BIO 168 Ch. 9 & 10 exam

Muscle tissue

• They make up nearly half of body’s mass

• Can transform chemical energy (ATP) into directed mechanical energy, which is capable of exerting force

Prefixes for muscle

Myo-, mys-, and sarco-

Ex: sarcoplasmm - muscle cell cytoplasm

3 types of muscle tissue

• skeletal

• smooth

• cardiac

Where are muscle fibers found?

Elongated muscle cells found in skeletal and smooth muscle, but not cardiac

4 Important functions of skeletal muscle in the body

1. Movement

2. Posture + body position

3. Stabilize joints

4. Generate heat

Skeletal muscle: connective tissue sheaths

• Each skeletal muscle, as well as each muscle fiber, is covered in connective tissue

• Support cells and reinforce whole muscle

Epimysium

• Most external

• Dense irregular connective tissue surrounding entire muscle; may blend with fascia

Perimysium

• Intermediate

• fibrous connective tissue surrounding fascicles (groups of muscle fibers)

Endomysium

• Most internal

• fine aerolar connective tissue surrounding each muscle fiber

Skeletal muscle attachments

• Direct (fleshy) attachments: epimysium fused to periosteum of bone or perichondrium of cartilage

• Indirect attachments: connective tissue wrappings extend beyond muscle: tendons and aponeurosis

Fascicles

• A discrete bundle of muscle cells, segregated from the rest of the muscle by a connective tissue sheath

• Surrounded by perimysium

Muscle fiber (cell)

• An elongated multinucleate cell; it has a straited (banded) appearance

• Surrounded by endomysium

• Contains glycosomes for glycogen storage and myoglobin for O2 storage

• Specialized structures within sk muscle cells: myofibrils, SR, T-tubules

Sarcolemma

Muscle fiber plasma membrane

Sarcoplasm

Muscle fiber cytoplasm

Myofibrils

• Densely packed, rod-like elements. A single muscle fiber can contain 1000s. Accounts for ~80% muscle cell volume

• Features: striations, sarcomeres, myofilaments, molecular comp of myofilaments

Myofibril striations

Stripes formed from repeating series of dark and light bands along length of each myofibril

• A band - darker region

• H zone - lighter region in middle of dark A band

• M Line - line of protein (myomesin) that bisects H zone vertically

• I band - lighter regions

• Z disc - coin shaped sheet of proteins on midline of I band

Sarcomere

• Smallest contractile unit (functional unit) of muscle fiber

• Contains A band with half of an I band at each end. Consists of area between z-discs

• Individual sarcomeres align end to end along myofibril

Actin myofilaments

• Thin filaments

• Extend across I band and partway in A band

• Anchored to z discs

• Have active sites for myosin head attachment during contraction

• Two F actin strands twist together to form a thin filament

Myosin filaments

• Thick filaments

• Extend length of A band

• Connected at M line

• Heavy chains intertwine to form tail

• Light chains form myosin globular head

Tropomyosin and troponin

Regulatory proteins bound to actin

• Cover and expose myosin binding sites on actin

• Troponin binds to calcium ions to remove tropomyosin from actin

Titin

• Elastic myofilament

• Holds thick filaments in place; helps recoil after stretch; resists excessive stretching

Dystrophin

• Links thin filaments to proteins of sarcolemma

Nebulin, myomesin, and C proteins

Bind filaments or sarcomeres together

• maintain alignment of sarcomere

Sarcoplasmic Reticulum (SR)

Network of smooth ER tubules surrounding each myofibril

• Most run longitudinally

• Terminal cisterns from perpendicular cross channels at A-I band junction

• SR functions in regulation of intracellular Ca2+ levels

• Stores and releases Ca2+

T-tubules

• Tube formed by protrusion of sarcolemma deep into cell interior

• Increase muscle fibers surface area greatly

• Lumen continuous with extracellular space

• Allow electrical nerve transmissions to reach deep into interior of each muscle fiber

• Tubules penetrate cells interior at each A-I band junction between terminal cisterns

Triad

Area formed from terminal cistern of one sarcomere, T-tubule, and terminal cistern of neighboring sarcomere

• When an electrical impulse passes by, T tubule proteins change shape, causing SR proteins to change shape, causing release of calcium into cytoplasm

