Chapter 12 Muscle Flashcards
Flexor
Decrease angle at the joint
Extensor
Increase angle at the joint
Agonist
Prime mover
Antagonistic muscles
perform opposite actions at the same joint (flexors and extensors)
Epimysium
surround skeletal muscles
Perimysium
divide fascicles
Endomysium
surround muscle fibers
Light bands:
I bands (actin)
Dark bands:
A bands (myosin)
Dark bands in the middle of I bands
Z-lines (discs)
Where a motor neuron stimulates a muscle cell
neuromuscular junction
Area of the sarcolemma that is stimulated by a somatic motor neuron with the neurotransmitter: acetylcholine
Motor end plate
Motor unit = _____ + ______
a motor neuron + all of the muscle fibers it innervates
Graded contraction
varied contraction strength by the recruitment of muscle units
Part of the A band with no thin myofilament overlap
H band
Contraction is the pulling together of _______ towards the midline of the sarcomere (________)
Pulling together of Z-discs towards H band
Basic subunit of the muscle, runs from one z-line to the next
Sarcomere
Titin
Connect the z-line to the M line, contribute to the force of contraction due to elastic recoil
M line
Center of the A band, hold down thick filaments
Sliding Filament Theory
A bands do not shorten, they move closer together
I bands shorten, but thin filaments do not
FILAMENTS DO NOT CHANGE IN LENGTH
Thin filaments slide towards the H band, which shortens/disappears
Myosin’s 2 globular heads with _____ and _____ binding sites
actin-binding
ATP-binding
Cross bridges
form between actin and myosin, slide the filaments and contract the muscle
Power stroke
Release of P after ATP is split in the myosin head, causes a conformational change and hinges the head to pull the actin fiber towards the center (H band)
____ ATP molecules are required for each power stroke
2
- one is split, P is released and head hinges
- the other is bound to release the actin for rebinding
Blocks cross bridges
Tropomyosin
Troponin complex
Troponin I: inhibits binding of mysosin
Troponin T: binds to tropomyosin
Troponin C: binds to calcium
Stores calcium when muscle is at rest
Sarcoplasmic reticulum (SR)
specifically: terminal cisternae
Calcium is actively pumped into the ________ after contraction by ______
Sarcoplasmic reticulum; SERCA (sarcoplasmic/endoplasmic reticulum calcium ATPase)
Conduct action potentials along the muscle fiber by responding to calcium channel opening
T tubules
Quick contraction and relaxation after a single electrical shock
Twitch
A second shock applied shortly after the first that “piggybacks” the twitch
Summation
Time between the stimulus and contraction
Latent period
What are the attachment points of skeletal muscles to bones?
Both ends of a skeletal muscle are attached to bone by tough tendons.
What happens when a skeletal muscle contracts?
When a skeletal muscle contracts it shortens. This contraction places tension on the tendons moving the associated bone at a joint.
What is the muscle insertion?
The muscle insertion is the point where the bone that moves is attached during muscle contraction.
What is the muscle origin?
The muscle origin is the point where the bone that does not move is attached during muscle contraction.
In a muscle movement in which direction does movement occur?
Movement occurs toward the origin of the muscle.
What is the function of flexor muscles?
Flexor muscles decrease the angle between two bones at a joint.
What is the function of extensor muscles?
Extensor muscles increase the angle between two bones at a joint.
What is the agonist or prime mover in muscle movements?
The agonist or prime mover is the main muscle responsible for movement in a given direction.
What are antagonists in muscle movements?
Antagonists are flexors and extensors that act on the same joint to produce opposite actions.
What is the action of an extensor muscle?
Increases the angle at a joint.
What is the action of a flexor muscle?
Decreases the angle at a joint.
What is the function of an abductor muscle?
Moves a limb away from the midline of the body.
What is the function of an adductor muscle?
Moves a limb toward the midline of the body.
What is the action of a levator muscle?
Moves insertion upward.
What is the action of a depressor muscle?
Moves insertion downward.
What does a rotator muscle do?
Rotates a bone along its axis.
What is the function of a sphincter muscle?
Constricts an opening.
What surrounds skeletal muscles and what is its composition?
Skeletal muscles are surrounded by a fibrous connective tissue called epimysium.
What is the role of perimysium?
Perimysium is connective tissue that subdivides the muscle into fascicles.
What is the structural composition of a fascicle?
A fascicle is subdivided into muscle fibers also known as myofibers which are surrounded by endomysium.
What organelles are present in muscle fibers?
Muscle fibers have the same organelles found in other cells.
What is the plasma membrane of a muscle fiber called?
The plasma membrane of a muscle fiber is called the sarcolemma.
What is a distinct feature of muscle fibers regarding nuclei?
Muscle fibers are multinucleated.
Describe the striation pattern in muscle fibers.
Muscle fibers are striated consisting of light bands known as I bands and dark bands known as A bands.
What are Z-lines (or Z-discs) in muscle fibers?
Z-lines are dark lines in the middle of the I bands.
What are motor end plates?
Motor end plates are specialized regions of the muscle fiber membrane at a neuromuscular junction where nerve impulses trigger muscle contractions.
What is a motor unit?
A motor unit consists of a motor neuron and all the muscle fibers it innervates.
What is a neuromuscular junction?
A neuromuscular junction is the site where a motor neuron stimulates a muscle fiber.
What is the motor end plate?
The motor end plate is the area of the muscle fiber sarcolemma where a motor neuron stimulates it using the neurotransmitter acetylcholine (ACh).
What happens to ACh after it is released at the neuromuscular junction?
ACh diffuses across the synaptic cleft and binds to receptors at the crests of the folds of the sarcolemma in the motor end plate.
What is the effect of ACh binding to the receptors on the muscle fiber?
Binding of ACh to receptors causes ligand-gated channels to open allowing for the simultaneous diffusion of Na+ (sodium) and K+ (potassium) ions in opposite directions.
What is the primary ion movement that causes the end plate potential?
The inflow of Na+ predominates leading to local depolarization known as the end plate potential.
How does the end plate potential lead to action potential generation?
The end plate potential spreads to the troughs of the motor end plate folds which are rich in Na+ channels leading to the generation of an action potential that travels along the muscle fiber.
What is a motor unit?
A motor unit consists of a single motor neuron and all the muscle fibers it innervates.
What is the significance of motor units in muscle contraction?
Motor units are important because they allow for the coordinated contraction of multiple muscle fibers leading to effective muscle movement.
Describe the role of ligand-gated channels in muscle contraction.
Ligand-gated channels open in response to ACh binding allowing Na+ and K+ ions to flow which generates the end plate potential and leads to action potential.
Explain the process from ACh release to muscle fiber action potential.
ACh is released at the neuromuscular junction diffuses across the synaptic cleft binds to receptors on the motor end plate leading to ion channel opening Na+ influx local depolarization (end plate potential) and subsequent action potential generation that triggers muscle contraction.
What is a motor unit and what does it consist of?
A motor unit consists of a single motor neuron and all the muscle fibers it innervates. This means all the muscle fibers that contract together when the motor neuron is activated.
What is meant by graded contractions in muscle physiology?
Graded contractions refer to variations in contraction strength that occur due to the activation of different numbers of motor units. This allows for a range of force output depending on the demand placed on the muscle.
