Muscle Metabolism Quiz

Podcast

Play an AI-generated podcast conversation about this lesson

Questions and Answers

What is the process of breaking down glucose into pyruvate called?

Glycolysis (correct)

Fermentation

Krebs cycle

Oxidative phosphorylation

Pyruvate is converted into lactic acid during aerobic respiration.

False

What is the final electron acceptor in the electron transport chain?

Oxygen

After exercise, muscles need oxygen to replenish _ and convert lactic acid back to __.

myoglobin, glucose Signup and view all the answers

Match the types of muscle fibers with their characteristics:

Slow oxidative fibers = Do not fatigue easily; endurance activities Fast oxidative-glycolytic fibers = Largest fibers with a fast contraction cycle Fast glycolytic fibers = Primarily use anaerobic respiration Signup and view all the answers

What is produced when pyruvate undergoes fermentation?

Lactic acid Signup and view all the answers

Slow oxidative fibers are also known as fast twitch fibers.

False Signup and view all the answers

What is the main metabolic mode of slow oxidative fibers?

Aerobic respiration Signup and view all the answers

Where is dystrophin located in muscle cells?

At the inner surface of the plasma membrane Signup and view all the answers

The sliding filament model states that the filaments change in length during muscle contraction.

False Signup and view all the answers

What is the role of troponin in muscle contraction?

Troponin binds Ca2+ and moves tropomyosin off the myosin-binding sites on actin. Signup and view all the answers

The attachment of myosin to actin to form a __________ is crucial for muscle contraction.

cross-bridge Signup and view all the answers

Which of the following proteins is NOT part of the dystrophin-glycoprotein complex?

Tropomyosin Signup and view all the answers

Calcium ions are primarily stored in the mitochondria of muscle fibers.

False Signup and view all the answers

What is the significance of the optimal sarcomere length?

It provides sufficient filament overlap for maximal tension generation. Signup and view all the answers

The ____________ restores the negative membrane potential after depolarization.

repolarization Signup and view all the answers

What initiates the release of calcium ions for muscle contraction?

Action potentials from somatic motor neurons Signup and view all the answers

Which ion channels are responsible for depolarization during an action potential?

Voltage-gated sodium channels (VGNCs) Signup and view all the answers

Myosin must bind to a new ATP molecule to release the thin filament.

True Signup and view all the answers

Match the terms with their definitions:

Dystrophin = Links actin filaments to the dystrophin-glycoprotein complex Sarcoplasmic reticulum = Stores calcium ions in muscle cells Tropomyosin = Blocks myosin-binding sites on thin filaments Myosin = Thick filament that pulls the thin filament during contraction Signup and view all the answers

Which component is crucial for transmitting tension from muscle fibers to tendons?

Dystrophin Signup and view all the answers

During contraction, the ___________ band narrows as the Z-discs move together.

I Signup and view all the answers

What happens during the power stroke of the contraction cycle?

Myosin pulls the thin filaments toward the M-line. Signup and view all the answers

The Na+-K+ pump moves potassium ions out of the cell.

False Signup and view all the answers

What is the role of voltage-gated K+ channels during repolarization?

They allow potassium ions to flow out, restoring resting membrane potential. Signup and view all the answers

Action potentials can cause simultaneous responses in muscle fibers that are not part of the same motor unit.

False Signup and view all the answers

What enzyme destroys acetylcholine at the neuromuscular junction?

acetylcholinesterase Signup and view all the answers

The process of returning calcium ions to the SR after muscle contraction involves _____.

ATPases Signup and view all the answers

Match the type of muscle contraction with its description:

Isotonic Contraction = Muscle changes length while maintaining constant tension Isometric Contraction = Tension generated is not sufficient to move a load Concentric Contraction = Muscle shortens to decrease joint angle Eccentric Contraction = Muscle lengthens while resisting a load Signup and view all the answers

What happens to calcium ions when a muscle action potential passes?

Calcium ions are released from the sarcoplasmic reticulum into the sarcoplasm. Signup and view all the answers

A twitch contraction occurs due to multiple action potentials firing at once.

