatigue?

Low oxygen levels Signup and view all the answers

What is the function of nebulin in muscle contraction?

To stabilize thin filaments Signup and view all the answers

During which type of respiration does the body primarily generate ATP at rest or during moderate exercise?

Aerobic respiration Signup and view all the answers

What occurs during the refractory period in muscle fibers?

Muscle fibers lose excitability and cannot respond Signup and view all the answers

What happens to the Z line during muscle contraction?

It moves closer together Signup and view all the answers

What describes the phenomenon known as treppe?

Increased strength of contraction with repeated stimulation Signup and view all the answers

Which statement is true about the sliding filament mechanism?

The H zones decrease in size Signup and view all the answers

At peak levels of exertion, what percentage of ATP is provided by mitochondria?

33% Signup and view all the answers

What initiates the contraction cycle in muscle fibers?

Calcium ion release from the sarcoplasmic reticulum Signup and view all the answers

What type of contraction is described as a brief contraction of all muscle fibers in a motor unit?

Twitch Signup and view all the answers

What is the primary source of ATP for muscles during two minutes of maximal activity?

Anaerobic glycolysis Signup and view all the answers

What occurs after the myosin head pivots during the contraction cycle?

Another ATP binds to the cross bridge Signup and view all the answers

Which of the following is NOT required to form cross-bridges during muscle contraction?

Dissociation of actin and tubulin Signup and view all the answers

What is true about calcium ion concentration in a relaxed muscle fiber?

It is low in the sarcoplasm and high in the SR Signup and view all the answers

What characterizes incomplete tetanus?

Muscle produces peak tension with rapid cycles of contraction and relaxation Signup and view all the answers

What is the primary cause of tetanus?

A toxin that enhances calcium ion permeability in muscle cells Signup and view all the answers

What does recruitment in muscle physiology refer to?

An increase in muscle tension from activating more motor units Signup and view all the answers

Which type of contraction occurs when muscle fibers produce tension without shortening?

Isometric contraction Signup and view all the answers

Which type of muscle fiber is characterized by large diameter and less resistance to fatigue?

Fast glycolytic fibers Signup and view all the answers

What condition refers to an increased muscle tone associated with enhanced tendon reflexes?

Spasticity Signup and view all the answers

In the context of muscle physiology, what is the Cori cycle primarily concerned with?

Recycling of lactate into glucose and pyruvate in the liver Signup and view all the answers

What describes hypertonia?

Increased muscle tone Signup and view all the answers

What primarily connects the smooth muscle cells in single-unit smooth muscle tissue?

Gap junctions Signup and view all the answers

Which of the following is a defining feature of multi-unit smooth muscle?

Stimulation by individual motor neurons Signup and view all the answers

Which of the following muscles is NOT typically classified as multi-unit smooth muscle?

Muscle walls of the stomach Signup and view all the answers

Which mechanism initiates the contraction in smooth muscle cells?

Interaction of calcium with calmodulin Signup and view all the answers

Which statement best describes the contractile nature of smooth muscle?

Smooth muscle exhibits plasticity, adjusting to various lengths. Signup and view all the answers

What role does smooth muscle play in the male reproductive tract?

Facilitating sperm mobility Signup and view all the answers

What is a consequence of limited oxygen availability in skeletal muscle?

Decreased mitochondrial efficiency Signup and view all the answers

Which condition is an autoimmune disorder affecting the neuromuscular junction?

Myasthenia Gravis Signup and view all the answers

What is a major characteristic unique to multi-unit smooth muscle?

Requires excitation by its own motor neuron terminal Signup and view all the answers

Which function is NOT typically attributed to smooth muscle?

Facilitating rapid voluntary muscle movement Signup and view all the answers

What initiates contraction in smooth muscle cells?

Calcium ions binding to calmodulin Signup and view all the answers

Which of the following statements about smooth muscle plasticity is correct?

Plasticity allows smooth muscle to maintain tension at varying lengths. Signup and view all the answers

Which type of muscle cell is described as uninucleate and lacking sarcomeres?

Single-unit smooth muscle Signup and view all the answers

What is the primary role of the sarcolemma in a muscle fiber?

Plasma membrane of the muscle fiber Signup and view all the answers

Which of the following correctly describes myofibrils?

Contractile organelles of muscle cells Signup and view all the answers

What structure is formed by a T-tubule and two terminal cisterns?