How does muscle control relate to motor unit recruitment?
Muscle control relates to motor unit recruitment as the strength of a contraction increases with the recruitment of additional motor units. Smaller motor units are activated for precise control while larger motor units are recruited for greater force.
What is the innervation ratio for the eye muscles and what does it indicate?
The innervation ratio for the eye muscles is one neuron to 23 muscle fibers. This low ratio indicates a high degree of control and precision in movement which is essential for eye function.
How do larger stronger muscles differ in motor unit structure compared to finer muscles?
Larger stronger muscles may have motor units that consist of thousands of muscle fibers leading to greater force production but less precise control. Finer muscles have smaller motor units with fewer muscle fibers allowing for more precise and delicate movements.
Describe the trade-off between control and strength in muscle contractions.
There is a trade-off between control and strength in muscle contractions; finer control requires smaller motor units with fewer muscle fibers while greater strength is achieved through larger motor units that control more fibers potentially reducing precision.
What are myofibrils and their significance in muscle fibers?
Myofibrils are densely packed subunits within muscle fibers that contain thin and thick myofilaments running the length of the muscle fiber. They are essential for muscle contraction and contribute to the organization of the muscle’s structure.
What produces striations in muscle fibers?
Striations in muscle fibers are produced by the arrangement of thick (myosin) and thin (actin) filaments. The striations differentiate into light (I bands containing only actin) and dark (A bands containing myosin and overlapping actin) bands.
What proteins primarily compose the I bands within muscle fibers?
The I bands within muscle fibers primarily consist of the protein actin which is a component of the thin filaments.
What are A bands and what do they contain?
A bands are the dark regions of striated muscle fibers that contain all of the thick filaments (myosin) and overlapping thin filaments (actin). They are crucial for the contraction mechanism in muscle.
Explain the structural relationship between myofibrils and muscle fiber function.
Myofibrils are key structural components that allow for muscle contraction. They are organized in a way that aligns thick and thin filaments enabling the sliding filament mechanism which is responsible for muscle contraction when stimulated.
What is the thick filament in muscle contraction and what do they overlap with?
The thick filament in muscle contraction is primarily composed of the protein myosin and it overlaps with some thin filaments.
What are H bands and where are they located in the sarcomere?
H bands are the regions located in the center of the A band of the sarcomere where there is no overlap with thin filaments.
What are Z discs and where are they located?
Z discs are lines found in the center of each I band and demarcate the boundaries of a sarcomere.
What is the sarcomere and what is its significance in muscle contraction?
The sarcomere is the basic subunit of striated muscle contraction running from one Z disc to the next. It is essential for muscle contraction and is where the sliding filament mechanism occurs.
What is the role of titin protein in the sarcomere?
Titin is a protein that runs from the Z disc to the M line within the sarcomere. It contributes to the force of contraction due to its elastic recoil properties and helps in refolding unfolded elements.
Where are M lines located and what is their function?
M lines are found in the center of each A band of the sarcomere. Their primary function is to hold down the thick filaments (myosin) in place.
What is the three-dimensional arrangement of sarcomeres in muscle fibers?
In three dimensions sarcomeres arrange themselves in a hexagonal pattern to form the structure of muscle fibers.
What happens to the sarcomere during muscle contraction according to the Sliding Filament Theory?
During muscle contraction sarcomeres shorten. The A bands do not shorten but move closer together while the I bands do shorten. Neither the actin (thin) nor the myosin (thick) filaments change in length but the thin filaments slide toward the center.
Which bands shorten during muscle contraction and which do not?
The I bands shorten during muscle contraction while the A bands do not shorten but become closer together.
What occurs to the thin filaments during contraction according to the Sliding Filament Theory?
During contraction the thin filaments slide toward the center of the sarcomere facilitating shortening.
What is the significance of the hexagonal arrangement of sarcomeres in muscle development and function?
The hexagonal arrangement of sarcomeres allows for efficient force generation and contraction in muscle fibers aiding overall muscle function.
Discuss the importance of both thick and thin filaments in muscle contraction.
Thick filaments (myosin) and thin filaments (actin) are crucial for muscle contraction. Thick filaments have cross-bridges that bind with thin filaments facilitating the sliding mechanism that allows muscles to contract.
How does the structure of the sarcomere relate to its function?
The structure of the sarcomere with its organized arrangement of thick and thin filaments and distinct bands enables it to efficiently generate force and shorten during muscle contraction through the sliding filament mechanism.
What is the H band in muscle contraction?
The H band is a region in the sarcomere of striated muscle that appears lighter than surrounding areas because it contains only thick filaments (myosin) and no thin filaments (actin). The H band shortens or may disappear during muscle contraction as the thin filaments slide past the thick filaments.
What is the sliding filament theory?
The sliding filament theory explains how muscles contract by the sliding of thin filaments (actin) past thick filaments (myosin). During contraction the cross-bridges formed between actin and myosin filaments allow the filaments to slide over one another resulting in muscle shortening.
What are cross bridges in muscle contraction?
Cross bridges are the connections formed between the myosin heads of thick filaments and binding sites on the actin of thin filaments during muscle contraction. These interactions are crucial for the contraction process as they allow the sliding mechanism to occur.
What are the two types of myofilaments involved in muscle contraction?
The two types of myofilaments are: 1. Thick filaments composed primarily of the protein myosin. 2. Thin filaments composed primarily of the protein actin along with regulatory proteins tropomyosin and troponin.
What are thick filaments composed of?
Thick filaments are primarily composed of the protein myosin. Each myosin protein has two globular heads that contain binding sites for actin and ATP located at the ends of hinge segments that extend from the myosin tail.
What role do tropomyosin and troponin play in muscle contraction?
Tropomyosin and troponin are regulatory proteins associated with thin filaments (actin). At rest they block the binding sites on actin for myosin preventing contraction. Upon muscle stimulation calcium ions bind to troponin causing tropomyosin to move and expose myosin-binding sites on actin allowing contraction to occur.
How does the myosin head function as an ATPase during muscle contraction?
The myosin head functions as an ATPase by hydrolyzing ATP into ADP and inorganic phosphate (P i). This ATP splitting provides the energy necessary for the myosin head to bind to actin and initiate the power stroke.
What happens during the power stroke in muscle contraction?
During the power stroke the myosin head after binding to actin releases inorganic phosphate (P i) which triggers a conformational change that ‘cocks’ the myosin head. This pulling motion draws the thin filament (actin) toward the center of the sarcomere resulting in muscle contraction.
What occurs after the power stroke during muscle contraction?
After the power stroke ADP is released from the myosin head and a new ATP molecule binds to the myosin. This binding causes the myosin head to release from the actin filament allowing it to reset and be ready for another cycle of contraction.
What is the significance of ATP in muscle contraction?
ATP is essential for muscle contraction as it provides the energy needed for the myosin heads to perform the power stroke and it is necessary for the release of myosin from actin after the power stroke. Without ATP muscles cannot contract or relax effectively.
What occurs during the repercussive splitting of ATP in muscle contraction?
During muscle contraction the ATP molecule is split into ADP and inorganic phosphate which facilitates the re-cocking and reattachment of the myosin head to a new position on the actin filament.
What is the Sliding Filament Theory?
The Sliding Filament Theory explains how muscles contract. It posits that myosin heads attach to actin filaments pulling them inward which causes the sarcomeres to shorten and consequently the muscle fibers to contract.
