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Questions and Answers

What neurotransmitter is primarily involved in the communication between motor neurons and muscle fibers?

Acetylcholine (ACh) (correct)

Serotonin

Dopamine

Norepinephrine

What separates the axon terminal and the muscle fiber at the neuromuscular junction?

Neuron gap

Cell membrane

Axon barrier

Synaptic cleft (correct)

Which type of ion channels are opened by chemical messengers such as neurotransmitters?

Voltage-gated ion channels

Mechanically gated ion channels

Temperature-sensitive channels

Chemically gated ion channels (correct)

What is the role of action potentials in neurons and muscle cells?

They enable the cells to change resting membrane potential voltages. Signup and view all the answers

What is contained within the synaptic vesicles in the axon terminals?

Acetylcholine (ACh) Signup and view all the answers

What does the neuromuscular junction (NMJ) consist of?

Axon terminal, synaptic cleft, and junctional folds Signup and view all the answers

What occurs after the release of acetylcholine at the neuromuscular junction?

End plate potential generation Signup and view all the answers

What is the function of junctional folds in the neuromuscular junction?

To increase surface area for ACh receptors Signup and view all the answers

What initiates the decision to move, leading to activation of muscle fibers?

Brain activation Signup and view all the answers

Which of the following best describes the effect of voltage-gated ion channels?

They respond to changes in membrane potential. Signup and view all the answers

What is one primary function of muscles?

Produce movement Signup and view all the answers

Which connective tissue sheath surrounds the entire skeletal muscle?

Epimysium Signup and view all the answers

What is the smallest contractile unit of a muscle fiber called?

Sarcomere Signup and view all the answers

How does the T tubule function in muscle fibers?

It allows electrical impulses to penetrate deep into the muscle fiber. Signup and view all the answers

What type of muscle is known as 'voluntary muscle'?

Skeletal muscle Signup and view all the answers

Which protein forms the thick filaments in a muscle sarcomere?

Myosin Signup and view all the answers

What role does titin play in the muscle structure?

Holds thick filaments in place and helps recoil after stretch Signup and view all the answers

During muscle contraction, which filaments slide across one another?

Actin and Myosin Signup and view all the answers

What is the main consequence of Duchenne muscular dystrophy?

Muscle-destroying diseases leading to loss of coordination Signup and view all the answers

What is the primary role of myoglobin in muscle fibers?

Stores oxygen for energy production Signup and view all the answers

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

To store and release calcium ions Signup and view all the answers

Which of the following best describes myofibrils?

Densely packed, rodlike elements within muscle fibers Signup and view all the answers

What kind of muscle contraction generates heat as a byproduct?

Concentric contraction Signup and view all the answers

Where are the active sites for myosin head attachment located?

On actin G subunits Signup and view all the answers

What occurs during the working (power) stroke of muscle contraction?

Myosin head pivots and pulls the thin filament. Signup and view all the answers

Which phase of the muscle twitch involves increasing tension due to cross bridge formation?

Period of contraction Signup and view all the answers

What is the result of ATP not being present after death?

Leads to rigor mortis due to permanent cross bridge formation. Signup and view all the answers

What type of contraction occurs when a muscle changes in length while moving a load?

Concentric contraction Signup and view all the answers

Which mechanism recruits more muscle fibers to enhance contraction strength?

Motor unit recruitment Signup and view all the answers

During which phase of muscle contraction does the muscle return to its resting state?

Period of relaxation Signup and view all the answers

What is the primary source of ATP during high-intensity, short-duration activities?

Direct phosphorylation of ADP by creatine phosphate Signup and view all the answers

What occurs in a scenario of wave summation due to increased stimulation frequency?

Twitch contractions become stronger with rapid stimuli. Signup and view all the answers

What is the metabolic process that occurs in the absence of oxygen leading to lactic acid formation?

Glycolysis Signup and view all the answers

What type of muscle contraction does not change the length of the muscle but generates tension?

Isometric contraction Signup and view all the answers

What happens during excess postexercise oxygen consumption (EPOC)?

Lactic acid is converted back to glucose. Signup and view all the answers

What distinguishes fused (complete) tetanus from unfused (incomplete) tetanus?

Fused tetanus results in a smooth, sustained contraction. Signup and view all the answers

What physiological change primarily causes muscle fatigue?

Ionic imbalances within the muscle cell Signup and view all the answers

What initiates the action potential (AP) in muscle fibers?

A change in membrane voltage to reach threshold Signup and view all the answers

What occurs during the repolarization phase of an action potential?