Sliding filament model of contraction

• The activation of cross bridges to generate force

• Shortening occurs when tension generated by cross bridges on thin filaments exceeds forces opposing shortening

• Contraction ends when cross bridges

• Cross bridge attachments form and break several times, each time pulling thin filaments a little closer toward center of sarcomere in a racheting action

• Causes shortening of muscle fiber with: I bands shorten, Z discs become closer, H zones disappear, A bands move closer to each other

T or F: During contraction, sarcomeres shorten

True

T or F: During contraction, A bands shorten

False

T or F: During contraction, I bands shorten

True

T or F: During contraction, the distance from one Z disc to the next Z disc decreases

True

T or F: During contraction, the M line disappears during a maximal contraction

False

T or F: During contraction, the H zone disappears during maximal contraction

True

T or F: During contraction, the Thin filaments slide past the thick filaments

True

T or F: During contraction, the thick and thin filaments overlap each other more

True

T or F: During contraction, the thick and thin filaments both shorten

False

How do motor neurons stimulate skeletal muscle fibers to contract?

• Decision to move is activated by the brain, signal is transmitted down spinal cord to motor neurons which then activate muscle fibers

• Neurons and muscle cells are excitable cells capable of action potentials (APs). They are capabale of changing resting membrane potential voltages

• AP crosses from neuron to muscle cell via the neurotransmitter acetylcholine (ACh)

Ion channels

Play major role in changing of membrane potentials

Two classes:

• Chemically gated: opened by chemical messengers such as neurotransmitters. Ex: ACh receptors on muscle cells

• Voltage-gated: open or close in response to voltage changes in membrane potential

Anatomy of motor neurons and NMJ

• SK muscles are stimulated by somatic motor neurons

• Axons (long extensions of motor neurons) travel from CNS to sk muscle

• Each axon divides into many branches as it enters muscle

• Axon branches end on muscle fiber, forming NMJ or motor end plate. Each muscle fiber has one NMJ with one motor neuron

• Axon terminal (end of axon) and muscle fiber are separated by gel-filled space called synaptic cleft

• Stored within axon terminal are membrane bound synaptic vesicles that contain ACh

• Infolding of sarcolemma called junctional folds contain millions of ACh receptors

• NMJ consists of axon terminals, synaptic cleft, and junctional folds

4 big steps that must occur for skeletal muscle to contract

1. Events at NMJ

2. Muscle fiber excitation

3. Excitation-contraction coupling

4. Cross bridge cycling

6 Events of the muscular junction

1. AP arrives at axon terminal of the motor neuron

2. Calcium ions enter the axon terminal through voltage-gated channels

3. Influx of calcium ions causes the release of ACh from the axon terminal

4. ACh diffuses across the synaptic cleft from the axon terminal to the sarcolemma

5. Chemically gated channels open, ions pass into and out of the muscle fiber creating a graded potential

6. ACh is broken down by acetylcholinesterase

Generation of an AP across the sarcolemma

• Resting sarcolemma is polarized, meaning a voltage exists across membrane

• Inside of cell is negative compared to outside

• AP is caused by changes in electrical charges

1. Generation of end plate potential

2. Depolarization

3. Repolarization

Generation of an AP across the sarcolemma: 1. End plate potential

• ACh released from motor neuron binds to ACh receptors on sarcolemma

• Causes chemically gated ion channels (ligands) on sarcolemma to open

• Na+ diffuses into muscle fiber. Some K+ diffuses outward but not much.

• Because Na+ diffuses in, interior of sarcolemma becomes less negative (more positive)

• Results in local depolarization called EPP

Generation of an AP across the sarcolemma: 2. Depolarization

Generation and propagation of an AP

• If EPP causes enough change in membrane voltage to reach critical level called threshold, voltage-gated Na+ channels in membrane will open

• Large influx of Na+ through channels into cell triggers AP that is unstoppable and will lead to muscle fiber contraction

• AP spreads across sarcolemma from one voltage-gated Na+ channel to next one in adjacent areas, causing that area to depolarize

Generation of an AP across the sarcolemma: 3. Repolarization

Restoration of resting conditions

• Na+ voltage-gated channels close, and voltage gated K+ channels open

• K+ efflux out of cell rapidly brings cell back to initial resting membrane voltage