How many ATP molecules are required for each power stroke in muscle contraction?
Two ATP molecules are required for each power stroke: one ATP is necessary for the myosin head to bind to actin and a second ATP is required for the myosin head to detach from actin after the power stroke.
What role does F-actin play in muscle contraction?
F-actin is a polymerized form of globular G-actin subunits composed of 300-400 units twisted in a double helix. It is crucial for providing the structural framework upon which myosin can act to produce muscle contraction.
What is the function of tropomyosin in muscle contraction?
Tropomyosin blocks the binding sites on actin for myosin preventing cross-bridge formation when calcium levels are low.
Describe the troponin complex and its components. What are their functions?
The troponin complex consists of three proteins: Troponin I which inhibits myosin binding to actin; Troponin T which binds to tropomyosin; and Troponin C which binds to calcium ions. The binding of calcium to Troponin C causes a conformational change that ultimately leads to muscle contraction.
What is the role of calcium ions in muscle contraction?
Calcium ions (Ca2+) are released inside the muscle fiber when stimulated. They bind to Troponin C leading to a series of structural changes that remove tropomyosin from the binding sites on actin allowing myosin heads to attach and initiate contraction.
What happens to cross-bridges during a normal muscle contraction?
During a normal muscle contraction only a fraction of cross-bridges are attached to actin at any given time which allows for repetitive cycles of muscle contraction and relaxation.
Explain the process of power stroke in muscle contraction. What is its significance?
The power stroke is the action where the myosin head moves and pulls the actin filament causing the sarcomere to shorten. This process is significant as it is the fundamental action of muscle contraction.
How are muscle contractions regulated?
Muscle contractions are regulated primarily through the action of troponin and tropomyosin which respond to the presence or absence of calcium ions to control the interactions between actin and myosin.
What is the role of troponin and tropomyosin in muscle contraction?
Troponin and tropomyosin are regulatory proteins that control access to the myosin binding sites on actin filaments. In the absence of calcium ions (Ca2+) tropomyosin blocks these binding sites preventing myosin from forming cross-bridges with actin. When Ca2+ is released from the sarcoplasmic reticulum it binds to troponin causing a conformational change that moves tropomyosin away from the myosin binding sites allowing cross-bridge formation and muscle contraction.
What is the sarcoplasmic reticulum (SR) and its role in muscle contraction?
The sarcoplasmic reticulum (SR) is a modified form of the endoplasmic reticulum found in muscle cells that serves to store calcium ions (Ca2+) when the muscle is at rest. The majority of calcium is stored in the terminal cisternae of the SR. Upon stimulation of a muscle fiber Ca2+ is released from the SR through calcium release channels known as ryanodine receptors which is critical for the initiation of muscle contraction. After contraction Ca2+ is actively pumped back into the SR to allow the muscle to relax.
What are transverse tubules (T tubules) and their significance in muscle contraction?
Transverse tubules (T tubules) are narrow membranous tunnels that extend from the sarcolemma (the muscle cell membrane) into the interior of the muscle fiber. They open to the extracellular environment through pores in the cell surface and are crucial for conducting action potentials deep into the muscle fiber. T tubules are located close to the terminal cisternae of the sarcoplasmic reticulum and contain voltage-gated calcium channels (dihydropyridine or DHP receptors) that respond to membrane depolarization facilitating the release of Ca2+ necessary for muscle contraction.
Describe the process of stimulating a muscle fiber and the role of acetylcholine.
The stimulation of a muscle fiber begins with the release of the neurotransmitter acetylcholine (ACh) from the motor neuron at the neuromuscular junction. ACh binds to receptors on the sarcolemma of the muscle fiber leading to depolarization of the muscle membrane and the initiation of action potentials. This electrical signal then travels along the T tubules and triggers the release of calcium ions (Ca2+) from the sarcoplasmic reticulum ultimately leading to muscle contraction.
What happens to calcium ions (Ca2+) at the end of a muscle contraction?
At the end of a muscle contraction calcium ions (Ca2+) are actively transported back into the sarcoplasmic reticulum (SR) by calcium pumps. This process reduces the concentration of Ca2+ in the cytosol leading to the re-binding of tropomyosin to the myosin binding sites on actin filaments. Consequently this stops the formation of cross-bridges between myosin and actin allowing the muscle to relax.
Explain the importance of voltage-gated calcium channels in muscle contraction.
Voltage-gated calcium channels specifically the dihydropyridine receptors (DHP receptors) are crucial for muscle contraction as they respond to changes in membrane potential during action potentials. Located in the T tubules these channels detect the depolarization of the muscle cell membrane and trigger the opening of ryanodine receptors in the sarcoplasmic reticulum leading to the rapid release of calcium ions (Ca2+) necessary for initiating the contractile process.
What are end plate potentials and how are they produced?
End plate potentials are the changes in membrane potential at the motor end plate of a muscle fiber caused by the binding of acetylcholine (ACh) to nicotinic receptors on the muscle cell membrane leading to a localized depolarization.
What is the relationship between action potentials and muscle contractions along the sarcolemma and T tubules?
Action potentials travel along the sarcolemma and into the T tubules which triggers the release of calcium from the sarcoplasmic reticulum initiating muscle contraction in an all-or-none fashion.
How do voltage-gated calcium channels function in the excitation-contraction coupling process?
Voltage-gated calcium channels in the T tubules undergo a conformational change when activated by the action potential which couples to ryanodine receptors on the sarcoplasmic reticulum leading to the opening of calcium release channels.
What happens when calcium is released during muscle contraction?
Calcium that is released from the sarcoplasmic reticulum binds to troponin C causing a change in the shape of the troponin-tropomyosin complex which exposes binding sites on actin for myosin heads.
What occurs during muscle relaxation after contraction?
Muscle relaxation occurs when action potentials cease leading to the closure of calcium release channels. The SERCA pumps in the sarcoplasmic reticulum actively transport calcium back into the SR reducing calcium availability for binding to troponin C.
What role does the Sarcoplasmic Endoplasmic Reticulum Calcium ATPase (SERCA) play in muscle relaxation?
SERCA pumps are responsible for moving calcium back into the sarcoplasmic reticulum through active transport helping to reduce the intracellular calcium concentration and facilitate muscle relaxation.
What effect does the absence of calcium have on muscle contraction?
In the absence of calcium there is not enough calcium available to bind to troponin C which results in tropomyosin blocking the binding sites on actin preventing myosin from attaching and leading to muscle relaxation.
Define a muscle twitch and how it relates to electrical stimulation.
A muscle twitch is a brief contraction and relaxation of a muscle fiber triggered by a single electrical stimulus. The strength of the twitch can increase with the intensity of the electrical stimulus up to a maximum level.
What is the relationship between stimulus voltage and the strength of muscle twitch?
As the voltage of the electrical stimulus increases the strength of the muscle twitch also increases until a maximum contraction force is reached.
What are the definitions of summation and tetanus in muscle contractions?
Summation refers to the increased strength of contraction that occurs when stimuli are repeated closely together leading to increased calcium availability. Tetanus refers to a sustained muscle contraction resulting from a high-frequency stimulation that prevents muscle relaxation between stimuli.
What is ‘summation’ in the context of muscle contractions?
