skeletal muscle tissue components

What is this?

• Epimysium

• outer layer encircling entire muscle

• Dense irregular connective tissues

What is this?

• perimysium

• Surrounds groups of 10 to 100+ muscle fibres

  • separates them into bundles (muscle fascicles)

• Dense irregular tissue

Endomysium

• penetrates interior of each muscle fascicle

• Separates muscle fibres from one another

• Mostly reticular fibres

What is a tendon?

• epimysium, perimysium and endomysium continuous w/ connective tissues

  • Extends beyond muscle fibres to form tendon attaching muscle to periosteum

Aponeurosis

• when connective tissues extends as a broad, flat sheet

What are somatic motor neurons

• Neurons that stimulate skeletal muscle to contract

• Axon extends from brain/spinal cord to group of skeletal muscle fibres

Diameter and length of mature skeletal muscle

• diameter: 10-100 micrometers

• Length 10cm or as long as 30cm (12 inch)

What is sacrolemma

• plasma membrane of muscle fibre

• Multiple nuclei of skeletal muscle fibre located beneath

• Has sarcoplasm

T tubules

• Thousands of tiny tube shaped invaginations of sarcolemma

  • form T tubules

• tunnel from surface to Center of fiber

• Filled with interstitial fluid

• Ensures an action potential excites all parts of muscle Fiber at once

Where does muscle action potential travel

• travel along sarcolemma and through T tubules

  • quickly spread throughout muscle fiber

Sacroplasm

• cytoplasm of muscle fibre

• Substantial amount of glycogen

• Contains myoglobin

Myoglobin

• red coloured protein

• Only found in muscle

• Binds oxygen molecules that diffuses into muscle from interstitial fluid

• Releases O2 when needed by mitochondria for ATP production

Myofibrils

• little threads in sacroplasm

• Contractile organelles of skeletal muscle

• ~2 micrometers in diameter

• Extend length of muscle fibre

• Make skeleton appear striated

Sarcoplasmic reticulum (SR)

• fluid-filled system of membranous sacs

• Encircles each myofibril

• releases calcium during muscle contraction

What is this

• Terminal cistern

• Dilated end sacs of the sacroplasmic reticulum

What is a triad

• T tubule and two terminal cisterns on either side

What does the sacroplasmic reticulum store and release

• stores calcium ions when muscle fibres relaxed

• Release Calcium ions from terminal cisterns

  —> triggers muscle contraction

Filaments/myofilaments

• smaller protein structures in myofibrils

• 2 thin filaments:1 thick

• Thin filaments

  • 8nm in diameter

  • 1-2 micrometers long

  • Composed of protein actin

• thick filaments

  • 16nm in diameter

  • 1-2 micrometers long

  • Composed of protein myosin

What is this?

• sacromeres

• Compartments/basic functional units of myofibril

Z Discs

• narrow plate shaped regions o dense protein

• Separate each sacromere

A band

• darker middle part of sacromere

• Extends entire length of thick filaments

• Toward each end of A Band is a zone of overlap of thick & thin filaments

I Band

• lighter, less dense area containing rest of thin filaments

What create striations of muscle

• Alternating I and A bands

H bands

• Center of each A band

M line

• supporting proteins holding thick filaments tgt at Center of H band

What are the 3 kinds of proteins in myofibril

• contractile proteins

  • generate force during contraction

  • Myosin + actin

• regulatory proteins

  • switch contraction processes on and off

  • Tropomyosin + troponin (part of thin filament)

• structural proteins

  • keep thick & thin filaments in proper alignment

Myosin

• main component of thick filaments

• Functions as motor protein in all 3 types of muscle tissue

• 300 molecules of myosin form one thick filament

• Each molecule shaped like 2 golf clips twisted together

• Myosin tails and heads

Motor proteins

• pull various cellular structures

• Converts chemical energy in ATP to mechanical energy

  • production of force

    
    

What are the 2 binding sites on myosin

• actin binding site

• ATP binding site

  • functions as ATPase—> hydrolyzes ATP→ energy for muscle contraction

Actin

• main component of thin filament

• Molecules form actin filament twisted into helix

• Each molecules has a myosin binding site where myosin head attaches

Tropomyosin

• strands of Tropomyosin cover myosin binding site of actin in relaxed muscle

Troponin

• hold Tropomyosin strands in place

• calcium ions bind to troponin

  → troponin changes shape

  → moves Tropomyosin away from myosin binding sites on actin

  
  

