Muscular System 5.2

Muscular System 5.2

​Myocyte (muscle Cells)

​​​Cells are multinucleated

• ​sarcolemma

  • ​The plasma membrane that covers the cell

• T-tubule

  • extension of the sarcolemma

    • the rapid transmission nerve impulse

• ​Sarcoplasmic Reticulum

  • ​Contains Ca⁺ ions

  • ​surrounds myofibrils

​Myofibril of Myocyte

• ​Myofilaments

  • ​Protein filaments

  • Form in repeating bands to thick and thin filaments

    • ​I band (light) #2

      • ​Z disc/line

    • ​A band (dark) #1

      • ​M line

      • H band

​Sarcomere

​

• ​a structural unit of a myofibril

  • ​thick filament

    • ​Myosin ATPase enzyme

  • ​Thin filament

    • ​actin filament (actin protein)

​Myosin and Actin Filament

• ​myosin

  • ​myosin heads

  • Form cross bridges

• ​Actin

  • ​Inactive-troponin is inside the tropomyosin (lack Ca⁺)

  • ​active- troponin is outside​ tropomyosin (ca⁺ is present)

​Cross-bridge Cycle (Sliding Filament theory )

1. ​The active site is exposed as Ca⁺ binds to troponin

2. ​myosin head attaches creating a cross-bridge

3. 3.​ Power Stroke the phosphate head bends releasing ADP and phosphate

4. ​ATP attaches to myosin head detaching the cross-bridge

5. ​Myosin returns to resting position

​Motor Neurons

• ​Specialized brain cell located in the spinal cord and brain

  • ​Upper - brain

  • ​Lower- spinal cord

• ​Transmit impulses

  • ​Efferent (exits)

  • ​Afferent (arrives)

Anatomy of a Motor Neuron

• ​Dendrites

  • ​receive signals

• ​Cell Body

  • ​Axon hillocks-convergent signals and fires impulse

• ​Axon

  • ​impulse travels to targeted tissues

• ​Axon terminal

  • neuromuscular ​junction site

​Neuromuscular Junction

• ​Synaptic cleft

  • ​the area between the axon terminal of the nerve cell and the sarcolemma

  • ​Filled with interstitial fluid

​Neurotransmitter of Motor neurons

• ​ACH

  • ​acetylcholine

  • ​Stored in the axon terminal

• ​attaches to the the protein receptors on the sarcolemma of the muscle cell

  • ​Cell opens Na⁺ Channels

​Sarcolemma Action potential

• ​Na⁺ channels open

  • ​cell membrane depolarizes

• ​T tubule

  • ​Opens Ca⁺ Channels open in sarcolemma ER

• ​Ca⁺ binds exposing troponin binding site for myosin heads

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​ALL OR NONE PRINCIPLE

​

​

​Motor Units

• Muscle fibers that controlled by a single motor neuron

  • Small motor units

    • ​precise movements

      • ​facial expressions

  • ​large motor units

    • ​gross movements

      • ​ lifting an object

​Types of Muscle Fibers

• ​Slow-twitch (slow-oxidative)

  • ​endurance and fatigue resistance

  • ​Aerobic

    • ​mitochondria producing ATP

• ​Fast-twitch (fast glycolytic)

  • ​quick response

    • ​anaerobic

      • ​glycolysis

​Muscle endurance and Fatigue

• ​Fatigue

  • ​loss of muscle contractibility

    • ​fuel depletion

    • ​Electrolyte loss

    • ​central fatigue

      • ​ammonia concentrations

• ​Endurance

  • ​Max oxygen uptake

    • ​rate of oxygen (VO_2max) consumption plateaus

  • ​increases with consistent aerobic excercise

​EPOC

• Excess ​post Oxygen consumption

  • ​oxygen debt

  • ​exercise depletion of ATP

​Muscular Strength

​dependent on multiple factors

​

• ​muscle size (more myosin cross-bridges)

• ​fascicle arrangement

  • ​pennate muscles

• ​size of motor units

• ​multiple motor unit summation

• ​temporal summation

  • ​summative wave frequency

• ​length-tension

• ​fatigue