Summation refers to the phenomenon where a second twitch occurs immediately after the first causing the two twitches to overlap and partially piggyback each other resulting in a stronger overall contraction.
Define the latent period in muscle contraction.
The latent period is the time between the application of a stimulus and the beginning of the muscle contraction during which excitation-contraction coupling occurs leading to the attachment of myosin cross-bridges to actin.
What are graded contractions and how do they occur?
Graded contractions are variations in the strength of muscle contractions that occur when more muscle fibers are recruited. As the intensity of stimulation increases more motor units are activated leading to stronger contractions until all fibers are engaged.
Explain incomplete tetanus.
Incomplete tetanus happens when the frequency of electrical shocks applied to muscle fibers increases resulting in decreased relaxation time between twitches. This leads to a sustained muscle contraction but some relaxation does occur.
What is complete tetanus?
Complete tetanus is a condition that occurs when the frequency of electrical shocks is so high that there is no relaxation between muscle twitches resulting in a smooth sustained contraction.
What happens when electrical shocks are increased in frequency regarding relaxation time?
Increasing the frequency of electrical shocks results in decreased relaxation time between twitches leading to more sustained muscle contraction.
Define twitch in muscle physiology.
A twitch is a quick all-or-nothing response of a muscle fiber to a single electrical stimulus characterized by a brief period of contraction followed by relaxation.
Describe the process of excitation-contraction coupling.
Excitation-contraction coupling is the physiological process in which an electrical stimulus (action potential) causes the muscle fiber’s membrane to depolarize leading to a series of events that result in the release of calcium ions from the sarcoplasmic reticulum which then allows for the attachment of myosin cross-bridges to actin filaments and ultimately muscle contraction.
What is the role of calcium ions in muscle contraction?
Calcium ions play a critical role in muscle contraction by binding to troponin on the actin filaments causing a conformational change that moves tropomyosin away from the myosin-binding sites allowing myosin heads to attach to actin and initiate contraction.
What factors influence the strength of a muscle contraction?
The strength of a muscle contraction is influenced by factors such as the frequency of stimulation the number of motor units recruited (motor unit recruitment) and the length-tension relationship of the muscle fibers.
What is tetanus in the context of muscle contraction?
Tetanus refers to a smooth sustained contraction of muscle fibers that occurs when there is a high-frequency stimulation of the muscle preventing relaxation between twitches.
What causes tetanus to occur in vivo?
Tetanus occurs in vivo due to asynchronous activation of motor units where some motor units twitch as others relax leading to continuous contraction of the whole muscle.
How does recruitment strengthen muscle contractions during tetanus?
Recruitment refers to the process of activating additional motor units to strengthen muscle contractions making the overall force produced by the muscle greater.
What is the staircase effect observed in muscle contraction?
The staircase effect known as treppe occurs when a fresh muscle is stimulated with several shocks at maximum voltage leading each successive twitch to be progressively stronger until a maximum is reached.
What is the significance of the force-velocity curve in muscle contractions?
The force-velocity curve illustrates that for muscles to contract effectively the force generated must exceed opposing forces; it also depicts the trade-off that greater force typically results in slower contraction speeds.
Define isotonic contraction.
Isotonic contraction is a type of muscle contraction where the muscle shortens while generating force maintaining a constant tension throughout the movement.
What are the two types of muscle contraction mentioned in the text?
The two types of muscle contractions mentioned are isotonic contractions (muscle shortens while lifting a constant load) and isometric contractions (muscle length remains unchanged while generating force).
What is treppe and how does it occur?
Treppe is a phenomenon observed when a muscle is stimulated with several shocks at maximum voltage resulting in an increase in the strength of each twitch creating a staircase-like graph.
What happens to the number of muscle fibers activated as voltage increases during stimulation in vitro?
As voltage increases the number of muscle fibers activated also increases until a maximum number of muscle fibers are stimulated.
In the context of motor unit activation what is asynchronous activation?
Asynchronous activation refers to the firing pattern of motor units where not all motor units contract simultaneously allowing for smooth and sustained muscle contractions without complete relaxation.
What happens during muscle contraction when the tension produced is greater than the load?
Muscle fibers shorten when the tension produced is just greater than the load.
What is a concentric contraction?
A concentric contraction occurs when a muscle fiber shortens as the force exerted by the muscle is greater than the load it is acting against.
What is an eccentric contraction?
An eccentric contraction occurs when a muscle may actually lengthen even though it is contracting if the load is too great.
What is the purpose of eccentric contractions?
Eccentric contractions allow for the controlled lowering of a weight gently after a full concentric contraction.
What is an isometric contraction?
An isometric contraction occurs when muscles cannot shorten despite exerting a certain tension because the load is too great.
Can isometric contractions be voluntary?
Yes isometric contractions can be voluntary.
What is the Series Elastic Component (SEC) in muscle contraction?
The Series Elastic Component consists of noncontractile parts of the muscle and tendons that must be pulled tight when muscles contract to allow movement of the insertion toward the origin.
What are the characteristics of tendons in relation to muscle contraction?
Tendons are elastic resist distension and can snap back to resting length. They also absorb some of the tension as muscles contract.
Why must noncontractile parts of the muscle and tendons be pulled tight?
Noncontractile parts of the muscle and tendons must be pulled tight when muscles contract to facilitate movement of the insertion towards the origin.
How does the elasticity of tendons affect muscle contractions?
The elasticity of tendons allows them to absorb some of the tension generated by muscles during contractions providing a buffer against excessive load.
What is the definition of ‘insertion’ in muscle anatomy?
Insertion refers to the point at which a muscle attaches to the bone that it moves typically moving toward the muscle’s origin.
What are the properties of tendons?
Tendons are elastic meaning they can stretch but also resist distension and snap back to their resting length after being stretched.
How do tendons function during muscle contraction?
Tendons absorb some of the tension generated when muscles contract helping to transfer force from the muscle to the bone.
What are the four key factors that determine muscle strength?
- Number of fibers recruited to contract. 2. Frequency of stimulation. 3. Thickness of each muscle fiber (thicker fibers are stronger). 4. Initial length of the fiber at rest.
What is the ideal resting length for striated muscle fibers?
The ideal resting length is the length at which striated muscle fibers can generate maximum force allowing for the best interaction between actin and myosin.
What is the relationship between sarcomere length and muscle tension?
Muscle tension is maximal when sarcomeres are at their normal resting length. If sarcomeres are stretched beyond this length tension decreases due to fewer interactions between myosin and actin.
What happens to muscle tension when sarcomere length is increased?
Increasing sarcomere length decreases muscle tension because there are fewer interactions between myosin and actin eventually leading to a point where no tension can be generated.
What happens to muscle tension when sarcomere length is decreased?
Decreasing sarcomere length also leads to decreased muscle tension primarily because the fibers become shorter and thicker resulting in increased fluid pressure and distance changes.
What is the significance of fluid pressure in muscle contraction?
Increased fluid pressure can affect muscle contraction by influencing the ability of the muscle fibers to shorten and generate force.
What is the role of frequency of stimulation in muscle contraction?
The frequency of stimulation affects the rate at which muscle fibers are activated influencing overall muscle strength and the generation of tension.
What percentage of ATP is used by myosin ATPase in the sarcomere for contraction?
