muscle physiology

muscle structure

epimysium is a dense irregular connective tissue that surrounds the entire muscle.

• It is the outermost layer of connective tissue, encircling the entire muscle and providing structural integrity.

• It helps to separate the muscle from surrounding tissues and organs.

• The epimysium is continuous with the tendons, which attach the muscle to bones, allowing for the transmission of force generated by the muscle.

perimysium

• surrounds muscles fascicles in skeletal muscles

• contains blood vessels, nerves, collagen

muscles fascicle

• bundle of skeletal muscles surround by the perimysium

muscle fiber

• wrapped by endomysium

• made of myofibrils

  • contractile unit of skeletal muscles

blood capillaries

• line muscle fibers and bring blood supply to the muscles

• can increase this through exercise

t tubules and terminal cisterns make up a triad

• t tubules are projections of the sarcolemma which carry MAPs throughout the muscle fiber and terminal cisterns are part of the SR which releases Ca+2 to elicit muscle contraction

• carry muscle action potentials (MAPs)

• allow muscle action potentials to travel within the myofibrils in the muscle fibers (integration)

• triggers calcium release

  sarcoplasmic reticulum

• orange “netting” in the third image

  

muscle organization

myofibril

• sarcomere

  • includes thick and thin filaments

• z disc

  • boundaries of the sarcomere (2 ends)

• m line

  • middle of the sarcomere

• titin filament

  • structural filament

• a band

  • shows length of the thick filament

  • never changes

  • zones of overlap and h bands can change during contraction

  • h band is the area where there is only tick filament and shortens during a contraction( thin filament comes closer to the center)

• i band

  • also decreases during contraction

  • zone of only thin filament

    

z disc passes through the center of each i band

thick filament (myosin)

• contains 2 myosin heads

  • contain actin binding sites and ATP binding sites

  • bind to actin binding sites on the thin filaments which allows for the pulling and contracting of muscle fibers

  • contains an ATPase enzyme that breaks down ATP

• m lines attach the thick filaments

• many myosin proteins join together to make up a thick filament

thin filaments (actin)

• helix shape

• myosin bind sites covered by tropomyosin during rest (stops contraction)

• troponin

  • control whether tropomyosin covers binding sites or not

  • binding site for calcium to start an action potential (shortening of sarcomere)

sliding filament theory

• binding of myosin heads to the actin to shorten the sarcomere and bring the z discs closer together

• contracted until h and i bands are no longer present

• THICK (a band) AND THIN FILAMENTS NEVER CHANGE IN LENGTH

muscle contraction cycle

• binding between thick and thin is called a cross bridge

  • many cross bridges are formed

  • pull thin and thick filaments past each other (power stroke)

• release is caused by ATP binding to the myosin head

force length relationship

sarcomere length and force produced by the muscle during a contraction

• optimal length is the maximal force that can be produced

• less myosin binding heads available during non optimal lengths

neuromuscular junction (NMJ)

where the nervous system meets the muscle

• synaptic end bulb

  • end of the nerve

• motor end plate

• when the electrical impulse from a somatic motor neuron reaches the synaptic end bulb, it triggers the voltage gated calcium channels to open

  • synaptic vesicles move toward the synapse and through exocytosis releases acyltocholine

• movement of ions causes action potential

muscle in resting state vs contracting state

red arrows show action potential travelling down the sarcolemma

• calcium release channels and voltage gated channels change shape allowing calcium to be released from the sarcoplasmic articulum into the cytoplasm

calcium is the purple circles

• calcium then triggers troponin to cause an action potential

ATPase pump is constantly pumping calcium into the sarcoplasmic articulum from the cytosol/ sarcoplasm

motor unit (MU)

1 motor neuron and its muscle fibers it supplies (about 150)

1 motor neuron action potential = 1 MAP occurs all muscle fibers in that MU simultaneously

muscles involved in fine motor movements have less fibers per MU

how is force controlled and produced

1. the rate of incoming motor neuron action potentials to each MU

faster = more force

2. number of motor units recruited

weaker MU enlisted first and stronger ones involved if we need more force for a certain task

• example: milk carton taking more strength to lift than expected

wave summation

• summation occurs when the second action potential comes before the first finishes

  fused tetanus

• no relaxation between action potential

types of muscle fibers

• slow oxidative fibers

  • high myoglobin (binds oxygen)

  • dark red

  • capillarization

  • high mitochondrial size/ amount

  • favour oxidative metabolism (aerobic) to be more fatigue resistant

  • generate force more slowly

• fast oxidative-glycolytic fibers

  • biggest fiber type

  • high myoglobin

  • dark red

  • high capillarization

  • high glycogen

  • participate in both aerobic and anaerobic metabolism

  • (intermediate between slow and fast glycolytic)

• fast glycolytic fibers

  • low myoglobin

  • less capillarization and mitochondria

  • white

  • high glycogen

  • quick and strong movements

    

relationship between MU and fiber type

• all muscle fibers within a motor unit will be the same type

• with progressive MU for bigger tasks, MU are recruited in order of slow oxidative, fast oxidative-glycolytic, fast glycolytic

• maximal contractile activities require all fiber types

main types of muscle contractions

1. concentric (shortening)

2. eccentric (lengthening)

3. isometric (no movement)

types of muscle growth

1. hypertrophy

muscles increase in size

2. hyperplasia

more muscle fibers

main functions of muscles

1. movement

2. stability and posture

3. substance transport and storage

   sphincters, stomach, bladder, heart, blood vessels, skeletal muscles

4. thermogenesis

   generating heat via muscle contraction

main properties of muscles

1. Electrical excitability
• Autorhythmic electrical signals (i.e., digestive tract, heart) or chemical
stimuli (i.e., neurotransmitters, hormones) can stimulate a muscle
action potential which produces muscle contraction
2. Contractility
• Muscle shorten for movement to occur
3. Extensibility
• The ability for muscle tissue to stretch (i.e., stomach, heart)
4. Elasticity
• Bounce back to original shape and length

appearance of types of muscular tissue

skeletal is somatic

cardiac and smooth are autonomic

• cardiac muscles have intercalated discs joining neighbouring fibers