Physiology 2130 Lecture Notes - Lecture 5: Active Transport, Rigor Mortis, Tetanic Contraction
29 May 2018
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Module 5 – Outcomes
Describe the structural components of skeletal muscle, including muscle fibers, myofibrils, myofilaments, sarcomere, and
the arrangement of the thin and thick myofilaments.
• Whole muscles are made up of bundles of fasciculi
o Which are surrounded by white connective tissue called perimysium
• Each fascicule is made up of groups of muscle cells or fibres
• Each muscle cell/fibre contains many bundles of myofibrils
• Each myofibril contains thin and thick myofilaments
o Thin myofilaments contain mostly the protein actin along with troponin
and tropomyosin
o Thick filaments contain the protein myosin
• Muscle cells/fibres are one of the few cells in the body with more than one nucleus
• Muscle cells are surrounded by sarcolemma – the muscle cell membrane
o The AP is transmitted over this
• The sarcolemma has small tube-like projections called transverse tubules (or T tubules)
that extend down the cell
o These T tubules conduct the AP deep into the cell where the contractile
proteins are located
• With the muscle cell are long cylindrical myofibrils that contain the contractile
proteins of the muscle
o The thin and thick filaments
• The myofibrils are surrounded by the sarcoplasmic reticulum (SR): a mesh like
network of tubes containing calcium ions (Ca+)
o Which are essential for contraction
• At either end of and continuous with the SR are the terminal cisternae: a
membranous enlargement of the SR which is close to the T tubule (where the AP
travels)
• The region from one Z disk to another is called a sarcomere
o This is the smallest functional contractile unit of the muscle cell
Describe the structure of the thin filament and its associated proteins.
• Made up of tropomyosin and actin
• Tropomyosin is made up of Troponin A, T and C
• Different troponin proteins interact to move tropomyosin on top of actin or to move tropomyosin back onto actin
• Each actin molecule contains a special bind site for the other contractile protein myosin
• Many actin molecules are strung together like beads on a string to form the backbone of the thin myofilaments
• Long strands of tropomyosin are found on the thin filaments
o These proteins cover the binding sites for myosin when the muscle is at rest
• The thin myofilament rests on top of the thick myofilament
Describe the structure of the thick filament.
• Made up of a myosin tail and globular myosin head
• The myosin heads can interact with the actin on the thin filament only in the presence of calcium
• This interaction causing contraction
Describe the sliding filament theory and the interaction of thin and thick filaments.
• The interaction between actin and myosin leads to muscle contraction
• Process
o When the head of the myosin molecule attaches to the binding site on actin and forms a cross bridge, the
myosin undergoes a change in shape
o This change in shape causes the myosin head to swing, producing the power stroke
o This power stroke propels or slides the actin filament past the myosin
• Note: the thick and thin filament do not change in size
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Document Summary
The region from one z disk to another is called a sarcomere: this is the smallest functional contractile unit of the muscle cell. Describe the structure of the thin filament and its associated proteins. Long strands of tropomyosin are found on the thin filaments: these proteins cover the binding sites for myosin when the muscle is at rest. The thin myofilament rests on top of the thick myofilament. Describe the structure of the thick filament: made up of a myosin tail and globular myosin head. The myosin heads can interact with the actin on the thin filament only in the presence of calcium. Describe the sliding filament theory and the interaction of thin and thick filaments. Define excitation-contraction coupling and describe the sequences involved: excitation-contraction coupling: the process by which an ap in the cell membrane excites the muscle cell to produce a muscle contraction.