About 70% of ATP is used by myosin ATPase in the sarcomere for contraction.
What percentage of ATP is used by pumps in skeletal muscle and what are those pumps?
The remaining 30% of ATP is used by pumps specifically: a) Ca2+ pump to actively return calcium to the sarcoplasmic reticulum (SR) and b) Na+-K+ pump for membrane repolarization and maintaining electrochemical gradients.
What is the primary energy source for skeletal muscles at rest and during mild exercise?
At rest and during mild exercise the primary energy source for skeletal muscles is the aerobic respiration of fatty acids.
What sources of energy do skeletal muscles rely on during heavy exercise?
During heavy exercise skeletal muscles rely on glycogen stores and blood glucose for energy.
How do GLUT4 channels function in skeletal muscle during exercise?
As exercise intensity and duration increase GLUT4 channels are inserted into the sarcolemma to allow more glucose to enter the muscle cells.
What type of metabolism do skeletal muscles rely on during the first 45 to 90 seconds of moderate to heavy exercise?
Skeletal muscles rely on anaerobic metabolism during the first 45 to 90 seconds of moderate to heavy exercise.
What is the purpose of anaerobic metabolism at the beginning of intense exercise?
Anaerobic metabolism at the beginning of intense exercise allows time to increase oxygen supply.
What is maximal oxygen uptake also known as and why is it important?
Maximal oxygen uptake is also called aerobic capacity or VO2 max. It determines whether a given exercise is light moderate or heavy for an individual.
What factors influence an individual’s maximal oxygen uptake (VO2 max)?
Maximal oxygen uptake (VO2 max) can be influenced by factors such as cardiovascular fitness muscle oxygen utilization and individual conditioning.
Why is ATP important for muscle contraction?
ATP is important for muscle contraction as it provides the necessary energy for the myosin ATPase to bind with actin and facilitate the contraction process in addition to powering calcium and sodium-potassium pumps.
What factors influence a person’s metabolic rate during exercise?
A person’s metabolic rate during exercise is influenced by age sex body size and athletic training.
How does metabolic rate differ between males and females?
Metabolic rate is generally greater for males compared to females due to differences in muscle mass and hormonal influences.
Which age group tends to have a higher metabolic rate during exercise?
Younger individuals typically have a higher metabolic rate during exercise compared to older individuals.
What is the range of maximal oxygen uptake (VO2 max) per kilogram of body weight during exercise?
Maximal oxygen uptake (VO2 max) ranges from 12 ml O2 per minute per kg of body weight to 84 ml O2 per minute per kg of body weight.
What is the lactate threshold?
The lactate threshold also known as the anaerobic threshold is the exercise intensity at which a noticeable increase in blood lactate levels occurs.
At what percentage of VO2 max does the lactate threshold typically occur?
The lactate threshold typically occurs at about 50% to 70% of VO2 max.
What is the effect of exercise on glucose levels in the body?
During exercise the need for glucose increases leading to a drop in blood glucose levels.
What are GLUT4 receptors and their role during exercise?
GLUT4 receptors are glucose transporters that increase in number in the plasma membrane during exercise facilitating greater glucose uptake by the muscles.
How does the liver respond to decreased blood glucose levels during exercise?
The liver compensates for decreased blood glucose levels by providing more glucose through hydrolysis of glycogen and gluconeogenesis.
What is oxygen debt?
Oxygen debt refers to the excess post-exercise oxygen consumption that must occur to restore the body to its pre-exercise state.
What happens to oxygen reserves during exercise?
During exercise oxygen is withdrawn from reserves in hemoglobin and myoglobin to supply oxygen to tissues that are warmed by exercise.
Why is oxygen needed to metabolize lactic acid produced during anaerobic exercise?
Oxygen is needed to convert lactic acid back to pyruvate through aerobic processes helping to clear lactic acid from the blood.
What is the main purpose of breathing rate elevation after exercise?
To repay the oxygen debt incurred during exercise.
What is phosphocreatine and its role in skeletal muscle metabolism?
Phosphocreatine is a high-energy compound used to quickly regenerate ATP from ADP during high-energy demands as ATP may be consumed faster than it can be created via cellular respiration.
What enzyme is responsible for the conversion of ADP to ATP using phosphocreatine?
Creatine kinase (CK) or creatine phosphokinase (CPK).
Where is creatine produced and how can it be obtained?
Creatine is produced by the liver and kidneys and it can also be obtained through diet primarily from meat and fish.
How are phosphocreatine stores replenished?
Phosphocreatine stores are replenished at rest allowing the muscle to recover its high-energy phosphate reserves.
How can creatine supplements affect muscle phosphocreatine levels?
Creatine supplements can increase muscle phosphocreatine levels aiding performance in short-term high-energy exercise.
What is a potential risk of long-term creatine supplementation?
Long-term use of creatine supplements may lead to liver damage.
Describe the process of ATP regeneration during muscle activity when ATP levels fall.
When ATP levels fall ADP combines with inorganic phosphate (P i) from phosphocreatine facilitated by the enzyme creatine kinase to quickly regenerate ATP.
Why is phosphocreatine important during intense exercise?
Phosphocreatine is important because it provides an immediate source of energy to regenerate ATP allowing for continued muscle contraction during short bursts of high-intensity activities.
What happens to creatine levels during rest periods?
During rest creatine levels can be replenished allowing for recovery of phosphocreatine stores used during exercise.
What is phosphocreatine and how does it relate to exercise?
Phosphocreatine is a high-energy compound that can increase muscle phosphocreatine levels thus aiding in short-term high-energy exercise. It provides a rapid source of energy during intense physical activity.
What are the potential effects of long-term use of phosphocreatine supplements?
Long-term use of phosphocreatine supplements may lead to liver damage.
What is creatine kinase (CK) and why is it significant?
Creatine kinase (CK) also known as creatine phosphokinase (CPK) is an enzyme that is measured in blood samples to help diagnose conditions such as myocardial infarction (heart attack) muscular dystrophy and rhabdomyolysis.
What is the significance of different isoenzymatic forms of creatine kinase?
Different isoenzymatic forms of creatine kinase such as CK-MM and CK-MB are significant because they indicate the source of muscle damage; CK-MM is released from skeletal muscles while CK-MB is released from damaged heart muscles.
What are slow-twitch muscle fibers?
Slow-twitch muscle fibers also known as type I fibers have a slower contraction speed can sustain contractions for long periods without fatigue possess a rich capillary supply contain more mitochondria and respiratory enzymes and have high myoglobin content.
What is the oxidative capacity of slow-twitch fibers characterized by?
The oxidative capacity of slow-twitch fibers is characterized by their ability to efficiently use oxygen for energy production allowing for prolonged exercise and endurance activities.
What are fast-twitch muscle fibers and how do they differ from slow-twitch fibers?
Fast-twitch muscle fibers contract quickly and are designed for short bursts of strength or speed; they fatigue more quickly compared to slow-twitch fibers which are adapted for endurance. Fast-twitch fibers are often categorized into two subtypes: Type IIa (oxidative) and Type IIb (glycolytic).
What are the key physiological characteristics of fast-twitch fibers?
Fast-twitch fibers have fewer mitochondria lower myoglobin content and a higher rate of glycogen storage which allows for rapid energy production but results in quicker fatigue.
What role does myoglobin play in muscle fibers?