K+ efflux brings the cell to resting voltage Signup and view all the answers

What is the significance of the refractory period in muscle fibers?

It prevents overstimulation and allows recovery Signup and view all the answers

How does excitation-contraction coupling occur?

AP spreading down into T tubules triggers Ca2+ release Signup and view all the answers

What is the role of calcium ions (Ca2+) in muscle contraction?

They bind to troponin, allowing myosin to attach to actin Signup and view all the answers

What happens when intracellular Ca2+ concentrations are low?

Tropomyosin blocks active sites on actin Signup and view all the answers

What occurs during the calcium-induced calcium release in muscle fibers?

Calcium release from the sarcoplasmic reticulum triggers contraction Signup and view all the answers

What is the primary function of the Na+-K+ pump after depolarization?

To restore ionic balance to resting state Signup and view all the answers

What triggers the release of calcium ions during excitation-contraction coupling?

AP propagation along T tubules Signup and view all the answers

What prevents myosin heads from attaching to actin during muscle relaxation?

Tropomyosin blocking the active sites Signup and view all the answers

What is the outcome of the cross bridge cycling process in muscle contraction?

Shortening of the sarcomere occurs Signup and view all the answers

What happens to calcium ions after the nervous stimulation ceases?

They are pumped back into the sarcoplasmic reticulum Signup and view all the answers

What initiates muscle contraction beyond the action potential?

Binding of calcium to troponin Signup and view all the answers

What is the primary energy source for slow oxidative muscle fibers during contraction?

Aerobic pathways Signup and view all the answers

How does the load on a muscle affect its contraction speed?

Contraction speed decreases with heavier loads. Signup and view all the answers

Which type of muscle fiber is best suited for short-term, intense activities?

Fast glycolytic fibers Signup and view all the answers

What role do myofilaments play in smooth muscle contraction?

They interact by the sliding filament mechanism. Signup and view all the answers

What is the main structural characteristic that differentiates smooth muscle fibers from skeletal muscle fibers?

Number of nuclei Signup and view all the answers

How does calcium contribute to smooth muscle contraction?

Calcium binds to calmodulin instead of troponin. Signup and view all the answers

Which of the following describes the energy efficiency of smooth muscle contraction?

It can maintain contraction for prolonged periods at a lower energy cost. Signup and view all the answers

Which feature of smooth muscle allows it to respond to stretch?

Stress-relaxation response Signup and view all the answers

What is the significance of the gap junctions in smooth muscle fibers?

They allow for synchronized contractions. Signup and view all the answers

Which two factors primarily influence the regulation of smooth muscle contraction?

Nerves and hormones Signup and view all the answers

In smooth muscle cells, what components primarily contribute to the contraction mechanism?

Actin and myosin interactions. Signup and view all the answers

What determines the percentage of each type of muscle fiber in an individual's muscles?

Genetics Signup and view all the answers

Study Notes

Events at the Neuromuscular Junction

Motor neurons control muscle contraction.
The neuromuscular junction (NMJ) is the synapse between a motor neuron and a muscle fiber.
The NMJ consists of the axon terminal, the synaptic cleft, and the junctional folds.
The axon terminal contains synaptic vesicles that store acetylcholine (ACh).
Junctional folds of the sarcolemma contain ACh receptors.
When an action potential arrives at the axon terminal, it triggers the release of ACh.
ACh diffuses across the synaptic cleft and binds to ACh receptors on the sarcolemma.
This binding causes depolarization of the sarcolemma, generating an end-plate potential.
The end-plate potential triggers an action potential in the muscle fiber.
The action potential spreads along the sarcolemma and down the T tubules, stimulating the release of calcium ions from the sarcoplasmic reticulum (SR).

Muscle Fiber Excitation

Muscle fibers are excitable cells that can change their resting membrane potential.
Depolarization of the sarcolemma is caused by the influx of sodium ions (Na+) through voltage-gated Na+ channels.
Repolarization is caused by the efflux of potassium ions (K+) through voltage-gated K+ channels.
The refractory period is the time during which the muscle fiber cannot be stimulated again.
The Na+-K+ pump restores the ionic conditions of the resting state.

Excitation-Contraction Coupling

Excitation-contraction (E-C) coupling is the process by which the action potential is linked to the sliding of the myofilaments, causing muscle contraction.
The action potential triggers the release of calcium ions (Ca2+) from the sarcoplasmic reticulum (SR).

Events at the NMJ - Summary

The events leading to muscle contraction start with a signal from the nervous system.
The signal travels down the motor neuron as an action potential.
This signal arrives at the axon terminal of the motor neuron and causes the release of acetylcholine (ACh).
ACh binds to receptors on the sarcolemma of the muscle fiber, triggering a series of events that lead to the contraction of the muscle fiber.