False Signup and view all the answers

What is the name of the period where a muscle fiber cannot respond to a new action potential?

refractory period Signup and view all the answers

Muscle tone is primarily maintained by _____ contractions of alternating motor units.

involuntary Signup and view all the answers

What is the outcome of increased frequency of action potentials?

It increases muscle tension. Signup and view all the answers

Isotonic contractions result in the muscle maintaining its length while generating tension.

False Signup and view all the answers

What connects the triads to the sarcolemma?

voltage-gated calcium channels (VGCCs) Signup and view all the answers

The time between the stimulus and the onset of muscle action is called the ____ phase.

latent Signup and view all the answers

Muscles primarily generate ATP from creatine phosphate at all times, regardless of their activity level.

False Signup and view all the answers

How does troponin function in muscle contraction?

It moves tropomyosin off myosin-binding sites. Signup and view all the answers

What is the deepest layer of muscle tissue called?

Endomysium Signup and view all the answers

Aponeuroses are thin structures that connect muscles to bones.

False Signup and view all the answers

What type of motor neurons regulate voluntary muscle contraction?

somatic motor neurons Signup and view all the answers

The cytoplasm of muscle cells is called __________.

sarcoplasm Signup and view all the answers

Match the following structures with their descriptions:

Myofibrils = Contractile protein threads in muscle cells Sarcoplasmic reticulum = Specialized endoplasmic reticulum in muscle cells T-tubules = Inward folds of the sarcolemma Myoglobin = Oxygen-binding protein in muscle cells Signup and view all the answers

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

Store and release calcium ions Signup and view all the answers

Muscle fibers can divide and increase in number as a response to strength training.

False Signup and view all the answers

What are the two types of filaments in a sarcomere?

thick and thin filaments Signup and view all the answers

The thick filaments in a sarcomere are made of __________.

myosin Signup and view all the answers

Which structure releases calcium ions during muscle contraction?

Sarcoplasmic reticulum Signup and view all the answers

The I-band in a sarcomere contains only thick filaments.

False Signup and view all the answers

What is the significance of myoglobin in muscle cells?

It binds oxygen for cellular respiration. Signup and view all the answers

Hypertrophy is a __________ to increased mechanical stress.

response Signup and view all the answers

Which protein stabilizes and connects the sarcomere?

Dystrophin Signup and view all the answers

Match the proteins involved in muscle contraction with their functions:

Myosin = Motor protein that pulls thin filaments Actin = Structural protein that forms thin filaments Troponin = Regulatory protein that binds calcium Tropomyosin = Blocks myosin binding sites on thin filaments Signup and view all the answers

What is the primary function of skeletal muscle tissue?

To contract and move bones Signup and view all the answers

Cardiac muscle tissue is responsible for regulating the gastrointestinal tract.

False Signup and view all the answers

What is the scientific study of muscular tissue called?

myology Signup and view all the answers

Muscular tissue is ____________ excitable.

electrically Signup and view all the answers

Which type of muscle is primarily responsible for moving blood through the body?

Cardiac muscle Signup and view all the answers

Match the type of muscular tissue with its function:

Skeletal = Moves bones Cardiac = Pumps blood Smooth = Regulates passage of substances Signup and view all the answers

Muscular tissue can stretch and still maintain its strength.

True Signup and view all the answers

The dense irregular connective tissue that wraps muscles is called __________.

epimysium Signup and view all the answers

Study Notes

Muscular Tissue

Three types of muscular tissue: Skeletal, Cardiac, and Smooth
- Skeletal muscle tissue contracts to move bones and stabilize body positions
- Cardiac muscle tissue contracts to move blood through the heart
- Smooth muscle tissue contracts to regulate passage of substances through the body
All muscles generate heat during contraction.

