Chapter 9: Muscle Anatomy and Physiology

Which types of muscle tissue are elongated and muscle fibers

Skeletal and smooth

Skeletal Muscle

• longest of all muscle

• has striations

• voluntary

• Multi nucleated

Cardiac muscle

• Cardiac,

• striated

• Involuntary

Smooth Muscle

• Visceral

• Non-striated

• Involuntary

• Hollow organs ( large arteries, bladder, stomach

Skeletal Muscle anatomy

• Nerve and blood supply

• connective tissue sheaths

• attachments

Epimysium

Dense irregular connective tissue surrounding entire muscle

Perimysium

fibrous connective tissue surrounding fascicles ( group of muscle fibers)

Endomysium

Fine areolar connective tissue surrounding each muscle fiber

Attachments

Direct: epimysium fused to periosteum of bone or perichondrium of cartilage

Indirect: connective tissue wrappings extend beyond muscle as rope-like tendon

Myofibril

long, cylindrical, contractile organelles within muscle cells

Sarcolemma

muscle fiber plasma membrane

Sarcoplasm

Muscle fiber cytoplasm ( contains many glycosomes ( glycogen storage, myoglobin O2 storage)

Sarcoplasmic Reticulum

• Stores and releases Ca 2+

• SR functions in regulation of intracellular Ca 2+ levels

• surrounds skeletal muscle fiber

Sarcomere

• smallest contractile unit of muscle fiber

• consists of area between z discs

Actin myofilaments

• thin filaments

• anchored to Z discs

• Extend across I band and pathway in A band

Myosin myofilaments

• thick filaments

• connected at M line

• extend length of A band

Muscle Fiber Contraction

• Muscle cells are excitable cells capable of action potentials

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

Ion channels in muscle contractions

Chemically gated ion channels - opened by chemical messengers such as neurotransmitters Ex) ACh receptors on muscle cells

Voltage- gated ion channels- open or close in response to voltage changes in membrane potential

Skeletal muscles are stimulated by…

somatic motor neurons

Neuromuscular Junction ( skeletal muscle)

The region where the somatic motor neuron contacts the skeletal muscle, consists of axon terminals and the underlying junctional folds of the sarcolemma

Synaptic cleft

muscle fiber are separated by gel-filled space

Events at the Neuromuscular Junction

1. AP arrives at axon terminal

2. Voltage-gated calcium channels open, calcium enters motor neuron

3. Calcium entry causes release ACh neurotransmitter into synpatic cleft

4. ACh diffuses across to ACh receptors (Na+ chemical gates) on sarcolemma

5. ACh binding to receptors, open gates, allowing Na+ to enter resulting in end plate potential

6. Acetylcholinesterase degrades ACh

Generation of an Action Potential across the Sarcolemma ( Muscle fiber excitation)

1.Generation of end plate potential

2.Depolarization

3.Repolarization

End plate potential

ACh released from motor neuron binds to ACh receptors on sarcolemma

Causes chemically gated ion channels on sarcolemma to open

Na+ diffuses into muscle fiber ( K+ diffuses outward)

Because Na+ diffuses in, interior of sarcolemma becomes less negative

results in local depolarization

Depolarization in skelteal muscle ( sarcomere)

• Generation and propagation of an Action potential

• end plate potential causes enough change in membrane voltage to reach critical level called threshold, Lyigand gated ion Na+ channels in membrane will open

• the influx of Na+ through channels into cell triggers AP that is unstoppable and will lead to muscle fiber contraction, this causes a series of voltage gated sodium channels opening all along the t-tubules

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

Repolarization

restoration of resting conditions

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

K+ effux 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+ out, K+ in)

Excitation Contracting coupling

AP is propagated along sarcolemma and down into T tubules, where voltage- sensitive proteins in tubules stimulate Ca 2+ release from SR leading to sliding of myofilaments

Cross Bridge Cycling

at higher intracellular Ca2+ concentrations ca2+ binds to troponin, troponin changes shape and Move tropomyosin away from myosin-binding sites ( it blocks active sites on actin), Myosin head bind to actin forming cross bridge. Once Nervous stimulation ceases ca2+ is pumped back into SR, ending contraction

Cross bridge cycle steps

1) Cross bridge formation: Energized myosin head attaches to an actin myofilament, forming a cross bridge ADP and Pi are attached at this time