Structural proteins in muscle

• contribute to alignment, stability, elasticity and extensibility of myofibrils

• Key proteins: Titin, a-actinin, myomesin, Nebulin & dystrophin

Titin

• gigantic

• Third most plentiful skeletal muscle protein

• stabilizes position of thick filaments

• Centralizes A bands

• elastic

  • allows sacromere to return unharmed to resting length after it has been stretched/ muscle contracted

a-actin

• Z Discs contains a-actin

• Bind to actin molecules of thin filaments and to titan

Myomesin

Form M lime

• Holds thick filaments in alignment at M line

Nebulin

• long, nonelastic protein wrapped around entire length of each filament

• Anchors thin filaments to Z Discs

• Regulates length of thin filaments during development

Dystrophin

• Acts as molecular shock absorber

  • help transmit tension generated by sacromeres to tendons

• links thin filaments to integral membrane proteins of sacrolemma

• Reinforce sacrolemma

When muscle contracted, what happens to bands

• H and I narrow/shorten

  • disappear when muscle maximally contracted

• Thin filament ts slide inward and meet at Center of sacromere

briefly list the steps of the contraction cycle (4)

• ATP hydrolysis

• Attachment of myosin to actin

• Power stroke

• Detachment of myosin from actin

ATP hydrolysis

• myosin ATP binding site hydrolizes ATP to ADP

• Energy generated stored in myosin head

• Myosin energized

  → myosin head is perpendicular (proper position to bind to actin)

Attachment of myosin to actin

• Energized myosin attaches to myosin binding site on actin

  • referred to as cross bridge

  • Phosphate group released

  
  

Power stroke

• position of myosin head goes from 90 to 45 degrees

  • pulls thin filament toward Center of sacromere

    → generates force

• power stroke uses energy from step 1

• ADP released after power stroke

Detachment of myosin from actin

• at end of power stroke, cross bridge remains until myosin binds to another molecule of ATP

What is excitation contraction coupling

• events linking excitation (muscle action potential) to contraction (sliding of filaments)

• Occurs at triads of skeletal muscle fibre

What are the two groups of integral membrane proteins linking T tubules and terminal cisterns?

• voltage gated Ca²+ channels

  • located in T tubule membranes

  • Voltage sensors triggering opening of voltage gated Ca²+ release channels

  • Prevent Ca²+ from leaving SR

• Voltage gated Ca²+ release channels

  • in terminal internal membrane of SR

What happens when skeletal muscle fibre is excited? (Process of excitation-Contraction Coupling)

1) action potential travels along T tubule

2) Voltage gated Ca²+ channels detect change in voltage

—> go through change in shape

—> causes Ca²+ release channels to open

3) Large amounts of Ca²+ flow into sarcoplasm around thick and thin filaments

4) calcium binds with troponin and changes shape

—> pulls Tropomyosin to move away from myosin binding sites on actin

5) myosin binds to actin to form cross bridges

—> muscle fibre contracts

Ca²+ ATPase pumps

• in Terminal cisternal membrane

• Use ATP to transport Ca²+ from sacroplasm back to SR

• As long as release gates are open, more Ca leaves SR than brought back In by ATPase pumps

calsequestrin

• molecules of protein

• Bind to Ca²+ allowing more Ca²+ to be stored in SR

What happens to troponin and tropomyosin after muscle contraction

• Ca²+ released from troponin

• Tropomyosin covers myosin binding sites on actin

• Muscle fibre relaxes

Length-Tension relationship

• forcefulness of muscle contraction depends on length of sacromeres before contraction

  • optimum length is 2.0-2.4 micrometers→ allows max contact + tension

  • longer/shorter sacromeres than optimal →less Zone overlap→ fewer myosin heads make contact w/ thin filaments

• Max tension: zone of overlap b/w thick & thin filaments extends from edge of H zone to one end of thick filament

Neuromuscular junction (NMJ)

• where muscle action potentials arise

• Synapse between somatic motor Neuron and skeletal muscle Fiber

Synapse

• region where communication occurs between two neurons

Synaptic cleft

Separates two cells

• communicates with neurotransmitter

Part of NMJ

What is at the end of a motor neuron

• axon terminal

• Divides into cluster of synaptic end bulbs

  
  

Synaptic vesicles

• in cytosol in each synaptic end bulb

• Hundreds of synaptic vesicles

• Contain ACETYLCHOLINE (neurotransmitter)

Motor end plate

• muscular part of NMJ

• 30-40 million acetylcholine receptors

Briefly list the steps of nerve impulse eliciting muscle action potential

• release of acetylcholine

• Activation of ACh receptors

• Production of muscle action potential

• Termination of ACh activity

Release of acetylcholine

• nerve impulses at synaptic end bulb stimulate voltage gated channels on synaptic end plates to open

• Ca²+ more concentrated in extracellular fluid

  → travels in thru channels into synaptic end bulbs

• Ca²+ stimulates synaptic vesicles undergo exocytosis

  → ACh released into clef

  → travel from motor neuron to motor end plate

Activation of ACh receptors

• 2 ACh bind to receptor on motor end plate

  → opens ion channel in ACh receptor

• when channel open, cations (Na+) flow across membrane

Production of muscle action potential

• inflow of Na+ makes inside of muscle Fiber more positive

  → triggers muscle action potential

• 1 nerve impulse= 1 muscle action potential

• Muscle Action potential propagates along sarcolemma/T tubules

  • SR releases stored Ca²+

Termination of ACh activity

• effect of ACh binding is brief bc ACh broken down by AChE

• AChE located near motor end plate membrane

Briefly lost the 3 ways muscle fibres produce ATP

• Creatine phosphate

• Anaerobic glycolysis

• Aerobic respiration

Creatine phosphate

• excess ATP created when muscle relaxed

  • used to synthesize creatine phosphate

  • Creatine steals P from ATP, holds onto it t

• Amino acid Energy rich molecule in muscle fibres

• Creatine Synthesized in liver kidneys and pancreas then transported to muscle fibers