Myoglobin is a protein in muscle cells that binds oxygen and facilitates its delivery to the mitochondria for aerobic respiration playing a crucial role in endurance and aerobic exercise capacity.
What conditions can creatine kinase measurement help diagnose?
Creatine kinase measurement can help diagnose conditions such as myocardial infarction (heart attack) muscular dystrophy and rhabdomyolysis among others.
How does creatine kinase release differ between skeletal muscles and heart muscles?
Damaged skeletal muscles release the CK-MM isoenzyme while damaged heart muscle releases the CK-MB isoenzyme which helps clinicians determine the source of muscle damage during diagnoses.
What is the impact of a rich capillary supply on slow-twitch muscle fibers?
A rich capillary supply in slow-twitch muscle fibers enhances their oxygen delivery supporting aerobic metabolism and prolonging endurance performance.
What are slow-twitch fibers and where are they commonly found?
Slow-twitch fibers are also known as red fibers and are commonly found in postural muscles. They have a slower contraction speed and are more resistant to fatigue.
Describe the characteristics of fast-twitch type IIX fibers.
Fast-twitch type IIX fibers have a faster contraction speed fatigue quickly are thicker and have fewer capillaries mitochondria and respiratory enzymes. They also have less myoglobin. These fibers are associated with a higher glycogen storage and are classified as fast glycolytic fibers that can metabolize anaerobically.
What is the ATP consumption rate in fast-twitch fibers?
Fast-twitch fibers have the greatest rate of ATP consumption which leads to rapid depletion of ATP and phosphocreatine.
What are fast-twitch type IIA fibers and how do they differ from IIX fibers?
Fast-twitch type IIA fibers are fast-twitch fibers with high oxidative capacity also known as fast oxidative-glycolytic fibers. They are more resistant to fatigue compared to type IIX fibers.
What factors influence the percentage of slow-twitch and fast-twitch fibers in muscles?
The percentage of slow-twitch and fast-twitch fibers in individuals’ muscles varies greatly due to genetic factors and training. Different individuals may naturally have different distributions of these fibers.
What are some general characteristics that differentiate Type I Type IIA and Type IIX muscle fibers?
Type I fibers (slow oxidative) are red have a small diameter and are fatigue-resistant. Type IIA fibers (fast oxidative-glycolytic) are red have a larger diameter than Type I and possess high oxidative capacity. Type IIX fibers (fast glycolytic) are white have the largest diameter and fatigue quickly.
Why are slow-twitch fibers referred to as red fibers?
Slow-twitch fibers are referred to as red fibers due to their high myoglobin content which gives them a red appearance and enables better oxygen storage and delivery for oxidative metabolism.
What is the main energy source for fast-twitch glycolytic fibers?
Fast-twitch glycolytic fibers primarily utilize glycogen as their energy source and can perform anaerobic metabolism.
What is the relationship between muscle fiber type and athletic performance?
Athletic performance may be influenced by an individual’s muscle fiber composition with different sports favoring different fiber types. Endurance athletes may benefit from a higher percentage of slow-twitch fibers while sprint or power athletes may benefit from a higher percentage of fast-twitch fibers.
List the features compared in Table 12.3 of Muscle Fiber Types.
The table likely compares various features such as fiber type (Slow Oxidative Type I Fast Oxidative-Glycolytic Type IIA Fast Glycolytic Type IIX) their predominant color (red or white) their diameter fatigue resistance and metabolic characteristics.
What are the three muscle fiber types classified by Z-line thickness?
Small Intermediate Large.
What are the three categories of Z-line thickness in muscle fibers?
Small Intermediate Large.
What are the three levels of glycogen content in muscle fibers?
Low Intermediate High.
How does resistance to fatigue differ among muscle fiber types?
Resistance to fatigue is High in Slow-twitch fibers Intermediate in Fast-twitch fibers and Low in Fast-glycolytic fibers.
What is the relative abundance of fiber types in muscles?
6 (Small fibers) 7 (Intermediate fibers).
How does the number of capillaries vary among the muscle fiber types?
Many capillaries in Slow-twitch fibers Many in Intermediate fibers and Few in Fast-glycolytic fibers.
Describe the myoglobin content across different muscle fibers.
High in Slow-twitch fibers High in Intermediate fibers and Low in Fast-glycolytic fibers.
What type of respiration is associated with each muscle fiber type?
Aerobic respiration in Slow-twitch fibers Aerobic in Intermediate fibers Aerobic in Fast-glycolytic fibers.
How does oxidative capacity compare between muscle fiber types?
High in Slow-twitch fibers High in Intermediate fibers and Low in Fast-glycolytic fibers.
What are the glycolytic ability differences among the muscle fiber types?
Low in Slow-twitch fibers High in Intermediate fibers and High in Fast-glycolytic fibers.
What is the twitch rate for each type of muscle fiber?
Slow twitch in Slow-twitch fibers Faster in Intermediate fibers and Fastest in Fast-glycolytic fibers.
How does the myosin ATPase rate compare in muscle fiber types?
Low in Slow-twitch fibers Higher in Intermediate fibers and Highest in Fast-glycolytic fibers.
Define muscle fatigue based on exercise-induced characteristics.
Muscle fatigue is a reversible exercise-induced reduction in the ability to generate force.
What causes muscle fatigue during sustained maximal contraction?
The accumulation of extracellular K+ which reduces the excitability of the sarcolemma and T-tubules by decreasing the resting membrane potential.
List the factors contributing to muscle fatigue in various types of exercise.
- Reduced SR calcium release 2. Increased concentration of PO4 due to phosphocreatine breakdown 3. Lack of ATP hindering Ca2+ pumps 4. Decrease of muscle glycogen.
How does the calcium release from the sarcoplasmic reticulum (SR) contribute to muscle fatigue?
Reduced SR calcium release leads to decreased contractility of the muscle fiber.
What is the role of phosphocreatine in muscle function and fatigue?
Phosphocreatine breakdown increases phosphate concentration (PO4) which can disrupt muscle function and contribute to fatigue.
Explain how a lack of ATP affects muscle cells during fatigue.
A lack of ATP hinders Ca2+ pumps leading to impaired calcium handling and reduced muscle contraction.
What happens to muscle glycogen levels during prolonged exercise and its relevance to fatigue?
Decreased muscle glycogen levels contribute to fatigue as glycogen is a primary energy source for muscle contraction.
What is the buildup of ADP related to in muscle physiology?
The buildup of ADP (Adenosine Diphosphate) is associated with muscle fatigue during prolonged exercise. High levels of ADP indicate that the muscle is using ATP (Adenosine Triphosphate) rapidly leading to its depletion and the subsequent onset of fatigue.
What role does lactic acid accumulation play in muscle fatigue?
Lactic acid accumulation and a lower pH are often observed during muscle fatigue; however they may be more coincidental with fatigue rather than a direct cause of it. The relationship is complex and not fully understood.
Define ‘central fatigue’ and its relation to muscle fatigue.
Central fatigue refers to the fatigue of upper motor neurons in the central nervous system (CNS). It occurs before the muscles have fatigued sufficiently to limit exercise indicating that the brain’s signaling to the muscles may diminish before the muscles are physically incapable of continuing.
List the adaptations resulting from endurance exercise training.