• Refractory period: muscle fiber cannot be stimulated for a specific amount of time, until repolarization is complete

• Ionic conditions of resting state are restored by Na+-K+ pump

• Na+ that came into the cell is pumped back out, and K+ that flowed outside is pumped back into cell

Excitation-Contraction (E-C) Coupling

• Events that transmit AP along sarcolemma (excitation) are coupled to sliding of myofilaments (contraction)

• AP is propagated along sarcolemma and down into T tubules, where voltage sensitive proteins in tubules stimulate Ca2+ release from SR. This leads to contraction

• AP is brief and ends before contraction is seen

Cross bridge cycling

Cross bridge formation requires Ca2+

• At low intracellular Ca2+ concentration: tropomyosin blocks active sites on actin so that myosin heads cannot attach to it. Muscle fiber remains relaxed. In response, voltage-sensitive proteins in T tubules change shape, causing SR to release Ca2+ to cytosol

• At higher intracellular Ca2+ concentrations, Ca2+ binds to troponin. Troponin changes shape and moves tropomyosin away from myosin-binding sites. Myosin heads is then allowed to bind to action, forming cross bridges. Cycling is initiated, causing sarcomere shortening and muscle contraction

When nervous stimulation ceases, Ca2+ is pumped back into SR, and contraction ends

4 Steps of Cross Bridge Cycling

1. Cross bridge formation

2. The power stroke

3. Cross bridge detachment

4. Cocking of myosin head

4 Steps of Cross Bridge Cycling: 1. Cross bridge formation

Energized myosin head attaches to actin myofilament, forming a crossbridge

4 Steps of Cross Bridge Cycling: 2. Power stroke

Myosin head pivots and bends, pulling actin toward M-line, leaving myosin head in low energy state. ADP and P are released

4 Steps of Cross Bridge Cycling: 3. Cross bridge detachment

ATP binds to myosin, causing it to detach from actin. Cross bridge breaks

4 Steps of Cross Bridge Cycling: 4. Cocking of myosin head

Myosin hydrolyzes ATP to ADP & P. This causes the myosin head to return to its prestroke high energy position.

Whole skeletal muscle contraction

• Same principles apply to contraction of both single fibers and whole muscles

• Contraction produces muscle tension, the force exerted by a contracting muscle on an object

• Load is the opposing force to muscle contraction, the force exerted on the muscle by the weight of the object to be moved

• Force and duration of muscle contractions vary in response to stimuli of different frequencies and intensities

The motor unit

Each muscle is served by at least one motor nerve.

• Motor nerve contains axons of up to hundreds of motor neurons

• Axons branch into terminals, each of which forms NMJ with single muscle fiber

Motor unit consists of the motor neuron and all muscle fibers (four to several hundred) it supplies. 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 entire muscle

Muscle Twitch

• Simplest contraction resulting from a muscle fiber’s response to a single action potential from motor neuron

• Muscle fiber contracts quickly, then relaxes

• Twitch can be observed and recorded as a myogram. Tracing: line recording contraction activity

• 3 Phases: latent period, Period of contraction, Period of relaxation

3 phases of muscle twitch: 1. Latent period

Events of excitation-contraction coupling

No muscle tension seen

Events of excitation-contraction coupling are occurring

3 phases of muscle twitch: 2. Period of contraction

Cross bridge formation

Tension increases

Cross bridge cycling is occurring, from the onset of tension development to the peak of tension

3 phases of muscle twitch: 3. Period of relaxation

Ca2+ reentry into SR

Tension declines to zero

Muscle contracts faster than it relaxes

Number of cross bridges declines as calcium ions are actively transported into the SR

Muscle twitch differences

• Differences in strength and duration of twitches are due to variations in metabolic properties and enzymes between muscles

• Ex: eye muscles contraction are rapid and brief, whereas larger, fleshy muscles (calf muscles) contract more slowly and hold it longer

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

Isometric contraction

No shortening; muscle tension increases but does not exceed load

Load is not moved and muscles does not shorten or lengthen

Isotonic contraction

Muscle changes in length and moves load. They can be two types:

• Concentric contractions: muscle shortens and does work. Ex: biceps contract to pick up book