• When ATP needed, creatine kinase breaks down Creatine phosphate

  → P binds with ADP

  → Fast Formation of ATP

• Enough energy to contract maximally for 15s

What happens to creatine phosphate when contraction begins

• CK transfers phosphate group back to ADP to create ATP

• Formation of ATP from creatine phosphate is rapid

  → first source of ATP at beginning of muscle contraction

Anaerobic respiration

• creatine phosphate is depleted

  → glucose catabolized to generate ATP

• Glycosis (aerobic/anaerobic conditions) breaks down each glucose molecule into 2 ATP + pyruvic acids

• Pyruvic acid converted into lactic acid

  → yield 2 lactic acid + 2 ATP

  → liver cells convert lactic acid back to glucose

  
  

Aerobic respiration

• pyruvic acid enters mitochondria and undergoes aerobic respiration

• Yields more ATP than anaerobic but is slower

• Each glucose molecule yields 30 or 32 ATP

• supplies enough ATP during rest and light/moderate exercises

• Supplies nearly all ATP for exercises that are 1hour+

What are 2 sources of oxygen of muscular tissue

1) O2 diffusing into muscle fibers from blood

2) O2 released by myoglobin in muscle fibres

• binds to O2 when plentiful, releases when scarce

Motor unit

• somatic motor neuron + all muscle fibres it stimulates

• All muscle fibres of a neuron contract in unison

What does the total strength of contraction depend on?

• size of motor units and number activated at a given time

  
  

Twitch contraction

• brief contraction of all muscle fibres in a motor unit in response to a single action potential

• Lasts 20-200 msec

Myocgram

• record of muscle contraction

• Latent period:

  • muscle action potential travels along sacrolemma

  • Calcium ions released from sacroplasmic reticulum

• contraction period:

  • Ca²+ binds to troponin

  • Myosin binding sites on actin exposed

  • Cross bridges form

  • Tension at peak

• relaxation period:

  • Ca²+ actively transported back into SR

  • Myosin binding sites covered by Tropomyosin

  • Myosin heads detach from actin

Refractory period

• Period of lost excitability

  • if muscle fibre receives enough stimulation to contract→ loses excitability

Wave summation

• second muscle contraction after refractory period is stronger

• Stimuli arriving at diff times can make contractions stronger

Unfused tetanus vs fused

• Unfused: skeletal muscle Fiber can only partially relax between stimuli

• Fused: skeletal muscle stimulated at higher rate, can’t relax at all

• caused by additional Ca²+ while levels of Ca²+ are still elevated from first stimulus

Isotonic contraction

• tension in muscle remains almost constant while muscle changes its length

• for body movements and moving objects

• Eccentric and concentric

  
  

Concentric isotonic contraction

• great enough to overcome resistance of object to be moved

• Muscle shortens, pulls on another structure (tendon)

  • produce movement + reduce joint angle

Ex: picking up book from table → involves biceps brachii

Eccentric isotonic contraction

• length of muscle increases during contraction

• Ex: Lowering a book, walking downhill

Isometric contraction

• tension generated not enough to exceed resistance of object to be moved

• Muscle doesn’t change in length

• Ex: holding book with outstretched hand

• maintains posture

• Energy still used

Briefly types 3 types of skeletal muscle fibres

• slow oxidative fibres

• Fast oxidative-glycolysis fibres

• Fast glycolysis fibres

Slow Oxidative fibres

• Half of skeletal muscles

• appear dark red bc of many myoglobin & capillaries

• Many large mitochondria

  • ATP by aerobic respiration

• slow bc ATPase on myosin hydrolyze ATP slowly

  • take longer to reach peak tension

• resistant to fatigue

  • posture & running marathon

Fast oxidative-glycolytic fibres

• appear dark red bc of many myoglobin & capillaries

• Largest fibres

• Generate ATP by aerobic respiration

  • resistance to fatigue

• intracellular glycogen level high → generate ATP by anaerobic glycosis

• fast bc myosin heads hydrolize ATP fast

  • contract faster

Walking and sprinting

Fast glycolysis fibres

• low myoglobin content, mitochondria, & blood capillaries

  • white in colour

• large amount of glycogen

  • generate ATP mainly by glycolysis

• hydrolyze ATP rapidly → contact strongly and quickly

• intense movements of short duration

• strength training → inc size strength glycogen content

Why do skeletal muscles lose ability to undergo cell division

• hypertrophy (enlargement of cells) rather than hyperplasia (increase numbers in fibres)