Triad Signup and view all the answers

What is found within the sarcoplasm of a muscle fiber?

Glycogen and myoglobin Signup and view all the answers

Which structure is primarily responsible for the striated appearance of skeletal muscle fibers?

Myofibrils with thick and thin filaments Signup and view all the answers

What are the compartments called that contain overlapping thick and thin filaments within myofibrils?

Sarcomeres Signup and view all the answers

What is the primary function of the sarcoplasmic reticulum in muscle cells?

To store calcium ions Signup and view all the answers

Which type of filament is primarily involved in the contraction of muscle cells?

Both actin and myosin filaments Signup and view all the answers

What is the primary function of myosin within the sarcomere?

To act as a motor protein for muscle contraction Signup and view all the answers

Which band within the sarcomere contains only thin filaments?

I band Signup and view all the answers

What role does titin play in muscle fibers?

It provides elasticity and extensibility to myofibrils Signup and view all the answers

What is the function of the Z disc in a sarcomere?

To separate one sarcomere from another Signup and view all the answers

Which statement accurately describes the A band in skeletal muscle fibers?

It contains both thick and thin filaments Signup and view all the answers

What is characterized by rapid cycles of contraction and relaxation in muscle tension?

Incomplete tetanus Signup and view all the answers

What distinguishes the H zone within a sarcomere?

It contains only thick filaments Signup and view all the answers

Which proteins are classified as regulatory proteins of muscle fibers?

Tropomyosin and troponin Signup and view all the answers

What does muscle tone refer to?

The amount of tension in a muscle at rest Signup and view all the answers

Which type of muscle fiber is characterized by high fatigue resistance and small diameter?

Slow oxidative fibers Signup and view all the answers

What is the characteristic structure of the thin filament in muscle fibers?

A helix of actin protein strands Signup and view all the answers

What type of contraction occurs when muscle fibers lengthen while producing tension?

Eccentric contraction Signup and view all the answers

What is the primary purpose of dystrophin in muscle tissue?

To reinforce the sarcolemma and transmit tension Signup and view all the answers

What is the process called when lactic acid is converted back to pyruvic acid in the liver?

Cori cycle Signup and view all the answers

What condition refers to decreased muscle tone?

Hypotonia Signup and view all the answers

Which of the following describes an increased muscle tone without changes to tendon reflexes?

Rigidity Signup and view all the answers

Which statement about fast fibers is correct?

They have fewer mitochondria and large glycogen reserves. Signup and view all the answers

What is the main characteristic of isometric contraction?

Muscle tension is produced but no change in length occurs Signup and view all the answers

Which type of muscle contraction results in muscle fiber shortening?

Concentric contraction Signup and view all the answers

Which statement about tropomyosin at rest is true?

It is held in place by troponin. Signup and view all the answers

What is the role of calcium ions in muscle contraction?

They bind to troponin, allowing actin's active sites to be exposed. Signup and view all the answers

Which part of the contraction cycle involves the hydrolysis of ATP?

Myosin head energizing and orienting. Signup and view all the answers

What occurs to the H zones and I bands during muscle contraction?

They get smaller. Signup and view all the answers

Which statement about cross-bridges in muscle contraction is true?

They are segments of the thick filament that attach to actin. Signup and view all the answers

Which event marks the beginning of muscle contraction at the cellular level?

Calcium ions are released from the terminal cisternae. Signup and view all the answers

What happens when cross-bridges bind to actin in terms of muscle movement?

The thin filaments are pulled toward the M line. Signup and view all the answers

Which subunit of troponin binds directly to calcium ions?

Troponin C. Signup and view all the answers

What remains constant during muscle contraction according to the sliding filament mechanism?

The width of the A band. Signup and view all the answers

What is true about ATP during the contraction cycle?