• Eccentric contractions: muscle lengthens and generates force. Ex: laying a book down causes biceps to lengthen while generating a force

Providing energy for muscle contraction

ATP supplies the energy needed for the muscle fiber to:

• Move and detach cross bridges

• Pump calcium back into SR

• Pump Na+ out of and K+ back into cell after excitation-contraction coupling

Available stores of ATP depleted in 4-6 seconds

ATP is the only source of energy for contractile activities; therefore, it must be regenerated quickly

ATP is regenerated quickly by three mechanisms…

1. Direct phosphorylation of ADP by creatine phosphate (CP)

2. Anaerobic pathway: glycolysis and lactate formation

3. Aerobic pathway: glycolysis and aerobic respiration in mitochondria

Direct phosphorylation of ADP by creatine phosphate (CP)

• Creatine phosphate is a unique molecule located in muscle fibers that donates a phosphate to ADP to instantly form ATP

• Creatine Kinase is an enzyme that carries out transfer of phosphate

• Muscle fibers have enough ATP and CP reserves ti power cell for about 15 secs

Creatine phosphate + ADP → creatine +ATP

No oxygen use and 1 creatine per ATP

Anaerobic pathway: glycolysis and lactate formation

ATP can also be generated by breaking down and using energy stored in glucose

• Glycolysis: first step in glucose breakdown. Does not require oxygen, glucose is broken into 2 pyruvate molecules, 2 ATPs are generated for each glucose broken down.

• Low oxygen levels prevent pyruvate from entering aerobic respiration phase.

• Normally, it should enter mitochondria but there is no oxygen available. Bulging muscles impair oxygen delivery.

• Anaerobic glycolysis and pyruvate is referred to as lactate. Lactate is then diffused into the blood stream to be used as fuel in the liver, kidneys, and heart

• Anaerobic respiration doesn’t produce as much ATP as aerobic respiration, but produces ATP way faster

Aerobic pathway: glycolysis and aerobic respiration in mitochondria

• Produces 95% of ATP during rest and light to moderate exercise. Slower than anaerobic pathway

• Consists of a series of chemical reactions that occur in mitochondria and require oxygen. Breaks glucose into CO2, H2O, and large amount of ATP (32)

• Fuels used include glucose from glycogen stored in muscle fiber, then bloodborne glucose, and free fatty acids. Fatty acids are main fuel after 30 minutes of exercise

Energy systems used during exercise

• Aerobic edurance: length of time muscle contracts using aerobic pathways. Light to moderate activity which can continue for hours

• Anaerobic threshold: point at which muscle metabolism converts to anaerobic pathway

Muscle fatigue

Inability to maintain muscle tension

• Ionic imbalances (K+, Ca2+, Na+) disrupt membrane potential of muscle cell

• Increased inorganic phosphate from CP and ATP breakdown interferes with calcium release in SR

• Decreased ATP

• Increased Magnesium (interferes with T tubule proteins

• Decreased glycogen: lack of ATP is rarely a reason for fatigue, except in severely stressed muscles

Excess Postexercise Oxygen Consumption (EPOC)

Referred to as oxygen debt. For a muscle to return to its pre-exercise state, you require extra oxygen:

• Oxygen reserves are replenished

• Lactate is reconverted to pyruvate

• Glycogen stores are replaced

• ATP and creatine phosphate reserves are resynthesized

How is muscle fiber classified?

Two characteristics

1. Speed of contraction: slow or fast. According to speed at which ATPases split ATP. Pattern of electrical activity of motor neurons

2. Metabolic pathways used for ATP synthesis: Oxidative fibers (aerobic), Glycolytic fibers (anaerobic)

Can be classified as Slow oxidative fibers, fast oxidative fibers, or fast glycolytic fibers. Most muscles contain a mix of fiber types, resulting in a range of contractile speed and fatigue resistance.