Muscular Tissue Properties

Study of muscular tissue is called myology
Excitability
- Produce electrical signals called muscle action potentials
- Nerve tissue is also excitable
Contractility
- Muscle action potentials stimulate contraction
- Contractions generate tension on bones, leading to movement
Extensibility
- Tissue can be stretched without tearing
- Example: Smooth muscle around stomach
Elasticity
- Resting length is restored after stretching

Structure Of Skeletal Muscle

Cells of skeletal muscle tissue are called muscle fibres
- Elongated cells also known as myocytes
  - Cells contain bunched protein filaments called myofibrils
Muscle fibres + connective tissue + nerve and blood supply = muscle (an organ)
- Muscles are surrounded by connective tissue layers called the fascia
- Fascia physically groups muscles with similar functions together and provides passage for nerves and vasculature

Fascia: Layers Of Connective Tissue

Three layers of fascia
- Epimysium (most superficial): Dense irregular CT that wraps muscles
- Perimysium (intermediate layer): Dense irregular CT that wraps fascicles (bundles of muscle fibres)
- Endomysium (deepest layer): Mostly reticular fibres that wrap individual muscle fibres.

Fascia Forms Tendons

Tendons connect muscles to bones as a thick rope-like structure
Aponeuroses are a broad sheet-type of tendon
- Example: Occipitofrontalis muscle bellies connected by the epicranial aponeurosis

Blood And Nerve Supply Of Muscle

Muscular tissue requires access to oxygen-rich blood
- Muscular tissue requires a lot of oxygen and is extensively vascularized (highly supplied with blood vessels).
- Used to make ATP necessary for aerobic cellular respiration.
Skeletal muscles are also extensively innervated (supplied with nerves).
- Voluntary muscle contraction is regulated by somatic motor neurons.
- Axons branch from the spinal cord to muscles, typically one branch per muscle fibre.

Skeletal Muscle Fibre Structure

Muscle fibres start as immature cells called myoblast in the womb
- Cells fuse as they mature, forming large multinucleate cells.

Structure Of Skeletal Muscle Fibres

Plasma membrane of myocytes is called the sarcolemma
- Electrical signals run along this
- Sarcolemma folds inwards (invaginates) to form T-tubules.
Cytoplasm of myocytes is called the sarcoplasm
- Densely packed with myofibrils
- Rich in glycogen (carbohydrate energy store)
Sarcoplasm also contains myoglobin
- Only found in muscle cells.
- Binds oxygen at an Fe-containing centre called heme.
- Myocytes receive oxygen from both inside and outside the cell.

Sarcoplasm Contains Myofibrils

Myofibrils are long threads of contractile protein filaments (~2 nm diameter).
Regular pattern of overlapping filaments gives skeletal and cardiac muscle a striated (striped) appearance.

The Sarcoplasmic Reticulum (SR)

Specialized smooth endoplasmic reticulum in muscle cells
- Extensively folded around each myofibril
- Membrane folds are called cisternae (singular: cistern).
- Terminal cisternae specifically release Ca2+ to each T-tubule.
When 2 terminal cisternae meet a T-tubule, it forms a triad.
Muscle fibres do not divide, but can lay down new protein and enlarge (hypertrophy).

Muscular Hypertrophy

Increase in sarcoplasmic volume
Each muscle fibre increases the volume of cellular contents, especially myofibrils.
- Also: mitochondria, SR, etc.
Hypertrophy is a response to:
- Increased mechanical stress (e.g., weight-bearing exercise)
- Hormones (e.g., anabolic steroids)
- Disease (e.g., increased demand on a diseased heart)

Sarcomere Structure

Myofibrils are bundles of thread-like structures called myofilaments.
Each myofilament is made of contractile units called sarcomeres joined end-to-end
- Each sarcomere consists of overlapping thick and thin filaments.
  - Thick filaments extend from the midline (M-line) of the sarcomere and are made of myosin
  - Thin filaments extend from the ends (Z-discs) of the sarcomere and are made of actin.

Sarcomere Zones and Bands

Regions where the thick and thin filaments overlap and everything in between is called the A band.
Regions between the zones of overlap around the M-line are called the H zone (only thick filaments).
Regions between zones of overlap around the Z-discs are called the I band (only thin filaments).