- Increased local blood flow and density of arterioles and capillaries supplying more oxygen and nutrients.
- Increase in maximal oxygen uptake (VO2 max) and an increased lactate threshold.
- Improved ability to utilize fatty acids as fuel during exercise.
- Decrease in type IIX muscle fibers and an increase in type IIA muscle fibers leading to more endurance.
- Increase in the number of mitochondria and concentration of mitochondrial enzymes.
How does muscle mass change with aging?
With aging there is typically a reduction in muscle mass primarily in type II muscle fibers. This can be mitigated with resistance training which helps to maintain and build muscle.
What impact does aging have on capillary blood supply in muscles?
Aging results in a reduction in capillary blood supply to muscles. Endurance training can help improve capillary density and blood supply thereby enhancing muscle function and endurance.
What is muscle atrophy?
Muscle atrophy is the reduction in muscle size often due to disuse malnutrition or aging. It can lead to a decrease in strength and functionality of the affected muscles.
What types of muscle fibers are mentioned in the context of endurance training adaptations?
Type IIA (fast oxidative) muscle fibers increase as a result of endurance training while type IIX (fast glycolytic) muscle fibers typically decrease.
What is the role of mitochondria in muscle adaptation to training?
Mitochondria are the powerhouses of the cell that generate ATP. An increase in the number of mitochondria and the concentration of mitochondrial enzymes enhances the muscle’s ability to produce energy efficiently particularly during prolonged exercise.
What are some ways to combat muscle decline associated with aging?
Muscle decline due to aging can be combated through:
- Resistance training to counteract muscle atrophy and promote hypertrophy.
- Endurance training to improve capillary blood supply and overall cardiovascular health.
What is neurogenic atrophy and what causes it?
Neurogenic atrophy is the loss of muscle mass due to damage to the motor nerves that innervate the muscles. It can be caused by diseases such as Amyotrophic Lateral Sclerosis (ALS) or trauma.
What is disuse atrophy and when can it occur?
Disuse atrophy occurs when muscles are not used regularly to work against gravity leading to muscle weakness and shrinkage. This can happen when a person is bedridden for a prolonged period or has a limb in a cast.
What is the role of lower motor neurons in skeletal muscle control?
Lower motor neurons have cell bodies located in the ventral horn of the spinal cord or in the brain stem. They travel to the skeletal muscles via the ventral root of the spinal nerves and are influenced by sensory feedback from muscles and tendons as well as facilitatory or inhibitory effects from higher motor neurons.
Where are the cell bodies of lower motor neurons located?
The cell bodies of lower motor neurons are located in the ventral horn of the spinal cord or in the brain stem.
How are lower motor neurons influenced in their function?
Lower motor neurons are influenced by sensory feedback from muscles and tendons and by facilitatory or inhibitory effects from upper motor neurons located in the brain.
What are upper motor neurons and what is their function?
Upper motor neurons are interneurons located in the brain that contribute axons to the descending motor pathways that control lower motor neurons. They play a crucial role in initiating and modulating voluntary movement.
What happens to the fibers of upper motor neurons in the central nervous system (CNS)?
Some fibers of upper motor neurons cross in the central nervous system (CNS) to control the opposite side of the body.
What are the consequences of damage to motor nerves for muscle strength?
Damage to motor nerves can lead to severe declines in muscle strength resulting in conditions like neurogenic atrophy.
What are the differences between neurogenic atrophy and disuse atrophy?
Neurogenic atrophy is caused by damage to the motor nerves typically due to disease or trauma while disuse atrophy is caused by a lack of muscle activity often due to immobilization or inactivity.
How can sensory feedback affect lower motor neurons?
Sensory feedback from muscles and tendons provides critical information to lower motor neurons allowing them to adjust muscle contractions based on the position and movement of the body.
What is the significance of upper motor neurons in muscle control?
Upper motor neurons are significant because they help coordinate and regulate movements initiated by lower motor neurons thereby ensuring fluid and purposeful motions.
What are commissural tracts?
Commissural tracts are bundles of nerve fibers that connect the left and right hemispheres of the brain allowing for communication between them.
What does it mean for a tract to be ipsilateral?
Ipsilateral tracts are those that conduct signals on the same side of the body meaning that the origin and destination of the signals are both on the same side.
What does contralateral communication involve?
Contralateral communication refers to nerve signals that travel from one side of the body to the opposite side involving pathways that cross over such as the brain’s corpus callosum.
What are intrafusal fibers?
Intrafusal fibers are modified muscle fibers found within muscle spindles that do not have contractile units and primarily serve to sense changes in muscle length.
What are extrafusal fibers?
Extrafusal fibers are the regular muscle fibers responsible for muscle contraction comprising the majority of muscle tissue and providing force during movement.
What role do Golgi tendon organs play in muscle function?
Golgi tendon organs monitor the tension that muscles apply to tendons. They send signals to the spinal cord to inhibit motor neuron activity via interneurons reducing muscle tension when necessary.
How does a Golgi tendon reflex work?
When tension increases in a tendon the Golgi tendon organ sends sensory signals to the spinal cord. These signals activate an interneuron that inhibits a motor neuron through inhibitory post-synaptic potentials (IPSPs) consequently reducing tension in the muscle.
What is the function of muscle spindles?
Muscle spindles detect changes in muscle length (stretch) and their action potentials increase with the degree of muscle stretch. They are more abundant in muscles that require fine motor control.
How does stretching a muscle impact muscle spindles?
When a muscle is stretched the intrafusal fibers within muscle spindles are also stretched which generates action potentials that indicate the degree of stretch and contribute to the muscle’s sensory feedback.
What is amyotrophic lateral sclerosis (ALS)?
Amyotrophic lateral sclerosis (ALS) is a neurodegenerative disease that primarily affects motor neurons in the brain and spinal cord leading to muscle weakness paralysis and eventual loss of voluntary movement.
What parts of the nervous system are affected by ALS?
ALS affects both lower motor neurons located in the spinal cord and upper motor neurons found in the brain disrupting the communication between the brain and muscles.
What is the role of motor neurons in the spinal cord in relation to muscle atrophy?
Motor neurons in the spinal cord lead to muscle wasting or amyotrophy which occurs when these neurons degenerate and fail to stimulate muscle contraction leading to progressive muscle weakness and atrophy.
What symptoms are associated with the degeneration of motor neurons?
The degeneration of motor neurons produces progressive muscle weakness atrophy and spastic paralysis.
What is the typical prognosis in terms of survival for patients with motor neuron degeneration?
Death usually occurs from respiratory failure within five years from the onset of symptoms related to motor neuron degeneration.
What distinguishes cardiac muscle from skeletal muscle?
Cardiac muscle is striated like skeletal muscle but its fibers are shorter branched and connected via intercalated discs which are gap junctions that allow impulses to be conducted cell to cell.
What are sarcomeres?
Sarcomeres are the basic contractile units of striated muscles including both cardiac and skeletal muscles formed by myosin and actin filaments.
How does contraction occur in cardiac muscle?
Contraction in cardiac muscle occurs due to the formation of cross bridges between myosin and actin stimulated by calcium ions.
Describe the structure and function of calcium channels in cardiac muscle.
In cardiac muscle the voltage-gated calcium channels are not directly connected to calcium channels in the sarcoplasmic reticulum (SR). Calcium acts as a second messenger to open SR channels in a process known as calcium-induced calcium release.