It must be continuously regenerated to maintain contractions. Signup and view all the answers

Study Notes

Nebulin

Long, non-elastic protein
Wraps around the entire length of each thin filament

Actin Binding

Actin's active sites (myosin-binding sites) are blocked by tropomyosin molecules at rest

Tropomyosin and Troponin

Tropomyosin is held in place by troponin at rest
Troponin is a globular molecule with three subunits:
- Subunit 1 binds to tropomyosin
- Subunit 2 binds to G-actin
- Subunit 3 binds to calcium ions

Sliding Filament Mechanism

Z lines (Z disks) move closer together during contraction
The H zones and I bands get smaller during contraction
The width of the A band remains constant during contraction
The zone of overlap gets larger during contraction (more overlap between thin & thick filaments)

Contraction Initiation

Muscle fiber contraction starts when calcium ions are released from the terminal cistern of the sarcoplasmic reticulum
Thick filaments pivot toward the M line when cross-bridges bind to the active site of actin thin filaments

Active Site Exposure

Active sites on the actin thin filament become available (exposed) when calcium ions bind to troponin subunit
Troponin moves tropomyosin away from the myosin-binding sites of the thin filament once calcium ions bind

Contraction Cycle

Step 1: Myosin head hydrolyzes ATP and becomes energized and oriented
Step 2: Myosin head binds to actin, forming the cross-bridge
Step 3: Myosin head pivots, pulling the thin filament past the thick filament toward the center of the sarcomere (power stroke)
Step 4: Another ATP binds to the cross-bridge, causing the myosin head to detach from the actin. Then, the cycle repeats

Cross Bridges

Are portions of the thick filament (myosin head)
Act as ATPase enzymes during the contraction cycle of muscle
Generate force

Cross-Bridge Formation Requirements

ATP must be broken down by the enzyme ATPase
Calcium ions must be present in the sarcoplasm
Myosin binding sites (actin active site) must be exposed

Calcium Concentration in Muscle Cells

When a muscle fiber is relaxed, the concentration of Ca2+ in the sarcoplasm is very low, but the concentration of Ca2+ in the sarcoplasmic reticulum (SR) is very high.
The concentration of Ca2+ in the sarcoplasm rises rapidly during muscle contraction.

Energy Sources for Muscle Contraction

Creatine phosphate acts as an energy reserve in muscle tissue
The combined amounts of creatine phosphate and ATP provide enough energy for the muscle to contract maximally for approximately 15 seconds
Anaerobic glycolysis supplies enough ATP for muscles for two minutes of maximal activity
Aerobic respiration supplies enough ATP for muscles during periods of rest or moderate exercises (running, jogging, swimming, and walking)
At peak levels of exertion, mitochondria can provide only one-third (33%) of the ATP needed. The remainder comes from glycolysis.

Muscle Fatigue

Inadequate release of calcium ions from the SR
Depletion of creatine phosphate
Insufficient oxygen
Depletion of glycogen and other nutrients
Buildup of lactic acid
Failure of action potentials in the motor neurons to release enough Ach
Muscle fatigue occurs when energy reserves in a muscle are exhausted and lactic acid levels increase

Motor Unit

A somatic motor neuron and all the skeletal muscle fibers it stimulates

Twitch

A brief contraction of all muscle fibers in a motor unit in response to a single action potential moving down the somatic motor neuron

Myogram

A record of muscle contraction

Myogram of Twitch Contraction

Latent period
Contraction period
Relaxation period

Refractory Period

A period that the muscle fiber loses its excitability and cannot respond to a second stimulus
Additional oxygen is required to metabolize the lactic acid produced during exercise during the refractory period

Treppe

Increased strength of a contraction that occurs when a second stimulus arrives after the muscle fiber has relaxed completely following the previous stimulus

Wave Summation

Increased strength of a contraction that occurs when a second stimulus arrives before the muscle fiber has relaxed completely

Tetanus

Complete tetanus (fused tetanus): a muscle that is stimulated so frequently that the relaxation phase is completely eliminated
Incomplete tetanus (unfused tetanus): a muscle producing peak tension with rapid cycles of contraction and relaxation
Tetanus is due to a toxin that makes the muscle cell membrane more permeable to calcium ions (Ca2+), causing powerful tetanic contraction.

Recruitment

Increase in muscle tension that is produced by increasing the number of active motor units

Muscle Tone

Small amount of tension in the muscle at rest due to weak, involuntary contractions of its motor units

Hypotonia

Decreased or lost muscle tone

Flaccid

A state of limpness in which muscle tone is lost

Hypertonia

Increased muscle tone

Spasticity

Increased muscle tone associated with an increase in tendon reflexes

Rigidity

Increased muscle tone in which tendon reflexes are not affected (tetanus)

Isometric Contraction

Type of contraction in which the muscle fibers produce tension but do not shorten nor lengthen (object cannot be moved)

Isotonic Contraction

Type of contraction in which the muscle fibers produce tension while muscle length is changed

Concentric Isotonic Contraction

Muscle fibers shorten (object is lifted)