Muscle Fiber Contraction: Cross-Bridge Cycling

At low intracellular Ca2+ concentrations, tropomyosin blocks the active sites on actin, preventing myosin heads from attaching.
When Ca2+ binds to troponin, it causes a conformational change that moves tropomyosin away from the myosin-binding sites on actin.
The myosin heads are then allowed to bind to actin, forming cross-bridges.
The cross-bridge cycle is initiated, which causes the sarcomere to shorten and the muscle to contract.
The four steps of the cross-bridge cycle are:
- Attachment: The myosin head attaches to an active site on the actin filament.
- Power stroke: The myosin head pivots, pulling the actin filament towards the center of the sarcomere.
- Detachment: The myosin head detaches after ATP binds to it.
- Reactivation: The myosin head hydrolyzes ATP, which reactivates it and allows it to bind to another active site on the actin filament.
When nervous stimulation ceases, Ca2+ is pumped back into the SR, and the muscle fiber relaxes.

Muscle Structure

Each skeletal muscle is made up of muscle fibers, which are long, cylindrical cells.
Muscle fibers contain many myofibrils, which are the contractile units of the muscle fiber.
Myofibrils are made up of a repeating array of sarcomeres, which are the basic contractile units of muscle.
Sarcomeres contain thick and thin filaments.
Thick filaments are composed of myosin.
Thin filaments are composed of actin.
The sliding filament model of muscle contraction states that the myosin heads bind to actin, causing the thin filaments to slide past the thick filaments, shortening the sarcomere and causing muscle contraction.
The sarcoplasmic reticulum (SR) is a network of membranous channels that surrounds each myofibril.
The SR stores and releases calcium ions (Ca2+).
T tubules are invaginations of the sarcolemma that extend deep into the muscle fiber.
T tubules allow the action potential to travel deep into the muscle fiber, allowing the release of Ca2+ from theSR.

Clinical Implications

Duchenne muscular dystrophy is a serious form of muscular dystrophy that causes muscle wasting and weakness.
DMD is caused by a mutation in the gene for dystrophin, a protein that links thin filaments to the sarcolemma.
DMD is inherited as a sex-linked recessive disorder, so it is primarily found in males.### Cross Bridge Formation
High-energy myosin head attaches to actin thin filament active site
Myosin head pivots and pulls thin filament toward M line (Power Stroke)
ATP attaches to myosin head, causing cross bridge to detach
Energy from ATP hydrolysis "cocks" myosin head into high-energy state

Rigor Mortis

Starts 3–4 hours after death, peaks at 12 hours
Intracellular calcium levels increase because ATP is no longer synthesized, so calcium cannot be pumped back into SR
This leads to cross bridge formation
Muscles stay contracted until muscle proteins break down, causing myosin to release

Muscle Twitch

Simplest contraction in response to a single action potential from motor neuron
Muscle fiber contracts quickly, then relaxes
Three phases:
- Latent period: excitation-contraction coupling, no muscle tension seen
- Period of contraction: cross bridge formation, tension increases
- Period of relaxation: Ca2+ reentry into S R, tension declines to zero
Muscle contracts faster than it relaxes

Graded Muscle Responses

Muscle contraction is smoother and strength varies depending on need
Graded muscle responses are controlled by:
- Frequency of stimulation (Temporal summation)
- Strength of stimulation (Motor unit recruitment)

Wave/Temporal Summation

Two stimuli received in rapid succession
Muscle fibers don't fully relax between stimuli, so twitches increase in force with each stimulus

Unfused/Incomplete Tetanus

Stimuli frequency increases, leading to smooth, sustained contractions
Further increase in stimulus frequency causes muscle tension to reach near maximum, resulting in a quivering contraction

Fused/Complete Tetanus

Stimuli frequency continues to increase, muscle tension reaches maximum
Contractions "fuse" into one smooth, sustained contraction plateau

Motor Unit

Consists of the motor neuron and all its muscle fibers (4 to several hundred)
Smaller the fiber number, the greater the fine control
Muscle fibers from a motor unit are spread throughout the whole muscle, so stimulation of a single motor unit causes only weak contraction of the entire muscle

Motor Unit Recruitment

Stimulus is sent to more muscle fibers, leading to more precise control.
Stimulus types:
- Subthreshold stimulus: no contraction seen
- Threshold stimulus: first observable contraction
- Maximal stimulus: maximum contractile force, all motor units recruited