Muscle Contraction

Muscles generate force by contraction
Three types of proteins involved in muscle contraction:
- Contractile proteins: shorten the sarcomere
  - Myosin: Motor protein (converts chemical potential energy in ATP to mechanical energy).
    - Each thick filament consists of ~300 myosin proteins.
    - Myosin “heads” extend radially from ends of thick filaments to contact thin filaments and pull thin filaments toward the M-line.
    - Each myosin head has:
      - An ATP-binding site
      - An actin-binding site
  - Actin: Cytoskeletal protein.
    - Long threads are twisted around one another to form helical thin filaments.
    - Have myosin-binding sites.
- Regulatory proteins: associate with thick and thin filaments to control contraction.
  - Troponin: Binds Ca2+  moves tropomyosin
  - Tropomyosin: Blocks myosin-binding sites on thin filaments.
- Structural proteins stabilize and/or connect the sarcomere and surrounding structures.
  - Dozens exist, including:
    - Titin: Large elastic protein that spans the M-line to Z-discs
      - Stabilizes the position of thick filaments.
    - Dystrophin: Connects thin filaments to integral membrane proteins in the sarcolemma.
      - Reinforces sarcomere structure
      - Transmits tension of sarcomeres to tendons

Sliding Filament Model

The sarcomere shortens as thin filaments slide over thick filaments.
Filaments do not change in length.

The Contraction Cycle

Myosin binds the thin filaments, pulls them into the M-line, then releases them.
This iterative binding and release is called the contraction cycle.

Myosin binds and hydrolyzes ATP:
- Energizes myosin and changes its conformation (cocked, like a gun). 2.Myosin binds thin filaments to form a cross-bridge. 3.Myosin pulls thin filaments toward the M-line:
- This conformational change is called the power stroke. 4.Myosin releases the thin filaments:
- Requires binding of a new ATP molecule to myosin.
- New cycle can begin.

The myosin-binding sites on the thin filaments are obscured by tropomyosin until troponin binds Ca2+.
- Ca2+ changes troponin conformation.
- Troponin moves tropomyosin off the myosin-binding sites on actin.
- Myosin can now form a cross-bridge.
Muscle contraction requires BOTH ATP and Ca2+

Sarcomere Changes During Contraction

As myosin pulls on the thin filaments, the Z-discs come together, causing the sarcomere to shorten.
- The H zone disappears.
- The I band narrows.

How Do Individual Sarcomeres Move Bones?

When sarcomeres shorten, this pulls on adjacent sarcomeres.
Tension is transmitted until the whole muscle fibre shortens.

Length-Tension Relationship

Amount of filament overlap matters:
If thick and thin filaments completely overlap at rest, myosin cannot generate tension effectively (no contraction):
- No room for thin filaments to slide.
If thick and thin filaments barely overlap at rest, myosin cannot generate much tension:
- Too few cross-bridges.
Therefore, there is an optimal sarcomere length for sufficient filament overlap to generate maximal tension.

Muscle Action Potentials

In most cells, the intracellular Ca2+ concentration is kept very low.
- Cells store Ca2+ in the sarcoplasmic reticulum.
- Cells export Ca2+ using membrane transporters.
Muscle fibres are electrically excitable.
- Signals from somatic motor neurons stimulate an action potential.
The neuromuscular junction (NMJ) is where neurons and muscles meet.
- Somatic motor neurons release chemical signals called neurotransmitters (e.g., acetylcholine)
  - Bind protein receptors on muscle cells.
  - Leads to an action potential in the muscle cell.

Muscle Action Potential Overview

Recall: The Na+-K+ pump keeps the inside of animal cells negative compared to the outside.
- This direct active transporter moves 3 Na+ out of the cell and 2 K+ into the cell per ATP hydrolyzed.
- Every cell maintains a negative resting membrane potential.
During an action potential, the membrane potential rapidly becomes positive:
- This is called depolarization.
Eventually, the cell needs to return to resting potential:
- The restoration of a negative membrane potential after depolarization is called repolarization.

Causes Of Membrane Potential Changes During Action Potentials

Plasma membrane transporters!
Voltage-gated ion channels:
- The signal that opens them is a change in membrane potential.
- They facilitate diffusion (ions will flow down their concentration gradients).