2) The power ( working) stroke: ADP and Pi are released and the myosin head pivots and bends changing to its bent low-energy state, pulling actin towards the M line ( In the absence of ATP myosin heads will not detach causing rigor mortis)

3)Cross bridge detachment: after adding ATP to myosin, the link between myosin and actin weakness and the myosin head detaches

4) Cocking of myosin head, As ATP is hydrolyzed to ADP, and Pi the myosin head returns to its prestoke high energy position

Rigor mortis

Due to ATP needed for cross bridge detachment, resulting in myosin head staying bound to actin causing state of contraction,

Intracellular calcium levels increase because ATP is no longer being synthesized so calcium cannot be pumped back into SR

Isometric contraction

no shortening; muscle tension increase but does not exceed load ( muscle neither shortens nor lengthens) Muscle tension matches external load

• isometric contractions, cross bridges generate force, but actin filaments do not shorten

• Myosin heads “ spin their wheels” on same actin-binding site

Isotonic contraction

Muscle shortens because tension exceeds load ( Muscle changes in length and moves load

• Actin filaments shorten and cause movement

Muscle twitch

simplmest contraction resulting from a muscle fiber repsonse to a single AP from motor neuron

The muscle twitch steps

1) Latent period: events of excitation-contraction coupling ( no muscle tension seen)

2) Period of contraction: cross bridge formation ( Tension declines to zero)

3) Period of relaxation: Ca2+ reentry into SR ( Tension declines to zero)

muscle tone

Constant, slightly contracted state of all muscles, due to spinal reflexes,

Concentric contractions

Muscle shortens and does work Ex) biceps contract to pick up a book

Eccentric contractions

Muscle lengthens and generates force Ex) laying a book down causes biceps to lengthen while generating a force

ATP providing Energy for 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

  • storage of ATP is depleted in 4-6 seconds

ATP regeneration quickly by three mechanisms

Direct phosphorylation of ADP by creatine phosphate

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 an enzyme that carries out transfer of phosphate

• the muscle fiber have enough ATP and CP reserves to power the cell for about 15 seconds

Anaerobic pathway

glycolysis and lactic acid formation

• breaking down and using energy stored in glucose

• Glycolysis: doesn’t require oxygen, glucose is broken into 2 pyruvic acid molecules, 2 ATP are generated for each glucose broken down

• In the absence of oxygen , pyruvic acid is converted to lactic acid

• lactic acid diffuses into bloodstream, used as fuel by liver, kidney and heart

• converts back to pyruvic acid or glucose by liver

Anaerobic respiration vs Aerobic Repiration

Anaerobic respiration yields only 5% as much ATP as aerobic Respiration, but produces ATP 2.5 times faster

-Aerobic Respiration produces 95% of ATP during rest and ligth to moderate exercise ( requires oxygen) ( 32 ATP produced)

Anaerobic threshold

point at which muscle metabolism converts to anaerobic pathway

Aerobic endurance

Length of time muscle contracts using aerobic pathways

Muscle Fatigue

Inability to contract muscle despite continued stimulation

due to K+, Na+ and Ca+ disrupting membrane potential of muscle

Why would Muscle fatigue happen>

Decreased ATP and increased magnesium ( magnesium can interfere with Voltage sensitive T tubule proteins

Decreased glycogen

Ionic imbalances ( K+, Na + and Ca+ disrupting membrane potential of muscle cell

Increased inorganic phosphate from CP and ATP breakdown may interfere with calcium release from SR

For a muscle to return to its pre-exercise state EPOC has to happen, what happens during EPOC

Excess Post exercise Oxygen Consumption

Oxygen reserves are replenished

Lactic acid is reconverted to pyruvic acid

Glycogen stores are replaced

ATP and creatine phosphate reserves are resynthesizes

factors that increase the force of skeletal Muscle Contraction

• High frequency of stimulation

• Large number of muscle fibers recruited

• Large muscle fibers

• Muscle and sarcomere stretched to slightly over 100% of resting length

Increase force of skeletal muscle contraction leads to

More cross bridges attached

Optimal sarcomere operating length

80%-120% of resting length

Slow oxidative muscle fibers are good for

low- intensity, endurance actvities Ex) maintaing posture

Fast oxidative muscle fibers

Medium- intensity activities Ex) sprinting or walking

Fast glycolytic muscle fibers

Short-term intense or powerful movements Ex) hitting a baseball

Aerobic Endurance Excercise

Ex) joining, swimming, biking leads to increased muscle capillaries, number of mitochondria, myoglobin synthesis

Resistance Exercise

Ex) weight lifting, isometric exercises

leading to Muscle hypertrophy ( increase in fiber size)

Increased mitochondria, myofilaments, glycogen stores, and connective tissue

increased muscle strength and size

Smooth Muscle

Found in wall so hollow organs, Respiratory , digestive, urinary, reproductive, circulatory ( not in the heart though!)