Size Principle

Motor units with the smallest muscle fibers are recruited first
Motor units with larger fibers are recruited as stimulus intensity increases
Largest motor units are activated only for the most powerful contractions

Muscle Tone

Constant, slightly contracted state of all muscles
Due to spinal reflexes: groups of motor units are alternately activated in response to input from stretch receptors in muscles
Keeps muscles firm, healthy, and ready to respond

Isotonic Contractions

Muscle changes in length and moves a load
Types:
- Concentric contractions: muscle shortens and does work (e.g., biceps contract to pick up a book)
- Eccentric contractions: muscle lengthens and generates force (e.g., laying a book down causes biceps to lengthen while generating a force)

Isometric Contractions

Load is greater than the maximum tension muscle can generate, so muscle neither shortens nor lengthens
Electrochemical and mechanical events are the same in isotonic or isometric contractions, but results are different:
- Isotonic: actin filaments shorten and cause movement
- Isometric: cross bridges generate force, but actin filaments don't shorten (Myosin heads "spin their wheels" on the same actin-binding site)

Providing Energy for Contraction

ATP is needed for:
- Moving and detaching cross bridges
- Pumping calcium back into the SR
- Pumping Na+ out of and K+ back into the cell after excitation-contraction coupling
ATP stores are depleted within 4-6 seconds
ATP needs to be regenerated quickly via three mechanisms:
- Direct phosphorylation of ADP by creatine phosphate (CP)
- Anaerobic pathway: glycolysis and lactic acid formation
- Aerobic pathway

Direct Phosphorylation of ADP by Creatine Phosphate (CP)

Creatine phosphate donates a phosphate to ADP to instantly form ATP
Creatine kinase is the enzyme involved
Enough ATP and CP reserves to power the cell for about 15 seconds
Creatine phosphate + ADP --> creatine + ATP

Anaerobic Glycolysis

ATP can also be generated by breaking down glucose
Glycolysis: first step in glucose breakdown
- Does not require oxygen
- Glucose is broken into 2 pyruvic acid molecules
- 2 ATPs are generated for each glucose broken down
At high intensity activity, oxygen is not available:
- Bulging muscles compress blood vessels, impairing oxygen delivery
In the absence of oxygen, pyruvic acid is converted to lactic acid
Lactic acid:
- Diffuses into the bloodstream
- Used as fuel by liver, kidneys, and heart
- Converted back into pyruvic acid or glucose by the liver
Yields only 5% as much ATP as aerobic respiration but produces ATP 2½ times faster

Aerobic Respiration

Produces 95% of ATP during rest and light to moderate exercise
Slower than anaerobic pathway
Breaks down glucose into CO2, H2O, and large amounts of ATP (32)
Fuels:
- Glucose from glycogen stored in muscle fiber
- Bloodborne glucose
- Free fatty acids (main fuel after 30 minutes of exercise)

Energy Systems Used in Sports

Aerobic endurance: length of time muscle contracts using aerobic pathways (light-to-moderate activity that can continue for hours)
Anaerobic threshold: point at which muscle metabolism converts to anaerobic pathway

Muscle Fatigue

Physiological inability to contract despite continued stimulation
Possible causes:
- Ionic imbalances: changes in K+, Na+, and Ca2+ levels disrupting membrane potential of muscle cells
- Increased inorganic phosphate (Pi) from CP and ATP breakdown may interfere with calcium release from SR or hamper power
- Decreased ATP and increased magnesium: magnesium levels increase as ATP levels drop, interfering with voltage-sensitive T tubule proteins
- Decreased glycogen
Lack of ATP is rarely a reason for fatigue, except in severely stressed muscles

Excess Postexercise Oxygen Consumption (EPOC)

Muscle needs extra oxygen to return to its pre-exercise state:
- Oxygen reserves are replenished
- Lactic acid is reconverted to pyruvic acid
- Glycogen stores are replaced
- ATP and creatine phosphate reserves are resynthesized
All replenishing steps require extra oxygen
Formerly referred to as "oxygen debt"

Force of Muscle Contractions

Depends on the number of cross bridges attached, which is affected by:
- Number of muscle fibers activated: more fibers, stronger contraction
- Size of muscle fibers: larger fibers, stronger contraction
- Frequency of stimulation: higher frequency, stronger contraction
- Length-tension relationship: optimal muscle length leads to strongest contraction
  - Too short or too stretched: fewer cross bridges can form, weaker contraction
  - Optimal length: maximum cross bridge formation, strongest contraction

Muscle Fiber Types

Muscle fibers are categorized based on their energy source and contraction characteristics:
- Oxidative fibers use aerobic pathways for ATP synthesis.
- Glycolytic fibers use anaerobic glycolysis for ATP synthesis.