• All fibers in one motor unit are the same type

• Genetics dictate individual’s percentage of each

Slow oxidative fibers

Red Slow Ox 🐮 Low intensity, edurance activites like maintaining posture or running marathon

• High myoglobin, mitochondria, capillaries, red color

• Aerobic ATP synthesis

• Slow speed of contraction, myosin ATPase acitivity, rate of fatigue

• Low glycogen stores

Fast oxidative glycolytic

FOG; Medium intensity activities like sprinting or walking

• High mitochondria, capillaries, myoglobin, pink color

• Aerobic pathway (some anaerobic glycolysis)

• Intermediate fatigue, glycogen stores

• Fast contraction, myosin ATPase activity

Fast Glycolytic fibers

Too Fast White Sugar; Short term intense or powerful movements like hitting a baseball (bicep or eye muscles)

• Fast speed of contraction, myosin ATPase activity, fatigue (most fatigueable)

• Few/low capillaries, mitochondria, myoglobin content, white in color

• Anaerobic glycolysis

• High glycogen stores

Load and recruitment: Load

Muscles contract fastest when no load is added

• The greater the load, the shorter the duration of contraction and the slower the contraction

Load and recruitment: Recruitment

The more motor units contracting, the faster and more prolonged the contraction

Smooth muscle

Found in walls of most hollow organs like respiratory, digestive, urinary, reproductive, circulatory (except in smallest of blood vessels) except heart

• packed into sheets of tightly packed fibers

• Logitudinal layer: run parallel to axis; contraction causes organ to shorten

• Circular layer: run around circumference; contraction causes lumen of organ to constrict

• Mix and squeeze substances of hollow organs

• Spindle shaped with only one nucleus and no striations

• Only has endomysium

• contains varicosities instead of NMJ. controlled by ANS

Contraction of smooth muscle

• They have less elaborate SR and no T tubules. Calcium instead comes from extracellular origins. Sarcolemma contains caveolae pouches that allow calcium influx

• Electrically connected by gap junctions

• Few thick filaments but a lot of myosin heads so still powerful

• No troponin (replaced by calmodulin) in thin filaments but does have tropomyosin

• Filaments arranged diagnolly and are anchored by dense bodies

• When contracted, it squishes

• Contraction regulated by ANS, hormones, and chemical signals

Prime mover (agonist)

Major responsibility for producing specific movement

Antagonist

Opposes or reverses particular movement

Synergist

Helps prime movers, adds extra force to same movement, reduces undesirable movement

Fixator

Type of synergist that immobilizes bone or muscle origin, provides stable base

What term describes the biceps brachii muscle during forearm flexion?

Prime Mover

Which term describes the biceps brachii muscle during forearm extension

Antagonist

A prime mover and its antagonist are located ________ of the joint across which they act.

On opposite sides

Muscle origin

Muscles attachment point on the bone that moves the least; usually proximal/medial attachment

Muscle insertion

Muscle attachment point on bone that moves the most; Usually distal or lateral attachment

Lever system components

• Lever

• Fulcrum

• Load

• Effort

Lever

Rigid bar (bone) that moves on a fixed point (fulcrum = joint)

Fulcrum

Fixed point; joint

Load

Resistance (anyhting that adds weight like bone, tissues, added weight) moved by effort

Effort

Force (supplied by muscle contraction) applied to lever to move resistance

Mechanical advantages (good things) of levers

• Moves a heavier load over a smaller distance

• Requires less muscular effort to move the load

• Achieved with a power lever that is strong

• Effort is applied to farther from the fulcrum than the load to be moved

Mechanical disadvantages (bad things) of levers

• Moves a lighter load over a great distance

• Effort is applied closer to the fulcrum than the load to be moved

• Achieved with a speed lever that is fast

• Greater muscular effort is required to move the load

First class lever

Seesaw or scissors

• Fulcrum is between load and effort

• Ex: neck extension, posterior neck muscles provide effort, atlantoccipital joint is fulcrum and facial skeleton is load

• Can be both advantage or disadvantage

Second class lever

Wheelbarrow or standing on toes

• Load is between fulcrum and effort

• Ex: effort is calf muscles, joints of the ball of foot are fulcrum, body weight is load

• Uncommon in body; Mechanical advantage

Third class lever

Tweezers, foreceps, most skeletal muscles

• Effort is applied between fulcrum and load

• Ex. effort exterted on proximal radius of forearm, fulcrum is elbow joint, load is hand and distal forearm

• Mechanical disadvantage (ideal for fast, large movements

Speed in mechanical disadvantage (speed levers)

Force is lost but speed and range of movement are gained

Speed in mechanical advantage (power levers)

Slower, but more stable. Used where strength is priority