Longitudinal layer

Fibers run parallel to long axis of organ

Contraction causes organ to shorten

Circular layer

fibers run around circumference of organ

Contraction causes lumen of organ to constrict

Differences between Smooth and Skeletal muscle Fibers

Smooth muscle fibers are spindle-shaped fibers ( thin and short

Skeletal muscle fibers are wider and much longer

Smooth muscle fibers are one nucleated and have no striations

Skeletal Muscle fibers are multinucleate and have striations

Skeletal Muscle fibers has multiple connective sheaths ( Epimysium, Perimysium, Endomysium)

Smooth Muscle fibers has only one connective tissue ( contains endomysium only)

Smooth muscle fibers contain varicosities ( bulbous swellings) of nerve fibers

Skeletal muscle fibers contain neuromuscular junctions

Skeletal muscle fibers are innervated by voluntary somatic nervous system

Smooth muscle fibers are innervated by involuntary autonomic nervous system

Skeletal muscle fibers have junctional folding vs Smooth muscle fibers have diffuse junctions

Smooth muscle has less elaborate SR, and no T tubules vs the opposite for skeletal muscle fibers

Sarcolemma contains pouch like infolding called caveolae ( contain numerous Ca2+ channels) skeletal muscle fibers rely on T-tubules for singnaling SR for source of Ca2+

Smooth muscle fibers are usually electrically connected via gap junctions whereas skeletal muscle fibers are electrically isolated

smooth muscle have myosin heads along entire length,

Smooth muscle has both thick and thin filaments

thick filaments are fewer in smooth muscle than skeletal muscle fibers

No troponin complex in smooth muscle but does contain tropomyosin smooth muscle ( calmodulin regulates calcium for smooth muscle, skeletal has troponin regulate calcium)

Thick and thin filaments arranged diagonally ( smooth muscle , contracts in a corkscrew manner)

Intermediate filament-dense body network

Dense bodies are the ones that anchor the filaments to sarcolemma on the smooth muscle while Skeletal muscle filaments are anchored by z discs

There is no presence of myofibrils

metabolism is mainly aerobic for smooth muscle and aerobic and anaerobic for skeletal

Contraction of Smooth Muscle

• Calcium is obtained mostly from extracellular space

• Calcium binds to calmodulin, not troponin

• Activated calmodulin then activates myosin kinase

• Activated myosin kinase phosphorylates myosin head activating ATPases

• Activated myosin forms cross bridges with actin of the thin filaments ( shortening begins

regulation of contraction in smooth muscle is done by

Neural regulation - neurotransmitter bind causes either graded ( local) potential or action potential

• Results in increases in Ca2+ concentration in sarcoplasm

• Response depends on neurotransmitter released and type of receptor molecules

Stopping smooth muscle contraction

Relaxation requires

Ca2+ detachment from calmodulin

-Active transport of Ca2 into SR and extracellularly

Dephosphorylation of myosin to inactive myosin

Smooth muscle contract without nerve signals due to

Reaction of hormones, oxygen levels or CO2 levels, they use G protein linked receptos to detect chemical changes

Unitary smooth muscle

• organized in large sheets, connected by gap junctions, uses pacemaker cells , less innerveration

• highly sensitive to stretch and chemical changes ( O2, hormones, ph)

• presence of pacemaker

• Rhythmic contractions

Multi-unitary smooth muscle fibers

• independent muscle fibers, not that many gap junctions

• richly supplied with autonomic nerve endings

• Responds to neural stimuli

Cardiac characterstics

only Endomysium connective sheath

Has myofibrils

has t-tubules

-presence of gap junctions in intercalated discs

no neuromuscular junctions

Has pacemakerExcitation or inhibition of nervous system stimulation

metabolism is Aerobic