Skeletal Muscle Fiber Types

Skeletal muscle fibers are classified into three types based on their speed of contraction and resistance to fatigue:
- Slow oxidative fibers are suitable for low-intensity, endurance activities like maintaining posture.
- Fast oxidative fibers are suitable for medium-intensity activities like sprinting or walking.
- Fast glycolytic fibers are best for short-term, intense, or powerful movements like hitting a baseball.
Most muscles contain a mixture of fiber types, resulting in a range of contractile speeds and fatigue resistance.
All fibers within a single motor unit are of the same type.
An individual's percentage of each fiber type is genetically determined.

Velocity and Duration of Contraction

Load: Muscles contract fastest when there is no added load.
- Increased load leads to shorter contraction durations.
- Increased load leads to slower contractions.
Recruitment: The more motor units contracting, the faster and more prolonged the contraction.

Smooth Muscle

Found in the walls of most hollow organs: respiratory, digestive, urinary, reproductive, and circulatory (except the smallest blood vessels) systems.
Most organs have two layers of smooth muscle fibers oriented at right angles to each other:
- Longitudinal layer: Fibers run parallel to the organ's long axis, causing shortening upon contraction.
- Circular layer: Fibers run around the organ's circumference, constricting the lumen during contraction.
Alternating contractions and relaxations of these layers mix and squeeze substances through the lumen of hollow organs.

Differences Between Smooth and Skeletal Muscle

Smooth muscle fibers:
- Are spindle-shaped, thin, and short compared to skeletal muscle fibers.
- Have one nucleus and lack striations.
- Lack connective tissue sheaths and only contain endomysium.
- Contain varicosities instead of neuromuscular junctions.
- Are innervated by the autonomic nervous system.
Smooth muscle has:
- Less elaborate SR and no T tubules.
- Pouchlike infoldings called caveolae on the sarcolemma that contain Ca2+ channels for rapid extracellular Ca2+ influx.
- Gap junctions that allow electrical depolarization to spread from cell to cell.
- Overlapping thick and thin filaments, but no striations or sarcomeres.
Other differences:
- Thick filaments are fewer and have myosin heads along their entire length (1:13 ratio of thick to thin filaments).
- No troponin complex, but contains tropomyosin.
- Thick and thin filaments are arranged diagonally, causing a corkscrew contraction.
- Intermediate filament–dense body network that resists tension.
- Dense bodies anchor filaments to the sarcolemma.

Contraction of Smooth Muscle

Mechanism of contraction:
- Slow, synchronized contractions due to electrical coupling through gap junctions.
- Some cells are self-excitatory and act as pacemakers.
- Contraction is similar to skeletal muscle in that:
  - Actin and myosin interact by sliding filament mechanism.
  - Increased intracellular Ca2+ level is the final trigger.
  - ATP energizes the sliding process.
- Contraction differs from skeletal muscle in that:
  - Most Ca2+ comes from extracellular space.
  - Ca2+ binds to calmodulin, not troponin, activating myosin kinase.
  - Phosphorylated myosin heads form cross-bridges with actin.
- Stopping smooth muscle contraction requires:
  - Ca2+ detachment from calmodulin.
  - Active transport of Ca2+ into the SR and extracellular space.
  - Dephosphorylation of myosin to inactive myosin.

Smooth Muscle Contraction Features

Energy Efficiency:
- Slow contraction and relaxation allows for prolonged contractions with minimal energy cost.
- Slower ATPases.
- Myofilaments can latch together to save energy.
- Most smooth muscle maintains a moderate degree of constant contraction (smooth muscle tone), made possible by aerobic respiration pathways.
Regulation of contraction:
- Controlled by nerves, hormones, or local chemical changes.
- Neural regulation:
  - Neurotransmitter binding causes either graded (local) potential or action potentials, increasing Ca2+ concentration in the sarcoplasm.
  - The response depends on the neurotransmitter and receptor type, with one neurotransmitter potentially having different effects on different organs.
- Hormones and local chemicals:
  - Some smooth muscle cells lack nerve supply.
  - Depolarize spontaneously or in response to chemical stimuli like hormones, high CO2, pH, and low oxygen.
  - Some smooth muscles respond to both neural and chemical stimuli.
Special features:
- Response to stretch:
  - Stress-relaxation response allows for temporary storage of contents in organs like the stomach and bladder.
- Length and tension changes:
  - Capable of contracting when between half and twice its resting length, allowing for organ volume changes.

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