Section 5: Muscles


Module 5

  1. Biological machines that utilize chemical energy from breakdown and metabolism of food to perform useful work
  2. 600+ muscles
  3. Perform 3 main functions
  4. Movement
  5. Heat production
  6. Body support, posture

Skeletal Muscle

  1. Whole muscles made up of bundles of fasciculi
  2. Each fascicle made up of groups cells or fibres
  3. Each muscle cell --> bundles myofibrils
  4. Each myofibril contains thick and thin myofilaments
  5. Thin <-- protein actin with troponin and tropomyosin
  6. Thick <-- myosin protein

--> interaction thin and thick = muscle contraction

--> fasciculi surrounded by white connective tissue called perimysium

Structure Skeletal Muscle

  1. Muscle cells surrounded by sarcolemma (muscle cell membrane)

--> action potential transmitted on

  1. Sarcolemma has small tube-like projections

--> called transverse tubules

--> extend down into cell

--> conduct action potential deep into cell where contractile proteins are located

  1. Within muscle cell are long cylindrical myofibrils that contain contractile proteins (thick and thin)
  2. Myofibrils surrounded by sarcoplasmic reticulum (SR)

--> mesh-like network of tubes containing calcium ions (Ca2+) (essential for contraction)

  1. Terminal cisternae (membranous enlargement of SR)

--> either end and continuous SR

--> its close to T-tubule (action potential travels)

Thin Myofilament

  1. Composed of predominately of globular protein --> actin
  2. Each actin has special binding site for myosin
  3. Actins strung together like beads on a necklace and then twisted to form a backbone of thin myofilaments
  4. Tropomyosin <-- long protein strands on thin myofilaments

--> muscle at rest, proteins cover binding sites of myosin

  1. Troponin <-- regulatory protein

A --> binds to actin

B --> binds to tropomyosin

C --> binds with Ca2+

  1. at rest troponin complex holds tropomyosin over myosin binding sites

Thick Myofilament

  1. Made of protein myosin
  2. Protein has long, bendable tail and two heads that can each attach to myosin binding sites on actin
  3. Heads have site that can bind and split ATP --> splitting of ATP that releases energy to myosin that powers contraction of the muscle
  4. Many myosin molecules arranged form one thick filament

Actin/Myosin Relationship

  1. Arranged in repeating pattern
  2. Each group thin myofilaments extends outward in opposite directions from central Z disc --> where they are anchored
  3. Groups thick myofilaments extend outward from central M line --> where they are attached
  4. Myofilament parallel to length of myofibril and muscle cell
  5. Sarcomere : region from one Z disc to another --> smallest functional contractile unit
  6. Under microscope gives striated appearance

A bands <-- thick filaments as dark bands

I bands <-- thin filaments with light bands

Muscle Contraction

  1. Interaction between actin and myosin leads to contraction
  2. Head myosin attaches binding site of actin --> forms cross-bridge, myosin undergoes change in shape
  3. Change in shape cause myosin head to swing --> producing power stroke
  4. Power stroke --> slides actin past myosin

*Thick/thin do NOT shorten during contraction*

Excitation Contraction Coupling & Muscle Contraction

  1. Process action potential excites muscle cell to produce muscle contraction
  2. Action potential at NMJ spread over sarcolemma and down T-tubules into core of muscle cell --> produce muscle contraction
  3. Action potential travels close to SR and opens Ca2+ channels --> releases Ca2+ from terminal cisternae of SR
  4. Ca2+ will bind to troponin C on thin myofilaments causing tropomyosin to uncover myosin binding sites on actin
  5. Myosin attaches to actin and power stroke occurs

Relaxation of Muscle

  1. Action potential stop, Ca2+ no longer diffuse out of SR
  2. Special Ca pumps, pumps Ca2+ back into SR (against concentration grad) --> requires ATP
  3. Tropomyosin covers myosin binding sites
  4. Myosin unable binds --> relax muscle no power strokes

Actin-Myosin and ATP cycle

  1. Splitting ATP to adenosine diphosphate (ADP) and inorganic phosphate releases energy to myosin and prepares myosin head for activity
  2. Formation cross-bridges occur when Ca2+:
  3. have been released from SR by Action Potential, binds Troponin C
  4. rolls tropomyosin off myosin binding sites
  5. Power stroke occurs when myosin heads bend and slides the thin myofilaments of actin over thick myofilaments of myosin
  6. ADP and P molecules released from head
  7. New molecule ATP binds to myosin heads

Two ways Muscles Alter Force Contraction

  1. Recruit Motor Units
  2. Summation Twitch Contractions

Motor Unit

  1. Is a motor neuron and all muscle cell/fibres causes contact
  2. One motor neuron contact several muscle cells --> ever muscle cell only one motor neuron

Large --> 200 cells

Small --> Few cells

Motor Unit Recruitment

--> Progressive activation of motor units resulting in more forceful contraction

Muscle Twitch

  1. Simplest and smallest contraction
  2. Result of an action potential in motor neuron
  3. Will cause excite cell and release Ca2+ from SR --> very small contraction
  4. Varies 10-100ms --> AP = 2ms
  5. Latent period due to all events at NMJ
  6. Can increase force contraction by increasing # action potential/second that travel down nerve (frequencies)
  7. High frequencies

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