Eccentric Isotonic Contraction

Muscle fibers lengthen (object is dropped)

Cori Cycle

Cycle in the liver in which lactic acid is converted to pyruvic acid and glucose

Fast Fibers

Fast glycolytic fibers (FG)
- Large diameter
- Produces powerful contraction
- Less resistant to fatigue
- Glycolytic fibers
  - Have fewer mitochondria and large glycogen reserves
  - Use anaerobic respiration
- Low concentration of myoglobin (White meat)

Slow Fibers

Slow oxidative fiber (SO)
- Small diameter
- Most resistant to fatigue
- Oxidative fibers
  - Have many mitochondria
  - Use aerobic respiration
- High concentration of myoglobin (Red meat)
- Produce slow contraction

Aerobic Endurance Activities

Most of the muscle's energy is produced in mitochondria
Krebs cycle is a main source of ATP (36 ATPs)
No oxygen debt
Oxygen is required
Dependent on slow oxidative fibers
Fatigue resistant (sustained contraction for hours)

Anaerobic Endurance Activities

Most of the muscle’s energy is produced in the cytoplasm by glycolysis
Glycolysis is a main source of ATP (2 ATPs)
Oxygen debts are common
Dependent on fast glycolytic fibers
Oxygen is NOT required
Muscle fatigue quickly (not able to have sustained contraction for hours)

Rigor Mortis

A condition in which muscles are in a state of rigidity 3-4 hours after death and last about 24 hours
The mechanisms of rigor mortis is due to calcium ions leaking out of the SR (allowing myosin heads to bind to actin) and muscle fibers running out of ATP (causing the cross-bridges cannot detach from actin)
Rigor mortis disappears as proteolytic enzymes from lysosomes digest the cross-bridges

Post-Exercise Oxygen Consumption

After prolonged strenuous exercise has stopped, heavy breathing will often continue for several minutes in order to provide the oxygen needed to:
- Convert the lactic acid produced during exercise back into glycogen
- Resynthesize creatine phosphate
- Replace oxygen displaced from muscle myoglobin

Types of Smooth Muscle

Single-unit visceral smooth muscle
Multi-unit smooth muscle

Single-Unit Smooth Muscle

Action potentials are initiated in response to neurotransmitters, hormones, or an auto-rhythmic signal
The action potential spreads throughout the tissue by moving through gap junctions that connect all the muscle cells together within the tissue.
Single-unit smooth muscle is found in the skin and in tubular arrangements that form part of the walls of small arteries and veins and of hollow organs such as the stomach, intestines, uterus, and urinary bladder

Multi-Unit Smooth Muscle

Muscle cells have few gap junctions with neighboring cells and thus must be excited by their own motor neuron terminal
Found in the walls of large arteries, in airways to the lungs, in the arrector pili muscles that attach to hair follicles, in the muscles of the iris that adjust pupil diameter, and in the ciliary body that adjusts focus of the lens in the eye.

Smooth Muscle Cell Characteristics

Uninucleate
Lack sarcomeres
Thin filaments are attached to dense bodies
Transmitting the contractile forces from cell to cells throughout the muscle tissue
Involuntary control (pacesetter cells)

Smooth Muscle Functions

Altering the diameter of the respiratory passageways
Moving food materials along the digestive tract
Moving sperms in the male reproductive tract and oocytes in the uterine tract
Expelling the fetus by contraction of the wall of the uterus
Acting as a sphincter

Smooth Muscle Contraction

Smooth muscle contracts when calcium ions Ca2+ interact with calmodulin, which activates the enzyme myosin light chain kinase, enabling myosin heads to attach to actin.

Plasticity

Ability of smooth muscle to function over a wide range of lengths

Stress-Relaxation Response

Ability of single-unit smooth muscle fibers to maintain a constant tension in the muscle despite changes in length

Muscle Growth

Individuals lifting weights build larger muscles because skeletal muscles increase the number of myofibrils but not the number of cells

Mitochondrial Activity

Mitochondrial activity in skeletal muscle cells is usually efficient, but can have limited ATP production if there is limited O2 availability

Myasthenia Gravis

Autoimmune disorder that targets the ACh receptors at the NMJ and ultimately reduces the number of available receptors

