11.2 MUSCULAR SYSTEM

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Exoskeleton

Exoskeleton: Internal skeleton surrounding and protecting most of the body surface of animals (insects)

Facilitate movement by:

  • Providing an anchorage for muscles
  • Acting as levers

In a lever there is an effort force, a pivot and a resultant force

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Antagonistic Pairs of Muscles

Antagonist Pair: When one muscle contracts the other muscle relaxes

Skeletal muscles occur in Antagonistic Pairs

Produces opposite movements in a joint such as in the elbow joint when triceps extend the forearm and the biseps flex the forearm

Image result for antagonistic muscles animation 

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Human Elbow

Joint: Point where bones meet

  • Joints allow bones to move in relation to each other this is known as articulation
  • Articulated joints have a similar structure: cartilage, synovial fluid and joint capsule

Cartilage: Tough smooth tissue that covers the regions of bone in the joint

  • Prevents contact between regions of bones that might rub - prevents friction
  • Absorbs the shock that might have caused bones to fracture

Synovial Fluid: Fills a cavity in the joint between the cartilages on the ends of bones

  • Lubricates joints to help prevent the friction that would have ocurred if the cartliages were dry and touching

Joint Capsule: Tough ligamentous covering to the joint.

  • It seals the joint and holds the synovial fluid and helps prevent dislocation

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Joint Types

Synovial Joint

  • Bone to bone joints
  • Self-contained capsule area that contains a lubericant (synovial fluid)
  • Ends of bones are coated with cartilage to cushion any bone to bone contact

Hinge Joint

  • Allows only two movements: Flexion (bending) & Extension (straightening)
  • Example: Knee

Pivot Joint

  • Allows rotation movement

Ball and Socket Joint

  • Greater range of movement: Flex, extend, rotate and move sideways and back
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Striated Muscle Structure

  • Muscles are attached to bones and move them
  • Composed of muscle fibres: Bundles of muscle cells
  • Muscle fibres are longer than typical cells
  • Multinucleated
  • Sacrolemma (single plasma membrane) surrounds each muscle fibre
  • Sacroplasmic Reticulium: Conveys singal to contract all parts of muscle fibres at once and stores calcium
  • Many Mitochondria between myofibrils to provide ATP for contractions

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Myofibrils

Muscle fibres contain many myofibrils. The myobibrils have alternating light and dark bands

Myofibrils are made of:

  • Repeating units of light and dark bands
  • Z line in the centre of each light area
  • Sacromere: Functional unit of the myofibril. Part of the myofibril between one Z line and the next

The pattern of light and dark bands in sacromere is due to the arrangement of:

  • Thin Actin Filaments
  • Thick Myosin Filaments.  

Myosin Filaments:

  • Occupy the centre of the sacromere
  • Each myosin fillament forms crossbridges with actin fillaments during muscle contraction
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Structure of a Sacromere

  • Myosin fibres: Thick with head-like structures
  • Actin fibres: Thin
  • Both are proteins
  • Myosin cannot become shorter because it is one continuous protein
  • Actin can slide towards the centre of the sacromere and therefore shortens
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Muscle Contraction Overview

  • Myosin filaments pull Actin filaments inwards towards the centre of the sacromere
  • Myosin heads bind to special sites on Actin creating cross-bridges
  • Each Sacromere shortens
  • As the Sacromere shortens the overall length of the muscle fibre shorterns

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Actin Filaments: Relaxed and Contracted Muscles

1. Motor neuron stimulates muscle

2. Calcium ions are released from the Sacroplasmic Reticulium into muscle fibre

3. Calcium binds to troponin and causes Actin to change shape

4. The changed shape of Actin causes tropomyosin to move exposing the binding sites on Actin

5. When the Myosin heads binds to Actin, the filament swievels to the centre of the sacromere forming cross-bridges

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Myosin Filaments: Relaxed and Contracted Muscles

1. Myosin heads form cross-bridges and then attach to binding sites on Actin

2. ATP binds to Myosin heads which cause them to break cross-bridges and detach from binding sites

3. ATP is hydrolised to ADP and Phosphate provides energy for Myosin heads to change angle and swievel outwards away from sacromere centre

4. Heads attach to the binding side of Actin that are further from the centre of the sacromere than the previous site (new cross-bridges)

5. ADP is released and the heads push the Actin filament inwards towards the centre of sacromere

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Stopping Muscle Contraction

Muscle contraction stops when the motor neuron stops sending signals to the muscle fibres

Calcium ions are pumped back into the Sacroplasmic Reticulum

The Actin binding sites are covered

Muscle fibres relaxes

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Explain Muscle Contraction

  • Muscle fibres contain repeating units called sacromeres
  • Sacromeres contain Actin and Myosin filaments
  • Actin filaments are thin and Myosin filaments are thick
  • An arriving action potential causes the release of Calcium from the Sacroplasmic Reticulium
  • Calcium binds to troponin causing troponin and tropomyosin to move exposing binding sites on Actin to which the Myosin heads bind forming cross-bridges
  • ATP binds to Myosin heads breaking the cross-bridges
  • ATP is the hydrolised into ADP and Phosphate
  • The energy from hydrolised ADP allows the Myosin heads to change shape
  • Myosin heads bind to exposed binding sites on Actin forming new cross-bridges
  • Myosin filaments move the Actin filaments to the centre of the sacromere
  • The sliding of the Actin filaments shortens the sacromere
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Contraction in Electron Micrographs

Relaxed Sacromere:

  • Z lines farther apart
  • Light bands are wider
  • Sacromere is longer
  • M line can be seen in the centre
  • More visible light bands around the M line

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Aequorin and Calcium Contraction

Ashley and Ridgway

Studied the role of Calcium in the relationship between nerve impulses and muscle contraction

Aequorin: Calcium binding bioluminescent protein

When Calccium binds to Aequorin it emits light. The timing of the light emission between the arrival of an electrical impulse and the contraction of a muscle fibre.

This action is consistent with the release of Calcium from the Sacroplasmic Reticulum

The light emissions are detected and recorded using specialised microscopes and cameras

Researchers have also used fluorescent dyes to visualise and measure the movement of Actin and Myosin

Aequorin and Fluorescent dyes only emit light for a few nano seconds therefore they are ideal to measure the rapid movements of muscle cells

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