Voltage-Gated Sodium (Na+) Channels (VGNCs)

Allow Na+ ions to enter the cell.
- Only open when a change in membrane potential occurs.
- When acetylcholine binds, it opens some Na+ channels, causing slight depolarization.
- This depolarization because they let in positive Na+ ions to make the membrane potential more positive.

Repolarization

Is caused by voltage-gated K+ (potassium) channels (VGKCs).
- VGKCs are slower to open in response to membrane potential changes.
- Once open, K+ flows rapidly out of cells.
- Restores resting membrane potential.
- VGNCs close as membrane repolarizes.

Excitation-Contraction Coupling

How do action potentials stimulate muscle contraction?
- The action potential travels along the sarcolemma to voltage-gated Ca2+ channels (VGCCs) at T-tubules.
- Here, triads formed with terminal cisternae of the SR are physically connected to the sarcolemma because the VGCCs plug Ca2+ release channels in the SR membrane.
Action potentials open VGCCs at triads:
- This releases and opens the Ca2+ release channels of the SR.
  - Ca2+ spills into the sarcoplasm and binds troponin.
Troponin moves tropomyosin off the myosin-binding sites on thin filaments.
- Muscle contraction occurs!
During contraction:
- The Ca2+ release channels increase the intracellular Ca2+ concentration by ~10X.
When the muscle action potential has passed:
- VGCCs in the sarcolemma close
- SR Ca2+ release channels close and reassociate with the VGCCs at triads
- Ca2+-ATPases actively pump Ca2+ back into the SR, and others pump Ca2+ out of the cell.
  - Muscle can now relax.
This process is called excitation-contraction coupling.
At the NMJ, acetylcholine is removed and destroyed by an enzyme called acetylcholinesterase.

Control Of Muscle Tension

Usually, 1 action potential = 1 contraction.
More frequent action potentials = more tension.
Each somatic motor neuron axon can form multiple NMJs with muscle fibers.
- A motor unit is 1 somatic motor neuron + all the skeletal muscle fibres it synapses with (average = 150).

Large Muscles And Motor Units

Large muscles have many motor units distributed throughout the muscles.
All muscle fibres in a motor unit contract and relax synchronously.
A twitch contraction is the contraction generated in all skeletal muscle fibres of one motor unit due to one action potential.

Twitch Contraction Phases

Twitch contractions proceed in three phases:

Latent period (2 msec):
- Delay between stimulus (e.g., electrical stimulation) and muscle action.
  - Action potential is moving through sarcolemma; Ca2+ is being released from SR. 2.Contraction period (10–100 msec):
- Cross-bridges form and sarcomeres shorten.
- Maximum tension develops. 3.Relaxation period (10–100 msec):
  - Ca2+ pumped back into SR.
  - Myosin detaches from actin.
  - Tension decreases.

If a muscle fibre is in the middle of responding to an action potential, it cannot respond to a new action potential simultaneously.
- Temporarily unresponsive to new signals.
- This short time period is called a refractory period.

Muscle Tone

Recall: all fibres within a motor unit contract simultaneously.
- But within a muscle, not all motor units will be working at the same time.
  - Functions to prevent muscle fatigue.
  - Helps make movements smooth rather than jerky.
For large muscles (e.g., biceps brachii), weak motor units work first; stronger motor units work second.
- This process is called motor unit recruitment.

Muscle Tone: Stabilizing Positions

Some of our skeletal muscles produce enough tension to stabilize positions but not enough to move bones (e.g., postural muscles in the neck).
- Small involuntary contractions of alternating motor units  slight stiffness of muscle.
- This is called muscle tone.

Types Of Muscle Contractions

Isotonic contractions: constant tension in muscle as it changes length.
- Concentric: Muscle shortens to decrease the angle around a joint.
  - Example: Biceps brachii contract to pick a book up.
- Eccentric: Muscle resists a load as it lengthens.
  - Example: Biceps brachii lengthens as you slowly put a book down.
Isometric contractions:
- Tension generated is not sufficient to overcome the resistance of the load  bones do not move.
- Examples: Holding a book out to someone or holding plank.
- Function to stabilize many joints during movement.