Muscle Structure

Sarcolemma is the muscle cell membrane.
Sarcoplasm is the cytoplasm of a muscle cell and stores glycogen and myoglobin.
Sarcoplasmic reticulum (SR) is a network of channels around each myofibril and stores calcium ions (Ca2+).
Transverse tubules (T tubules) are tunnels from the surface to the center of a muscle fiber.
Terminal cisterns are expanded ends of the SR, forming a triad with a T tubule.
Myofibrils are the contractile organelles of muscle cells.
Microfilaments are smaller proteins within myofibrils, including thin and thick filaments.
Sarcomere is the basic functional unit of a myofibril, containing both thin and thick filaments.
Z line is a plate-shaped region separating sarcomeres.
M line links thick and thin filaments laterally.
A band is the darker middle area of a sarcomere, containing both thick and thin filaments.
I band is the lighter area containing only thin filaments.
H zone is the narrow central area of the A band containing only thick filaments.
Zone of overlap is the dark region where thin and thick filaments overlap.

Muscle Proteins

Myosin is the main protein in thick filaments and functions as a motor protein.
Actin is the main protein in thin filaments, arranged in a helix.
Tropomyosin and troponin regulate muscle contraction.
Titin is a large protein extending from the Z disk to the M line, contributing to elasticity.
Myomesin is part of the M line.
Nebulin is a non-elastic protein wrapped around thin filaments.
Dystrophin reinforces the sarcolemma and transmits tension to tendons.

Muscle Contraction

Active sites on actin are blocked by tropomyosin at rest.
Calcium ions bind to troponin, causing it to move tropomyosin away from active sites.
Myosin head hydrolyzes ATP and becomes energized, binding to actin.
Power stroke: the myosin head pivots, pulling thin filaments toward the center of the sarcomere.
Another ATP binds to the myosin head, causing it to detach from actin, and the cycle repeats.

Muscle Fiber Types

Fast fibers (FG): Large diameter, produce powerful contractions, fatigue quickly, use anaerobic respiration, low myoglobin (white meat).
Slow fibers (SO): Small diameter, highly resistant to fatigue, use aerobic respiration, high myoglobin (red meat).
Intermediate fibers: Have characteristics of both fast and slow fibers.

Muscle Tension

Treppe: gradual increase in tension after repeated stimuli.
Wave summation: successive stimuli increase the tension before relaxation is complete.
Incomplete tetanus: rapid cycles of contraction and relaxation, producing peak tension.
Complete tetanus: sustained contraction with no relaxation.
Recruitment: increasing the number of active motor units to increase muscle tension.
Muscle tone: slight tension in resting muscles due to involuntary contractions.
Hypotonia: Decreased or lost muscle tone.
Flaccidity: Limpness with loss of muscle tone.
Hypertonia: Increased muscle tone.
Spasticity: Increased muscle tone with increased tendon reflexes.
Rigidity: Increased muscle tone with unaffected tendon reflexes.

Muscle Contractions

Isometric contraction: Muscle tension increases, but length remains constant (no movement).
Isotonic contraction: Muscle tension remains constant, but length changes.
Concentric isotonic contraction: Muscle shortens (lifting).
Eccentric isotonic contraction: Muscle lengthens (lowering).
Cori cycle: Lactic acid is converted to pyruvic acid and glucose in the liver.

Smooth Muscle

Single-unit smooth muscle: Cells are connected by gap junctions and contract as a unit, found in walls of hollow organs.
Multi-unit smooth muscle: Cells are less connected and contract independently, found in large arteries and airways.
Characteristics of smooth muscle: Uninucleate, lack sarcomeres, thin filaments attach to dense bodies, involuntary control.
Functions of smooth muscle: Regulating airway diameter, moving food through the digestive tract, moving reproductive fluids, acting as sphincters.
Smooth muscle contraction:: Calcium ions bind to calmodulin, activating myosin light chain kinase.

Additional Points

Muscle tissue can undergo stress-relaxation response: Allowing smooth muscle to function over a wide range of lengths.
Weightlifting increases myofibrils: Not the number of cells.
Mitochondrial activity in skeletal muscle: Typically efficient, but limited ATP production with limited oxygen availability.
Myasthenia Gravis: Autoimmune disorder affecting ACh receptors at the neuromuscular junction, reducing the number of available receptors.

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Description

Test your knowledge on muscle contraction mechanisms, including the roles of nebulin, tropomyosin, and troponin in muscle fibers. This quiz covers essential concepts like the sliding filament theory and contraction initiation processes.