What is excitation-contraction coupling in cardiac muscle?
Excitation-contraction coupling in cardiac muscle refers to the physiological process where an electrical signal (excitation) leads to the release of calcium ions which ultimately triggers muscle contraction.
What is the significance of intercalated discs in cardiac muscle tissue?
Intercalated discs are critical in cardiac muscle because they contain gap junctions that facilitate direct electrical connections between adjacent cardiac cells allowing efficient transmission of electrical impulses.
What are some consequences of upper motor neuron damage?
Damage to upper motor neurons can lead to symptoms like increased muscle tone spastic paralysis and exaggerated reflexes due to loss of inhibitory control over lower motor neurons.
What are the key structural features of smooth muscle?
Smooth muscle does not have sarcomeres but contains large amounts of actin and myosin. The actin filaments are long and attached to dense bodies or regions of the plasma membrane. Myosin filaments are stacked vertically allowing for more extensive formation of cross bridges with actin.
How does the arrangement of actin and myosin in smooth muscle impact its ability to contract?
The unique arrangement of actin and myosin in smooth muscle allows for contraction even when the muscle is greatly stretched providing functional resilience.
What initiates excitation-contraction coupling in smooth muscle?
Excitation-contraction coupling in smooth muscle begins with a sharp rise in intracellular calcium concentrations.
Where does the calcium for contraction in smooth muscle primarily come from?
Most of the calcium that triggers contraction comes from outside the cell across the plasma membrane when voltage-gated calcium channels are opened although some calcium comes from the sarcoplasmic reticulum (SR).
What role does calmodulin play in smooth muscle contraction?
In smooth muscle calcium binds to calmodulin (as there is no troponin) which activates myosin light chain kinase (MLCK). This kinase then phosphorylates myosin light chains leading to the formation of cross bridges and muscle contraction.
What is the function of myosin light chain kinase (MLCK) in smooth muscle?
Myosin light chain kinase (MLCK) phosphorylates the myosin light chains in smooth muscle facilitating the interaction between actin and myosin to promote contraction.
Compare and contrast skeletal cardiac and smooth muscle regarding their structural characteristics.
Skeletal muscle is striated with organized sarcomeres cardiac muscle is also striated but has intercalated disks for deep structural connections while smooth muscle is non-striated and lacks sarcomeres but has long actin filaments attached to dense bodies.
How do the myosin filaments in smooth muscle differ from those in skeletal muscle?
In smooth muscle myosin filaments are stacked vertically and can form more extensive cross bridges with actin compared to myosin filaments in skeletal muscle which are part of organized sarcomeres.
What are the implications of smooth muscle’s ability to contract over a wide range of stretch?
The ability of smooth muscle to contract effectively even when stretched significantly allows for functions such as peristalsis in the gastrointestinal tract and regulation of blood vessel diameter.
What is the significance of the absence of troponin in smooth muscle contraction?
The absence of troponin in smooth muscle means that muscle contraction is regulated by calcium binding to calmodulin instead which is a different mechanism compared to the troponin-tropomyosin system in skeletal muscle.
What is the structural arrangement of myosin in striated muscle?
In striated muscle myosin is arranged in sarcomeres.
How is actin arranged in striated muscle compared to smooth muscle?
In striated muscle actin is arranged in sarcomeres while in smooth muscle actin is not arranged in sarcomeres.
What is the relationship of actin and myosin in smooth muscle?
Smooth muscle has more actin than myosin. Actin inserts into dense bodies and the plasma membrane.
Describe the development of the sarcoplasmic reticulum (SR) in cardiac muscle.
Cardiac muscle has a well-developed sarcoplasmic reticulum and transverse tubules.
What is the level of development of the sarcoplasmic reticulum and transverse tubules in smooth muscle?
Smooth muscle has a poorly developed sarcoplasmic reticulum and lacks transverse tubules.
What proteins are found in the thin filaments of striated and smooth muscle?
Striated muscle contains troponin in the thin filaments while smooth muscle contains calmodulin which activates myosin light-chain kinase when bound to Ca 2+.
Where does calcium (Ca 2+) come from in striated muscle?
In striated muscle Ca 2+ is released into the cytoplasm from the sarcoplasmic reticulum.
How does calcium (Ca 2+) enter the cytoplasm in smooth muscle?
In smooth muscle Ca 2+ enters the cytoplasm from the sarcoplasmic reticulum and extracellular fluid.
What is the source of calcium (Ca 2+) for contraction in cardiac muscle?
In cardiac muscle Ca 2+ enters the cytoplasm from extracellular fluid sarcoplasmic reticulum and perhaps mitochondria.
What is necessary for muscle contraction in striated muscle?
Striated muscle cannot contract without nerve stimulation; denervation results in muscle atrophy.
Can smooth muscle contract without nerve stimulation?
Yes smooth muscle can contract without nerve stimulation and maintains tone in the absence of nerve signals.
Where do action potentials originate in the heart?
Action potentials in the heart originate in pacemaker cells.
What is the role of visceral smooth muscle in the body?
Visceral smooth muscle is responsible for involuntary movements within the internal organs such as the intestines and bladder. It produces pacemaker potentials which are crucial for initiating rhythmic contractions.
What are pacemaker potentials?
Pacemaker potentials are spontaneous electrical changes that occur in certain types of smooth muscle enabling them to generate rhythmic contractions without direct nervous stimulation.
What is the effect of denervation on visceral smooth muscle?
Denervation of visceral smooth muscle results in hypersensitivity to stimulation meaning that the muscle is more responsive to neurotransmitters and stimuli than it would be if it were intact.
How are muscle fibers in visceral smooth muscle stimulated and what is unique about this process?
Muscle fibers in visceral smooth muscle can be stimulated independently as they do not form connections through gap junctions allowing for individual control of muscle contractions.
What role do gap junctions play in cardiac muscle?
In cardiac muscle gap junctions are present at intercalated discs allowing for electrical coupling between adjacent cardiomyocytes. This enables coordinated contractions throughout the heart.
Are gap junctions present in smooth muscle? If so where?
Yes gap junctions are present in most smooth muscles facilitating communication between smooth muscle cells and allowing for coordinated contractions in tissues such as the gastrointestinal tract.
What is the importance of intercalated discs in cardiac muscle?
Intercalated discs are crucial in cardiac muscle as they contain gap junctions and desmosomes. Gap junctions allow for rapid electrical signaling between cells enabling synchronized heart contractions while desmosomes provide mechanical stability.
Compare the connectivity of muscle fibers in smooth muscle to that in cardiac muscle.
In smooth muscle fibers are not interconnected by gap junctions allowing for independent stimulation. In contrast cardiac muscle fibers are interconnected via gap junctions at intercalated discs promoting synchronized contractions.
What is the clinical relevance of understanding pacemaker potentials in smooth muscle?
Understanding pacemaker potentials in smooth muscle helps in the management of conditions related to motility disorders as treatment can aim to regulate contractile activity in organs such as the intestines.
Describe the physiological consequences of hypersensitivity in denervated visceral smooth muscle.
Hypersensitivity in denervated visceral smooth muscle can lead to abnormal contractions or spasms potentially causing symptoms like pain cramps or motility disorders due to overstimulation by normal physiological signals.
